Robert Pěnička

Martin Saska
email: martin.saska@fel.cvut.cz

Christophe Reymann
email: christophe.reymann@laas.fr

Simon Lacroix
email: simon.lacroix@laas.fr

Reactive Dubins Traveling Salesman Problem for Replanning of Information Gathering by UAVs

We introduce a novel online replanning method for robotic information gathering by Unmanned Aerial Vehicles (UAVs) called Reactive Dubins Traveling Salesman Problem (RDTSP). The considered task is the following: a set of target locations are to be visited by the robot. From an initial information gathering plan, obtained as an offline solution of either the Dubins Traveling Salesman Problem (DTSP) or the Coverage Path Planning (CPP), the proposed RDTSP ensures robust information gathering in each given target location by replanning over possible missed target locations. Furthermore, a simple decision making is a part of the proposed RDTSP to determine which target locations are marked as missed and also to control the appropriate time instant at which the repair plan is inserted into the initial path. The proposed method for replanning is based on the Variable Neighborhood Search metaheuristic which ensures visiting of all possibly missed target locations by minimizing the length of the repair plan and by utilizing the preplanned offline solution of the particular information gathering task. The novel method is evaluated in a realistic outdoor robotic information gathering experiment with UAV for both the Dubins Traveling Salesman Problem and the Coverage Path Planning scenarios.

I. INTRODUCTION

This paper presents a new robust approach for solving the robotic information gathering by Unmanned Aerial Vehicles. In this task, a set of locations is to be visited by the robot to collect sensory data. The proposed method called Reactive Dubins Traveling Salesman Problem (RDTSP) is an online approach that uses the UAV's onboard computational resources for replanning and decision making to ensure that all given target locations are visited, regardless of possible disturbances during the information gathering task, that can lead to missing measurements from some of the target locations.

The first and typical application of the proposed method arises in the Coverage Path Planning (CPP) [?] scenarios where a predefined region is given, and the task is to find appropriate waypoints (sensor measurement locations) such that the whole area is scanned by the onboard sensor. For the aerial robotics, a simple zigzag path (further also called 'sweeping' path) is usually sufficient as there is no need for avoiding obstacles during the coverage mission.

The second considered scenario is the information gathering specified as the Dubins Traveling Salesman Problem (DTSP) [?], where its multi-robot case is shown in [?] for Dubins vehicle, which we use as a simple model of considered UAV for constructing smooth constant speed paths. Contrary the CPP, the DTSP uses a predefined set of target locations, and the task is to find a minimal length path over all target locations. Notice that after finding the appropriate waypoints in the area, the CPP is a special case of the TSP or in our case a special case of the DTSP.

Using UAVs for obtaining sensory data in such pre-defined locations in the environment is always influenced by uncertainties in robot motion, due to wind disturbances and imprecision of the dynamic model, among other factors. Another source of uncertainty is introduced into the system by imperfect sensors used for the particular application. All these aspects together may cause that some places of interest are not perceived, or the obtained information is not sufficient. Often it is possible to detect online (during the mission) that a required location was not visited properly by identification of a deviation from the pre-planned trajectory, or that the obtained sensory measuring is not valid. The current approach is to collect all the locations with identified missing sensory information after the mission and to plan an additional flight over these places to complement the data. The proposed RDTSP, however, identifies the missing target locations online and applies the repair plans during the mission.

The typical solution of the information gathering mission either for CPP or DTSP is done offline prior to the mission, and during the execution, a simple trajectory following is used. However, defining the path for sweeping scenarios is rather simple by using the zigzag path, the solution of NP-hard DTSP is computationally demanding. The optimal solution of the DTSP (for a given sampling of heading angles of Dubins vehicle) can be obtained using a transformation of the DTSP into an Asymmetric TSP (ATSP) [?] and then solved by e.g. Concorde TSP solver [?]. Therefore the further introduced RDTSP method uses the offline precalculated path for the information gathering and replans over the missed target locations such that the new repair part of the plan (generally a constrained solution of DTSP over missed locations) is inserted into the original plan.

The proposed Reactive Dubins Traveling Salesman Problem is an online algorithm that uses the predefined path from either CPP or DTSP and ensures the visit of all target locations by replanning over the missed target locations. Since the optimal solution of the DTSP is computational demanding, the proposed RDTSP method utilizes the precomputed plan and inserts the repair plan over the missed target locations into the existing plan between adjacent target locations. The existing approaches for the information gathering and surveillance by UAVs [?], [?], [?], [?] focus on the planning of the whole mission offline (as a solution of the DTSP) and do not consider possible replanning over missed target locations. The proposed VNS-base DTSP planner keeps planning during the mission and minimizes the overall path length not only by considering a different sequence of visiting the missed targets, but also by finding the location of applying the repair plan. Furthermore, a decision making approach is used to determine when the replanning is applied to revisit the missed locations and also to determine which target locations are considered as missed.

The remainder of this paper is organized as follows. The problem statement of the proposed RDTSP is specified in the next section. The overall system architecture is introduced in section III. Section IV describes the proposed method for RDTSP. First experimental results are presented in V and section VI concludes the paper.

II. PROBLEM STATEMENT

The proposed Reactive Dubins Traveling Salesman Problem is based on the idea of utilizing the offline computed initial plan for information gathering in the online iterative improvement of the repairing plan that covers the missed target locations. The RDTSP method uses an initial path P init which is a feasible Dubins path for information gathering over the required target locations. We expect the target locations S = {s 1 , • • • , s n } to be ordered according to the occurrence inside P init .

For the purpose of RDTSP for the targeted Unmanned Aerial Vehicle, we use the kinematic model of Dubins vehicle [?]. The state of the vehicle q = (p, θ) T = (x, y, θ) T is described by the position p = (x, y) ∈ R 2 and the heading angle θ ∈ S 1 . We expect a constant velocity v of the information gathering vehicle that is controlled by input u to either go straight or turn.

q = ṗ θ =   ẋ ẏ θ   = v   cos θ sin θ u ρ   , u ∈ [-1, 1] . ( 1
)
The Dubins vehicle (1) is specific for its minimal turning radius ρ which can be, for the considered UAV, derived from the equation of circular motion ρ = v 2 d /a max , where v d is desired constant speed and a max is the maximal acceleration allowed by the UAV. The distance L d (q σi , q σj ) between two states q σi and q σj of the Dubins vehicle is then the shortest Dubins maneuver out of the six possible maneuvers [?].

For the RDTSP we expect an online deployment where the replanning of the whole initial path after missing some locations is not possible due to the fact that the DTSP is NPhard, thus very computationally demanding for a large number of target locations. Instead, we propose the VNS-based method that uses the initial DTSP plan, and try to find a Dubins path over the missed locations such that the path can be inserted into the original plan between two adjacent locations.

After starting the execution of the initial plan, the current position inside the plan is described by an index number c ∈ (1, n) that belongs to the location s c which is the last location that has been attempted to visit. The set S m contains k missed target locations. A solution of the RDTSP can be described as a permutation Σ = (σ 1 , • • • , σ k+2 ) to visit the missed locations in S m , where σ 1 > c is location on the initial plan somewhere in the future, σ k+2 = σ 1 + 1 is the location where the repair plan connects back to the initial plan, and s σi ∈ S m for i ∈ (2, k + 1) being the missed target locations.

