Model for BGA and CSP reliability in automotive underhood applications

Fine-pitch ball grid array (BGA) 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 plastic ball grid array (PBGA) packages has been evaluated in the automotive underhood environment. Experimental studies indicate that the coefficient of thermal expansion (CTE) as measured by thermomechanical analyzer (TMA) typically starts to change at 10-15°C lower temperature than the T_g specified by differential scanning calorimetry (DSC) potentially extending the change in CTE well into the accelerated test envelope in the neighborhood of 125°C. High T_g substrates with glass-transition temperatures much higher than the 125°C 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 were derived on ceramic ball grid array (CBGA) assemblies, with predominantly solder mask defined (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 the non solder mask defined (NSMD) pads in thermal fatigue. The thermal mismatch on CBGAs is much larger than PBGA assemblies. Crack propagation in CBGAs 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, 17, and 23mm size plastic BGAs with NSMD pads and 63Sn37Pb on high-T_g printed circuit boards. The data has been benchmarked against Darveaux´s data on CBGA assemblies. Experimental matrix also encompasses the effect of bis-maleimide triazine (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.