PCS Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, U. K.

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1 @ EDP Sciences, Les Ulis DOI: /jp4: Dynamic properties of solders and solder joints C. R. Siviour, D. M. Williamson, S. J. P. Palmer, S. M. Walley, W. G. Proud and J. E. Field PCS Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, U. K. Abstract : The high strain rate mechanical properties have been measured for three types of solder : 63% Sn 37%Pb, 96.5Sn 3.5 Ag and 95. 5Sn 3. 8Ag 0. 7Cu. Bulk properties were measured using a split Hopkinson pressure bar (SHPB) at strain rates from 500 to 3000 s'. A novel method bas been designed and used to measure the shear strength of solder joints at low and high rates, using an Instron and SHPB respectively. 1. INTRODUCTION The recent massive expansion of the portable electronics industry has made dynamic testing of solder joints a priority. There are a number of reasons for this. Modem surface mount technology means that the solder joints, and the substrate beneath them, have to provide a physical support, as well as electrical connections. Also, telephones and computers are increasingly mobile, and therefore prone to experience more severe shocks, for example from being dropped, than in the past. These devices often undergo thermal cycling, due to heating of the device when in use, which induces rapid ageing of the solder, characterised by grain growth [l], and diffusion of metals from the substrate into the solder [2]. Grain growth is known to contribute to weakening of materials [3, 4], and the other metals act to embrittle the joint. Finally, the introduction of lead free solders means that it is necessary to evaluate the strengths of both traditional and lead-free solders for future use [5]. Failure can be categorised into three types : those where the solder joint yields or breaks, those where the joint disconnects from the circuit board, and those where a break occurs inside the board. The following paper will outline recent experimental progress in the study of the first two types. There is very little published experimental research on the high strain rate properties of solder : we know of only three papers [6-8]. Many papers have however, been published on the quasi-static, or creep behaviour, e.g. réf. [9j. This is an important area of study, since solder creeps considerably, even at room temperature [10]. However dynamic events, such as occur in the dropping of a mobile phone on the floor can, can produce strain rates locally of the order 1000 s' [11,12]. Research that has been performed in shear and compression shows that there is a very strong dependence on strain rate and température.-1 This paper outlines measurements of the bulk properties of solder between 500 s and 3000 s-l using a split Hopkinson Pressure bar (SHPB). Experimental procedures were also developed to allow the shear properties of solder joints to be measured, in an Instron at quasistatic rates, and dynamically in a SHPB. 2. EXPERIMENTS AND RESULTS The solder was supplied in two forms. For the shear and spall experiments, PCBs were supplied with an array of solder joints of mm diameter, spaced mm apart. The joints consist of balls of solder attached to copper pads on a polymer substrate. These joints had been subjected to the standard heating and cooling profile used in PCB manufacture. The material for the bulk mechanical experiments was provided as flux-free solder wire. This was made into Hopkinson bar specimens by melting it into a silica glass tube. The tube of molten solder was then cooled rapidly by immersion into a bath of cold water. This formed a solder rod, which was then machined into specimens of 6mm diameter and 2mm or 5mm length. The radial inertia for solder

2 specimens of this size was calculated as being negligible at strain rate of3000 s' (thé rate used in these tests), using the analysis ofgorham [13, 14]. maximum 2.1Bu!kmateriat The Hopkinson bar used was made of Inconel rods instrumented with semiconductor strain gauges. Inconel was used because its acoustic impedance is only a weak function of temperature [15]. Three solder compositions were tested, all three being eutectics. Their melting points are shown in table 1. Composition Melting temperature/ C 63% Sn 37% Pb % Sn 3. 5% Ag % Sn 3. 8% Ag 0. 7% cru 217 Table 1. Melting temperatures of the solders studied. All three compositions were tested at three strain rates at room temperature. The rates were nominally 500 s'\ 1000 s'', and 3000 S. I. Stress equilibrium was checked in all the experiments, and was found to be achieved at around 10 as after the start of the loading pulse. The tin-lead solder was also studied art-40 C and +60 C. This was achieved by heating or cooling helium gas, which was then passed into a small chamber surrounding the specimen and the ends of the Hopkinson bars. The temperature in the chamber was measured using a thermocouple. The strain gauges were shielded from the helium gas as they are very temperature sensitive. The temperature gradient in the bars was such that the temperature change at the gauges was very small. The gauges are part of a potential divider circuit, which is arranged to make their output insensitive to temperature changes. Figure 1 shows stress strain curves for the bulk solder at room temperature. The SnPb solder shows elasto-plastic behaviour with a distinct yield point. The other two compositions examined show neither of these and also no noticeable strain rate dependence. Figure 2 shows that the tinlead solder exhibits strong temperature dependence. Both the strain rate and temperature dependencies are linear over the region studied. SEM micrographs, figure 3, show that the microstructure of the SnPb solder is quantitatively and qualatively different to that in the other two types. The SnPb solder bas distinct tin and lead grains, whilst the other two have small precipitates, but no larger structure s-1 : æ : : æ : looloo 4035s-i 011' " 045 s : : J True strain True strain Figure la. Stres strain curves for SnPb solder. Figure lb. Stress-strain curves for SnAg solder.

