Jean-François Silvain
email: jean-francois.silvain@icmcb.cnrs.fr

Loïc Constantin

Jean-Marc Heintz

Sylvie Bordère

Lionel Teulé-Gay

Yongfeng Feng Lu

Jean-Luc Diot

Renaud De Langlade

Emilien Feuillet

Loic Constantin

Lionel Teule-Gay

Controlling interfacial exchanges in liquid phase bonding enables formation of strong and reliable Cu-Sn soldering for high-power and temperature applications

Keywords: Transient liquid phase bonding, Intermetallic, Cu-Sn, Diffusion, Mechanical characterization

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

and reliable high-temperature Cu-Sn soldering by modifying the interfacial exchange during a transient liquid phase bonding (TLP) process. Our approach relies thus on altering internal phenomena (diffusion and transport of reactive species) rather than classical external TLP bonding parameters (e.g., time, temperature, and pressure). By adding a Cu3Sn coated layer between Cu and Sn before the TLP process, a fast dissolution of Cu in liquid Sn is achieved, altering undesired Cu6Sn5 scallop grains impingement and promoting their uniform growth within the liquid. A bonding and pore formation mechanism of the solder with or not the Cu3Sn coated layer is proposed based on experimental and theoretical analysis. The developed TLP joint possesses a shear stress resistance of more than 80 MPa with a thermal cycle endurance

Introduction

The current trend in power electronics requires using solder technologies that can handle high power densities, high junction temperatures (superior to 200 °C), and withstand significant thermomechanical stresses during multiple on/off cycles [START_REF] Kassakian | The future of electronics in automobiles 13[END_REF]. Yet, classical packaging technologies utilize soft soldering and wire bonding, which suffer from low thermomechanical resistance, limiting the power module performances [START_REF] Madjour | Silicon Carbide market update: from discrete devices to modules PCIM Europe[END_REF].

In the late 1950s, J. F. Lynch et al. introduced the transient liquid phase (TLP) bonding as a hybrid process, combining diffusion bonding and brazing [START_REF] Lynch | Brazing by the Diffusion-Controlled Formation of a Liquid intermediate Phase Welding[END_REF][START_REF] Jung | A Review on Recent Advances in Transient Liquid Phase (TLP) Bonding for Thermoelectric Power Module Reviews on advanced materials science[END_REF]. Since then, the TLP bonding has drawn significant consideration as a promising method of producing high-temperature solder joints [START_REF] Li | Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process P.A. Agyakwa[END_REF][START_REF] Harris | Selecting Die Attach Technology for High-Power Applications Power Electronics[END_REF][START_REF] Heuck | Aging of new Interconnect-Technologies of Power-Modules during Power-Cycling[END_REF][START_REF] Bordère | Understanding of void formation in Cu/Sn-Sn/Cu system during transient liquid phase bonding process through diffusion modeling[END_REF]. The TLP bonding relies on using an intermetallic compound (IMC) as a bonding medium. During the high-temperature bonding, the IMC turns liquid, which is then progressively consumed. The TLP process uses a binary system composed of either two metals or metal alloys with different melting points. A well-known system is the copper-tin (Cu-Sn), where Sn is the low-melting-point interlayer, inserted between two high-melting-point Cu layers. The Cu-Sn system has been intensively studied due to its affordability, the low Cu-Sn melting points, and the widespread usage of Cu in electronic packaging [START_REF] Chin | Are Intermetallic in Solder Joints Really Brittle? Electronic Components and Technology Conference[END_REF]. According to the Cu-Sn phase diagram, Cu6Sn5 and Cu3Sn are the two main IMCs which form and grow under the TLP bonding conditions [START_REF] Massalski | Binary alloy phase diagrams[END_REF][START_REF] Fürtauer | The Cu-Sn phase diagram, Part I: New experimental results[END_REF]. In addition, if Sn fully transforms into Cu3Sn, the solder joint becomes thermodynamically stable (up to 350 °C). Also, Cu3Sn has a much higher remelting temperature (676 °C) than the process temperature making the Cu-Sn system of great interest [START_REF] Heuck | Aging of new Interconnect-Technologies of Power-Modules during Power-Cycling CIPS Nuremberg[END_REF].

Previous reports have shown that the growth kinetics and morphology evolutions of Cu-Sn IMCs during TLP soldering depend upon the process parameters (e.g., heating rate, bonding temperature, compressive stress, etc.), and the initial thickness of the Sn interlayer [START_REF] Flötgen | Wafer bonding using Cu-Sn intermetallic bonding layers Microsyst[END_REF][START_REF] Chui | Formation of nearly void-free Cu3Sn intermetallic joints using nanotwinned Cu metallization[END_REF]. W.L. Chui et al. demonstrated that from 240 °C to 280 °C and in the early stages of the process, the growth of Cu6Sn5 dominates [START_REF] Chui | Formation of nearly void-free Cu3Sn intermetallic joints using nanotwinned Cu metallization[END_REF]. Next, when the liquid Sn (Sn(l)) is fully consumed, the Cu3Sn layer grows at the expense of the previously formed Cu6Sn5. It was observed that the Cu6Sn5 grows into a rounded grains morphology called scallops from both Cu/Sn bonding interfaces [START_REF] Ma | Scallop formation and dissolution of Cu-Sn intermetallic compound during solder reflow[END_REF]. Later, such grains development led to a Cu6Sn5 impingement before the total Sn consumption, thus forming pores in the solder joint [START_REF] Madjour | Silicon Carbide market update: from discrete devices to modules PCIM Europe[END_REF][START_REF] Bordère | Understanding of void formation in Cu/Sn-Sn/Cu system during transient liquid phase bonding process through diffusion modeling[END_REF][START_REF] Bosco | Critical interlayer thickness for transient liquid phase bonding in the Cu-Sn[END_REF]. Naturally, pores degrade the IMC's thermal and mechanical properties and deteriorate its reliability [START_REF] Brincker | Strength and reliability of low temperature transient liquid phase bonded Cu-Sn-Cu interconnects Microelectronics Reliability[END_REF]. It has been shown that high temperature, fast heating rate, thin Sn foil, and intimate contact between layers help to decrease the number of pores but cannot suppress them [START_REF] Flötgen | Wafer bonding using Cu-Sn intermetallic bonding layers Microsyst[END_REF][START_REF] Chui | Formation of nearly void-free Cu3Sn intermetallic joints using nanotwinned Cu metallization[END_REF]. Recently, O. Mokhatari reviewed the current TLP soldering state-of-the-art and concluded that voids' formation is yet to be understood during the process but must be prevented and reduced to enhance the attractiveness of the method and effectiveness of the solder joint [START_REF] Mokhtari | A review: Formation of voids in solder joint during the transient liquid phase bonding process -Causes and solutions Microelectronics Reliability[END_REF]. Although these results, forming pore-free, robust, and durable high-temperature and power IMC joints remains challenging.

