ACKNOWLEDGEMENTS

First of all, I would like to thank Dr. Mohammed Cherkaoui for providing me an opportunity to work under his supervision. His support, guidance and motivation have always been a great inspiration during the entire work.

I would like to thank Dr. Karim Inal, Dr. Olaf Van der Sluis, Dr. El Mostafa Daya, Dr.

Raphael Pesci, Dr. Suriyakan Kleitz and Mme. Stephanie Blanc who accepted my request to be the part of my PhD committee. I strongly appreciate their suggestions and remarks.

I would like to thank the whole techno-group of Schlumberger, Paris, for providing me an opportunity to work on such a challenging project. I would like to thank Jean-Sebastien Lecomte for his help in Nanoindentation, Claude

Guyomard for the die design, Olivier Naegelen for the sample casting and Marc Wary for the polishing and etching processes. I am also thankful to Abderrahim Nachit and Ammar for their help in the electrical characterization of the solder alloys.

I am thankful to Dr. Min Pei for his help in the coarsening models and David Macel for his support in wettability testing.

My experience at University of Lorraine was made more pleasant by my colleagues Wei, Mathieu, Bhasker, Malek and Liaqat, Sajid, Rafiq, Armaghan, Aamer, Jawad, Khuda Bux, Mohsin and Fahd. I would like to thank all of them.

I would like to offer special thanks to my brother Muhammad Arif for all his help and support in editing and organizing my thesis.

I would like to thank the whole team of University of Lorraine who helped me in everything to make this period more interesting and enjoyable. Ag 3 Sn

RE RE RE K Q d d t T R T RE d d RE RE K K R E RE Q Q RE

PILE-UP AREA CALCULATIONS

c c A b h pu i A a i c ic i i b A h Ch a r op pu S E A A op pu F H A A 8.7 CREEP CHARACTERIZATIONS BY NANOINDENTATION b t CHAPTER 9

WETTABILITY TESTING

Wettability of Solders

Wetting is the behavior of a liquid to wet a solid surface. This behavior is very important to the interconnection of electronic packages, especially in soldering, for making highly strong bonding between different solid components. In order to attain successful soldering, a certain degree of wetting of the molten solder on the solid substrate surface is required. So generally, a wetting or solderability test is used to measure,

The initial wettability of the component termination materials

The wetting properties of a newly developed solder A proper metallurgical bond is always necessary to form before analyzing the wetting performance of any solder alloy. This bond, of course, varies for different substrates. An interfacial reaction takes place which forms certain amount of IMCs or in some cases an Inter Metallic Layer (IML) at the two interfaces. These IMCs or IML works as an adhesion layer between the substrate and the solder and hence keep them firmly together.

The wettability of any lead-free solder alloy varies with changing its chemical composition and the substrate. For SnPb, the wettability is good over many metallic substrates including copper. Thus, changing from SnPb to lead-free, wettability is an important concern. The substitute must have sufficient wettability under severe service conditions and for different soldering processes such as reflow and wave soldering.

Wettability Measurement Methods

There are mainly 2 methods for the wettability measurements; Spread-area test (good for reflow soldering) and the wetting balance test (good for the wave soldering). They are described below.

Spread-Area Test

A solder disc (e.g. 6 mm in diameter and 1 mm in thickness) is coated with flux and melted, and then allowed to solidify on a substrate (e.g. Cu). When a bond is formed, the free energy is reduced and hence the solder changes its shape [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]]. This change in shape causes an increase in the contact area which indicates the wetting behavior of solder. In some cases, the ratio of the as-bonded area to this new area (after soldering) is taken as the wettability of solder.

This wetting is given by wetting (or contact angle), c , and is calculated using the Young Dupre's equations as 9.1 and 9.2 [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]

Cos

where SF , LF , and SL refer to the surface tensions of the substrate/flux, liquid (solder)/flux, and substrate/liquid (solder) interfaces, respectively, as shown in Figure 9.1 [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]]. For a good wetting, the contact or wetting angle should be small (i.e. LF , and SL smaller and SF larger). The spread area test is a good approximation for the reflow soldering process. The solder disc, in the spread area test, has a similar shape to the layer of solder paste by screen-printing or in similar process. Besides, the solder disc in the spread area test and the solder paste in reflow soldering undergo the same process of heating. They are heated above the melting point and are then allowed to solidify.

Wetting Balance Test

Wetting balance test is the second important technique to evaluate the solder wettability.

