Effects of aging and underfills on mechanical-drop tests of SnAgCu PBGA (plastic ball grid array) packages on ImAg PCB (printed circuit board)

The effects of aging and underfills on the drop test of lead-free (SnAgCu or SAC) PBGA packages on ImAg (immersion silver) PCB are investigated. Two different underfills are studied. These underfills are dispensed on the PCB to fill the gaps between the BT (bismaleimide triazene) substrate of the PBGA, the Sn3wt%Ag0.5wt%Cu solder balls and paste and the PCB. In addition, two more sets of samples (without underfull) are tested (one is with SAC solder balls and paste and the other is with Sn37wt%Pb solder balls and paste), which serve as controls. The sample size for each of the 4 different sets of samples is 24 and half of them (12) are subjected to temperature aging (125°C for 500 hours). The test condition is based on JESD-B111 (the acceleration = 2900G and the impact duration = 0.3ms). A strain gauge is used to measure the deformation of the test board during impacts. The true characteristic life, true mean time to failure (MTTF), and true Weibull slope of the PBGA interconnects under certain confidence requirements have also been determined. Failure analysis has been performed and examined on some of the failed samples. The results presented in this study should be useful for understanding the effects of aging and underfills on the lead-free PBGA interconnect reliability under drop conditions.