Here, instead of simply relying on external TLP bonding parameters (e.g., temperature, time, and pressure) to fabricate void-free, strong, and reliable Cu-Sn solder joints, we proposed to modify the interfacial exchange between Cu and a SnAg3.8Cu0.7 (SAC) filler metal. As previously reported, adding a layer between a metal and filler metal influences the nucleation, growth, and morphology of IMCs [START_REF] Liashenko | Phase growth competition in solid/liquid reactions between copper or Cu3Sn compound and liquid tin-based solder[END_REF][START_REF] Liashenko | Initial stage of isothermal wetting of bulk Cu6Sn5, Cu3Sn and Cu substrates by liquid Sn[END_REF]. Therefore, in a similar fashion, we altered the growth of Cu6Sn5 scallops by inserting a Cu3Sn coated layer between Cu and SAC during the TLP bonding. We showed that the deposition of a Cu3Sn layer between Sn and Cu before the TLP bonding process allows suppressing grains impingement, leading to a pore-free Cu-Sn solder joint. A bonding and pore formation mechanism of the solder having or not the Cu3Sn coated layer is proposed based on microstructural, elemental, chemical analysis, and previously reported kinetic and thermodynamic simulation diffusion of the TLP process [START_REF] Bordère | Understanding of void formation in Cu/Sn-Sn/Cu system during transient liquid phase bonding process through diffusion modeling[END_REF]. Finally, the mechanical and thermomechanical properties of the bonded materials having the Cu3Sn coated layer were studied and compared to a classical SAC solder joint. Adding a Cu3Sn layer enables modifying internal phenomena during the bonding process and holds great promise to remove the bottleneck of the TLP method.

Experimental part

Deposition of the Cu3Sn coated layer:

The deposition of a thin Sn layer on top of the Cu substrates (Goodfollow TM , 99.9 purity, 30×30×3 mm 3 ) was carried out by radiofrequency sputtering (Plassys TM MP 700 at ambient temperature). The Sn target (Neyco TM target Sn, 99.99%) was powered by a 13.56 MHz radiofrequency supply with a fixed value of 15 W. Every 30 min, the deposit was impinging by Argon (Ar) ions (Anatech Ion gun) during 30 s with an accelerator voltage of -50 V. The deposition rate was estimated to be about 0.4 m/h. The coated Sn film was then annealed at 250 °C for 10 min under a reducing atmosphere composed of Ar and dihydrogen (H2, 5%), which gave rise to a Cu3Sn layer on top of the Cu substrate.

Formation of the bonded assembly:

Bonded materials were fabricated by combining a substrate of Cu or Cu/carbon fibers (CFs) composites having or not a Cu3Sn coated-layer with a SnAg3.8Cu0.7 solder sheet (SAC, Materon TM , thickness = 20 m). The Cu composites were fabricated using a uniaxial hot pressing at 650 °C under 50 MPa for 30 min. More information on the Cu/CF composites fabrication can be found elsewhere [START_REF] Mokhtari | A review: Formation of voids in solder joint during the transient liquid phase bonding process -Causes and solutions Microelectronics Reliability[END_REF]. A Lamination Press (MTI Corporation TM VHP-5T-4 500 °C) was used to combine the different layers. The applied pressure and temperature were fixed to 10 kPa and 270 °C, respectively.

Mechanical and thermomechanical testing of the Cu-Sn joint:

The double notch shear (DNS) test was selected to measure the mechanical properties of the IMCs because of its simplicity of implementation. The DNS test follows the ISO standard 11003-2 using an Instron machine (Model 3369) with a 1 mm/min displacement speed and a maximum loading charge of 50 kN. The notch's diameter and length were fixed to 0.3 and 2.5 mm, respectively, with a distance between two notches of 2.5 mm. The thickness and size of the Cu substrate were 5 and 15 mm, respectively. The deformation behavior and stresses were simulated by a stress analysis model using COMSOL® finite element software. The Cu substrates were modeled with an elastoplastic equation, whereas the IMC joint stays in its elastic domain.

Microstructural characterization:

The microstructure of the joints was characterized using scanning electron microscopy (SEM, VEGA II SBH, TESCAN TM ). Auger electron spectroscopy (AES) was performed using a Nano sonde AUGER PHI TM 710 equipment with an analysis diameter of about 50 nm. The SAC was etched with a solution of hydrochloric acid and water (50:50) for 10 s.

Results and discussions

Preparation of the Cu3Sn layer on Cu substrates.

The Cu-SAC system undergoes various phases and microstructural evolutions during the TLP soldering process. As observed by others, Cu6Sn5 forms first, which then transforms into Cu3Sn [21]. However, the Cu6Sn5 grows in the form of scallop, inducing grains impingement and pores in the solder layer, thus deteriorating its mechanical and thermomechanical properties. It has been reported that during a wettability measurement, a Cu3Sn layer modified the nucleation, growth, and morphology of Cu6Sn5 scallops when inserted between Cu and Sn [START_REF] Liashenko | Initial stage of isothermal wetting of bulk Cu6Sn5, Cu3Sn and Cu substrates by liquid Sn[END_REF].

Particularly, in the presence of a Cu3Sn layer, the Cu6Sn5 phase preferentially grows inside the Sn(l) rather than at the Cu/Sn(l) interfaces. In addition, Cu3Sn has a high melting point compared to Sn or SAC, making it a pertinent candidate to alter the interfacial exchange. Likewise, we synthesized a Cu3Sn layer between Cu and SAC to suppress the scallops grain impingement.

The Cu3Sn layer was obtained by first depositing a thin Sn layer onto a Cu substrate, followed by an annealing treatment at 250 ºC for 10 min, as illustrated in Figure 1a. The thickness of the Cu3Sn layer was set to 1 μm to avoid forming a thick seal. Figure 1b shows a top view of the Cu3Sn film after the annealing treatment. It can be observed that the Cu substrate is homogeneously coated with Cu3Sn, having a grain size of about 1 μm. Figure 1c shows a crosssectional view of the Cu3Sn film on the Cu substrate, where a significantly rough film is obtained. A SAC filler metal layer was used to bond two Cu substrates having or not the Cu3Sn coated layer, as illustrated in Figure 2a. Once bonded via the TLP process, at 270 ºC for 120 min and under 10 kPa, the samples were cut and mechanically polished for microstructural observations. As can be observed in the low magnifications backscattered electron (BSE) micrographs in Figure 2b, the bonded Cu-SAC-Cu has numerous macropores along the solder joint. Figure 2c shows higher magnification BSE micrographs of the Cu-SAC-Cu bonded layer where threecolor contrasts can be identified. In this detection mode, Cu appears darker than Cu3Sn, which is darker than Cu6Sn5. The BSE observations suggest, thus, the formation of a thick Cu6Sn5

Microstructural evolution of the Cu-SAC soldering having or not a Cu3Sn coating layer during the TLP process.

To understand the effect of the Cu3Sn coated layer on phase evolutions and pore formation, the bonding process was monitored at different stages of the TLP process. Figure 3a shows SEM micrographs of a bonded Cu-SAC-Cu sample at 270 ºC for 0, 30, and 90 min and at a constant pressure of 10 kPa. It can be observed that during the heating ramp (t = 0 min) and without the Cu3Sn coated layer, Cu6Sn5 scallops nucleated at the Cu-Sn(l) interfaces. Next, the scallops grew (t = 30 min) until they intercepted the grains from the opposite bonding front (t = 90 min) and formed a layer across the initial SAC. However, the synthesized Cu6Sn5 bonding layer is mixed with unreacted SAC domains. O. Y. Liashenko et al. observed a similar discontinuous Cu6Sn5 layer and assimilated it to the scallop grains impingement [START_REF] Liashenko | Phase growth competition in solid/liquid reactions between copper or Cu3Sn compound and liquid tin-based solder[END_REF].

