Mitigation of thermal fatigue failure in fully underfilled lead-free array-based package assemblies using partial underfills

A design of experiments was conducted to determine the thermal cycling reliability of full and partial capillary flow underfilled lead-free chip scale packages (CSPs), the tests were carried out based on the JEDEC JESD22-A104C standard. Two kinds of representative fast cure and reworkable underfill materials are used in this study, and CSPs without underfill were also tested for comparison. The test results show that both two underfill materials reduce the thermal cycling reliability of CSP assemblies. The underfill with high glass transition temperature (Tg) and storage modulus yielded better performance, of course, the CTE is also very critical to the thermomechanical fatigue performance, but its effects is not so obvious in this study due to the similar CTE of used underfills. In addition, the negative effect of partial underfill is smaller than that of full underfill. Failure analysis shows that the dominant failure mode observed is solder cracking near package and/or PCB pads.