Improving the reliability of quad flat no-lead packages through test & structural optimization

An integrated stress based reliability criterion has been proposed, with application aiming at predetermining and improving the moisture/reflow reliability of QFN packages. A brief account on the fundamental concepts of moisture modeling has been discussed to provide greater comprehensiveness in relation to analyzing moisture induced failure, such as during the course of solder reflow. Key strategies on implementing these concepts in the integrated stress analysis have also been discussed. In the current setup, actual test statistics from moisture/reflow reliability is coupled to mechanical simulation based on the finite element method. Correspondingly, an engineering test parameter termed as the stress endurance weightage (SEW) has been developed. SEW provides a comparative measure of stress overload along the epoxy molding compound (EMC)-die paddle interface of the QFN devices under study. The stress state addresses both structural and material (molding compound in particular) implications on device integrity and it serves as a risk indicator of possible interface delamination for the current QFN test vehicle. A clear advantage of this indicator is the establishment of a common reference across all analyzes, regardless of variations in package geometry as well as material selection. The test methodology was implemented on in-house QFN packages ranging from body sizes of 4×4mm to 8×8mm. Good correlations were achieved in separate validations of moisture/reflow reliability conducted under IPC/JEDEC moisture sensitivity (MSL) level 2 and 3 across QFN packages of different body sizes. With the development of a test chart based on SEW parameter, design of experiments (DOE) can be better refined prior to a full-blown execution, relieving burdens on extensive and inefficient testing.