On the contrary, for the Cu-Cu3Sn-SAC-Cu3Sn-Cu bonded sample (i.e., Cu3Sn coating on Cu), the reaction between the Cu3Sn coated layer and Sn(l) led to preferential Cu6Sn5 grains nucleation within the liquid. The reaction occurred as early as Sn tuned liquid (t = 0 min). The coalescence of the Cu6Sn5 crystal (t = 30 min) produced a uniform and continuous layer across the solder joint (t = 90 min). Only residual SAC can be observed in the vicinity of the Cu3Sn layer, as shown in Figure 3b.

In addition, the Cu6Sn5 nuclei in Sn are faceted crystals, which significantly differ from the scallop grains at the Cu/Sn(l) interfaces (Figure 3a and Figure 3b). The top of the SAC layer was thus etched to gain more insight into the growth of the faceted grains, as illustrated in In the previous section, we observed that a Cu3Sn layer deposited onto the Cu substrate altered the growth of Cu6Sn5 grains. Instead of growing in scallops, the Cu6Sn5 phase grew in faceted crystals inside the Sn(l). AES maps and line profiles were performed across the Cu-SAC and Cu-Cu3Sn-SAC interfaces after being heated at 270 ºC for 10 min and under 10 kPa, to understand the role of the Cu3Sn coated layer on the Cu6Sn5 growth mechanisms as shown in

Figure 4a.

The comparison of AES maps (Figure 4b) and line profiles (Figure 4c) of the bonded samples show, in both cases, the presence of a ~ 1 μm thick Cu3Sn interphase and Cu6Sn5 scallops. In other words, the Cu3Sn coated layer does not prevent the growth of Cu3Sn interphases during the TLP process and does not suppress the scallop grains but restricts their nucleation and growth. The Cu line profile in Figure 4c shows that owing to the Cu3Sn coated layer; the scallop size is fewer than 0.5 μm. Conversely, without the Cu3Sn coated layer, the scallops developed for more than 1 µm.

Additionally, a smooth transition between Cu6Sn5 and Sn is observed in the absence of the Cu3Sn coated layer. This transition suggests forming a metastable diphasic domain with a highvolume fraction of Cu6Sn5 and implies nucleation in progress from Cu to liquid SAC (Figure 4c). On the contrary, with a Cu3Sn coated layer, a sharp transition between Cu6Sn5 and Sn is observed suggesting the termination of scallop nucleation at the Cu-Sn(l) interfaces and promoting Cu6Sn5 crystal growth in liquid SAC.

The bonding mechanism of Cu-SAC joints having or not the Cu3Sn coated layer.

The microstructural, elemental, and chemical analysis of the Cu-SAC bonding via the TLP process and previous studies were then correlated to propose a bonding mechanism, including the Cu3Sn coated layer effect. Note that the following mechanisms rely on the experimental analysis presented in this paper and previously published kinetics and thermodynamics simulations [START_REF] Bordère | Understanding of void formation in Cu/Sn-Sn/Cu system during transient liquid phase bonding process through diffusion modeling[END_REF]. The simulations are based upon Fick's diffusion law of the different elements and phases present in the bonding process and corroborated to interfacial mass transport equations. Herein, these mechanisms are used to explain the phase evolutions, pore formation, and the effect of a Cu3Sn coated layer pre-exists.

In the early stage of the bonding process (t = 0 min), the phases in contact with Sn(l) dissolve (i.e., Cu or Cu3Sn), supersaturating Sn(l) with Cu (pathway 1). Under these metastable conditions, Cu6Sn5 grains nucleate and grow, as illustrated in Figure 5a-1 and Figure 5a-2.

However, the nucleation, growth, and morphology of the Cu6Sn5 are severely altered by the Cu3Sn coated layer. In the absence of a Cu3Sn coated layer, Cu6Sn5 scallops nucleate at the Cu/Sn(l) interfaces (Figure 3a), while with a Cu3Sn coated layer, Cu6Sn5 faceted crystals nucleate preferentially within Sn(l) (Figure 3b). These different growth mechanisms result from a faster Cu dissolution from Cu3Sn than from pure Cu in Sn(l), as reported by Liashenko et al. [START_REF] Mokhtari | A review: Formation of voids in solder joint during the transient liquid phase bonding process -Causes and solutions Microelectronics Reliability[END_REF]. Hence, a low dissolution/nucleation ratio promotes the growth of Cu6Sn5 scallops in the vicinity of the dissolving interfaces. On the contrary, a high dissolution/nucleation ratio encourages the development of Cu6Sn5 faceted crystals within Sn(l). Naturally, with the Cu3Sn coated layer, the growth of Cu6Sn5 nuclei is faster in Sn(l) than at the Cu/Sn(l) interfaces, giving rise to a grain size discrepancy in the early stage of the bonding process as observed in Figure 4b andFigure 4c.

After 30 min, Cu6Sn5 and Cu react to form a thin Cu3Sn layer where it was not yet present, as illustrated in Figure 5b-1. Then, Cu6Sn5 grains grow under a quasi-saturation condition via grain boundary (pathway 2) and volume (pathway 3) diffusions, as demonstrated by S. Bordère et al. [START_REF] Bordère | Understanding of void formation in Cu/Sn-Sn/Cu system during transient liquid phase bonding process through diffusion modeling[END_REF]. The diffusion mechanisms generate a transport of species (pathway 1) and vacancies (pathway 4) in Sn(l), which then drive the growth of Cu6Sn5 and the formation of pores, respectively. Later, the pressure applied to the sample (i.e., 10 kPa) allows filling pores created during the transport of vacancies. Additionally, the Oswald ripening is another mechanism favoring Cu6Sn5 faceted crystal growth within the Sn(l). In both cases, grain boundary and volume diffusions are the limiting mechanisms and related to Cu6Sn5 growth. In other words, a similar volume fraction of Cu6Sn5 must be obtained regardless of the presence of the Cu3Sn coated layer and matches our previous volume fraction measurement in Figure 3a and Figure 3b.

Then, large Cu6Sn5 grains coalesce and produce an IMC layer (t = 90 min). Without the Cu3Sn coated layer, an early grain impingement occurs because of the Cu6Sn5 scallops growth from both bonding sides. Later, this impingement constrains the further development of Cu6Sn5 into a fixed volume, creating pores where the liquid phase remains (Figure 3a and Figure 5c-1).

On the contrary, with the Cu3Sn coated layer, the growth of Cu6Sn5 arises inside Sn(l), avoiding grain impingement and volume restriction, thus preventing pores (Figure 3b and Figure 5c-2).

Finally, after Sn(l) consumption, Cu3Sn grows at the detriment of the previously formed Cu6Sn5 (120 min). The phase evolution is driven by volume diffusion in which pores size and distribution are unaffected. As a result, without the Cu3Sn coated layer, the IMC has macropores (Figure 3a and Figure 5d-2), whereas, with the Cu3Sn coated layer, the IMC is pore-free (Figure 3b and Figure 5d-2 and(d-1) -(d-2) 120 min.

Comparison of the mechanical and thermomechanical properties of TLP and classical

Cu-SAC soldering.

