Conformal coating is applied to circuit cards to provide a dielectric layer on an electronic board. This layer functions as a membrane between the board and the environment. With this coating in place, the circuit card can withstand more moisture by increasing the surface resistance or surface insulation resistance (SIR). With a higher SIR board, the risk of problems such as cross talk, electrical leakage, intermittent signal losses, and shorting is reduced.
This reduction in moisture will also help to reduce metallic growth called dendrites and corrosion or oxidation. Conformal coating will also serve to shield a circuit card from dust, dirt and pollutants that can carry moisture and may be acidic or alkaline.
There are several types of conformal coating materials and the selection of one for your application must consider several variables.
In today's manufacturing environment with no-clean fluxes being used for many products, there are concerns about applying conformal coating over the flux residues. In fact, many of the suppliers don't recommend this step, stating that residues can reduce adhesion, potentially resulting in delamination, and that it creates micro-condensation conditions; which can be more detrimental than no conformal coating
The application where the coating will be used should also be considered. For example:
The automotive industry specifies conformal coatings to protect circuitry from gasoline vapor, salt spray and brake fluid. The use of electronic systems in vehicles is growing rapidly, and as such, the use of conformal coatings is becoming vital to ensure long term reliability. PCB coatings are used in applications both under the hood (e.g. engine management systems), and in passenger compartments (e.g. onboard computers).
The aerospace industry with its high reliability requirements is also a viable application for different coatings. The environmental requirements of the aerospace industry where rapid compression and decompression can affect the performance of circuitry, necessitates the use of conformal coatings. The coatings are used in both pressurized and depressurized areas.
Both fresh and salt water environments will attack electronic circuitry. Coatings are ideal for the protection of equipment used for these applications, which can range from under the dash of high performance boats, to exterior equipment used on larger maritime systems.
Similarly, in the medical industry there are numerous areas where a conformal coating will be required for environmental protection: Tool protection while in storage to prevent corrosion; pacemakers, where it is vital to ensure continuous performance and even food carts in hospitals. The ideal conformal coating will have performance requirements that include good electrical characteristics, low moisture permeability, good chemical resistance and mechanical integrity. It must adhere to the printed wiring board and mix of component surfaces.
There are several choices of both conventional and new materials available for use as coatings. Understanding your end use application is vital in making the appropriate selection. For example, an acrylic coating might not be the ideal choice for an automotive application, due to the high temperatures involved and exposure to moisture or petroleum residues. A better choice would be a silicone coating, which has a usable operating range of -55A�C to +200A�C and offers resistance to high humidity environments. A UV cured coating may not be the best option if the assembly being coated has high-profile components. A possible result is that shadowing can leave uncured coating which compromises the reliability of the PWB.