Bismaleimide (BMI) adhesives are used in the manufacture of flexible printed electronic circuit boards and other electronic devices and in applications where high temperature resistance is required. The cured adhesives have a similar molecular structure as polyimides with similar high temperature resistance. They are suitable for long term exposure to temperatures up to 400 °F (200 °C) and for short term exposure up to 450 °F (230 °C).
BMI adhesives offer noticeable improvements in processability over polyimides. The latter are obtained by polycondensation of dianhydrides with diamines resulting in high-molecular precursor polymers that are soluble in some solvents. The subsequent imidization creates the typical imide structures. This process leads to the final linear, non-soluble polymers with high thermal resistance. For this reason, the shape of the polymer article must be defined prior to imidization because they cannot be melted afterward. On the other hand, bismaleimides are low molecular resins (usually in the form of a dry powder) having already the imide structure. These monomers undergo various polyaddition reactions with themselves as well as with other co-monomers. In contrast to polyimides, the polyaddition reactions of bismaleimide does not produce linear polymers but three-dimensional, crosslinked, thermosets.
Bismaleimides are seldom used in their pure, solid powder form. In most applications, they are blended with reactive co-monomers, like aromatic amines, vinyl and allyl compounds, and allyl phenol compounds. These mixtures have usually better processability. They are paste-like liquids and can be cast to the desired shape. Often solvents are added to improve the processability. These adhesives are normally cured for several hours at temperatures between 300 and 350 °F (150 -175 °C) under pressure. Afterward, they are post-cured at or near the maximum service temperature without pressure.
BMI adhesives are suitable for joining metals (steel, stainless steel, aluminum etc.), glass, ceramics and some plastics. They exhibit very high thermal resistance, and very high solvent resistance. However, the BMI adhesives currently available are quite rigid and have rather low peel strength. Solvent-free bismaleimide adhesives are often sold as film adhesives. These films consist of a dry layer of bismaleimide cast onto a (polyester) carrier film. The processing of these film adhesives is often much simpler than that of adhesive pastes, since the adhesive can be cut to the desired shape and then applied to the bond area. After cutting the film to the required shape, the adhesive film is placed on the bond area and the carrier film is peeled off. Then the second bond area is pressed against the adhesive and the ahesive is subseqently cured by heating to 300 to 350 °F (150 - 175 °C). The strength of BMI film adhesives is comparable to the paste-like BMI adhesives – in some cases the films perform even better. Film adhesives are prefereble to pastes when used on flat, smooth bond areas.