Wire bonding characterization for small MLP footprint

With the industry trend moving towards miniaturization and cost reduction, leadless molded lead frame based package such as MLP/QFN has become the package of choice due to its good thermal performance, flexibility in matching any leaded footprint, fast turn-around time and low tooling up cost. As a result, the MLP is also highly favored by low power discrete products. However, low power discrete products requires chip size to be as large as possible or dual chips to be packaged within a unit, thereby constraining the lead frame design especially when package sizes go below 2times2mm. This constraint to lead frame design robustness impacts wire bondability and throughput. Moreover, the use of larger diameter wire and multiple bonds to a single lead on small MLP footprint is a major challenge. This paper gives an account of the challenges encountered at wire bond on small MLP footprint where the DAP is poorly anchored causing the DAP to shift or bounce and resulting in poor bond integrity such as ball non-stick or smashed bonds and the ensuing wire bond process characterizations and evaluations that the authors went through to improve the 1^st bond integrity on bouncing and shifting die attach paddle (DAP). The evaluation includes window top plate design, evaluation of various bonding sequences; lead frame based pre-tape material used, and bond parameter optimization. The output of the evaluations were measured through assembly level reliability (ALR) data such as ball shear, wire pull, intermetallic formation, inspection for cratering damage and also the visual appearance of the bonds