All these products vary from soft and elastic to hard and rigid. Specific gravity is from 0,1 for some foams to more than 2 for bulk-filled resins. Syntactic foams are between 0,5 and 0,8. Before the hardening (curing) process, potting compoundsare liquids, pastes or even powders. Some cure by atmospheric exposure (one-part systems), others by the addition of a catalyst (two-part systems).

To avoid the formation of bubbles, it is frequently necessary to pour small quantities, de-gas under low vacuum (a few Pa), pour again, and so on.

True foam products foam and rise in place (the “debubbling” operation is not applicable).When closedmoulds are used, the quantity of initial product shall be carefully calculated to produce the specified final volume and density. Moulds equipped with overflow vents are also used. The hardening of potting compounds shall be done under a certain cure schedule,which can be as simple as a few hours exposure at room temperature. Sometimes a simple heat treatment is necessary. In other cases, post-curing under vacuum is required. When curing is by atmospheric moisture, the curing time depends on the accessibility of air; thick samples need a longer time. Foams are cured simultaneously with the rising process, and the rate of both should be matched to produce the specified density, implying good control of the curing-temperature profile. When fillers are used, they shall be carefully dried and shall be kept dry until and during mixing.

Some catalysts used in potting compounds and sealants have corrosive effects on metals (for example dibutyl-tin-dilaurate on copper).Most of them are in one way or another harmful to man and safety precautions are necessary. The need for correct debubbling procedures is stressed above. The viscosity of the mixture to be poured should be low enough to permit a good flow in intricate devices. Most potting compounds and sealants have a limited shelf life and pot life depending on the conditions of storage and use.

Except when special fillers are used, potting compounds and sealants have a rather low thermal conductivity and overheating of enclosed parts can occur in powered devices.

• Vacuum exposure of potting and sealant materials leads to problems analogous to those of conformal coatings , i.e. contamination of the vicinity and possibility of corona effect due to release of gases in cracks and voids when these products are used in the presence of strong electrical fields. Closed-cell foams contain gases (CO2 or freon), which normally take a very long time to evolve even under space vacuum: some foams can be considered for space insulation up to 5 kV. Contamination of the vicinity by potting materials is sometimes diminished by a postcure under vacuum or by an “egg-shell” varnish applied as a thin layer over the potted module.
• Radiation exposure of potting and sealant materials is normally minimal, since they are mostly used inside modules.
• Temperature effects shall be considered. On the low side, potting and sealants shrink and becomemore rigid, their damping ability gets worse and internal stresses rise, particularly in potting cured at high temperatures. On the high side, chemical degradation can occur, particularly around power-dissipation component. Silicones have the best high-temperature properties (class 180). Thermal cycling due to the switching on and off of equipment can lead to cracking and debonding.

• Araldite CY205,
• Epikote 828/Versamite 140,
• RTV 566,
• RTV S691,
• Solithane 113,
• Stycast 1090,
• Stycast 2850FT