Journal of SMT Article


Authors: Mudasir Ahmad, Jennifer Burlingame, and Cherif Gui
Company: Technology and Quality Group, Cisco Systems, Inc.
Date Published: 1/31/2009   Volume: 22-1

Abstract: The conversion to lead-free Ball Grid Array (BGA) packages has raised several new assembly and reliability issues. One reliability concern becoming more prevalent is the increased propensity of pad cratering in Printed Circuit Boards (PCBs) [1, 2].

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 and shock 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 occur due to a weaker PCB structure 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 in detail, along with calibration and test results comparing a broad spectrum of PCB attributes. 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 pull temperatures and multiple reflows, to study the effect of temperature on dielectric mechanical strength characteristics. Ultimately, these metrics can be used to optimize PCB design, thus mitigating pad cratering potential.

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