Silver filled thermoplastic adhesive with ultra high conductivity for flip chip attach

Electrically Conductive Adhesive (ECA) has the ability to form a highly flexible electrical bond that can withstand the large strain due to the thermal mismatch between the Si chip and organic circuit board to accomplish Direct Chip Attach (DCA). Major challenges to accomplish DCA are two: First, is the poor resistivity of ECA that causes unacceptably large variation in the resistance. For example, a variation of over 15-30% is measured for Ag-epoxy ECA compared to ∼3% for solder. Second, the ECA bond has to be highly flexible with fine feature dispensability at chip-to-board distances of >50 μm to withstand the thermal stresses. To achieve high flexibility, the polymer content must be large compared to the Ag powder. Although, higher conductivity, better than solder, have been obtained for ECA; the amount of Ag powder is above 90% by weight which degrades the adhesion, fine-feature screenability and bond flexibility. Furthermore, epoxy-based ECA will be rigid due to their high cross-linked density. We will describe a thermoplastic based ECA that has 100,000 fold higher conductivity than commercially available ECA at similar fractions of Ag powder. At <32% by volume of Ag, the conductivity is 25% better than solder. Application to DCA using Flip Chip Attach will be discussed in the light of flexibility of the bond, fine-feature screenability, and adhesion to Au surface.