Take a quick look at the photograph on your right. If you are in the business of PCB manufacturing with conformal coatings, does it frighten or concern you at all? Does the magnified coating appear to truly “conform” to the surfaces evenly? And if not, what effect, if any, might this have on the protection of your PCBs?
First of all, don’t panic. What you see in the photograph is not unusual and is not necessarily a problem. It is important, however, that if you are in the business of applying conformal coatings, you should have a clear understanding of some of the unique challenges they present.
A basic definition of a conformal coating is a coating, which is applied to the PCB surface and “conforms” to the various irregularities and contours of components at an even and consistent thickness. This of course means that, as an example, applying 50 microns of dry coating would be measured at 50 microns on flat sections, vertical sections, and corners of components. Since nearly all commercial conformal coatings are applied as liquids, it is in fact quite challenging if not impossible to apply a coating in a truly uniform thickness across all the areas.
Challenges to Uniformity of Thickness
Conformal coatings are applied using multiple methods including
- manual atomized spray
- “fan” or non-atomized spray
- selective atomized spray
Each of these approaches has its own unique challenges to achieving a uniform thickness, but in general the following factors can and will affect uniformity:
- Coating Solids and Viscosity
Lower solids and lower viscosity coatings will tend to flow off of vertical surfaces more readily and solidify more slowly. This can lead to very even thickness on horizontal regions but thin areas on more vertical areas. High solids and viscosities can have an opposite effect, with better adhesion to angled areas but lack of flow and consistency in flatter areas.
- Carrier Solvent Volatility
Most conformal coatings are carried and dissolved in industrial solvents or water. The volatility of the carrier fluids can affect thickness and uniformity. Slower evaporating fluids will decelerate drying and solidification and lead to more flow off of vertical components. Fast evaporating solvents can help avoid excess flow but can lead to skinning or “orange peel” thickness variations on flat areas.
- Desired/Required Coating Thickness
Conformal coatings are generally expected to dry in the range of 25 to 75 microns thickness. Attempts to coat at 75 microns or higher can lead to “puddling” in corners or between components at excessive thicknesses. Coating on the low end at 25 microns and below will often result in the coating flowing to below minimum thickness on top corners of components.
Related article: The Importance of The Conformal Coating Thickness
- Surface Tension of the Coating
The surface tension of the coating is a value to compare the energy required to allow the liquid to flow over a surface evenly. The lower the surface tension of the liquid, the more likely it is to flow out evenly. Organic solvents tend to have much lower surface tension values compared to water; thus, solvent-borne products tend to be more capable of resulting in an evenly wetted surface and thickness.
- Surface Energy of the PCB and Components
Surface energy is a measurement of the ability of any surface or material to pull a liquid to form an even coating layer. It is in some ways the opposite of liquid surface tension in that higher values of surface energy result in better and more even wetting of the coating. The ideal situation for coating uniformity is high surface energy of the solid and low surface tension of the coating. It is common that plastic surfaces of PCB components will have low or inadequate surface energy due to mold release or plasticizing agents covering their surface. This often leads to poor wetting on tops and corners of components.
Related article: What is PCB Surface Energy and How Does It Affect Conformal Coating
Strategies for Achieving Uniform Thickness
As discussed earlier, it is essentially impossible to achieve a completely uniform coating thickness using a liquid conformal coating. This is really not an issue in most cases as long as the thinnest of areas is capable of maintaining above the minimum thickness for your application.
In extreme cases, lack of uniformity can lead to failures. Too thin on vertical surfaces and top component corners can lead to a lack of protection from liquids and gases. Too thick and puddling on inside corners or due to excessive viscosity can result in inadequate drying and curing and then possible cracking during thermal or mechanical shock cycling.
Below are some practical suggestions to consider when addressing conformal coating uniformity:
- Problems on Horizontal Surfaces
Solutions to uneven coating on horizontal surfaces include using more or slower evaporating thinners to allow formation of a more even surface, and cleaning or treating board surfaces to increase surface energy.
- Problems on Sloped or Vertical Surfaces
Solutions to issues on sloped surfaces include use of higher solids or higher viscosity coatings, employing atomized spray to facilitate solidification time, and application of multiple thinner layers of coating. In the most extreme cases, it is also possible to spot coat problem areas with a compatible gel coating that adheres to sloped surfaces.
- Problems on Inside Corners and Between Tightly Spaced Components
As discussed, it is common for coatings to flow into inside corners and between tightly spaced components, resulting in excessively thick areas. Actions that minimize flow, such as higher solids and viscosity, can help. It can also be effective to apply multiple thinner coating layers to allow each layer to dry and adhere faster and without flowing.
As a supplier of electronics protection polymers and coatings, Chase Corporation and our employees can help you with an unbiased approach to evaluating your application and process. We’ll show you how to maximize efficiency, minimize cost, and improve product reliability. Our outstanding manufacturing and technical support groups can provide your organization with reliable global supply, unmatched quality, and superior technical support.