Surface Mount International Conference Proceedings


Author: R. Wayne Johnson
Company: Auburn University
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: Generally, in the process of reflowing flip-chip solder bumps during assembly, a low-residue no-clean flux is employed. After reflow, the flip-chip assembly is under-filled with an epoxy-based encapsulant and cured. Manufacturers have been concerned about the potentially harmfull effects of the flux residues on the underftll properties. Most low-residue fluxes are comprised of rosin or other organic acids. These residual acids can act as polymer-chain terminators when the epoxy is crosslinked, potentially reducing the adhesion Tg, and modulus of the encapsulant in the vicinity of the flux residues. To resolve this concern, new fluxes comprising synthetic organic acids that crosslink during reflow have been developed. These polymer fluxes have the additional benefit of also crosslinking with the epoxy undefilll encapsulant. When these polymer fluxes are used in flip chip assemblies in conjunction with commercially-available underfill encapsulant, a more robust flip-chip assembly is created. The polymer fluxes also have the potential to entirely supplant the tedious and expensive post-reflow underfilling requirement. Keywords: Flip chip, flux, reflow, assembly, underfill, polymer, encapsulant, solder bump

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