The invention relates to a method for utilizing one or more B-stageable or pre-formed underfill encapsulant compositions in the application of electronic components, most commonly
chip scale packages (CSP's) to substrates. One such composition comprises a
thermoplastic resin
system comprising a phenoxy resin, an expandable
polymer sphere or thermosetting composition, optionally an
epoxy resin such as higher molecular weight
epoxy resin, a
solvent, an
imidazole-anhydride catalyst or comparable latent catalyst, and optionally, fluxing agents and / or
wetting agents. The underfill encapsulant may be B-stageable to provide a
coating on the substrate or component that is smooth and non-tacky. In an alternative embodiment, the underfill encapsulant is a pre-formed film. In both embodiments the expandable filler material expands upon the application of higher temperatures to form a closed-
cell foam structure in the desired portion of the
assembly. The method of applying the underfill application of the underfill to a component or substrate, attachment of the component and substrate, and heating of the
assembly to a temperature sufficient to cause the expandable
thermoplastic or thermosetting composition to foam. A second pre-applied underfill composition containing an
epoxy resin, an anhydride curing agent, and catalyst may also be applied, either separately or in conjunction with the foamable underfill. The second composition acts as a
pressure sensitive adhesive and may be applied selectively to parts of the CSP, for example to the solder bumps. The
pressure sensitive adhesive property of the composition provides sufficient tack in order to hold the electronic
assembly together during the assembly process.