The mechanical properties of the developed TLP joints (Figure 6a) were then tested and compared to traditional soldering. Two TLP joints were made with a SAC sheet and are composed of either a Cu6Sn5 layer having Cu3Sn interphases (Figure 6b) or a fully reacted Cu3Sn layer (Figure 6c). On the other hand, the classical solder was elaborated using a SAC The reliability of the solder joint depends upon the number of thermal cycles it can endure.

Heating and cooling an electronic module create thermal stresses at the joint/metal interface leading to delamination and failure. Also, thermal stresses are significant when materials on both sides of the soldering have different coefficients of thermal expansion (CTE). Therefore, to generate sufficient thermal stresses in the assembly during thermal cycling, Cu matrix composites were fabricated and bonded. One side of the assembly is composed of a Cu matrix reinforced with 30 vol.% of CFs, while the other side is composed of a Cu matrix reinforced with 40 vol.% of CFs, as illustrated in Figure 8a. The CTEs of the Cu/30 vol.% CFs and Cu/40 vol% CFs composites are 14.10 -6 K -1 and 10.10 -6 K -1 , respectively. A 100 μm Cu layer was placed between the composite and SAC to assure the bonding. Once bonded, the assemblies were inserted in a climatic chamber and subjected to open-air passive thermal cycles, as illustrated in Figure 8a. The duration of a cycle was set 120 min, with a first plateau at 180 °C for 20 min and a second plateau at -45 °C for 20 min.

Figure 8b

presents the mechanical shear behavior of the Cu-Cu3Sn-Cu6Sn5-Cu3Sn-Cu TLP and classical solder after 0, 500, and 1200 thermal cycles. It can be observed that the mechanical shear resistance of classical SAC joint decreases from 30 MPa at t0 to 6 MPa at t1200, corresponding to an 80 % decrease in resistance. Figure 8c shows that cracks formed inside the solder joint, and delamination occurred after only 500 thermal cycles. In opposition, the TLP joint has a resistance of up to 45 MPa with a minor decrease of its mechanical resistance after 1200 cycles. The slight tensile strength reduction from 45 to 35 MPa is attributed to decreased Cu/CFs composite shear strength (CTE mismatch between Cu and CFs). Figure 8d shows SEM micrographs of the TLP solder layer after 0, 500, and 1200 cycles. It can be noticed that after 500 cycles, the initial Cu6Sn5 layer transformed to Cu3Sn, and the joint is free of defects. The successive deformation of the composite assembly during the cycling generates vertical cracks, as seen during the DNS test. The vertical cracks allow withstanding significant thermomechanical stresses and thus preserving the assembly integrity. The IMCs assemblies manufactured with the Cu3Sn coated layer possess then excellent thermal cycling reliability.

Conclusion

In summary, we showed that adding a Cu3Sn coated layer between Cu and SAC positively altered the nucleation, growth, and morphology of IMCs during the TLP soldering process. The Cu3Sn coated layers restrained the growth of Cu6Sn5 scallops in the vicinity of the Cu3Sn-SAC interfaces avoiding scallops grains impingement and pores formation. We argued that inserting a Cu3Sn layer between permitted a fast dissolution of Cu in Sn(l), promoting Cu6Sn5 nucleation within Sn(l), rather than at the Cu/Sn(l) interfaces, thereby forming a dense, robust and reliable joint. As demonstrated, the TLP solder having a Cu3Sn layer withstood more than 80 MPa and

  The parameters used for that simulation are the following: density, dCu = 8960 kg/m 3 , dIMC = 8900 kg/m 3 ; Young modulus ECu = 100 GPa, ECu = 110 GPa ; Poisson coefficient, Cu = 0.35, Cu = 0.33 and yield point; Cu = 100 MPa. The thermomechanical of the bonded layer were evaluated using in a climatic chamber (CTS Clima Temperatur Systeme TM , T-40/25). The assemblies were inserted in the chamber and subjected to open-air passive thermal cycles. A cycle duration was set to 120 min, with a first plateau at 180 °C for 20 min and a second plateau at -45 °C for 20 min.

Figure 1 .

 1 Figure 1. (a) Illustration of the Cu3Sn coated layer synthetized on Cu. SEM micrographs of the

2 .

 2 Effect of the Cu3Sn coated layer on the Cu/SAC soldering microstructure.

  layer and thin Cu3Sn interphases as a result of the Cu and SAC reaction. An AES line profile was performed across the bonding layer to confirm the formation of Cu6Sn5 and Cu3Sn phases, as shown in Figure 2d. The measured Cu and Sn atomic percentages (at.%) suggest the following sequence: Cu, Cu3Sn, Cu6Sn5, Cu3Sn, and Cu and corroborate the BSE observations. An equivalent microstructure is also obtained on the Cu-Cu3Sn-SAC-Cu3Sn-Cu sample, as shown in Figure 2e and Figure 2f. The volume fractions of the Cu6Sn5 and Cu3Sn phases appear similar regardless of the Cu3Sn coated layer presence and measured to be about 90 and 10 vol.%, respectively. However, in the absence of the Cu3Sn coated layer, macropores formed inside the Cu6Sn5 phase (Figure 2b and Figure 2c), while only a few micropores formed close to the Cu3Sn interphases with a Cu3Sn coated layer as can be seen on the low and high magnification SEM in Figure 2e and Figure 2f, respectively.

Figure 2 .

 2 Figure 2. (a) Illustration of the Cu-Sn soldering having or not the Cu3Sn coated layer, (b) and

Figure 3c .

 3c Figure 3c.It is observed that the grains have a polyhedron structure with a hexagonal base, as

Figure 3 .

 3 Figure 3. (a) BSE micrographs of the Cu-SAC-Cu bonded samples at 270 ºC and 10 kPa for 0,

Figure 4 .

 4 Figure 4. (a) SEM micrograph of the Cu-SAC and Cu-Cu3Sn-SAC interfaces elaborated at

  ).

Figure. 5 .

 5 Figure. 5. Illustrations of the Cu-SAC bonding during the TLP process having (column 1)

  paste, as illustrates in Figure 6d. The use of a solder past led to a 100 μm thick SAC seal, in which Cu6Sn5 scallops grew at the Cu/SAC interfaces due to the absence of a Cu3Sn coated layer as shown in Figure 6e.

Figure 6 .

 6 Figure 6. (a) Illustration of the TLP solder having : (b) Cu6Sn5 layer with Cu3Sn interphases

Figure 7c

 7c Figure 7cshows the results of the DNS test for the different bonded layers. As can be observed,

Figure 7 .

 7 Figure 7. (a) Schematic of the DNS test and (b) simulation of the stresses induced during the

Figure 8 .

 8 Figure 8. (a) Illustration of the thermal cycling process of bonded Cu/CF(40 vol.%) -Cu/CF(30

  Figure legend:

Figure 1 .

 1 Figure 1. (a) Illustration of the Cu3Sn coated layer synthetized on Cu. SEM micrographs of the

Figure 2 .

 2 Figure 2. (a) Illustration of the Cu-Sn soldering having or not the Cu3Sn coated layer, (b) SEM

Figure 3 .

 3 Figure 3. (a) SEM micrographs of the Cu-SAC-Cu bonded samples at 270 ºC and 10 kPa for

Figure 4 .

 4 Figure 4. (a) SEM micrograph of the Cu-SAC and Cu-Cu3Sn-SAC interfaces elaborated at 270

Figure. 5 .

