Impact of Molding Compound CTE and Mechanical Constraint on Solder Joint Fatigue Life in QFN and BGA Packages under Thermal Cycling

The fatigue life of solder joints has long been a critical reliability focus on the communications industry, particularly for devices utilizing Quad Flat No-Lead (QFN) and Ball Grid Array (BGA) packaging. With the advent of 5G technology, the increasing demands for high power output, miniaturization, and energy efficiency in RF systems have significantly intensified the thermal loading conditions within electronic devices. These evolving operational requirements result in more frequent and wider temperature fluctuations during daily usage, imposing greater thermal and mechanical stress on solder interconnects and thereby posing heightened challenges to their long-term reliability.In this paper, a QFN-packaged RF power amplifier (PA) and a fine-pitch BGA-packaged eMMC components are selected as representative study objects, to enhance the understanding of solder joint fatigue life at the product level through a combined experimental and simulation-based approach.In the experimental phase, a design of experiments (DOE) methodology was employed to systematically vary the CTE of the molding material used in the packages, thereby assessing the impact of different thermal expansion mismatches on solder joint life. Additionally, a mechanical fixture simulating actual installation scenarios was designed, wherein the test PCB was mounted using screws to replicate a "fixed" condition and compared against a "free" (unconstrained) setup to evaluate mechanical constraint effects.To further investigate stress behavior under thermal cycling, finite element analysis (FEA) was conducted to identify stress concentration zones on the PCB during high and low temperature conditions. The simulation results were validated through physical measurements using strain gauges.