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Pad Cratering Under BGAs on PCBs
Wednesday, July 15, 2009 | Mudasir Ahmad, Jennifer Burlingame, Cherif Guirguis — Cisco Systems

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 for pad cratering on Printed Circuit Boards (PCBs).

Lead-free solder joints are stiffer than tin-lead solder joints, and lead-free compatible (Phenolic-cured) PCB dielectric materials are more brittle than the FR4 (dicy-cured) PCB materials typically used for eutectic assembly processes. These two factors, coupled with the higher peak reflow temperatures used for lead-free assemblies, 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 to quantify the propensity for pad cratering in different PCB materials is presented. Gage repeatability and reproducibility studies to fully develop the test method were performed. Several different design variables, such as PCB material, resin content, solder alloy, number of reflows, pad size and shape were studied with a range of material sets. The test method was refined to develop a comparative metric that can be used to rank-order different PCB materials and pad size combinations.

Introduction

Mechanical bend and shock tests are routinely performed on surface mount BGA assemblies to ensure that they can sustain anticipated production, handling and end use conditions. The strains and strain rates applied to BGA assemblies during bend and shock testing can lead to a variety of failure modes in the vicinity of the BGA solder joints. Figure 1 shows the failure modes that are known to occur in assemblies subjected to mechanical loading conditions.

The prevalence and distribution of these failure modes depend on several factors, including the solder metallurgy used, the package type, construction, component-to-PCB pad size ratio and PCB materials. Usually, multiple failure modes occur concurrently, at different strain and strain rate levels.

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Figure 1: Different Failure Modes Occurring in a Printed Circuit Board Assembly.


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