Reliability of Lead-Free LGAs and BGAs: Effects of Solder Joint Size, Cyclic Strain and Microstructure

An accelerated thermal cycle experiment comparing similarly constructed area array devices representing Land Grid Array (LGA) and Ball Grid Array (BGA) technology with 0.254, 0.30, and 0.40mm diameter SAC305 solder balls was performed. The devices were subjected to three thermal cycle conditions in order to promote 2nd level solder fatigue. Failure data was compared using Weibull analyses. The results show that time to failure is highly influenced by the package pitch and thermal cycle temperatures in a manner predicted by simple mechanics. However, there were instances in which the effect of solder ball size did not fit the traditional solder joint reliability model in which increasing solder joint standoff height improves reliability (i.e. more cycles to failure). A theory is proposed that substantial differences in SAC305 solder joint Sn grain morphology may explain, at least partially, the discrepancies and evidence to support this theory is presented.