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Solvent-free fluxing underfill film for electrical interconnection

A variety of underfill materials as film-and paste-types have been developed for electronic package applications, as the die/pitch becomes thinner and finer, recently. The paste-based adhesion such as isotropic conductive pastes (ICPs) is hard to precisely control the required amount, resulting in surplus resins at edges and a laborious process. Common film-based adhesion is deficient in flux, limited to non-oxidizable electrode elements, like Au stud bumps. This film formation also requires solvents or additional UV-curable resins. Here, to overcome these drawbacks, we generated flux-infiltrated adhesive films via a solvent-free, thermally-induced film formation, based on our previous research involved with fluxing underfill pastes. Thermoplastic materials and heat-curable epoxy resins were utilized for newly designed films as a matrix and an adhesive, respectively. They were mixed together at a temperature between the thermoplastic melting and the beginning of epoxy cure to prevent solidification and reaction. The melt-mixture was coated and at the end cooled down onto plastic release films. This solvent-free, facile film fabrication is a promising pathway in terms of convenient processing conditions allowing a solvent-free process during film production. This film also provides a simultaneous fluxing/underfill function during reaction at a temperature where solder components are melted. This solvent-free non-conductive film (NCF) showed good solder wettability and transmittance ∼90%. Toward this end, the combination of thermoplastics and curable resin facilitates flux-adhesive film formation without a solvent and thus has potential to be used for a wide range of electronic package applications.