National Starch and Chemical Investment Holding Corporation

David Peard

Jayesh P Shah

Paul L Morganelli

Douglas Katze

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.

Method of using pre-applied underfill encapsulant

A method for utilizing one or more pre-formed underfill compositions in the application of surface mount components, most commonly chip scale packages (CSP's), to substrates for use in electronic devices. The pre-formed underfill of the invention is applied directly to the top and/or sides of the CSP before the reflow process and softens during reflow to flow across the circuit/board gap. One underfill composition utilized for this method comprises a thermoplastic film system that provides a coating on the component that is smooth and non-tacky. The film may be applied selectively to parts of the CSP such that it overhangs the top of the component and upon reflow flows over the edge of the CSP to form a connection with the substrate. A second pre-applied underfill composition or solder paste may be applied as an adhesive to provide sufficient tack in order to hold the electronic assembly together during the assembly process and to serve as a flux to facilitate solder wetting.

Growing community of inventors

Windham, NH, United States of America

David Peard