 5 Figure. 5. Illustrations of the Cu-SAC bonding during the TLP process having (column 1) or

Figure 6 .

 6 Figure 6. (a) Illustration of the TLP solder having: (b) Cu6Sn5 layer with Cu3Sn interphases

Figure 7 .

 7 Figure 7. (a) Schematic of the DNS test and (b) simulation of the stresses induced during the

Figure 8 .

 8 Figure 8. (a) Illustration of the thermal cycling process of bonded Cu/CF(40 vol.%) -Cu/CF(30

Funding Sources

No funding sources

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Abbreviations


Mutation rate estimates for 110 Y-chromosome STRs combining population and father-son pair data

Miguel Navascués, Concetta Burgarella

INTRO Y-chromosome microsatellites (Y-STRs) are typically used for kinship analysis and forensic identification as well as for inferences on population history and evolution. All applications would greatly benefit from reliable locus-specific mutation rates, to improve forensic probability calculations and interpretations of diversity data. However, estimates of mutation rate from fatherson transmissions are available for few loci and have large confidence intervals, due to the small number of meiosis usually observed. By contrast, population genetic diversity data exist for many more Y-STRs, holding unused information about their mutation rates. ) can be used to estimate the parameter θ=2Nμ=((1/H)-1)/2 for each locus. The ratio between estimates of θ from H for two loci (R H ) provides an estimate of the relative mutation rates between loci. A logistic regression was performed between the proportion of observed mutations in father-son transmissions (i.e. meiosis) and the predictive variables R H (for a reference locus), size of repeat motif (tri-, tetra-, penta-o hexanucleotide) and structure of repeat motif (simple or complex). Data were obtained from several publications. This regression was used to predict mutation rates on 110 Y-STRs loci. The error of this method was evaluated by simulations.

RESULTS

• Simple Y-STRs mutate faster than those with a complex structure (fig. 1)

• Repeat size influences mutation rate: μ tetra >μ hexa >μ penta ≈μ tri (fig. 1)

  Figure 1. Logistic regression curves in function of relative genetic diversity to a reference locus (R H ) and size and structure of repeat motif.

Figure 2 .

 2 Figure 2. Relative mean squared error (RelMSE) for mutation rate estimates in simulated datasets of meiosis and population data.

  Mutation rate estimates for 110 Y-STRs combining population and father-son pair data Concetta Burgarella 1,2 & Miguel Navascués 1,3

	repeat type	# of loci (with meiosis)	mean μ (meiosis)	mean μ (regression)
	tri+simple	18 (16)	9.57×10 -4	8.15×10 -4
	tri+complex	1 (1)	4.30×10 -4	4.75×10 -4
	tetra+simple	44 (37)	2.30×10 -3	2.64×10 -3
	tetra+complex 29 (17)	2.69×10 -3	3.11×10 -3
	penta+simple	8 (6)	5.78×10 -4	5.95×10 -4
	penta+complex 8 (1)	4.56×10 -3	5.11×10 -4
	hexa+complex 2 (1)	1.65×10 -3	1.56×10 -3

1 CNRS UMR 7625 Écologie et Évolution, Paris (France) 2 CIFOR-INIA, Madrid (Spain) 3 INRA, UMR CBGP (INRA/IRD/Cirad/Montpellier SupAgro) Montpellier (France) navascues@supagro.inra.fr

Table 1 .

 1 Average mutation rate estimates for each category of Y-STRs. Locus specific estimates for 110 loci will be presented elsewhere (article submitted to European Journal of Human Genetics)

	METHODS
	'Homozygosity' (H=Σp i 2


Amélie Beaudet

José Braga

Frikkie De Beer

Burkhard Schillinger

Christine Steininger

Vladimira Vodopivec

Clément Zanolli

Neutron microtomography-based virtual extraction and analysis of a cercopithecoid partial cranium (STS 1039) embedded in a breccia fragment from sterkfontein member 4 (South Africa)

Keywords:

more cercopithecoid remains than any other fossiliferous site in South Africa [START_REF] Heaton | Taxonomy of the Sterkfontein fossil Cercopithecinae: the Papionini of Members 2 and 4 (Gauteng, South Africa)[END_REF]. As yet, six papionin taxa (Parapapio jonesi, Pp. whitei, Pp. broomi, Papio izodi, P. robinsoni, Theropithecus oswaldi) and one colobine (Cercopithecoides williamsi) were recovered in its deposits [START_REF] Delson | Cercopithecoid biochronology of the African Plio-Pleistocene: correlation among eastern and southern hominid-bearing localities[END_REF][START_REF] Delson | Chronology of South African australopiths site units[END_REF][START_REF] Pickering | The context of Stw 573, an early hominid skull and skeleton from Sterkfontein Member 2: taphonomy and paleoenvironment[END_REF][START_REF] Heaton | Taxonomy of the Sterkfontein fossil Cercopithecinae: the Papionini of Members 2 and 4 (Gauteng, South Africa)[END_REF].

Among the primate assemblages from Sterkfontein, Brain (1981: 205) mentioned a monkey cranial fragment labelled STS 1039 and preliminarily assigned it to Cercopithecoidea indet. Heaton (2006: 440) provided a first anatomical description of this incomplete fossil specimen based on the meagre tooth evidence appreciable at the external surface of the breccia. However, as STS 1039 is embedded in a block of sedimentary breccia which masks most details of its morphology (Fig. 1A), no detailed study of this material has been performed so far and its taxonomic attribution remains uncertain. In order to reveal the presence of any possible diagnostic element preserved in the hardened sediment, we preliminary detailed the breccia block by using X-ray microtomography.

However, due to the extremely low contrast between "container" (the breccia) and "content" (bony and tooth elements), the record failed to provide reliable information of taxonomic value.

Accordingly, we tentatively experienced the use of advanced investigative techniques based on neutron microtomography (n-µCT), whose absorption profile (interaction mechanism of neutrons with matter) differs from X-rays because of the unique ability of neutrons to penetrate materials opaque to X-rays and by being more strongly attenuated by the organic material [START_REF] Kardjilov | New features in cold neutron radiography and tomography. Part II: applied energy-selective neutron radiography and tomography[END_REF][START_REF] Winkler | Applications of neutron radiography and neutron tomography[END_REF][START_REF] Sutton | Tomographic techniques for the study of exceptionally preserved fossils[END_REF][START_REF] Tremsin | High resolution neutron imaging capabilities at BOA beamline at Paul Scherrer Institut[END_REF].

MATERIALS AND METHODS

X-rays tube microfocus and synchrotron radiations contrasts between materials are attenuated due to the different natural absorption and scattering (e.g., Pair-production or Compton scattering principles) of X-rays and to the different electron cloud densities surrounding the atoms of the analyzed materials (thus depending of the element positions in the periodic system). Since the neutrons are only being absorbed or scattered by the nucleus of the atom, this principle does not apply to neutron-based analyses. The neutrons only interact directly with the nuclei of the atoms as being neutral particles, delivering contrast depending on the inner structure of the nuclei which often shows huge differences between neighbouring elements, and even between isotopes of the same element [START_REF] Kardjilov | New features in cold neutron radiography and tomography. Part II: applied energy-selective neutron radiography and tomography[END_REF][START_REF] Winkler | Applications of neutron radiography and neutron tomography[END_REF][START_REF] Tremsin | High resolution neutron imaging capabilities at BOA beamline at Paul Scherrer Institut[END_REF]. Hydrogen is a special case, which attenuates neutron beams by equal-mass scattering, while heavier mineral elements, usually constituting geological and paleontological material, are easily penetrated and additionally show high contrast levels between bone and matrix (breccias) [START_REF] Schwarz | Neutron tomography of internal structures of vertebrate remains: a comparison with X-ray computed tomography[END_REF][START_REF] Sutton | Tomographic techniques for the study of exceptionally preserved fossils[END_REF].

