Evolution of Fatigue Reliability of UF-Substrate Interfaces under High Temperature Exposure

Automotive electronics increasingly requires the use of high I/O packaging owing to applications requiring advanced computing in safety critical functions such as lane-departure warning, collision-avoidance systems, driver-alertness monitoring, adaptive cruise-control, and semi-autonomous navigation. Most electronics are present in front of the firewall in automotive underhood, hence subjected to high temperatures in the range of 100°C-200°C for sustained periods of vehicle use-life. Underfills needed to reinforce interconnects in FCBGAs have been shown to fail at the interface under thermal cycling prior to solder joint failure. Underfill-substrate interface reliability is not well understood. In this study underfills-substrates have been subjected to high temperature 100°C for a sustained period of upto 90 days. The samples have been subjected to cyclic loading till catastrophic failures with 4-point bend. Stress Intensity Factor (SIFs) and number of cycles (Nf) to failure has been measured to characterize fatigue crack growth delamination. The evolution of Paris power law fatigue constants has been studied over the aging period to understand the degradation of the underfills under sustained high temperature operation reflective of automotive underhood. A slope parameter is developed to categorize and rank the performance of the different underfills under cyclic bend loading.