Strain Rate and Cyclic Dependencies of PCBA Pad Crater Susceptibility
Authors: John McMahon, Brian Standing, Michael Thomson, Jim Wilcox, Derek Robertson, and Matt Kelly Company: Celestica Inc. and IBM Corporation Date Published: 1/31/2014
Abstract: Pad cratering has been widely recognized as a dominant mechanical failure mode in printed circuit board assemblies (PCBAs) that are subjected to SnAgCu solder alloy processing temperatures. This susceptibility is especially noted in cost competitive, phenolic-cured epoxy based composites, filled with ceramic particles or microclays. A Spherical Bend Test (SBT) program developed by the authors has identified a relationship between the principal rising strain rate associated with PCBA mechanical deformation and the maximum survivable strain for PBGA components. In this work, those results are compared to a similar new experiment using varied design parameters and multiple board materials with the intent of identifying a material independent descriptive equation. On one typical laminate material the experimentation is expanded beyond a single deformation excursion to failure in an attempt to define a surface of safe working strain over a range of cyclic exposures. The scope of this experiment covers a range of strain excursions bounded by strain rate on one axis and SBT cycle count on the other. The limits of this test parameter range are intended to represent strain rate excursions and cycle counts that might reasonably be expected to occur in normal manufacturing environments.
This paper describes the theory and practice of material selection, test vehicle design, assembly, and test methods used to generate data. Selected test results, meaningful relationships and the particulars of the failure modes determined by physical failure analysis are discussed.
Pad Crater, Fracture Mechanics, Spherical Bend Test, Cyclic Mechanical Testing