The Significance of Underfill on the IC Packages Subjected to Temperature Cyclic Loading

The melting point of Sn—Ag/Sn—Ag—Cu solders are higher than that of Sn—Pb solders by 30°C in particular. These in turn lead to significant divergences in plastic and creep behaviors. This research uses the validated finite-element analyses to study the elastic-plastic-creep behaviors of the 96.5Sn—3.5Ag, 95.5Sn—3.8Ag—0.7Cu lead-free solders, and the classical 63Sn—37Pb solder bumped wafer level chip scale package (WLCSP) on printed circuit board (PCB) assemblies subjected to four different temperature cycle tests. The behaviors of equivalent stress and equivalent total strain hysteresis loops are examined in the five key areas, which are the center, upper-right, upper-left, lower-right, and lower-left, at the outmost corner solder joint. It can be seen that: (1) for the elastic-plastic-creep behaviors to these three solder alloys, the range of the temperature loading is the most important factor to assess the reliability of the solder bumped WLCSP on PCB assemblies; (2) in the case of the same scale to the WLCSP and the WLCSP with underfill, the difference of the equivalent total strain range exceed an order of magnitude for all the three solder joints.