Zero‐stress Film Adhesive for Substrate Attach

Traditional B‐staged epoxy film adhesives have been used as substrate attach adhesive for hybrid circuits. The advantages of using film adhesive instead of paste adhesive are precise bond‐line, clean operation, ease of application for larger bonding area and possibility to automate the process for large volume production. Epoxy, in general, is more stable than polyurethane and is better in adhesion with no contamination problems in comparison with silicone. Traditional epoxy, however, has high bond strength and is therefore limited only to substrates with matched thermal expansion properties. Combining the advantages of excellent adhesion of epoxy and flexibility of rubbery substance, a low glass transition temperature (Tg) epoxy which is flexible in nature has been developed for bonding substrates with mis‐matched thermal expansion coefficient. The new epoxy adhesive is also designed to meet the requirements of MIL‐STD‐883C/5011 in application without sacrificing ionic purity, low outgassing, thermal conductivity and long‐term dielectric or conductive properties. The new flexible film adhesive, in both its insulating and conductive forms, has attracted new applications and designs that were not previously feasible. For example, the flexible film adhesive can be used to bond ceramic hybrids directly to a lower cost metal substrate such as aluminium or copper. Testing has been performed by users on the combination of alumina/aluminium for over 1000 thermal cyclings and shocks from −55 to 150°C for large area bonding (up to a maximum of 6 x 6 inches square). The cured flexible epoxy adhesive, while very pliable, also exhibits more than 1,000 to 2,000 psi lap‐shear strength (depending on the type of substrate) and withstands more than 15,000 g acceleration test.