Selection of chemistry as basis for best electronic protection

Figure 1: Comparison of properties of typical polymers for conformal coatings (left) and casting and potting resins (right)

When the chemistry is chosen, a compromise must always be reached between the different material properties in each case. Polyurethanes and polybutadienes are generally very versatile and have a good combination of material hardness and flexibility, chemical resistance and a broad temperature range in which they can be used. But if the electronics must be protected from mechanical forces or very aggressive chemicals, epoxides are the first choice. The extreme hardness of the material can also lead to problems, however. Stresses resulting during temperature fluctuations, for example, are transmitted directly to the component and can even damage it in the worst case. For this reason, epoxides are often used only for very specific applications in the electronics field. By contrast, polyolefins are characterised by their flexibility, which results in their outstanding performance in temperature shock testing, and by their high degree of hydrophobicity, which provides excellent protection against moisture and makes polyolefins the obvious choice for protecting electronic components with direct water contact. For applications exposed to high operating temperatures and also requiring a high level of material flexibility over the entire temperature range, silicones are the gold standard. However, silicones have a higher permeability to water vapour and corrosive gases in comparison with other polymers. Table 1 shows selected materials from each product group, divided into conformal coatings and casting resins.