Damage relationships for BGA reliability in harsh environments

Fine-Pitch BGAs and underfills have been used in benign office environments and wireless applications for a number of years, however their reliability in automotive underhood environment is not well understood. In this work, the reliability of fine-pitch PBGA packages has been evaluated in the automotive underhood environment. Experimental studies indicate that the CTE as measured by TMA typically starts to change at 10-15degreesC lower temperature than the Tg specified by DSC potentially extending the change in CTE well into the accelerated test envelope in the neighborhood of 125degreesC. High Tg substrates with glass-transition temperatures much higher than the 125degreesC high temperature limit are therefore not subject to the effect of high coefficient of thermal expansion close to the high temperature of the accelerated test. Darveaux's damage relationships [1992, 1995, 2000] were derived on CBGA assemblies, with predominantly SMD pads and 62Sn36Pb2Ag solder. In addition to significant differences in the crack propagation paths for the two pad constructions, SMD pads fail significantly faster than NSMD pads in thermal fatigue. The thermal mismatch on CBGA's is much larger than PBGA assemblies. Crack propagation in CBGA's is often observed predominantly on the package side as opposed to both package and board side for PBGAs. In the present study, crack propagation data has been acquired on assemblies with 15 mm, 17 mm, and 23 mm size plastic BGAs with NSMD pads and 63Sn37Pb on high-Tg printed circuit boards. The data has been benchmarked against Darveaux's data on CBGA assemblies. Experimental matrix also encompasses the effect of BT substrate thickness on reliability. Damage constants have been developed and compared against existing Darveaux Constants. Prediction error has been quantified for both sets of constants.