Effect of Thermal Cycling on Reliability of QFN Packages

Quad flat no-lead (QFN) packages are rapidly gaining popularity in the semiconductor industry. The chip in a QFN package, is bonded to a metal frame and wirebond interconnects form the first level of interconnects between the chip and the leadframe. Package leads do not extend down to the board. Instead, package contacts are available just below the package along the periphery. Owing to their format, QFN packages offer thin profile, low inductance, and low weight. However, the reduced standoff height of the QFN part is a concern for solder joint reliability under thermo-mechanical loads. Assessment of the thermo-mechanical reliability requires methods to accurately model the variance in the part and interconnect geometry including the presence of solder-voids and joint imperfections. FE models often assess reliability using nominal geometry of the assembly. In this paper, model creation based on x-ray micro-computed tomography data has been demonstrated. Molded QFN assembly was fabricated on FR4 board with SAC305 paste. The assembly was scanned using X-ray-CT system and the scanned data was converted into FE model. The model includes all the defects that were present in the solder interconnects. Fabricated parts were subjected to thermal cycle loading till failure. Cross-sectioning of the interconnects was performed to confirm the failure modes. Results from FE model and experimental testing were then compared and validated.