Thermoplastic films for adhesive bonding: hybrid microcircuit substrates

A study is reported of six generic thermoplastic films that were found to be suitable for substrate attachment and compliant to military specifications: polysulfone (PSF), polyethersulfone (PES), polyetherimide (PEI), polybutylenetephthalate (PBT), polycarbonate (PC) and polyetheretherketone (PEEK). Some of them yielded excellent adhesive strength at a bonding temperature as low as 240°C maintained at the interfaces for 15-60 s, allowing attachment of populated substrates with little risk of degrading the epoxy die-attach adhesive. Since no curing is required or possible, processing time is reduced to a few minutes from the hours required with epoxies. The adhesives have almost no detectable ion content, considerably better thermal stability, cleaner residual gas analysis, higher bond strength, and better electrical properties than an epoxy control. They also provide better bond-line control and easy reworking. The films can be stored for years at room temperature without deterioration, as opposed to the limited shelf life of epoxies, even refrigerated