Furthermore the considered Dubins vehicles requires determining respective heading angles at the target locations from Σ. The heading angles can be described as a vector Θ = (θ σ1 , • • • , θ σ k+2 ), where θ σ1 and θ σ k+2 are set to the values from the initial plan P init and the heading angles at the missed target locations need to be found together with the permutation of the missed locations Σ.

In other words we want to find the starting location inside the future part of the plan, from which the repair plan would be started, together with the sequence of visits to the missed locations such that the produced path has minimal length. The addressed RDTSP can be described as an optimization problem to minimize the length L path of the Dubins tour over the missed target locations:

minimize Σ,Θ L path = k+2 i=2 L d (q σi-1 , q σi ) subject to σ 1 = s ∈ (c + 1, n) , σ k+2 = σ 1 + 1.
(2)

III. SYSTEM ARCHITECTURE The software architecture of the proposed system is built on top of the system developed at Czech Technical University and University of Pennsylvania for our participation at the MBZIRC competition in Abu Dhabi (see [?] for a description of the initial version of the CTU system primarily designed for formation flying [?], [?], [?] and swarm applications [?], [?], [?]; experiments with the current version of the MBZIRC system can be found at http://mrs.felk.cvut.cz/projects/mbzirc). The core of the system, based on Robot Operating System (ROS), is a Model Predictive Control (MPC) algorithm capable of following any given trajectory feasible for UAVs (the MPC controller was built from our solution designed for flying in GPS-denied environment, which is capable of UAV stabilization and trajectory tracking using only embedded micro-controller [?], see http://mrs.felk.cvut.cz/projects/cesnet for deployment of the system in task of scanning of large historical buildings). The system allows giving a trajectory specified by a sequence of waypoints sampled at a predefined frequency, which is post-processed and smoothed using known UAV motion constraints. For precise trajectory following, a state estimation mechanism is integrated fusing data from a variety of sensors. GPS data, IMU output, an output from image processing from two cameras, a range finder for altitude measurement, and in some applications also Differential GPS can be used in the feedback of the controller. Figure 2 illustrates the components of the system relevant to the scope of this paper. The mission consists of visiting a predefined list of targets (positions where the sensory data have to be captured), which are collected into a sweeping trajectory or for which a predefined DTSP solution can be computed in advance and fed as an input to the system. The situation assessment node processes the sensory outputs as well as the current estimated UAV state to track the status of the current target in the plan. When flying over a target, this node will assess if the measurements are good enough and upon failure of the sensor reading or error in position exceeding an allowed threshold, declares the target as missed.

This status is passed to the decision making node, which gathers the list of missed targets. Positions of the missed targets are used by the VNS-DTSP node (we use VNS -Variable Neighborhood Search based DTSP) solving the defined RDTSP problem to repair the plan by optimizing a DTSP tour over these targets. Each time the VNS-DTSP node finds a better solution, in the form of a local plan update including at least one previously missed node, a decision is made whether to use it or wait for a potentially better one. When the updated plan is deemed good enough for its execution, it is fed back to the situation assessment node which waits for the UAV to be in a state that allows seamless switching to the new plan, which is then sampled, post-processed and sent to the controller.

IV. PROPOSED APPROACH FOR THE RDTSP

Because the Dubins TSP problem is NP-hard, the exact optimization scheme requires, in general, a long computation time and is not suitable to find good solutions in online settings. In fact, no algorithm is capable of finding optimal solutions to the DTSP (without sampling the heading angles) for very large number of points at this moment, which is the case of our predefined overall mission plan. Moreover, in most of the information gathering applications, a simple sweeping or DTSP trajectory over the required places of interest is preferred, and there is no need to change the overall plan completely. Therefore we seek only to locally repair the given plan by re-visiting the missed sensory locations during the mission.

As new missed targets may come in at any moment, and due to the anytime nature of the VNS algorithm, we need to be able to decide when to stop the optimization and switch to the current best plan. We control the VNS-DTSP scheme from the decision making node by providing:

• the current plan, serving as a blueprint for plan updates • a list of missed targets S m , to be inserted in the current plan • current position c inside the current plan The decision to stop optimizing the current set of missed targets and to execute the newly adapted repair plan is then taken based on three criteria: the proximity to the node at which the current plan update starts (the plan is updated just before reaching the repair plan start), the time elapsed since the current best solution was found, and based on number of missed locations together with chance of missing currently approaching location.

A. Reactive Dubins Traveling Salesman Problem

The proposed solution of the Reactive Dubins Traveling Salesman Problem (RDTSP) is based on the Variable Neighborhood Search (VNS) metaheuristic for combinatorial optimization [?]. The used VNS method iteratively switches between shake and local search procedures, where the shake randomly changes the actual best solution of the problem and local search then tries to find on such randomly created solution a new and better solution than the currently best one. VNS employs a predefined neighborhood structures N 1,...,lmax further described as an operation on the problem solution which is the planned repair path. A Randomized variant of the VNS (RVNS) is used, which tries iteratively a number of same random operations during the local search procedure.

For finding the appropriate heading angles at the missed target locations, we propose a sampling based approach where the heading angles θ i ∈ 0, 2π) are equidistantly sampled into m values. The heading angles are then determined by a graph search over all heading samples for location sequence Σ and the ones with the minimal path length are used.

The proposed VNS-based solution of RDTSP uses following neighborhood structures. The randomized shake procedure uses:

• Path Move (l=1) randomly selects part of the repair plan between s σ2 and s σ k+1 , and moves it to a different position inside the plan. • Path Exchange (l=2) works similar to the path move, but selects two random non overlapping parts of the plan between s σ2 , s σ k+1 and exchange their position. • Start Exchange (l=3) is a special neighborhood structure that randomly changes the location s σ1 where the repairing path starts which also changes the end of repairing plan to s σ k+2 = s σ1 + 1. To limit the computational demand and also to focus on prior start of the repairing path, the start exchange is limited by the maximal distance d max which is a number of locations ahead of the current one s c to which the s σ1 is randomly changed.

During one iteration of the local search procedure, the Randomized VNS tries one of the following neighborhood structures for a number of time that is equal to k 2 .

• Point Move (l=1,3) randomly moves one location inside the solution into different position inside the plan. • Point Exchange (l=2) selects randomly two target locations and switch their positions.

The VNS-based algorithm for the proposed RDTSP is summarized in Alg. 1. The planning algorithm is started every time S m contains at least one missed location and the local optimization keeps trying to improve the repair path until it is applied by the decision making node which clears the set of missed target locations S m .

