Metal coated polymer spheres for compliant fine pitch ball grid arrays

The further development of heterogeneous 3D integration involving electrical connections between layers of various materials demand compliant fine pitch interconnects to allow for the varying expansion as a function of temperature of the different materials involved. However, most available fine pitch interconnects, like micro bumps and copper pillars, are not particularly compliant whereas available compliant interconnects, like plastic core solder balls, are not fine pitch. In this work we present a novel process for achieving a fine pitch ball grid array comprised of compliant interconnects using singular 30 μm metal coated polymer spheres (MPS) in conjunction with nano-particle silver conductive ink. Results show that when an MPS attached to a chip is flip chip assembled onto a substrate with a 1.3 μm thick layer of wet conductive ink, the ink is drawn up along the MPS surface to such an extent that it a forms a bonding neck between the MPS and the chip surface. It also created a coating up to 1.5 μm on the MPS. Shear testing of the assembly gave values of up to 0.67 grams per MPS. This strength can be increased further by optimizing the process and to utilize such an interconnect in combination with underfill can result in sufficient reliability for several applications. The volume of ink required to form such bonds is in the same range as industrial ink jet nozzles can deliver. MPS was applied onto a substrate which had been jetted with a regular pattern of ink droplets. It was found that the MPS were observed to have had a certain extent of self-alignment along the jetted pattern.