Waterproofing a Printed circuit assembly (PCA) is to allow the circuit to have either flowing or stationary water on the surface of the board without causing any of the electronics to fail in the short or long term.
Preventing problems from water flowing on a circuit board is one of the most difficult problems in coating application. This is because any area not adequately covered by the coating is a potential shorting point for the electricity.
Also, waterproofing is regularly confused with moisture protection. Most coatings whether thin or thick film provide adequate moisture protection if applied reasonably. However, being able to protect a PCA from water where the board could be submerged and expected to work is a completely different problem.
“When waterproofing a printed circuit board using thin film technologies like conformal coatings, the critical factor is tip coverage. Get this wrong and the coating may as well not be there”
When considering waterproofing you need to consider the severity of the attack from the water (or liquid). For example, is the PCA going to be occasionally splashed or have condensation form on the surface in a small amount? Or, is the PCA to be submerged completely for long periods of its life and expected to function the whole time? Again, this can change the way you approach the waterproofing production process since other factors like costs for manufacture, material costs, reparability, heat dissipation, weight and size can also have important influence on the performance of the circuit.
In terms of options for waterproofing, the choices fall into two areas. That is thin film (liquid conformal coatings) and thick film (potting and encapsulating materials). The former gives benefits such as a lower weight coupled with less bulk, easier to repair and potentially less problems with heat dissipation. However, with waterproofing by thin film coverage of printed circuit boards the critical factor is tip coverage. Get this wrong and the coating may as well not be there. This naturally adheres to all areas of the printed circuit board and is truly homogeneous. For thin film coatings this is your best option for waterproofing the circuit. If this coating cannot be applied then the liquid conformal coatings can be used but extra care and attention is required for the tip coverage and special techniques for application are needed.
With thick film technologies such as potting and encapsulating the issue is less to do with providing a waterproofing coating since this is almost guaranteed. Put a PCA in a brick of epoxy resin and no water is getting to the circuit. However, you now have to consider all the issues of heat management, repair in the future, the additional weight of the device, the potential costs of material and the bulk that the potting compound can add to the device. Again, this is a balance between several key points and all must be weighed together to give the right option