SMTA International Conference Proceedings


THE IMPACT OF THERMAL CYCLE REGIME ON THE SHEAR STRENGTH OF LEAD-FREE SOLDER JOINTS

Authors: Milos Dusek, Martin Wickham and Christopher Hunt
Company: National Physical Laboratory
Date Published: 9/26/2004   Conference: SMTA International


Abstract: This paper describes work that was to undertake a comparison of accelerated test regimes for lead-free solders. Identical samples were subjected to six different regimes to investigate the effect of thermal excursions, ramp rates and temperature dwells. The most damage to joints was found to be caused by thermal cycling between -55 and 125°C, with a 10°C/min ramp rate and 5-minute dwells. Larger thermal excursions were shown to give faster results without compromising failure mode.

Similar degrees of damage to lead-free solder joints were experienced with thermal shock regimes with ramp rates in excess of 50°C/min. However, these regimes, although faster to undertake, appeared to cause different crack propagation modes than the thermal cycling.

There is a very little difference between SAC (95.5Sn3.8Ag0.7Cu) and SnAg (96.5Sn3.5Ag) solder alloys across all types of cycles. Thermal cycling causes cyclic strains and develops cracks in a similar way to natural usage and weakens the joint structure by cyclic fatigue. Thermal cycling can therefore be conveniently employed in accelerated testing of the joint, to assess alloy reliability. The purpose of this work is to assess various thermal cycle loading regimes in terms of fatigue damage induced in a Pb-free solder joint.

Although various thermal loading cyclic regimes have been developed over 40 years for Pb-based solders, these may not be applicable for lead-free alloys. As the thermalcycling is applied in wider temperature range than is generally the case in the field use, the loading is accelerated and an acceleration factor can be established. For Pb-free alloys, the acceleration factor may not simply be proportional to a temperature range as is the case for Pb-based alloys. Hence thermal-loading regimes used to evaluate reliability of solder joint have to be re-evaluated for Pb-free alloys.

Key words: reliability, lead-free, thermal cycling



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