For this month’s column, I’m going to examine the main methods for achieving the best outcome for your conformal coating process, ultimately increasing the protection afforded by your coating and lifetime of your product.  

Let’s start at the very beginning, at the design stage, factoring in careful consideration for coatings and highlighting the potential for problems during production. This will have a positive impact in the long term and avoid excessive time and expense, resolving problems at a later stage. Another important factor for successful coating is selecting the right coating for the job. This could be an acrylic, a polyurethane, a silicone, a UV cure, a two-part (2K) system, or even a water-based coating. Whilst the design of the board doesn’t directly affect the choice of coating material, it will determine which methods can be used to apply the coating. Certain materials, such as moisture curing, 2K or UV curable materials, are difficult to use in a dip process due to the material curing prematurely, so it could be said that the choice of coating is indirectly related to the board design. Similarly, board design will affect the production yield. Consider maximising the distance between components that must be coated and those that must not. The greater the distance between them, the more the production staff will thank you. As a rule, 2 mm is the absolute minimum clearance between coat/no-coat areas, but 5 mm will make an enormous difference in the coating cycle time and success.

Factors such as operating temperature range, temperature excursions (such as thermal shock and thermal cycling), corrosion, condensation, and resistance to chemicals, solvents, and water are all key in determining the successful outcome for a coating.

How Important is Coating Application?
Do not underestimate this element. Probably the single biggest influence on coating success is as a direct result of how well the coating is applied. The application method of a conformal coating is as important, if not more important, than the selection of the right material. I can’t stress this enough, because even the best coating, when applied poorly, may not provide the level of protection required or that it is capable of. Significant elements to consider include the thickness the coating is applied, the level of coverage achieved and how well the coating adheres to the board and its components. It is imperative to choose the best method for applying the coating.

There are numerous methods to apply coatings, which include hand applications such as brush, aerosol and atomised spray, and machine applications such as dip and selective coating. When applied correctly they form a thin film that conforms to the contours of the PCB and its components, as well as the exposed metal of component leads and solder joints. Typically, conformal coatings are applied in the 25–250-micron range.

The application method chosen for a particular assembly will depend upon which existing equipment is available to the manufacturer, the coating processes in use, the takt time (the average time interval between the start of production of one unit and the start of the next), and the design of the assembly. This includes those areas of the circuit, which must be coated and those which must not, connectors, switches and so forth. In addition, it is incredibly helpful to define don’t care areas, which may or may not be coated. The best application method will ensure that each board coated receives good coverage on all required metal surfaces at a sufficient thickness to afford protection against the environment. These requirements will change from board design to board design, and environment to environment, and invariably they need to be tested and verified ahead of the production run.

Coating Thickness
Achieving the correct coating thickness is essential; bear in mind that if the coating is too thick it can lead to entrapment of solvents which can result in blow-out bubbles, delamination, or lack of adhesion. Similarly, it can cause the coating to crack as it cures or as the result of changes in temperature, or due to mechanical shock and vibration. Conformal coatings should not be applied at thicknesses greater than necessary and should never exceed the maximum thickness they were designed to be applied at. 

Is it Necessary to Clean Prior to Coating?
The potential presence of residues on the substrate can have a critical impact on coating performance. If the substrate is not clean, the residues present may interfere with the curing mechanism, lead to poor adhesion of the coating to the substrate, and/or trap conductive/ionic materials under the coating. Without meticulous attention to preparation or pre-coat cleaning regimes, corrosive residues bridging the PCB’s conducting tracks can cause failures over time. It is important to test the reliability of no-clean flux residues from your assembly process with your coating process, to ensure compatibility and adequate performance in your design.