The algorithm is written for the online execution during the mission, where both the set of missed locations S m and the current index of location c are changing. During the deployment, the current index c increases which requires to change the considered staring locations s σ1 and ending locations s σ k+2 of the repair path to be s σ1 > c and s σ k+2 = s σ1 + 1 which is done by adjustStart on line 3. Whenever the c increases, the algorithm changes automatically the starting position to the one inside d max that produces the shortest repair path. Furthermore, the introduction of new missed locations into S m requires addition of the location to the existing repair plan (addToPath on line 5) which is done greedily. By this manner, the proposed VSN-base algorithm for RDTSP benefits from reusing the existing repair plan during the deployment and tries to optimize it to minimize the additional path length required to revisit the missed target locations.

V. EXPERIMENTAL RESULTS

The proposed RDTSP method has been tested in a realistic outdoor experiment with the UAV depicted in Fig. 3. The Dubins vehicle model is used for the hexarotor UAV to produce smooth paths over the target locations using constant speed trajectories that are preferred due to the precision of sensory measurements in the information gathering scenario. For both scenarios1 , the same configuration of the proposed RDTSP has been used. The maximal distance in which the VNS-based method considers the disconnection from the original plan and applying the repair path has been set to d max = 20. The nominal forward speed of UAV during the experiment has been set to constant speed v c = 2.5 m s . The RDTSP further uses the number of sampled heading angles m = 16 and turning radius of the Dubins vehicle ρ = 4m. The turning radius has been selected with respect to the maximal acceleration a max = 3 m s 2 of the used hexarotor UAV. The decision making node in the conducted RDTSP experiments has been used for determining whether the specified target locations were visited and also for determining when the calculated repair plan is applied. The comparison of the target locations position and UAV odometry was used to assess the missing of a target location with decision tolerance distance of 1m. Furthermore, to test the real replanning ability of the proposed method, the decision making used an artificially introduced probability p miss = 0.2 of missing the target location (which simulates an error in the sensory data measurements). Furthermore, the decision of applying the repair path (also done by the decision making node) has been set to a simple variant where the new plan is applied every time the UAV reaches a starting position of the repair path with two or more missing target locations.

A. RDTSP in coverage scenario

The conducted experiment of RDTSP in the Coverage Path Planning scenario uses a simple zigzag plan over the specified area with equidistantly sampled target locations (see Figure 4). During the experiment, a total number of six target locations were missed, either due to the real displacement of the UAV position from the original plan or due to the artificially introduced random missing of the target. The initial three targets, as shown in Fig. 5, were revisited by the first repair path and the other three by the second replanning, always by connecting the repair plan between two adjacent target locations on the original coverage plan.

B. RDTSP in DTSP scenario

In the RDTSP scenario with the initial path as the Dubins Traveling Salesman Problem, a total number of 20 target locations were specified. The initial plan, an open-loop version of the DTSP, was found by conversion of the DTSP into the ATSP and then optimally solved using the Concorde solver. The results of the experiment are depicted in Figure 6 and Figure 7, where the two missed target locations at the beginning of the mission are revisited by one repair path that is significantly shorter compared to the case of visiting the missed locations after the mission. The introduced online method minimizes the overall length by connecting the repair plan into the initially computed offline plan. Especially Figure 6, with the DTSP scenario, shows the benefits of the online replanning with lookahead distance, where the final path length is minimized by selecting the appropriate target location of connection the repair plan.

VI. CONCLUSIONS

In this paper, we introduced a novel method for information gathering by UAVs called Reactive Dubins Traveling Salesman Problem. The method uses a precomputed information gathering path from either the Coverage Path Planning for scanning of a given area or from the Dubins Traveling Salesman Problem where the length of the path over all given target locations is minimized. The proposed VNS-based online algorithm for the RDTSP uses the predefined path during the mission execution and ensures the robustness of information gathering task by replanning the initial plan for visiting possible missed target locations due to disturbances in sensory measurements or localization. The proposed method was verified in realistic outdoor information gathering experiments with hexarotor UAV and shows the robustness of the proposed online replanning approach in both CPP and DTSP scenarios. For the future work, we intend to investigate different strategies and improvements for the decision making, where the application of the repair plan is based on the real sensor measurements.

ACKNOWLEDGMENT

The presented work has been supported by the Czech Science Foundation (GA ČR) under research project No. 17-16900Y. The support of the Grant Agency of the Czech Technical University in Prague under grant No. SGS17/187/OHK3/3T/13 to Robert Pěnička is also gratefully acknowledged. The support of the Czech Ministry of Education, Youth and Sports and of the French Ministries of Foreign Affairs for the Czech-France MOBILITY project 7AMB16FR017 is greatly appreciated.

Fig. 1 :

 1 Fig. 1: Example of information gathering as a solution of Dubins Traveling Salesman Problem for multiple UAVs (MDTSP) on left, and the data collection as a camera detection of colored objects with number recognition (on right)

Fig. 2 :

 2 Fig. 2: Software architecture. Arrows represent messages, blocks ROS components. The dashed line symbolizes communication with lower level components.

Fig. 3 :

 3 Fig. 3: The hexarotor UAV used during the experimental verification of the proposed RDTSP in scenario with initial plan obtained as the DTSP (for more details see http://mrs.felk.cvut.cz/ecmr17rdtsp)

Fig. 4 :

 4 Fig.4: Initial coverage zigzag path plan between starting and ending positions with equidistantly sampled target locations. The positions of the UAV ('Traveled path by UAV') shows the odometry measurements of the UAV that includes the two applied repair plans to revisit the target locations missed during the mission.

Fig. 5 :

 5 Fig.5: Two repair plans applied by the RDTSP method during the execution of the coverage scenario.

Fig. 6 :Fig. 7 :

 67 Fig. 6: The initial Dubins Traveling Salesman Problem plan over specified target locations together with the odometry measurements of the UAV including the additional repair path over missed target locations.

  Algorithm 1: VNS based method for the RDTSP Input : Pinit -initial plan Input : dmax -start lookahead distance Input : lmax -maximal number of neighborhoods Input : m -Number of heading angles samples Changing Input: Sm -Set of missed locations Changing Input: c -actually passed location index in Pinit

		Output	: P -Actual RDTSP repair plan
	1	P ← createInitialPath(Sm) ;	// greedily
	2	while Repair plan is not applied do
	3	if c changes then
	4	adjustStart(P ,c) ;	// ensures sσ 1 > c
	5	if new missed target si ∈ Sm then
	6	addToPath(P ,si) ;	// greedily
	7	l ← 1
	8	while l ≤ lmax do
	9	P ← shake(P , l)
	10	P ← localSearch(P , l)
	11	if L path (P ) < L path (P ) then
	12		P ← P
	13		l ← 1
	14	else
	15		l ← l + 1

We refer to http://mrs.felk.cvut.cz/ecmr17rdtsp for more information and video from the outdoor experiment that further visualizes the real time performance of RDTSP method.


Ilyas Dchar

Cyril Buttay
email: cyril.buttay@insa-lyon.fr

SiC power devices packaging with a short-circuit failure mode capability

The failure mode of press-pack-type packages dedicated to SiC devices is experimentally analyzed in order to investigate their use for HVDC applications. Single SiC Schottky diode samples have been submitted to short-circuit conditions and continuous current flow test. The samples have been then characterized with optical and scanning electronic microscopy. Results from the experiments reveal that the press-pack structure offers a short-circuit failure mode with SiC devices, as it does for Si devices. The metallurgy involved is, however, quite different. Cu, Ni, Ag or Al foils are found to be suitable interface material between the package and the die to achieve a stable a short-circuit failure mode, providing the die is properly attached to a substrate.

