Effect of PCB thickness on solder joint reliability of Quad Flat no-lead assembly under Power Cycling and Thermal Cycling

Abstract

QFN packages gained popularity among the industry due to its low cost, compact size, and excellent thermal electrical performance. Although PCBs are widely used for QFN packages in handheld devices, some customers require it for heavy industrial application demanding thicker PCB. When an electronic device is turned off and then turned on multiple times, it creates a loading condition called power cycling. The die is the only heat source causing non-uniform temperature distribution. The solder joint reliability assessment of Quad Flat no-lead Package (QFN) is done using Finite element analysis (FEA) under two different loads. In this paper, the power cycling and thermal cycling act as a combined load. The reliability assessment is done to check stress distribution on PCB boards and solder joint. The life to failure is determined for QFN package assembly. Also, three different QFN boards were used for analysis and comparison has been done to investigate the impact of thickness and copper content of board on solder joint reliability under power cycling and thermal cycling. The mismatch in coefficient of thermal expansion (CTE) between components used in QFN and the non-uniform temperature distribution makes the package deform. Modeling of life prediction is usually conducted for Accelerated Thermal Cycling (ATC) condition, which assumes uniform temperature throughout the assembly. An assembly is also subjected to Power Cycling i.e. non-uniform temperature with the chip as the only source of heat generation. This work shows the performance of QFN package assembly under thermal and power cycle in combination and the stress distribution and plastic work for the package. The layered model analysis was done to investigate the impact of the FR4 layer and copper content in the PCB on the solder joint reliability. The comparative study between lumped and layered model has also done under power cycling and thermal cycling.