Silicone/Epoxy/Acrylic/Urethane/Paraxylene

Silicone

Silicone coatings range from elastoplastic which is a tough, abrasion-resistant to soft, elastomeric materials. Silicones are typically used in high temperature environments. It has good moisure and humidity resistance. It has good thermal shock resistance due to its flexibility and is also easy to apply and repair. Its moisture resistance is similar to urethane and acrylic and Dielectric withstand is lowered than for the other coatings (1100 volts/mil). Flexibility of coating allows for much thicker film build than comparable acrylic or urethane coating. Its typical temperature range is -65C to 200C.

Epoxy

Epoxy coatings are very hard, usually opaque, and good at resisting the effects of moisture and solvents. Epoxy is usually available as a two part thermosetting mixture and shrinks during curing leaving a hard difficult to repair film. It possesses excellent chemical and abrasion resistance but can cause stress on components during thermal extremes. Epoxy isquite easy to apply but nearly impossible to remove without damaging the components.

Acrylic

Acrylic coatings are typically solvent based. Thermoplastic lacquer base means that the coating is easy to be applied andrepaired. They are usually low cost, tough, hard, and transparent. It exhibits low moisture absorption and have short drying times. However, this type of coating does not demonstrate resistance to either abrasion or chemicals especiallypetroleum solvents and alcohol. It typical dielectric withstand is greater than 1500 volts and has a temperature range of -59C to 132C.

Urethane

Urethane coatings are hard and durable that has excellent resistance to solvents. It has similar moisture resistance to acrylic and silicone. Shrinkages during curing and hard film may stress the electronic components. It is difficult to applyand hard to be removed. Temperature range is quite similar to acrylic. However, its lack of reparability often prevent their use.

Paraxylene

Paraxylene coatings are highly uniform and yield excellent pin coverage. Their limitations include high cost, sensitivity to contaminants, and the need for a vacuum application technique.

Coating process

Before coating a printed circuit board, it must be cleaned and de-moisturized within 8 hours of conformal coating. De-moisturizing can be done in an oven with temperature set to 88 Celcius to 98 Celcius for 4 hours. Methods of coatinginclude spraying, brushing or dipping. Chemical vapor deposition is used to coat with paraxylene. Steps of a spray coating are as listed below.

a) Board is cleaned.
b) Protected areas like terminal pins, connectors are masked off or removed.
c) Coating is applied using a spray process on both sides of the PCB and its edges.
d) Coating is cured using oven according to the coating type.
e) Masking is removed and any removed parts are reassembled.
f) Board undergoes full production testing to ensure.
functionality of board is not affected by the process.