In summary, neutron radiography and n-µCT are bound to deliver contrast very different from Xrays, sometimes even complementary [START_REF] Sutton | Tomographic techniques for the study of exceptionally preserved fossils[END_REF]. In this study, neutrons help to distinguish between the otherwise very similar dense materials representing the matrix, maxillary bone, tooth enamel and dentine.

The block of breccia bearing the partial cranium STS 1039 (c. 13.5x7x3 cm) is permanently stored at the Ditsong National Museum of Natural History, Pretoria, South Africa. In 2014, the specimen has been detailed by n-µCT at the ANTARES Imaging facility (SR4a beamline) of the Heinz Maier-Leibnitz Center (FRM II) of Technische Universität München, Germany. The measurements were carried out according to the following parameters: the neutron beam originated from the cold source of the FRM II reactor, with an energy range mostly from 3 to 20 meV, a collimation ratio of L/D = 500 (ratio between sample-detector distance and collimator aperture) and an intensity of 6.4 × 10 7 n/cm 2 s. The final virtual volume was reconstructed with an isotropic voxel size of 75.0 µm. Because the detection of the tissue interfaces is based on attenuation at the boundary of a structure in both X-ray and neutron-based microtomography, we performed a threshold-based segmentation with manual corrections [START_REF] Spoor | Linear measurements of cortical bone and dental enamel by computed tomography: applications and problems[END_REF], as usually applied for X-ray acquisitions using the software Avizo v.7.0 (Visualization Sciences Group Inc.).

In order to compare STS 1039 to the reference dimensional record provided by [START_REF] Freedman | The fossil Cercopithecoidea of South Africa[END_REF] for the fossil Cercopithecoidea crania of South Africa, we measured the distance between the lateral surfaces of the maxillae anteriorly to the third premolar (meas. BP3) and anteriorly to the third molar (meas. BM3).

Following segmentation, the elements of each tooth row have been isolated and virtually extracted for visualization and quantitative assessment of their structural features (Figs. 1C,D and2). Besides the outer mesiodistal (MD) and buccolingual (BLm and BLd) crown diameters of the upper left M1 and M2 and of the right M3, a total of six linear, surface and volumetric variables describing internal tooth crown structure and tissue proportions were digitally measured or calculated on the unsmoothed virtual surfaces of the upper right M3, the best preserved tooth element of STS 1039 (Table 1). Before assessing their crown size, we virtually integrated the punctually damaged lateral enamel of both left M1 and M2. Intra-and inter-observer tests for measurement accuracy run by two observers revealed differences less than 5% (cf. [START_REF] Bondioli | Morphometric maps of long bone shafts and dental roots for imaging topographic thickness variation[END_REF][START_REF] Zanolli | The late Early Pleistocene human dental remains from Uadi Aalad and Mulhuli-Amo (Buia), Eritrean Danakil: macromorphology and microstructure[END_REF]. Scale-free three-dimensional relative enamel thickness [START_REF] Kono | Molar enamel thickness and distribution patterns in extant great apes and humans: new insights based on a 3-dimensional whole crown perspective[END_REF][START_REF] Olejniczak | Dental tissue proportions and enamel thickness in Neandertal and modern human molars[END_REF] Enamel thickness topographic distribution in the upper third molar was rendered at the outer enamel surface through a 3D map (where thickness increases from "thin" dark blue to "thick" red), which facilitates the site-specific and synthetic comparative assessment of this complex variable [START_REF] Macchiarelli | Virtual dentitions: touching the hidden evidence[END_REF][START_REF] Macchiarelli | From outer to inner structural morphology in dental anthropology. The integration of the third dimension in the visualization and quantitative analysis of fossil remains[END_REF][START_REF] Bayle | Three-dimensional imaging and quantitative characterization of human fossil remains. Examples from the NESPOS database[END_REF].

A comparative geometric morphometric (GM) analysis of the enamel-dentine junction (EDJ) of the right M3 was performed on the reconstructed virtual surface by placing six sets of 3D semilandmarks along the marginal ridges and transverse crests connecting the mesiolingual and mesiobuccal and the distolingual and distobuccal cusps, respectively, representing a total of 88 points [START_REF] Skinner | Enamel-dentine junction (EDJ) morphology distinguishes the lower molar molars of Australopithecus africanus and Paranthropus robustus[END_REF]. A between-group principal component analysis (bgPCA) was performed to examine overall shape variation of both the extant and fossil samples, and the taxonomically undetermined specimen STS 1039 was thus projected onto the shape space to identify its closest neighbours [START_REF] Mitteroecker | Linear discrimination, ordination, and the visualization of selection gradients in modern morphometrics[END_REF].

For comparative purposes, we used information from the literature and the original high resolution X-ray microtomographic record of a number of Plio-Pleistocene South African and extant cercopithecoid specimens representing a whole of 9 taxa (Table 2). For each variable (tissue proportions, enamel distribution maps and EDJ morphology), the fossil specimens that were used are notified by "X", while the number of individuals is given for the extant groups.

a Ditsong National Museum of Natural History, Pretoria.

b University of the Witwatersrand, Johannesburg.

c Muséum national d'Histoire naturelle, Paris.

d Royal Museum for Central Africa, Tervuren.

RESULTS

The specimen preserves both maxillary bones, the pterygoid bones and part of the left temporal bone, including the mastoid process and the external acoustic meatus, up to the level of the canine jugum (Fig. 1B). Even if only partially preserved, the maxilla apparently lacks the maxillary fossa extending from P3 to M3 commonly seen in Papio and suggests instead a relatively flat and smooth outline. The presence of a maxillary sinus, a structure usually found above the postcanine tooth roots in living macaques and fossil colobines [START_REF] Kuykendall | Presence of the maxillary sinus in fossil Colobinae (Cercopithecoides williamsi) from South Africa[END_REF][START_REF] Rae | Paranasal pneumatization in extant and fossil Cercopithcoidea[END_REF], cannot be assessed in STS 1039 because of incomplete preservation (Fig. 1B). The maximal external breadth across the maxillae measured anterior to the P3s (BP3) and to the M3s (BM3) corresponds to 37.5 mm and 45.7 mm, respectively ( h Makapansgat; male; M, male; F, female. All comparative data are from [START_REF] Freedman | The fossil Cercopithecoidea of South Africa[END_REF].

Together with the relatively complete elements of the right and left upper postcanine dentition, the crown of the left maxillary canine and a small root fragment of the right canine are also preserved (Fig. 1C,D). Because of some occlusal wear having slightly affected both third molars (stage 1B; [START_REF] Delson | Fossil colobine monkeys of the Circum-Mediterranean region and the evolutionary history of the Cercopithecidae (Primates, Mammalia)[END_REF] and the completely developed roots, STS 1039 certainly represents an adult, more likely a young adult individual.

