Reliability modeling and testing of advanced QFN packages

Since its introduction in 1998, the quad flat no-lead (QFN) package has been used widely in semiconductor industry for various applications. In the past few years, advanced QFN packages with large body sizes (>10 mm) and multi-row bottom I/O terminals (two or more rows) have emerged as a cost & performance competitive alternative to other BGA styles packages, particularly for small form factor products. The application space of advanced QFN packages is determined by the solder joint reliability when the packages are assembled on the printed circuit board (PCB). This paper presents a comprehensive study on the modeling and testing of board level reliability for selected advanced QFN packages. A parametric, three-dimensional (3D) finite element model was developed for the large body size, multi-row, and fine pitch QFNs on board with considerations of detailed pad design, realistic shape of solder joint and solder fillet, and non-linear material properties. The modeling predicted fatigue life was first correlated to thermal cycling test results using curve-fitting techniques. The model has the capability to predict the fatigue life of solder joint during thermal cycling test within a certain error range. Then, design for reliability and assembly analyses were performed to study the effects of various key package and PCB design parameters including package and die dimensions, material properties, and PCB pad design layout for both the signal and ground/thermal pads, and thermal cycling test conditions. The results from modeling and reliability testing were used to develop best practice design and assembly recommendations.