A Thermal Fatigue Failure Mechanism of 58Bi-42Sn Solder Joints

Surface mount components soldered with 58Bi–42Sn on printed circuit boards fell off after about 500 cycles of−45°C ↔ 100°C in a thermal chamber. Failure analysis revealed that the failure was caused by dissolving of Pb from either HAL (hot air leveled) board surfaces or component lead coatings into the Bi-Sn solder joints, forming 52Bi–32Pb–16Sn eutectic phase that melts at 95°C. The dissolving of Pb occurred primarily at reflow, when the Bi-Sn was molten. During the thermal cycling, the small amount (<7 volume % of a Bi-Sn solder joint) of the low melting eutectic phase provided liquid channels for fast atom transportation, causing exceptionally significant grain growth and phase agglomeration in the whole volume of the solder joint, which in turn caused dramatic reduction in mechanical strength of the solder joints. Solutions to the failure may be either using Pb-free coating on both board surfaces and component leads, or limiting the product service temperature to below 95°C.