Resins used for these applications can be of various chemical types. Epoxy resins have been widely used for many years – they are generally hard and tough and exhibit low shrinkage on cure. They are characterised by an excellent level of mechanical properties, good high temperature performance and good adhesion to a wide variety of substrates – chemical resistance is also good. The cross-linking or curing process generally takes place slowly especially where small volumes of resin are involved. Fast cure hardeners can be used but these generate much heat during cure, giving rise to a high exotherm which can damage electronic components and cause high mechanical stresses on both components and the circuit.