In this method, the coupon (for example Cu) is dipped into the molten solder, present inside the crucible at a temperature more than its melting point. The molten solder climbs up the Cu coupon due to wetting force exerted on it as shown in Figure 9.2 [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]]. As spread area test was a good approximation of reflow soldering, this wetting balance test is a good approximation of the wave soldering in which the substrate is brought into contact with the molten solder. The height of the solder onto the Cu coupon depends on many parameters including the soldering temperature, alloy composition and wetting time. Generally, the wetting

F Vg p

Where p is the perimeter of the coupon, LF the surface tension of the solder in contact with the flux, c the contact angle, the density of the solder, g the acceleration due to gravity, and V is the immersed volume. A typical wetting curve is shown in Figure 9.3 which represents different types of wettings for solders, in which the wetting time, t w , is the time at which the solder contact angle to the coupon is 90°, as shown at point B. Wetting occurring in a short wetting time is considered to be good. Hence, the wetting force and time of each solder system are revealed by the wetting balance test.

The wettability of lead-free solders becomes crucial when high solder joint reliability is needed. As presented in equation 9.4, the contact angle is one of the most important parameter in characterizing the solderability (wettability) of any lead-free solder. A smaller contact angle, c , and thus higher cos c is representing good wettability. Ideally, cos c can reach to 1 which makes c to be 0° which is the best possible wettability. But, in practice, it is somewhere in the range of 40 to 50°, as shown in Figure 9.4, depending on the temperature applied, flux used and many other experimental parameters. On the other hand, a contact angle of more than 90° is not recommended for good joining. A schematic of contact angle more than 90° is provided in In [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]], the impact of RE elements, mainly Ce and La, has been studied

for the wettability performance on SnPb, SnAg, SnCu and SAC. It has been reported that they can all be soldered successfully using the rosin mildly activated (RMA) flux.

The wettability of SnPb solder is quite good in comparison to SnAg, SnCu and SAC alloys. In any case, over the copper substrates, SAC alloys perform well whereas the RE dopings have shown significant improvements in the wetting performance of all SnAg, SnCu and SAC alloys [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]]. In most of the cases, the addition of RE elements help to reduce the wetting angle. It can be seen that the amount of RE elements needs to be optimized for the best wetting performance, Figure 9.6.

An extensive experimental study on wettability study on Sn3.5Ag eutectic and Sn-3.5Ag-RE doped alloys was made by [START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]. The wetting curves of SnAg, SnAg-0.25RE and SnAg-0.5RE are compared with that of SnPb in Figure 9.6. It can be seen that all four solders have roughly the same wetting time. However, the wetting force of SnPb is the highest among the four alloys. The effect of adding RE elements (mainly Ce and La) is clearly demonstrated by a significant increase in the wetting force of Sn3.5Ag-0.25RE alloy as getting closer to the SnPb solder. However, an excessive amount of RE addition can lower the wetting force. In the same study, this effect was also applicable to the wetting angle, such that SnAg-0.25RE has the lowest wetting angle among SnAg, SnAg-0.25RE and SnAg-0.5RE.

Sample Preparation And Experimental Conditions

The equipment used for the wetting balance test was METRONELEC as shown in Figure 9.7, a French manufacturer of soldering equipments. The copper foils used as substrates were 0.025mm thick and 12mm wide. They were purchased from the "Good Fellows", France, with a purity of 99.9%. The samples were cleaned with acetone and dried. Later on, they were dipped into 20 % HNO 3 solution for 20 seconds and then placed in the flux for about 1 second. The flux used was resin mildly activated (RMA) and was manufactured by "Metaux Blancs Ouvres" with fluxing code BC 310.15. The immersion depth into the crucibles, for the copper substrate, was 3mm as shown in Figure 9.8. The wetting time used was 10s and the wetting temperatures applied were 250°C and 260°C.

Cos

The angle as directly linked to surface tensions is thus representative of the wetting quality. For a smaller contact angle, and thus better wettability, LV should be as small as possible. This is the main function performed by adding the RE elements which decrease the surface tension and this reduces the contact angle as performed during the experiments. This is important to mention that the contact angle is only conceivable for the alloys in the molten state.

Meniscography Principle

As described earlier, after immersing the Cu coupon into the molten solder bath, the surface tensions are highest at the solder/flux interface. The measurement of the resultant force is representative of the meniscus and, consequently, of the wetting angle and of the solderability.

The principle set forth above is described in equation 9. Where is the mass density of the solder alloys, which is provided in Table 9.1, P is the perimeter of the coupon immersed into the solder crucible and g is the acceleration due to gravity (9.81 m/s 2 ). Thus F is the measured force from which the contact angle is deduced.