Introduction

Generally, the failure mode of a power electronic component can be classified as open-circuit failure or short-circuit failure. Components which fail to an open circuit are considered unsuitable for applications requiring series connection. Especially, in some highvoltage direct current (HVDC) applications, the modules must be designed such that when a failure occurs, the failed module keeps carrying the load current by the formation of a stable short circuit [START_REF] Gunturi | On the operation of a press pack IGBT module under short circuit conditions[END_REF], while the remaining modules share the blocking voltage. Such operation is referred to as short circuit failure mode (SCFM). One of the existing packaging technologies which has the ability to achieve SCFM is called press-pack. This technology was first developed for thyristors and GTOs, and, later, for IGCTs and IGBTs [START_REF] Sharifabadi | Design, Control, and Application of (a) (b) (c) (d) Conductive filament SiC MMC for HVDC Transmissions Systems[END_REF]. Press-pack modules are very different compared to the more conventional wire-bond modules. In press-pack, the die is in direct contact with metal parts which serve both as heat sinks and electrical terminals. The semiconductor is consequently cooled from both sides, which is a considerable advantage because the cooling area is doubled. Additionally, the housing is hermetic, which means that pollutants cannot enter and cause subsequent damage [START_REF] Sharifabadi | Design, Control, and Application of (a) (b) (c) (d) Conductive filament SiC MMC for HVDC Transmissions Systems[END_REF].

Moreover, this package type does not explode in the majority of failure cases [START_REF] Wakeman | Electromechanical evaluation of a bondless pressure contact IGBT[END_REF]. Press-pack packages offer high reliability because they have no soldering layers that can delaminate and no bond-wires that can lift-off or melt in case of very high surge currents. Thanks to the massive metal contacts, the faulty semiconductor enters into a safe SCFM: metals like silver (Ag) and aluminum (Al) are used in contact with the die to form low melting point eutectic alloys with silicon [START_REF] Lang | Short-circuit resistant IGBT module[END_REF]. The alloy thus formed has to carry the electrical current instead of the failed device. In HVDC application, this state has to be maintained until the next scheduled maintenance, which may be performed yearly [START_REF] Sharifabadi | Design, Control, and Application of (a) (b) (c) (d) Conductive filament SiC MMC for HVDC Transmissions Systems[END_REF]. Currently, the available press-pack components use silicon semiconductors. SiC power devices are very attractive from an electrical point of view and will probably replace silicon for high voltage applications in the future. SiC, however, require a complete redefinition of the SCFM mechanism, as it has very different physical properties compared to silicon. The challenge of this work is to investigate experimentally if it is possible to achieve SCFM capability with SiC semiconductors by using press-pack technology.

Experimental procedure

Sample preparation

To evaluate the behavior of SiC press-pack modules under extreme operating conditions, a simplified package has been implemented. A clamping fixture (Fig. 1) is used to perform destructive tests on bare SiC dies (here some 1200V SiC Schottky diodes, part ref. CPW4-1200S020B from Wolfspeed). The characteristics of the SiC dies are shown in Table 1. They have ~4 µm Al metallization on the top anode pad and ~1.4 µm Ni/Ag metallization on the cathode side.

Thickness

(mm) CPW41200S020B 1200 20 3.08x3.08 0.377 Table 1. Characteristics of the SiC dies used in this article.

Figure 2 shows a simplified drawing of the two tested structures. In the first one (Fig. 2a), a die is placed in a shallow pocket machined in the cathode contact part (Fig. 3a). A thin (200 µm) metal foil is then placed on top of the SiC die. This foil is 2.5 x 2.5 mm², and is made of Al, Ag, Cu or Ni (Goodfellows, 99.9% purity). These materials were selected because they may react with SiC when failure occurs [START_REF] Park | Kinetic control of silicon carbide/metal reactions[END_REF], [START_REF] Backhaus-Ricoult | Interfaces between SiC and Metals[END_REF]. Once the die and the foil are in contact, the container (Fig. 3b) is filled with some dielectric fluid (MIDEL 7131) to prevent arcing. Finally, the anode contact is applied, and pressed by tightening the force adjustment screw (Fig. 3c). As shown in Fig. 2a, the die and the metallization are pressed between two molybdenum pieces. This material was chosen because of its high melting point (2623 °C). In the second structure (Fig. 2b), the SiC die is attached to a DBC substrate using the Ag joint (Fig. 4). Then, we have used different metal foils for the top side of the dies.

Description of the bench for destructive tests

Short circuit tests are carried out using the test bench presented in Fig. 5 and6. A High voltage power supply (3 kVDC) charges a film capacitor C (1250 µF, 3000 V). The "auxiliary" IGBT is used in series with the diode to deliver the current pulse to the sample [START_REF] Dchar | Avalanche Robustness of SiC Schottky diode[END_REF]. In order to estimate the energy leading to device's failure, the capacitor voltage has been increased from a low value where the device is able to sustain the short circuit conditions up to the failure. The energy leading to the device failure is determined by measuring the current through and the voltage across the device using a Pearson current monitor model 4997 and a high voltage probe (PHV 662-L) from PMK.

Continuous current flow test

After destructive testing, samples showing fail-to-shortcircuit behavior are subjected to a continuous current flow (10 A) for 2 hours. The aim of this test is to evaluate the quality and stability of the SCFM. The electrical resistance between anode and cathode contacts is measured over time. If the resistance of the faulty device does not change over time (or if it decreases), then the short circuit is considered as stable. On the contrary, if the resistance increases significantly (more than 10% compared to the value immediately after failure) over time, then the short-circuit failure mode is considered as unstable. The measurement of the resistance (RCC) is carried out by 4-point method (Fig. 7). Fig. 7. Picture of the current passage test using 4-point probes method.

Failure analysis

After the continuous current flow test, the samples are removed from the clamping system. A first observation is made with an optical microscope (Zeiss Axio-Scope 1).

When the metal foil separates naturally from the chip, the contact surfaces are observed. When the metal foil remains attached to the SiC die, a microsection is performed. In both cases, a scanning electron microscope (SEM) with elemental analysis capability (EDS) is used for further analysis (Tescan Vega 3 with Bruker Quantax EDS probe).

Experimental Results

Press-Pack samples

Fig. 8 shows an example of the current and voltage recorded across one of the samples during the shortcircuit operation. The tested sample are based on the structure in Fig. 2a (the SiC die was in contact with a silver foil). Since diodes have a good surge current capability, large current values are required to trigger failure (here 1100 A for a 20 A-rated device). For the example below, the dissipated energy leading to the failure of the sample is estimated at 6.9 J (corresponding to an energy density of 76 J / cm 2 ).

Fig. 8. Voltage and currents waveforms of sample (silver) recorded during the short circuit test @ Tc=25°C.

