Authored By:
Mudasir Ahmad, Jennifer Burlingame, Cherif Guirguis
Technology and Quality Group
Cisco Systems, Inc.
San Jose, CA, USA
Summary
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) [1, 2].
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.
Conclusions
An easy to implement test method for characterizing pad cratering has been presented. This test method can be used in conjunction with the standard bend and shock tests to evaluate and improve the robustness of printed circuit board assemblies. Based on the study results, the key variables analyzed are rank ordered in the summary table below. New variables from the most recent study have been added and are highlighted in bold print. (Table 4)
A complete plot of all the most recent test results and standard deviations is shown in Figure 13. It is important to note that granularity of the test method is limited by the standard deviations for the given sample sizes. The test method appears to be able to detect differences in pull force values with a standard deviation range of about 100-500 grams. It is quite possible that other critical variables impacting pull force/pad cratering may not be detectable if they fall below the minimum granularity of the test method.
Initially Published in the IPC Proceedings
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