The green material effect on the board level reliability qualification of QFN packages

In the past few years, QFN packages with large body sizes and multi-row external terminals as well as small body devices with ultra-fine pitch pads have emerged as a cost & performance competitive alternative to other BGA style packages, particularly for small form factor products. Due to pb-free and halogen free conversion requirements, green (environmentally friendly) molding compounds have been developed for these components. These compounds have higher filler content generating lower CTE properties in order to reduce thermal stress in the QFN component during the SMT. The lower CTE molding compound may also impact thermal fatigue board level reliability. This paper presents a study evaluating the use of green molding compound and pb-free lead finishes on board level reliability for selected QFN packages. In this paper, temperature cycling tests (0 to 100°C) with a dwell time of 10 minutes were used for testing interconnect reliability at board level. Failure analysis using cross-sectioning and surface analysis techniques was undertaken to understand the failure mechanisms. Furthermore, 3D-CT X-ray (3-Dimensional computerize Tomography) techniques were also applied to inspect and evaluate the solder joint quality and failure modes on non-cross sectioned samples. The experimental results can be used to help develop a robust design for QFN packages from material viewpoint.