On comparative ground, the minimal estimate of the well-preserved left canine crown height (25.3 mm; Fig. 2A) is compatible with the South African male cercopithecoid condition reported by [START_REF] Freedman | The fossil Cercopithecoidea of South Africa[END_REF]. The third and fourth premolar, as well as the first molar crowns are damaged, most of their enamel being lost. While the roots of the left post-canine teeth are perfectly preserved, those of the right raw, from the third premolar to second molar, are broken (Fig. 2A,B). The postcanine dentition of STS 1039 significantly differs from the high-crowned and columnar-cusped specialized cheek teeth of Theropithecus [START_REF] Jolly | The classification and natural history of Theropithecus (Simopithecus) (Andrews, 1916), baboons of the African Plio-Pleistocene[END_REF][START_REF] Jablonski | Fossil Old World monkeys: the late Neogene radiation[END_REF][START_REF] Jablonski | Cercopithecoidea[END_REF]. Conversely, STS 1039 molar morphology is consistent with C. williamsi, especially by displaying a marked lingual median notch, and both South African fossil Papio and Parapapio taxa, which share flared crowns bearing relatively low bulbous cusps [START_REF] Freedman | The fossil Cercopithecoidea of South Africa[END_REF][START_REF] Szalay | Evolutionary history of the primates[END_REF][START_REF] Jablonski | Fossil Old World monkeys: the late Neogene radiation[END_REF].

The assessed crown dimensions of the left M1 and M2 (following virtual integration of the missing Taung, P. robinsoni (CO 134D, and D. ingens (SB 3, from Swartkrans, Skurweberg and Cooper's, P. angusticeps from Kromdraai A and Cooper's, G. major The use of advanced radiography techniques based on neutron microtomography (n-µCT) to inspect the breccia block embedding the partial monkey cranium from Sterkfontein proved to be successful. So far, n-µCT has been used to detail some vertebrate fossil specimens [START_REF] Schwarz | Neutron tomography of internal structures of vertebrate remains: a comparison with X-ray computed tomography[END_REF][START_REF] Grellet-Tinner | Description of the first lithostrotian titanosaur embryo in ovo with Neutron characterization and implications for lithostrotian Aptian migration and dispersion[END_REF] and to tentatively discriminate between enamel and dentine in highly mineralized hominid teeth [START_REF] Zanolli | Problems and limitations of X-ray microtomography for the endostructural characterization of fossil tooth tissues[END_REF]. However, at the best of our knowledge, this is the first time such highly promising analytical approach is applied to virtually disclose and 3D render the content of breccified primate remains from the South African hominin-bearing cave sites. Our experience with STS 1039 illustrates a potentially novel approach to solving the problems commonly encountered in such contexts, where as much of paleontological work centres on physically removing fossils from breccia blocks, a delicate and time-consuming activity which is not systematically free from the risk of damaging the embedded specimens. Before deciding, case by case, for any physical/chemical extraction-a decision which, anyhow, essentially relies on the specific research questions [START_REF] Schwarz | Neutron tomography of internal structures of vertebrate remains: a comparison with X-ray computed tomography[END_REF]-we note the ability of n-µCT in providing an immediate accurate picture of the whole elements contained in the blocks and in revealing sources of data previously impossible to access (e.g., bone morphology, enamel thickness across the crown, EDJ shape, etc.).

Accordingly, it is likely that advances in n-µCT technology will allow in a near future the extensive investigation of a variety of paleontological remains, including notably for the abundant South African hominin specimens embedded in breccia, for which laboratory X-ray analyses cannot grant the extraction of reliable structural information.

Figure 1 .

 1 Figure 1. The block of sedimentary breccia from Sterkfontein embedding the specimen STS 1039 (A). Virtual slice of the breccia (B) by neutron microtomography (n-µCT) showing the quality of the signal and the preservation state of the specimen, notably of the nasal cavity (a) and of the pterygoid fossa (b). The 3D reconstruction of the virtually extracted maxilla is shown in occlusal (C) and right lateral (D) views. Scale bars: 1 cm.

a

  Distance between the lateral surfaces of the maxillae anterior to P3 b Distance between the lateral surfaces of the maxillae anterior to M3

Figure 2 .

 2 Figure 2. (A) The virtually extracted right (r) and left (l) tooth rows of STS 1039 rendered in semitransparency in buccal (upper) and lingual view (lower). (B) 3D rendering of the upper RM3 showing the outer enamel and dentine (left), the enamel-dentine junction (centre), and the pulp cavity (right) in occluso-lingual view. Scale bars: 1 cm.

  enamel), as well as of the right M3 of STS 1039 are shown in the Table 4 and comparative analyses are illustrated in the Figure 3. In this comparative context, the molar dimensions of STS 1039 globally fit the proportions displayed by fossil Pp. broomi, Pp. jonesi, P. izodi and C. williamsi, and that of extant P. cynocephalus.

Figure 3 .

 3 Figure 3. (A) Occlusal surface model of an extant Papio upper M3 showing the crown length (MD diameter) and mesial (mBL) and distal (dBL) breadths. (B-D) Plots of the crown dimensions (in

Figure 4 .

 4 Figure 4. STS 1039 clearly reveals relatively thin enamel across the entire crown, the thickest areas being located on the buccal cusp apices and along the buccal marginal ridge. The C.

Figure 4 .

 4 Figure 4. Enamel thickness (mm) cartographies of the upper RM3 crown of STS 1039 compared with the condition revealed by some selected Plio-Pleistocene and extant (P. ursinus and Col.

Figure 5 .

 5 Figure 5. Between-group principal component analysis (bgPCA) of the Procrustes-registered shape coordinates of the EDJ morphology calculated for the upper RM3 of STS 1039 compared with the evidence from some Plio-Pleistocene and extant (Papio sp. and Col. sp.) specimens. The shapes at the extremities of the axes illustrate the morphological variation trends along each component. The light and dark grey convex hulls represent the variability expressed by modern Papio and Colobus, respectively.

Table 1 .

 1 Linear, surface and volumetric variables used for assessing external and internal tooth

	structural variation and tissue proportions
	Variables	Definition

  The fossil sample has been detailed by microtomography at the South African Nuclear Energy Corporation (Necsa), Pretoria, and at the Palaeosciences Centre in the University of the Witwatersrand, Johannesburg. Extant specimens were imaged at the platform Accès Scientifique à la Tomographie à Rayons-X (AST-RX), Paris, and using the equipment provided by the French Research Federation Fluides Energie

