Impact of Layer Thickness of QFN Package Mounted Printed Circuit Boards During Drop Testing
Sabne, Sayalee Pradeep
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In past studies, it was established that board level reliability of the packages depends mainly upon the design of the package and the soldering technology used for mounting the components. There are many methods to test the reliability, such as temperature cycling test, bend test, power cycling, etc. which have been studied and practiced. We need to consider the failure rate and failure mechanism both, to understand how specific loading conditions affect the failure of an assembly. In this study, PCB’s with variable thicknesses and QFN package mountings are subjected to drop test. It has been observed previously that the failure initiates at the solder joints in the form of crack initiation. Hence, the drop test simulation is carried out using ANSYS workbench with detailing of the solder joints. Under quasi-static conditions, assuming supports to be stationary, impact load gets converted into uniformly distributed load over the surface of the board. The forcing function is dependent on the acceleration of the drop and not on the height of the drop for this particular study. The stress resulting due to impact is analyzed and related to impact life of the package. The results give the idea about stress concentration and the amount of deflection of the board. To understand the effect of variable thickness of the board on the reliability of the package, the thickness of the PCB is varied by varying the thickness of individual layers or two from the PCB stack-up. Each assembly with different thickness of the PCB was tested and analyzed for failure. The study gives an insight on the reliability of the QFN packages under impact loading and ways to improve the reliability of the solder joints.