Improving the Vibration Reliability of SAC Flip-Chip Interconnects Using Underfill

In this study the vibration durability of Flip-Chip (FC) solder interconnects at room temperature (RT) was investigated. A specimen design, specially designed for vibration fatigue investigations at various temperatures, was modified for the use of FC components. Daisy-chain structures allow an in-situ electrical resistance monitoring in order to precisely detect failure events. A 20% increase of the initial resistance was used as the failure criterion. Two different solder alloys, SAC305 (Sn96.5 Ag3.0 Cu0.5) and Innolot®, as well as components either w/ or w/0 underfill material (UM) were used for the vibration fatigue investigation in this study. Destructive physical analysis (DPA) was conducted by means of cross-sectioning in order to inspect failure sites and crack propagation within the solder joints. Using the obtained time to failure (TTF) data of the experiments, reliability (Weibull) analysis was conducted and Weibull distribution parameters were derived. SAC305 solder alloy was found to be much more fatigue resistant against vibration loads compared to Innolot®. As expected, using an UM to enhance the physical strength of the FC solder joints, increased the characteristic life under vibration load by up to 13-fold.