	Specimen/Sample			Site/Provenance Description	Stored at	References	µCT acquisition	Tissue	Enamel	EDJ
								voxel size (µm)	proportions	maps
	Papio (P.) robinsoni		SK 555	Swartkrans	Male	Ditsong	Freedman, 1957;	77.5			X
				(Member 1)	muzzle	a	Jablonski, 2002
			SK 602	Swartkrans	Male	Ditsong	Freedman, 1957	86.6	X	X	X
				(Member 1)	maxilla	a
	Gorgopithecus (G.) maj	KA 192	Kromdraai A	Male	Ditsong	Freedman, 1957;	63.6		X	X
	or				cranium	a	Jablonski, 2002;
							Gilbert, 2013
	Cercopithecoides (C.) w	M 3055	Makapansgat	Male	Witsb	Maier, 1970;	78.7	X	X	X
	illiamsi				cranium		Jablonski, 2002
	Extant taxa (genus abbrev.)
	Papio (P.) sp.	n = 6		Ethiopia,	Male	MNHNc		32.7-104.5	4	1	6
				Zambia,	&	RMCAd
				Central	female
				African	crania
				Republic,
	Specimen/Sample			Site/Provenance Description Congo	Stored at	References	µCT acquisition	Tissue	Enamel	EDJ
	Colobus (Col.)	n = 3		Congo	Male	MNHNc		voxel size (µm) 32.7-66.2	proportions 2	maps 1	3
	sp.				&	RMCAd
	Fossil taxa (genus abbrev.)			female
	Parapapio (Pp.) broomi	STS 544	Sterkfontein	Maxilla crania	Ditsong a	Eisenhart, 1974;	50.2	X	X	X
				(Member 4)			Heaton, 2006
			MP 224	Makapansgat	Female	Wits b	Freedman, 1976;	81.6	X		X
					cranium		Jablonski, 2002
	Parapapio (Pp.) whitei	MP 221	Makapansgat	Male	Wits b	Freedman, 1976;	71.2		X	X
					cranium		Gilbert, 2013
	Parapapio (Pp.) jonesi	STS 565	Sterkfontein	Female	Ditsong a	Freedman, 1957;	71.9		X
				(Member 4)	cranium		Eisenhart, 1974;
							Heaton, 2006
	Papio (P.) angusticeps	KA 194	Kromdraai A	female	Ditsong a	Freedman, 1957;	74.0		X	X
					cranium		Gilbert, 2013

Réacteurs Matériaux et Transferts (FERMaT) available at the Centre Inter-Universitaire de Recherche et d'Ingénierie des Matériaux (CIRIMAT), Toulouse.

Table

2

. List of the Plio-Pleistocene and extant cercopithecoid specimens/samples detailed by Xray microtomography (µCT) and used for comparatively assessing external and internal maxillary morphology and tooth structural variation and tissue proportions

Table 3 .

 3 Table 3). For both BP3 and BM3 cross-sectional breadths, STS 1039 nears the condition expressed by Pp. jonesi and C. williamsi from Sterkfontein and Pp. antiquus from Taung. Conversely, in terms of absolute values and proportions, STS 1039 differs from both the larger G. major from Kromdraai A and D. ingens from Swartkrans, but better approximates the condition of extant P. ursinus males. Dimensions (in mm) of the maxillary breadth measured anterior to the P3 and to the M3 in STS 1039 and in some Plio-Pleistocene and extant cercopithecoid specimens/samples

	Taxon	Origin	n	BP3a	s.d.	n	BM3b	s.d.
	STS 1039	STSc		37.5			45.7
	Pp. broomi	STSc, MPh	3	41.3		2	49.5
	Pp. jonesi	STSc	2	37		2	47
	Pp. antiquus	TGe	1	36		3	48.3
	P. izodi	TGe				3	44.7
	P. robinsoni	SKd	6	46	8.3	4	53.3
	P. angusticeps	COg, KAf	2	41.5		2	46
	D. ingens	SKd	2	48		1	53
	G. major	KAf	2	47.5		1	54
	C. williamsi	STSc, MPh	2	34.5		2

  The tissue proportions were quantified in unworn crowns available in our comparative sample (Table2). Compared with Parapapio broomi, P. robinsoni, C. williamsi and extant Papio sp. the M3 of STS 1039 shows the lowest values for the total volume of the enamel cap (Ve) and the volume of the coronal dentine and pulp (Vcdp) (Table5), but significantly exceeds the estimates available for Colobus. In terms of both relative (RET) and average (AET) enamel thickness, STS 1039 closely approximates the pattern displayed by C. williamsi in showing a relatively thinenamel (Table5).See Table1for the meaning of the variables and Table2for the genera abbreviations.

	Table 5. Crown tissue proportions (including average and relative enamel thickness) of the M3 in
	STS 1039 (right side) and in some Plio-Pleistocene and extant cercopithecoid specimens/samples
	Taxon (specimen)	Ve (mm 3 ) Vcdp	Vc (mm 3 ) SEDJ	3D	AET	3D RET
				(mm 3 )		(mm 2 )	(mm)
	STS 1039	119.1		189.2	308.3	165.7	0.7		12.5
	Pp. broomi (STS 544)	191.8		238.0	429.8	215.0	0.9		14.4
	Pp. broomi (MP 224)	248.2		254.7	502.9	218.1	, from Kromdraai A 1.1	18.0
	and C. williamsi (AD 1326, M 631-655A-666, STS 252-350-347-361-392-394A, T 15) from P. robinsoni (SK 602) 343.2 447.0 790.3 321.2 1.1	14.0
	C. williamsi (M 3055) 129.9		196.3	326.2	177.6	0.7		12.6
	P. sp. mean (n = 4)	232.4		237.2	469.7	205.9	1.1		18.2
	Table 4. Crown length (MD diameter) and mesial (mBL) and distal (dBL) breadths (in mm) of the min. 163.6 159.0 322.6 167.4 1.0	17.3
	upper LM1, LM2 and RM3 measured in STS 1039
	max.	285.4	MD	281.0	BLm 566.4	231.5	BLd 1.2		18.8
	Col. sp. (n = 2)	49.8		58.0	107.8	81.1	0.6		16.0
	ULM1		8.4		9.6		9.0
	min.	49.5		52.5	102.0	74.7	0.6		14.3
	ULM2		10.5		11.1		10.1
	max.	50.1		63.5	113.5	87.6	0.7		17.7
	URM3		10.2		10.4		8.3
	The cartographies imaging enamel thickness distribution across the overall M3 crowns projected
	in different perspectives in STS 1039 and selected comparative specimens are presented in

Sterkfontein, Taung and Makapansgat are from

[START_REF] Freedman | The fossil Cercopithecoidea of South Africa[END_REF]

; statistics for extant P. cynocephalus and Col. polykomos are from

[START_REF] Swindler | Primate dentition. An introduction to the teeth of non-human primates[END_REF]

.

ACKNOWLEDGMENTS

The authors are indebted to S. Potze (Pretoria), B. Zipfel (Johannesburg), J. Cuisin (Paris), G.

Fleury (Toulouse), E. Gilissen and W. Wendelen (Tervuren) for having granted access to fossil and comparative material under their care. They also thank K. Carlson and T. Jashashvili (Johannesburg), L. Bam and J. Hoffman (Pretoria), G. Clément and M. Garcia-Sanz (Paris), C. Tenailleau and B. Duployer (Toulouse) for micro-X-ray tomographic acquisitions, J. Dumoncel and D. Ginibriere (Toulouse) for methodological support and contribution to data processing. For scientific discussion, they are especially grateful to E. Delson (New York), N. Jablonski (University Park), R. Macchiarelli (Poitiers and Paris), and F. Thackeray (Johannesburg). They thank two anonymous reviewers for their valuable comments that improved the quality of the paper. The French research federation FERMaT (FR3089), the National Research Foundation (NRF) and Department of Science and Technology (DST) of South Africa are acknowledged for providing access to micro-X-ray tomography facilities.