Accelerated thermal fatigue of tin-lead and lead-free solder joints

Description

The objective of this research was to establish the performance of SnPb and Pb-free solder joints in thermal fatigue, and to identify the effects of the ramp rate and temperature range in accelerated thermal cycling. Three accelerated thermal cycling (ATC) conditions with two ramp rates: 14 and 95°C/min; two temperature ranges: 0 to 100°C and -40 to 125°C were applied to resistor 2512 and PBGA 256 test vehicles constructed with either SnPb or a SnAgCu (SAC) solder alloy. In addition, the effect of the SAC solder reflow cooling rate was investigated using resistors. The ATC results were analyzed statistically to understand the effects of the temperature profiles and solder type on solder joint reliability. The overall effect of the ramp rate was relatively small for both SnPb and SAC solders. For the resistors, the SAC solder joints lasted longer than the SnPb solder joints when subject to the smaller temperature range, but were inferior at the larger temperature range. In contrast, the SAC solder joints in the PBGA test vehicles lasted longer than the SnPb joints regardless of the temperature range. The fatigue lives of solder joints were greatly reduced by an increased ATC temperature range.Finite element models were then used to simulate the stress and strain in solder joints subjected to the above three ATC test conditions. It is concluded that temperature gradients due to shock ATC conditions (95°C/min ramp rate) have a negligible effect on the stress and strain in resistor solder joints. The FE results are well correlated with ATC test results. Strain-based and energy-based thermal fatigue life prediction models using these FE results were able to correlate the ATC lifetimes accurately indicating the suitability of these life prediction equations and illustrating that the high ramp rate data could be correlated with the same equation as the low ramp rate data.

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