Characterization And Prediction Of Pad-Crater Fracture In Lead-Free Compatible PCBs
Authors: Siva P.V. Nadimpalli and Jan K. Spelt Company: Brown University and University of Toronto Date Published: 5/4/2011
ICSR (Soldering and Reliability)
Abstract: Epoxy-based printed circuit board (PCB) laminates that are compatible with higher lead-free reflow temperatures can be more brittle than earlier materials. These epoxies can have an increased propensity for pad-cratering in lead-free assemblies. In this study, padcrater fracture was characterized in terms of the critical strain energy release rate, Jci, measured at various mode ratios, (psi). Specimens were prepared from lead-free CTBGA package-PCB assemblies and fractured at low and high loading rates in various bending configurations to generate a range of mode ratios. The specimens tested at low loading rates all failed by pad cratering, while the ones tested at higher loading rates fractured in the brittle intermetallic layer of the solder. The Jci of pad cratering increased with the mode ratio of loading (or phase angle, psi), but was independent of surface finish (OSP and ENIG) and reflow profile (time above liquidus 60 s and 120 s). The generality of the J=Jci(psi) failure criterion to predict pad-cratering fracture was then demonstrated by predicting the fracture loads of single lap-shear specimens made from the same lead-free assemblies.