The challenge of overcoming wire sweep in ultra-fine-pitch wire bonded ball grid array packages

The constant pursuit to reduce silicon area, and accordingly silicon cost, continues to drive the bond pad size and pitch down to smaller dimensions as time moves forward. The reduction in bond pad pitch will require a progressive step reduction in wire diameter due to the smaller bonded ball size that is required for the smaller bond pad openings. As the silicon area is reduced, and wire diameters get smaller, the wires are typically getting longer and ever more susceptible to the molding stresses in ball grid array (BGA) packages, especially when multiple BGA's are molded simultaneously using a single cavity on a single substrate. This molding configuration while providing the most cost-effective substrate utilization causes higher wire sweep, thus increasing the opportunity of adjacent wires shorting to one another and causing device failure. To facilitate a smaller wire diameter in an existing package many things must be analyzed and experiments must be designed. The wire type and diameter, mold compound's formulation, molding process parameters, and wire bond layout design must be carefully considered to account for the increase in wire sweep expected with a smaller diameter wire. The methodology and experimentation used to address the packaging and assembly requirements of a smaller diameter wire will be discussed. The experiments demonstrate that wire type, wire looping profile, mold transfer speed, and mold compound filler size and distribution prove to be significant in reducing wire sweep.