Anisotropic conductive adhesive films for flip chip on flex packages

Miniaturization and high performance demand more and more flip chip and chip scale packages for consumer products. New packages require increased functionality with a reduction in overall size and weight. The traditional flip chip approaches using solder bumps pose an unacceptably high cost for low-end consumer products. Package technologies for integrated circuits with low to moderate I/O counts (below 150) are critical. A low cost and low profile flip chip on flex CSP package using anisotropic conductive adhesive film (ACF) was developed. This package has the flexibility to use the existing wire bonding pad configuration without adding prohibitive redistribution and wafer solder bumping costs, and also eliminates the need for under-chip encapsulation. Material research and evaluations were conducted to optimize the adhesive material for flip chip on flex applications. Anisotropic conductive adhesive film bonding processes were developed through design of experiments. Critical bonding equipment parameters and process conditions were identified. ACF bonding quality was characterized to adjust the bonding equipment co-planarity. A double layer epoxy film with the second layer loaded with Au plated polymer spheres was identified to be the best ACF material. Contact resistances of the ACF joints were monitored though multiple reflow and thermal-mechanical shock cycles. Various volume production approaches were also explored.