Wettability Measuring Parameters

Generally, there are 3 important parameters for the wettability measurements. These are the surface tension, wetting force and the contact (or wetting) angle. These parameters with required description and evaluation are provided below.

Flux-Solder Surface Tension

In order to determine the wettability in terms of wetting force and contact angles, it is necessary to calculate the surface tension, LV . The results for surface tension are given in RE is in excess (>0.25%), is mainly because of the oxidation resistance during wetting due to the strong affinity of RE to oxidize. Oxidation at the solder surface inhibits the molten solder from contacting with the solid substrate, which usually causes non-wetting behavior. But when the oxide is massive, or the oxide quickly generates, the flux cannot perfectly remove the oxides, and the wettability will degrade. This has also been confirmed by the work performed by [START_REF] Zhou | Investigation on properties of Sn 8Zn 3Bi lead-free solder by Nd addition[END_REF]]. They showed (after TGA analysis) that 0.1% of Nd is the optimum amount as the surface tension increases if Nd is more than 0.1% which finally increases the contact angle. Similarly [START_REF] Shi | Effects of small amount addition of rare earth Er on microstructure and property of SnAgCu solder[END_REF]] have suggested the maximum RE (Er) doping as 0.1-0.4% and [Wang el al. 2002] have demonstrated RE (Ce) as 0.25-0.5% in SnAg alloys. In the work performed by [START_REF] Noh | Effects of cerium content on wettability, microstructure and mechanical properties of Sn Ag Ce solder alloys[END_REF], they stated that RE is liable to oxidation [START_REF] Huang | Creep behavior of eutectic Sn-Ag lead-free solder alloy[END_REF] due to its strong affinity towards oxygen and have suggested 0.3 % of Ce as the optimum doping in SAC alloys. The

Summary

Extensive work has been performed on the wettability testing of SAC and SAC-La doped alloys at 2 different temperatures of 250°C and 260°C. It was noticed that increasing temperatures make better wettability for the same composition. This is dedicated to the decreasing effects of surface tension at elevated temperatures.

For the SAC-La doped alloys, the surface tension decreased due to RE dopings at both 250°C and 260°C as compare to the SAC alloy. RE doping further increases the wetting forces from 5.7 mN (SAC) up to 6.7 mN (SAC-0.25La). SAC-0.5La has smaller wetting force than SAC-0.25La which means an optimization of RE elements is to be specified for better alloy compositions.

Wetting or contact angles measurements were made on the basis of the surface tensions already evaluated. An appreciable decrease in the contact angles was investigated and once again SAC-0.25La has a better (smaller) contact angle than SAC and SAC-0.5La alloys.

I

  am extremely thankful to Dr. Raphael Pesci for all his contributions, support and motivation during the entire experimental work. fr attitude gave me the real motivation to work in a friendly environment. I am thankful to Dr. El-Mostafa Daya and Dr. Jean-Marc Raulot from LEM3, University of Lorraine, France for their enriching discussions and valuable remarks.

Figure 9 . 1 :

 91 Figure 9.1: Schematic diagram of the spread area test

  Figure 9.2: Coupon geometry for wetting balance test on molten solder alloy[START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]

Figure

  Figure 9.5.

Figure 9

 9 Figure 9.3: Variation of wetting force with time [METRONELEC]

Figure 9

 9 Figure 9.6: Wetting curves of Sn-3.5Ag-RE solders compared with Sn-Pb solder[START_REF] Wu | Properties of lead-free solder alloys with rare earth element additions[END_REF]

Figure

  Figure 9.7: METRONELEC wettability setup

Figures 9 .

 9 Figures 9.10-9.16 for different compositions and at different temperatures. Good reproducibility

Figure 9 .Figure

 9 Figure 9.10: SAC surface tension vs. wetting time at 250°C

  -9.20. The average values with 5% error bars are given in Figure 9.21.

Figure 9 .

 9 Figure9.17: SAC wetting force vs. wetting time at 250°C

  oxides is also confirmed during our EDS mapping on the free surface of unpolished specimens.

Figure 9 .

 9 Figure 9.22: SAC contact angle vs. wetting time at 250°C

  ], ..........................................................................(9.1)

	SF	SL	LF Cos	c
	c	SF	SL	.............................................................................(9.2)
			LF

Table 9 .

 9 1: Densities for SAC and SAC-La alloys

	Alloy	Density (mg/mm 3 )
	SAC	7.45
	SAC-0.05	7.453
	SAC0.25	7.46
	SAC-0.5	7.48

CHAPTER 6 MECHANICAL PROPERTIES