After this destructive test, the sample (still mounted in the clamping fixture) was biased with a 10 A current (continuous current flow test, as described in section 2.3). As shown in Fig. 9, the short circuit failure mode is found to be stable with a constant RCC value (139 mΩ). Table 2 summarizes the experimental results for the different samples. 5 different configurations are tested: 4 different foil compositions (Cu, Al, Ag and Ni). In addition, one test is performed without foil, with a direct contact between the die and the molybdenum parts. Some tests are duplicated to assess their reproducibility. 2. Summary of the results of the short-circuit testing from all the tested samples using structure of Fig. 2a. ESC is the dissipated energy leading to failure, Ri refers to the initial electrical resistance after SC test, and Rf refers to the final electrical resistance measured after current flow test. t is the duration of current flow test.

# Metal foil

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Press-Pack samples mounted on DBC substrate

In this section, the dies are attached to a DBC substrate (using silver sintering) instead of being clamped between the molybdenum contacts. Fig. 2b shows the tested stack. Table 3 shows the results. Except for Mo (i.e. no metal foil), the electrical resistance of all the test samples after the current passage test are very close to each other and are in the range of 72 mΩ to 167 mΩ. The SCFM is found to be stable. 3. Summary of the results of the short-circuit tests from all the tested samples using structure of Fig. 2b. The samples were then microscopically observed to analyze the location and extend of the failure. As an example, Figure 11 shows some images of sample #11. As it can be seen, a clear defect is located in the active area of the diode. A corresponding mark is also visible on the copper foil (which did not adhere to the die, as it is sometimes the case). SEM observations (Fig. 12) show that a crater formed in the SiC die. Only a few, thin conductive paths were found on this crater. There is no evidence that the metal from the foil reacted with SiC of flowed to create a strong conductive path.

# Metal foil

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Discussion

As shown in Table 2 and3, the samples tested without any metal foil, and thus in a direct contact with the molybdenum blocks, do not offer a stable SCFM. The contact resistance, initially high, increased further after a few minutes of the current flow test. All the samples tested with a metal foil offer a lower initial resistance. Some of them (samples #3, #5, #7 and #9), however, do not achieve stable SCFM.

The "clamped" samples (Table 2) are found to offer unsatisfying reproducibility, as their SCFM is sometimes stable, sometimes unstable. This is not observed with the DBC-mounted dies which are always found to offer a stable SCFM (albeit on a slightly smaller sample set). The difference in reproducibility between clamped and DBC-mounted samples may be due to the condition of the die after the short-circuit test: Most of the clamped dies were found to be shattered, with many fragments missing when dismounted from the molybdenum contacts (Fig. 13). For the DBC-mounted samples, on the contrary, all fragments were held in place thanks to the silver die attach (Fig. 11a). Another explanation is that for the DBCmounted samples, low-melting point metal was available from both sides of the die (metal foil on the top, silver die attach on the bottom) to help forming a conductive path.

Moreover, it should be noted that, contrary to the results presented in the literature for Si press-pack devices, we didn't observe a metallurgical reaction between the SiC and the metal foils. The conductivity after failure appears to be related to the presence of thin metal filaments through the failed die. Fig. 14 gives an example of destructive test on a sample (using silver foil) which achieved a stable short-circuit. Fig. 14 From the experiments of Table 2 and3, it was learned that the failure is always localized in a small area of the die, resulting in very high temperature, sufficient to create a crater in the SiC die. This crater seems to be the origin of the physical short circuit between the anode-cathode observed during the device failure (Fig. 15). In order to get a stable short-circuit, the crater walls have to be covered with with some conducting material. The use of sintered Ag joint for the SiC die attachment in combination with Ag, Ni, Cu and Al foils for the insert material has found to be sufficient to produce stable SCFM.

Conclusion

This paper investigates the possibility of using SiC semiconductors in press-pack package. Samples based on single diodes were tested to evaluate their SCFM capability. The tests have been followed by microscopic observations for failure analysis. The results obtained revealed that press-pack structure was able to offer a SCFM with a low initial electrical resistance (around a hundred milliohms). The challenge was to evaluate the stability of the short circuit after the failure of the chip. For that, we have performed aging tests by injecting a current into the faulty chips to observe the change in resistance over time. These tests showed that the Ag joint for the die attachment and Cu, Ni, Ag and Al foils for the top side insert material give a stable short-circuit.

Fig. 1 .

 1 Fig. 1. Picture of single chip press-pack clamp.

Fig. 2 .

 2 Tested samples. (a) The die and the metal foil are pressed between two contacts; (b) Sample constructed with DBC substrate.

Fig. 3 .

 3 Fig. 3. Assembly of the structure of Fig.2a. (a) The die is placed in a pocket of the bottom contact. A metal foil (Ag, Cu, Ni or Al) is placed on the top of the die; (b) the container is filled with Midel 7131 insulating oil; (c) the top contact is lowered and the pressure is adjusted using the force adjustment screw.

Fig. 4 .

 4 Fig. 4. Assembly of the structure of Fig.2b. (a) The die is attached to a DBC substrate using silver sintering; (b) The DBC is then placed in the clamping fixture (again, with a metal foil on the top of the die). Because of the size of the DBC, the container is not used.

Fig. 5 .

 5 Fig. 5. Circuit diagram of the short-circuit test system.

Fig. 9 .

 9 Fig. 9. Contact resistance of Ag based test sample.

Figure 10

 10 Figure 10 shows an image of the sample after short-circuit destructive test and current flow test. After tests, the silver metallization remains attached to the SiC die. As shown

  Fig. 11. Picture of sample #11 after short-circuit destructive test @8.6 J and current flow test (10 A). (a) SiC die; (b) Copper foil.

Fig. 12 .

 12 Fig. 12. SEM image of the crater formed in the SiC die.

Fig. 13 .

 13 Fig. 13. Test resulting in the shattering of the SiC die. During the continuous current flow test through the sample (#7), the area of contact was further reduced. This resulted in heating of the sample and unstable short circuit (Rf=524 mΩ).

  (a) shows the crater which was formed during the test. The EDX element map is shown in Fig.14 (b). In Fig.14 (c), the dark phase represents Si while the bright phase refers to Ag and Al. The material composition at point A is Si 11.8 at%, Ag 60.8 at%, Al 7.8 at% And C 4 at%. The composition of the materials at point B is Si 73 to%, Ag 0.85 at%, C 22 to% and Al 3.78 at%. From Fig.14 (d), it can be clearly seen that the walls of the crater are covered with silver and aluminum (bright phase). These filaments are probably responsible for the low contact resistance of the sample after failure.

Fig. 14 .

 14 Fig. 14. SEM images of sample (using silver foil) which achieved a stable short-circuit. (a) Image of the crater formed during the destructive test, (b) EDX element map (c) Material composition of points, (d) The conductive channel.

Fig. 15 .

 15 Fig. 15. Picture showing the location of the contact responsible for current conduction (10 A) after failure. Despite the low contact resistance (~ 100 mΩ), the small size of the contact area (~0.1 mm²) results in a very high power density, hence the light emission.

