METHODOLOGY TO CHARACTERIZE PAD CRATERING UNDER BGA PADS IN PRINTED CIRCUIT BOARDSAuthors: Mudasir Ahmad, David Senk, and Jennifer Burlingame
Company: Cisco Systems, Inc.
Date Published: 1/24/2008 Conference: Pan Pacific Symposium
In general, lead-free solder joints are stiffer than tin-lead solder joints, and some lead-free compatible PCB dielectric materials are more brittle than conventional tin-lead compatible PCB materials. These two factors, coupled with the higher peak reflow temperatures for lead free assembly, could transfer more strain to the PCB dielectric structure, causing a cohesive failure underneath the BGA corner pads.
The likelihood of pad cratering occurring in any given assembly depends on several factors including, but not limited to the BGA package size, construction and surface finish; and the PCB pad size, material and surface finish. Standard assembly level bend, shock and drop tests can be used to determine if the entire assembly can survive a given strain and strain-rate range without having any failures.
However, with these standard assembly level tests, it is difficult to determine if the failures occurred due to an unusually weak PCB dielectric/PCB pad size or a stiffer BGA package. It is critical to have a standardized test method that can be used to characterize and rank-order different PCB dielectric materials and PCB pad sizes.
In this study, an easy-to-implement test method is presented, along with results comparing known good and known bad PCBs. Different dielectric materials and pad sizes were evaluated to develop a comparative metric that can be used to rank-order different material/pad size combinations. The results were generated over different temperatures, to study the effect of temperature on dielectric mechanical strength characteristics. Finite Element Analysis was also performed to better understand the factors impacting the variation in results.
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