Model for BGA and CSP reliability in automotive underhood applications

Abstract

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 un- derhood environment is not well understood. In this work, the re- liability of fine-pitch plastic ball grid array (PBGA) packages has been evaluated in the automotive underhood environment. Exper- imental studies indicate that the coefficient of thermal expansion (CTE) as measured by thermomechanical analyzer (TMA) typi- cally starts to change at 10-15 C lower temperature than the specified by differential scanning calorimetry (DSC) potentially ex- tending the change in CTE well into the accelerated test envelope in the neighborhood of 125 C. High substrates with glass-tran- sition temperatures much higher than the 125 C high tempera- ture limit, are therefore not subject to the effect of high coefficient of thermal expansion close to the high temperature of the acceler- ated test. Darveaux's damage relationships (1)-(3) were derived on ceramic ball grid array (CBGA) assemblies, with predominantly solder mask defined (SMD) pads and 62Sn36Pb2Ag solder. In ad- dition 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 assem- blies. Crack propagation in CBGAs is often observed predomi- nantly 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 23 mm size plastic BGAs with NSMD pads and 63Sn37Pb on high- printed cir- cuit 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. Index Terms—Ball grid array (BGA), bis-maleimide triazine (BT), ceramic ball grid array (CBGA), coefficient of thermal expansion (CTE), non solder mask defined (NSMD), plastic ball grid array (PBGA), thermomechanical analyzer (TMA).