Acknowledgement

This work was supported by a grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program (ANE-ITE-002-01)


Wissam Sabbah

Faical Arabi

Oriol Avino-Salvado

Cyril Buttay

Loïc Théolier

Hervé Morel

Lifetime of Power Electronics Interconnections in Accelerated Test Conditions: High Temperature Storage and Thermal Cycling

We investigate the effect of three testing conditions (thermal shock, Rapid Temperature Changes -RTC -and high temperature storage) on the interconnects of a power electronic module. In particular, the mechanical strength of thick aluminium wirebonds is investigated and shows that while it is not affected by storage at 230 °C, it is much more sensitive to thermal cycling. Shock tests are found to be especially severe, despite having a smaller temperature swing than RTC. Regarding the die attach, no noticeable reduction in mechanical strength is found, regardless of the ageing conditions, and despite clear micro-structural evolutions.

Introduction

In many applications, such as aerospace or automotive, power electronics systems are expected to operate over a wide ambient temperature range (more than 200 °C temperature swings, [START_REF] Buttay | State of the art of high temperature power electronics[END_REF]). This is especially true for silicon-carbide (SiC) devices, as their maximum junction temperature is usually higher than that of their silicon counterparts.

Deep thermal cycling causes a high level of thermomechanical stresses in the packaging of these systems. Also, the high temperature encountered during the thermal cycle accelerates mechanisms such as the diffusion of chemical species at the interfaces, which in turn may cause weaknesses in the packaging.

As the reliability of power electronic modules was found to be unsatisfactory [START_REF] Johnson | A Joint IeMRC-EEPKTN Power-electronics Roadmap[END_REF][START_REF] Yang | An Industry-Based Survey of Reliability in Power Electronic Converters[END_REF], and as the thermal stresses are only expected to increase in the foreseeable future, there is a need to improve the current packaging technologies.

For example, regarding the die attach, many solutions are available, from Au or Zn-based solder to glasses or silver sintering [START_REF] Manikam | Die Attach Materials for High Temperature Applications: A Review, Components, Packaging and Manufacturing Technology[END_REF][START_REF] Khazaka | Survey of High-Temperature Reliability of Power Electronics Packaging Components, Power Electronics[END_REF]. Silver sintering is a promising technology, but it uses a very different process compared to soldering. Among solders, AuSn is an attractive lead-free replacement for High Melting Point (HMP) solders (which contain 95 % lead). The melting point of AuSn is limited (281 °C), but it is easier to implement than more demanding alloys such as AuGe or AuSi [START_REF] Manikam | Die Attach Materials for High Temperature Applications: A Review, Components, Packaging and Manufacturing Technology[END_REF].

Regarding the ceramic substrates, Si 3 N 4 has been identified as the most attractive material for reliable substrates [START_REF] Khazaka | Survey of High-Temperature Reliability of Power Electronics Packaging Components, Power Electronics[END_REF][START_REF] Pietranico | Characterisation of power modules ceramic substrates for reliability aspects[END_REF]. Indeed, its flexural strength (700 MPa) is almost double of that of Al 2 O 3 (450 MPa) or AlN (350 MPa) [START_REF]Bending strength of ceramics for curamik substrates[END_REF]. On the contrary, AlN is attractive for its high thermal conductivity.

Another weak point is the die topside interconnection, usually based on aluminium wirebonds. Aluminium is chosen because it is cheap (wires used in power electronics are much thicker than in microelectronics, 200-500 µm vs. 25-50 µm in diameter), it does not harden when cycled, and it is a good electrical conductor. There are, however, concerns regarding the reliability of the wires: the wire/die interface, in particular, generates thermo-mechanical stresses due to the mismatch in coefficient of thermal expansion (CTE) between a SiC die (≈ 4 ppm/K) and the aluminium wire (≈ 23 ppm/K). Thermal cycling tests listed in the literature range from 10 cycles between -196 and 500 °C [START_REF] Mustain | Evaluation of Gold and Aluminum Wire Bond Performance for High Temperature (500 degC) Silicon Carbide (SiC) Power Modules[END_REF] to 12000 cycles between 45 and 175 °C [START_REF] Kraft | Reliability of Silver Sintering on DBC and DBA Substrates for Power Electronic Applications[END_REF]. Another concern is the evolution of the microstructure at the interface between the wire and the copper layers of the ceramic substrate. Usually, these copper layers receive a Ni/Au finish. Au is often mentioned as causing kirkendall voiding with Al [START_REF] Khazaka | Survey of High-Temperature Reliability of Power Electronics Packaging Components, Power Electronics[END_REF]. However, some experimental tests [START_REF] Riches | Effectiveness of barrier layer metallisations in long term high temperature endurance tests on wire bond interconnections[END_REF] with Al wires bonded on NiAu-finished tracks only detected the occurrence of Al/Au Intermetallic Compound (IMC) after more than 7000 h at 250 °C.

In this paper, we focus on AuSn die attaches and aluminium wires. These interconnection technologies were selected for an aircraft application where reliability is the main concern. AuSn was chosen as a lead-free alternative to HMP solder alloys. The reliability of these interconnection technologies is evaluated in thermal cycling conditions as well as in high temperature storage (isothermal conditions). In the next section, we detail the test protocol. Then, in sections 3 and 4 we present the results regarding the wirebonds and die attach respectively. Our conclusions are given in section 5.

Test protocol

Preparation of the test vehicles

The test vehicles used in this article were prepared by Microsemi, using silicon carbide diodes (also from Microsemi, 4.79×4.79 mm 2 , 380 µm-thick with 5.7 µm Al as the top metal) and ceramic substrates sourced from two manufacturers: • Si 3 N 4 , Active Metal Braze (AMB) substrates from the company Kyocera (referenced to as "K" in the remaining of the article), with a 320 µm-thick ceramic layer, 300 µm-thick copper on one side and 200 µm on the other side; • AlN AMB Substrates from the company Dowa (referenced to as "D"), with 300 µm-thick copper layers (both sides) and 635 µm-thick ceramic. In both cases, the substrate pattern is identical (see fig. 1), and a Ni/Au ENIG plating was applied by the manufacturer (5 µm Ni and 50 nm Au). The SiC dice are soldered onto the substrates using Au80Sn20 solder alloy (melting point 281 °C).

Each diode is connected using 8 aluminium wirebonds (diameter 254 µm). In some cases (175 °C storage, described below), the test vehicles were split into 5 samples using a low speed diamond saw (Escil Labcut 150, with a Presi LM+ blade) to allow for more tests.

Accelerated Test Conditions

Two kinds of accelerated tests were performed:

• Storage tests (isothermal tests), in which the samples are exposed to a constant, elevated temperature over a long period of time. These tests are expected to trigger temperature-sensitive phenomena such as diffusion of chemical species. • Temperature cycling tests, where the samples are exposed to large temperature swings. This generates thermo-mechanical stresses in the samples, which trigger fatigue mechanisms. For the thermal cycling tests, two temperature profiles were chosen, to provide two largely different testing conditions:

• Rapid Temperature Change (RTC): -55 °C, maintained for 15 min, ramp-up at 10 °C/min up to 175 °C, 15 min at 175 °C and ramp-down at 10 °C/min, in a Climats Excal 1423-TE chamber. Each cycle lasts for 76 min. • Shock: -40/+125 °C temperature swing, in a twochamber system (Climats CTr series) which generates very fast temperature changes (estimated at 65 °C/min).

