Thermosetting adhesives preforms

Dielectric heating can be used to preheat powder, pellets, or preforms to remove water before processing the material. Dielectric heating can also be used to generate the heat necessary for curing polar, thermosetting adhesives, and it can be used to quickly evaporate water from a water-based adhesive formulation. 

Solid epoxy adhesives may also be fused or compacted into various shapes or preforms (see Fig. 13.2) such as sticks, rings, and beads.5 They can then be easily applied to a specifically shaped part. Similar to thermoplastic hot-melt adhesives, these thermosetting epoxy adhesives flow on the substrate by heating. However, the product will cure to an infusible thermoset stage with continued heating. Once cured, the product demonstrates properties similar to those of other structural epoxy adhesives. These solid epoxy adhesives are sometimes referred to as thermosetting hot melts. 

The bonding materials are generally (1) in the form of thermoplastic preforms that bond or seal as a hot-melt type of adhesive or (2) thermosetting paste which cures to a structural adhesive. 

Preforms, tape, or film adhesives, unlike the paste or liquid adhesives, are already in the solid state. They may be thermoplastic or thermosetting but, in both cases, may be purchased as sheets of various thicknesses and cut to size to accommodate specific device or part sizes. In using preforms, the electronics assembler avoids problems associated with the storage, handUng, and dispensing of liquid adhesives. Parts such as leadframes, lids, and substrates may also be purchased with pre-attached preforms, a convenience to the user, who then avoids having to cut and handle individual preforms. Most parts can also be stored for up to one year in ambient conditions without affecting their properties. 

First, in terms of their physical form, adhesives may be either pastes or preforms (also known as films or tapes). Pastes are semisolid materials, easily dispensed through a needle or applied by screen or stencil printing. Film adhesives are solid sheets of thermoplastic or partially cured (B-staged) thermosetting polymers that can be cut to size and generally used to attach large-area components, substrates, and lids. 

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