Parametric study of the QFN mounted PCB- Cu layer thickness effect on the stress induced during drop testing

• Published in: 2016 15th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm) pp. 1530 - 1536
• Main Authors: Sabne, Sayalee, Mahmood, Anik, Barua, Trina, Agonafer, Dereje, Kansara, Nachiket
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.05.2016
• Subjects: Acceleration; ANSYS simulation; board level reliability; Copper; Deformable models; Drop Test; Finite element analysis; multi-layered PCB; QFN package; Reliability; solder joint reliability; Soldering; Stress
• ISSN: 1087-9870; 2577-0799
• DOI: 10.1109/ITHERM.2016.7517730

In past studies [1], 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 [2] 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 layer thicknesses and QFN package mountings are subjected to drop test. The packages are custom made with probable high density end application. It has been observed previously that the failure initiates at the solder joints in the form of cracks. Also, the solder joints fail at two particular regions [6]. Hence, the drop test simulation is carried out using ANSYS workbench with detailing of the solder joints. 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 the possible impact life prediction models are suggested based on it. 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 layers on the reliability of the package, the thickness of individual layers is varied. As the core of the PCB needs to be constant in terms of material and composition, the thickness of only the outermost layers is altered. Each assembly with different thickness of the PCB layers 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.