Improving the deflection of wire bonds in stacked chip scale package (CSP)

The deformation of gold wire bonds during transfer molding of stacked chip scale package (CSP) can seriously cause wire crossover and shorting. The major challenges of the stacked CSP development are to reduce the wire sweep (deflection), and make the sufficient space clearance between the wires of first to second die. In this paper, M shape wire looping program is developed to increase the wire sweep resistance in the stacked CSP. Both linear elastic finite element analysis and experiments based on wire bonding and molding process evaluation are conducted. It is found that M shape looping program is much better than conventional normal wire shape in terms of wire sweep resistance after molding. X-ray and scanning electronic microscopy (SEM) can verify the improvement of wire deflection after chemical de-capsulation. It is believed that using M shape looping program can efficiently overcome the risk of wire shorting and improve the yield of wire bonds in high volume production of stacked CSP.