The samples are moved every 15 min. Regarding the storage conditions, a preliminary test was performed: some test vehicles were submitted to a "step stress", during which the temperature was increased from 170 to 270 °C by 20 °C steps every 24h. For each step, two samples were removed from the oven, and characterized (die shear test, wires No change in appearance is noticeable on the die or on the substrate finish.

pull and shear test, and microsections). No evolution could be detected from these mechanical tests. The micro-structure followed a classical evolution, with a coarsening of the phases, and the thickening of the interfacial intermetallical compounds of the solder joint, but no indication of advanced degradation was found. As the melting point of the AuSn alloy is 281 °C, it was decided to perform the ageing at a noticeably lower temperature: 230 °C. In parallel, a series of samples was stored at 175 °C to provide a comparison with the RTC cycling tests. For practical reasons, the 230 °C-storage tests were performed in a nitrogen atmosphere, while all the other tests (175 °C storage and all cycling tests) were performed in air. The composition of the atmosphere (air or nitrogen) was not found to have any particular consequence on the results. D-Type substrates failed early during the cycling tests (before the first 500 cycles for RTC and 2500 cycles for shock). This is not surprising, as AlN ceramic (used in D-type) is not as tough as Si 3 N 4 (used in K-type) [START_REF] Dupont | Effects of metallization thickness of ceramic substrates on the reliability of power assemblies under high temperature cycling[END_REF]. As a consequence, and because this article focuses on investigating the interconnections (die attach and wirebond), we only present cycling results for the K-type substrates. D-type and K-type results are presented for storage, as it induced no substrate failure.

Characterization

The tests and corresponding characterization were performed in two laboratories: Laboratoire Ampère (Lyon, France) for the storage tests, and IMS (Bordeaux, France), for the cycling tests. As a consequence, a special care was taken to ensure consistent results between both teams.

For each data point presented below, the samples were characterized as follows. At IMS, a sample with 5 dice was removed from the cycling chambers for mechanical testing, and a sample with two dice was taken for for microsection. At Ampère, only one sample was used, with 4 dice used for mechanical tests, and one for microsection.

For mechanical characterization, a Dage 3000 was used at IMS, with the following parameters: for die shear, a 100 kgf load cell, a tool speed of 100 µm/s, and a tool height of 70 µm; for the wire pull, a 1 kgf load cell and a tool speed of 250 µm/s. At Laboratoire Ampère, the test system was a XYZTEC Sigma Condor, with a 200 kgf load cell for die shear, a tool height of 80 µm, and a tool speed of 200 µm/s; for wire pull, a tool speed of 600 µm/s and a 10 kgf load cell. At Ampère, pull tests were performed on half of the wires only, the remaining half being submitted to wire shear test with the following parameters: 10 kgf load cell, tool speed 200 µm/s, tool height 50 µm.

Wirebonds

The evolution of the pull strength of the aluminium wires during the tests is presented in Fig. 3. It appears that while the strength drops rapidly for the cycling tests, it only shows a moderate reduction during the storage tests (except a 0.1 kgf drop in the first 24 h). The actual pull strength for the cycled samples is even lower than plotted in Fig. 3, as some wires detached in the cycling chamber and could not be pulled (these wires were taken into account in the evaluation of the failure modes, below). This is summarized in Fig. 4. This figure represents the distribution of the failure modes of the pulled wires, for each test condition (shock, RTC, 230 °C storage). For the shock tests, a clear shift is visible: at the beginning, most ruptures occur in the wire. Then, the attach of the wire becomes weaker, and finally it is so weak that the wires even detach before the pull test. These are marked as "Failed" in Fig. 4b.

For the RTC tests, the trend is similar, albeit slower, and with a period where "Foot" failures are predominant. For storage, however, no failure of the attach (or failure in the oven) has been observed yet. Storage results show little evolution between t=0 and 2400 h. Pull failure modes in storage conditions are distributed between "Wire" and "Foot" failures. As most of the failures occur close to the "ankle" of the wire, it can sometimes be difficult to discriminate between "Foot" and "Wire" failure modes. Nevertheless, the trend seems to be a shift towards more "wire" failures as the storage test progresses (the same trend is observed for sample types D and K, which are considered together in Figs 3 and4).

The stability of the wire/die and wire/substrate interfaces in conditions of thermal storage is demonstrated in Fig. 5, which presents the shear strength of the wire feet, either on die, Ktype substrate or D-type substrate. Except a ≈20% drop in shear strength at the beginning of the test for wire attaches on type-D substrate, there is no noticeable evolution of the shear strength over time. Furthermore, all ruptures occur in the wirebond material rather than at the wire/die interface ("Attach") as depicted in Fig. 6. This indicates that in the absence of thermal cycling, the attach remains strong.

Die Attach

Regarding the die attach, no obvious drop in mechanical strength has been observed: in cycling test (shock and RTC), the die shear strength exceeded the tester capability (100 kgf) for all samples. The shear tester used for the stored samples has a higher capability (200 kgf). A few failures were observed (Fig. 7), but in all cases, the initial failure mode was related to a fracture of the die. Therefore, it is difficult to conclude whether the observed failures are related to ageing. Some cross-sections of K-type samples before testing and during ageing are presented in Figs. 8 and9. They show that the solder layer is uniform. Only a few voids could be seen in the solder joint, which is consistent with the X-ray analyses of the same samples presented in [START_REF] Arabi | Power electronic assemblies: Thermo-mechanical degradations of gold-tin solder for attaching devices[END_REF]. The composition of the various phases was obtained by comparing EDS measurements with the phase diagrams in [START_REF] Zhu | Au-sn bonding material for the assembly of power integrated circuit module[END_REF]. The evolution of the microstructure with time is consistent with [START_REF] Zhu | Au-sn bonding material for the assembly of power integrated circuit module[END_REF]: a thick interfacial IMC forms at the bottom, in contact with the Ni layer. The fine microstructure visible in Fig. 8a coarsens with ageing, with a clear phase growth in Fig. 8b andc. Cross sections of a sample which underwent 3000 RTC cycles are given in Fig. 10. They show some noticeable difference with the stored samples: many cracks have developed in the solder layer, and some of them tend to propagate in the Ni and Cu layers; hillocks form at the copper/solder interface (Fig. 10a), which is no longer flat as for stored samples (Fig. 9).

Discussion and Conclusion

Wirebonds

The storage tests, although performed at high temperature (230 °C) did not demonstrate a reduction in mechanical strenght. This was expected on the die side, where the Al wires are bonded to a Al pad. The same results are observed on the substrate side, despite the different metals in contact (NiAu finish on copper). The pull tests (Fig. 3) show a slow reduction in breaking force, which is most likely associated with a reduction in the wire tensile strenght (increase of "wire" failures in Fig. 4).

