Evaluation about Solder-IMC Crack of Fine Pitch BGA Package

Abstract

FBGA is widely adopted as AP and memory package for mobile devices such as a smart phone and a smart watch. FBGA is attached to PCB assembly during SMT process. Double side SMT is preferred to increase the battery area for slim design and long life of mobile device. Solder-IMC crack is in-process reliability failure occurs during 2nd side SMT. The buried via is often used in double side SMT PCB is filled with filler material of CTE is highly mismatched to surrounding PCB laminates (FR-4). Tensile stress is applied to BGA pad by this CTE mismatch at high temperature and causes the solder-IMC crack. In this paper, we evaluate the factors of solder-IMC crack related to PCB and BGA. It is found that the length of buried via related to PCB thickness influences the solder-IMC crack. But if the location of buried via is not aligned to the location of BGA pad, solder-IMC crack does not occur. In the case of buried via and BGA pad are aligned each other, the failure rate of solder-IMC crack can be lowered by controlling O2 level that is less than 1000ppm and by applying solderball with high Ag content for SAC alloy. The temperature cracking solder-IMC interface is measured. The result shows that the solder-IMC crack occurs before solder joints are fully melted. The solder joint with low Ag content having broad melting temperature range is relatively weaker than high Ag having near eutectic point. It is recommended to design the PCB with buried via which avoids the location of the BGA pad and this is the good solution to reduce the failure rate of solder-IMC crack to near zero.