3 <II J Z r 10s-' s-1 = 1--ll00 -' = tdc zozo o True strain True strain Figure le. Stress-strain curves for SnAgCu solder. Figure 2. Stress-strain curves for SnPb solder, at 950 i : 30s''. Figure 3a. SEM micrograph of SnPb solder. Figure 3b. SEM micrograph of SnAgCu solder. Figure 3e. SEM micrograph of SnAg solder.

4 2. 2 Solder joint shear experiments For the quasi-static experiments, an Instron loading machine was used, allowing strain rates from 10-5 to 0' s-l to be studied. The apparatus is illustrated in Figure 4. A PCB was mounted in the jig and the brass blade above moved down at a chosen speed. The load to shear the solder ball was recorded. The dynamic tests were carried out in a second jig, figure 5, which could be held in an SHPB. The PCB was held on the input bar, and the blade on the output bar. The bars and jig were made of Dural. The force trace obtained from the output bar gave a direct measurement of the force transmitted by the solder bail into the blade. The balade could shear either 1 or 2 balls at any time. Typical output traces are shown in figure 6, and a full set of quasi-static and dynamic results in figure 7. Wo Figure 4. Apparatus for quasi-static testing of solder Figure 5. Jig for dynamic testing of solder joints joints. 1 M z. SnAg 2, Onejoint 30 joints.snagc 50 0 SnPb 50' LL , 10-, O. C) Time 1 ps Blade speed (m/s) Figure 6. Force time curves for dynamic shear of SnAg Figure 7. Shear force supported by solder joints. solder joints.

5 3. DISCUSSION 3.1Bu)kresutts The three types of solder studied exhibit a number of differences. Whilst the SnPb solder is elasto-plastic and strongly strain rate dependent, the other solders are not. The specimens were prepared from the melt using the same cooling regime. The SEM micrographs of the three grain structures show that the SnPb solder is very different from the other types. This may explain the difference in the results. 3.2 Shear resutts From figure 7 it would appear that the shear strength is strain rate dependent, although the relationship is not a simple one, as a polynomial line of best fit is required on the logarithmic plot. At low strain rates the solder is able to flow easily, as any dislocations (or other defects) have time to activate and move. Solder is a very ductile material that can flow under its own weight. However, higher strain rates do not allow enough time for the defects to respond and the dislocations become entangled, thereby stiffening the material and giving rise to the observed increase in shear strength. For the SnAgCu material, there is an increase up to a blade speed of m s' and then a decrease. The dynamic experiments also show that the SnAgCu joints are weaker at high rates than either the SnPb or the SnAg. This is tentatively explained in terms of a change from shear of the material to failure of the interface between the material and the PCB. This was supported by post experiment observations, but needs further study. It should be noted that while in the dynamic shear experiments the bimetallic solder joints were stronger than the trimetallic ones, the SnPb material itself was weaker than either the SnAg or the SnAgCu. This may be because of grain size effects : whilst the joints were produced with similar grain sizes, the equipment was not available to produce the bulk material in this way. Instead the bulk material samples were produced at the same cooling rate, which was observed to give different grain sizes. 4. CONCLUSIONS Methods have been developed to study the shear strength of solder joints over a wide range of strain rates. The compressive strength of solder bas also been tested at impact strain rates over a range of temperatures. The results from these experiments are believed to be governed by the different grain structures of the materials studied. Further studies to measure the range of shear and bulk properties are being carried out. Experiments on bulk specimens of different grain sizes are also being performed. The results from these experiments will allow modelling of the response of portable electronic equipment to high rate loading to be carried out. Acknowledgements This research is part of a collaborative research program between Cambridge, the National University of Singapore (NUS), and the Institute of Mocroelectronics (IME) Singapore. E. H, Wong is thanked for his support as co-ordinator. R Flaxman and R Smith are thanked for their help in preparing the bulk solder specimens, and GB Siviour for some of the SHPB tests. CR Siviour would like to thank the EPSRC, QinetiQ and the Worshipful Company of Leathersellers for their financial support.

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