For cycled samples, the reduction in mechanical strength is much faster and much more dramatic, with most wires eventually falling before the pull test. Furthermore, after an initial distribution of failure modes comparable to that of stored samples ("Wire" and "Foot" failures, see Fig. 4), the failures concentrate at the wire attaches. Because this is not observed on stored sam- ples, it can be concluded that the thermo-mechanical stresses are the main cause for the reduction in mechanical strength.

Furthermore, samples submitted to shock cycling degrade much faster than those submitted to RTC, despite the smaller temperature swing (-40/+125 °C for shocks vs. -55/175 °C for RTC). This means that the rate of the temperature change is very important. Either time-dependent phenomena are involved (visco-plastic deformation), or the rate in temperature change is high enough with the shock tests to cause inhomogeneous temperature distribution in the sample (for example, the wires could heat and cool faster than the dies and substrates, which would amplify the thermo-mechanical stresses). Finite-Elements simulations would be needed to validate or invalidate these hypotheses. A comparison with active power cycling, which generates even faster temperature changes (but with a lower amplitude) might also help. This kind of cycling is often used to assess the reliability of the wirebonds, as it concentrates the thermo-mechanical stresses around the die (which dissipates the power). Based on the results presented in [START_REF] Schmidt | Novel wire bond material for advanced power module packages[END_REF], failure is expected to occur for around 10000 cycles with a temperature swing of ∆T = 160 °C, which is consistent with the 6500 cycles observed in our case for the shock tests (∆T = 165 °C).

In any case, reducing the sensitivity of the interconnections to the thermal cycling would involve the standard techniques [START_REF] Ciappa | Selected failure mechanisms of modern power modules[END_REF]: using a larger number of smaller-diameter wires, to reduce the stresses at the wire attach area; place a spacer with a CTE closer to that of Al on top of the die (for example with a NiAu finish, as it was found to produce reliable attaches); use a "glob top" to encapsulate the wire foot. The wire could also be changed for AlCu wire [START_REF] Schmidt | Novel wire bond material for advanced power module packages[END_REF] or ribbon.

Die Attach

No significant variation of the mechanical strength of the die attach was observed during any of the tests: for cycling tests, all attaches exceeded the capability of the equipment (100 kgf, equivalent to 43 MPa for a 4.79×4.79 mm 2 die). For storage tests, a few samples did not sustain 200 kgf (the limit of the second shear tester), but all failure cases occurred above 100 kgf, with a fracture of the die (which might have been caused by slight misalignment or denting of the shear tool, chipping of the die, or actual ageing of the assembly).

Microsections were found to be much more instructive. As expected, the microstructure coarsens with storage time (Fig. 8). No continuous interfacial IMC appears to form at the solder/die interface. A much thicker, continuous IMC forms at the solder/substrate interface, but it does not cause any voiding or delamination (which is consistent with the shear test results).

Although they remain strong, the solder joints of cycled samples are very different (Fig. 10: they contain many voids and cracks, some extending to the copper layer of the substrate; their copper layer exhibits "hillocks" (Fig. 10a), while that of stored samples (Fig. 9) remains flat. Such hillocks have been reported to appear during thermal cycling for DBA substrates, which use Aluminium instead of copper [START_REF] Kraft | Reliability of Silver Sintering on DBC and DBA Substrates for Power Electronic Applications[END_REF]. As AuSn solder is harder than the classical HMP alloys, it can be assumed that a large part of the thermo-mechanical stresses caused by thermal cycling is transferred to the copper, resulting in its plastic deformation. On the contrary, Figs. 10b and10c show that the size and distribution of the phases in the solder layer are not noticeably affected by cycling, as they are similar for a cycled and stored sample which spent approximately the same time at 175 °C. Obviously, the temperature has a much more pronounced effect, as can be seen in Fig. 8b, which has a coarser microstructure and thicker interfacial IMCs after 672 h at 230 °C.

Conclusion

The structure investigated in this paper (SiC dice soldered on a substrate using AuSn, and connected with thick Al wirebonds) was found to have an excellent reliability regarding continuous operation at high (230 °C) temperature. Regarding thermal cycling, the weakest point was found to be the wirebonds, which started failing after only 1000 thermal shocks (-40/+125 °C). The comparison with wider but slower thermal cycles indicates that wirebonds are very sensitive to the rate of temperature change. On the contrary, thermal cycling caused visible damage in the solder layer, but we observed no reduction in its mechanical strength. Providing its thermal resistance remains low enough for the application, this solder attach is expected to survive many more cycles before failure.

Figure 1 :

 1 Figure 1: Photograph of the test vehicles. Two configurations were used (based on the same substrate, the dice being mounted on one side or the other): 5 dice or 2 dice.

Figure 2 :

 2 Figure 2: Photographs of one of the test samples before storage and after 672 h at 175 °C in air. The only visible change is the darkening of the greed marking.No change in appearance is noticeable on the die or on the substrate finish.

Figure 3 :

 3 Figure 3: Wire pull as a function of time spent at maximum temperature. The error bars correspond to the standard deviation of the measurements.

Figure 4 :

 4 Figure4: Failure mode for the pulled wires. (a) location of the failure: "Foot" failmode corresponds to a rupture within the aluminium wire, at the ankle (or foot) area. "Attach" corresponds to a rupture of the die/wire or the substrate/wire interface. Tests marked as "Failed" correspond to wires which were no longer attached after cycling, and therefore could not be pulled.(b) results obtained after cycling (shock and RTC) and storage (230 °C) tests.

Figure 5 :

 5 Figure 5: Wire shear strength as a function of storage time (at 230 °C). The error bars correspond to the standard deviation of the measurements. No clear evolution is visible, except a small drop at the beginning for the substrate side on substrate type D.

Figure 7 :

 7 Figure 7: Die shear strength as a function of storage time. The test equipment used for cycled samples is limited to 100 kgf, and no failure could be measured. For stored samples, the test were performed up to 200 kgf, but that was still insufficient to remove most of the dies; all of the failures were due to die fracture, not to the solder layer. Dots plotted at 200 kgf correspond to dies which survived the test.

Figure 8 :

 8 Figure 8: Cross sections of the die attach before testing (a), after 672 h (b), after 2417 h (c) at 230 °C. Phase compositions were identified using EDS spot measurements

Figure 9 :

 9 Figure 9: Low-magnification cross-section of the die attach, showing the flat copper/solder interface (a wirebond attach is also visible in this image). This sample was stored at 230 °C for 504 h.

Figure 10 :

 10 Figure 10: Cross section of the die attach after 3000 RTC cycles (corresponding to 750 h at 175 °C). A clear coarsening of the copper surface is visible, forming hillocks. Many cracks are visible in the AuSn layer. (b) is an enlargement of the area highlighted in (a). (c) is a cross section of a sample stored at 175°C for 840 h (approximately the same time spent at 175 °C as 3000 RTC), which shows a microstructure which is comparable with (b).

Preprint submitted to ElsevierJuly 3, 2017

Acknowledgement

This work was funded as part of the French GENOME-PREMICES PIA initiative. The authors would like to thank Microsemi for supplying the test vehicles used in this article.