Denis Barbini Ph.D. & Michael Meilunas
Universal Instruments Corporation
Conklin, NY, USA
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.
A strain rate approach for analyzing the ATC reliability data appears to better describe the results than a strain range approach. Using the strain rate approach it has been shown that SAC305 solder joints approximating LGA-like joint qualities (shape, height and microstructure) can be more reliable than larger BGA-like SAC305 solder joints for a given strain rate.
Evidence from this experiment and from literature suggests that the 'small' SAC305 solder joints tend to develop an interlaced twinned Sn grain morphology which has significantly different thermal-mechanical properties than the single grain or beach ball morphology commonly formed in BGA-sized solder joints. These findings suggest that low-profile packages like LGA can be quite reliable in ATC testing - if the proper microstructure forms.
Initially Published in the IPC Proceedings