A Conformal coating is a protective chemical coating or polymer film 25-75µm thick (50µm typical) that ‘conforms’ to the circuit board topology. Its purpose is to protect electronic circuits from harsh environments that may contain moisture and or chemical contaminants. By being electrically insulating, it maintains long-term surface insulation resistance (SIR) levels and thus ensures the operational integrity of the assembly. It also provides a barrier to air-borne contaminants from the operating environment, such as salt-spray, thus preventing corrosion.
Some typical application examples include high-reliability defence, automotive and aerospace, where coatings are used to protect against various combinations of moisture, aggressive chemicals and vapours, salt sprays, large temperature variations, mechanical vibration, and even organic attack (e.g. fungus). The protective nature of conformal coatings also means that they not only protect, but also serve to enhance product reliability and thereby reduce the potential cost and damaging effects of early field failures.
As such their use is becoming increasingly common in consumer and domestic applications that can be susceptible to environmentally-induced field failure; including portable devices comprising fine pitch, densely populated assemblies, such as mobile phones, through to washing machines.
How do conformal coatings work?
A conformal coating is a protective barrier that shields sensitive electronic components against harsh environmental conditions such as moisture, chemicals and debris. They are not designed to be a total sealant. Conformal coatings are a breathable protective layer that will protect against the particular environment requirement but will also allow any moisture trapped in the circuit board to escape.
The particular advantages of conformal coatings can be summarised as follows:
- Insulating properties allow a reduction in PCB conductor spacing of over 80%
- Can help eliminate the need for complex, sophisticated enclosures
- Light weight
- Completely protect the assembly against chemical and corrosive attack
- Eliminate potential performance degradation due to environmental hazards
- Minimise environmental stress on a PCB assembly
Ideally, conformal coatings should exhibit the following characteristics:
- Simple application
- Easy removal, repair and replacement
- High flexibility
- Protection against thermal and mechanical shock
- Protection against environmental hazards including: moisture, chemicals and other corrosive elements
How do you apply Conformal Coating?
To be effective, a conformal coating should completely cover the assembly and provide good coverage of sharp edges (components and component leads) and other contours (solder joints for example). It may even be essential to coat the edges of the board, as these will not even have any solder resist present.
The effectiveness of the coating is, to some degree, influenced by the efficiency of the application method, which also governs the type of coating used (some suppliers offer dedicated versions of the same coating product to suit different application methods).
There are essentially four main ways of applying a conformal coating:
- Dipping – limited to materials that do not cure quickly by moisture, oxidation or light
- Selective robotic coating – such as Asymtek, PVA or DIMA. All coating types can be used if the correct dispense head is selected
- Spraying – hand spray using a spray booth or aerosol can. All coatings can be applied in this way
- Brushing – requires extremely proficient and skilled operators in order to be suitable for production purposes
Finally you will have to consider the curing method determined by the coating selected, air dry, oven dry or UV light cure. The liquid coating should thoroughly wet all surfaces and cure without leaving surface defects. Epoxies are especially sensitive to surface defects. Epoxies can also shrink while setting and may lose adhesion as a result In addition; excessive shrinking during cure can place severe mechanical stresses upon circuit components.
The supplier, to create the special formulation required for a particular application, should be able to modify all of these properties. Other characteristics can also be enhanced. Accessory chemicals and solvents should only be blended with base polymer resins by the coating manufacturer. Plasticisers can also improve mechanical flexibility, and selected solvents in conventional coating formulations can improve adhesion of the cured film.
How much coating will I use and what will the cost be?
This is obviously related to how many boards you process but there is a simple calculation that can give you a reasonably accurate prediction. Solvent based coatings should be applied at 50µm dry thickness; UV coatings will be between 100µm and 200µm, depending on application method.
The calculation is therefore as follows:
Volume per PCB (mL) = [width (mm) X length (mm) X dry thickness (mm) X 100 / %solid] / 1000
Cost per PCB (£/PCB) = [cost (£/L) X Volume per PCB (mL)] / 1000