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.