Failure Site Transition During Drop Testing of Printed Wiring Assemblies

This paper is part of an ongoing effort to develop a test methodology to examine the durability of surface mount interconnects under impact loading conditions. Literature indicates that as the drop height is increased, there is a transition in the failure site from the ductile solder to the brittle intermetallic. Not much work has been done to understand this phenomenon. This study considers damage accumulated in the interconnects in terms of the local strain in the printed wiring assembly (PWA), local strain rate and component acceleration. The advantage is that the results are less dependant on structure and loading, because damage is quantified in terms of specimen response rather than the loading. A simple test specimen is fabricated to concentrate the study on interconnect failure mechanisms. An instrumented, repeatable test setup is developed to conduct high speed bend tests and drop tests on the specimen. All tests are replicated twice for proof of consistency of the test data. The paper presents the results of the high speed bend tests. As expected, the durability of the specimen decreases monotonically with PWA strain. On the other hand, the durability first increases and then decreases as the PWA strain rate increases. Failure analysis shows a transition in the failure site from solder to intermetallic. It is hypothesized that for a given package design, the rate dependent material properties determine the partitioning of the strain energy of deformation. Yield stress of the solder and fracture toughness of the intermetallic are identified as the key parameters. More tests are needed to understand this rate dependent strain energy partitioning. The end goal is to develop a consistent, accurate and generic methodology for ranking the impact durability of different surface mount interconnects technologies.