Long-term thermal fatigue testing of solder joints and related fatigue life predictions

In order to investigate the damage behaviour of SAC 305 and Innolot solder joints under long-term thermal fatigue stress, field cycle investigations were started in 2009, the status of which had already been reported after 4800 cycles [1]. The conditions for the field cycle (FC) are 21/93 °C, 6 hours cycle time. Meanwhile, the field-cycle tests have been continued after approx. 8 years with a cycle status of 10500 cycles. Further withdrawal points were at a cycle status of 6500 cycles, i. e. 5 years cycle time and 8500 cycles (approx. 6% years cycle time). The solder joints damages occurring during this period were analyzed by metallographic analyses. Of the analysed components DPAK, D2PAK, SO8, SOT23, SOT223, TQFP64 and various leadless ceramic components, the latter have been found to have suffered the most severe damage, of which the paper is exposed. Completely cracked SAC soldered joints could be observed for R1206 from approx. 6500 FC and for R0805 from approx. 10000 FC, whereas for R0603 at this cycle state typical cracking of the stand-off occurred. For Innolot, cracks in the standoff did not appear until 10500 FC. However, cracks at the large angle grain boundaries under swelling tensile stress were observed for this material at an early stage, but these did not lead to electrical failure. These cracks as well as ceramic fractures under test cycle stressing -40/150°C have shown that a reliability prediction based on creep fatigue is not sufficient for Innolot, since other and different mechanisms occur under field and test loads. In addition to the results on questions of reliability in field and transformation relationships to test loading, which are essential for appropriate testing, some questions of microstructural processes of thermal fatigue of SAC and Innolot are discussed and some relationships to the FE-based reliability prediction based on continuum mechanics are established.