One of the most common customer support calls we receive is about solving conformal coating application issues. Given the number of variables involved in a conformal coating process (e.g. coating formula, viscosity, substrate variations, temperature, air mix, contamination, evaporation, humidity, etc.), it’s no surprise that issues come up frequently. Let's take a look at some of the more common problems that can crop up when applying and curing conformal coatings, potential causes, and associated cures.
- Dewetting - This is caused by contamination on the substrate that is incompatible with the coating. The most likely culprits are flux residues, process oils, mold release, and fingerprint oils. Dewetting can be recognized simply by observing areas of good coating application next to areas where the coating beads up and moves away from the contaminated area. Think of it as putting a drop of detergent in oily water – the oil immediately moves away from the single drop. Cleaning the substrate thoroughly prior to coating application will resolve this.
Photo courtesy of NPL - Delamination - This is characterized by an area of the coating losing adhesion to the substrate, which can cause the coating to lift from the surface. Though there are a couple of common causes for this problem, one main cause is again a contaminated surface. Usually, you will only notice delamination after the part is out in the field, as it is not immediately observed in the majority of cases. Proper cleaning can prevent delamination. Another cause is insufficient tack time between coats. In this scenario, the solvent would not have had the proper time to flash off prior to the next coat. Ensuring adequate time between coats is a must.
Photo courtesy of NPL - Air bubbles - Can be caused by a coating layer not leveling and adhering to the substrate surface, causing air to be trapped underneath. As this air rises through the coating, it creates small bubbles. Some bubbles collapse to create concentric rings in the shape of a crater (for larger caters, see “Fisheyes” section below). Another cause can occur during brushing. If the operator is not extremely careful, the brushing action can entrain air bubbles into the coating, which results in the same cratering effect.
Photo courtesy of NPL
- More bubbles and voids - The majority of bubbles are caused by a solvent that has been trapped, which ultimately vapors through the coating layers. If the coating layer is too thick, if the coating goes into accelerated (heat) cure too quickly, or if the coating solvent evaporates too quickly, the coating surface may skin over while there is still solvent underneath that is vaporizing up. All of these situations can lead to bubbles in the top layer.
Photo courtesy of NPL
- Fisheyes - These are small circular areas, highlighted by “craters” in the center, and are usually seen during spray application or shortly thereafter. This effect can be caused by entrained oil or water in the sprayer air system and is commonly seen when using shop air. Prevention comes in the form of a well-maintained filtration system which scrubs any oil or moisture from entering the sprayer.
Photo courtesy of NPL
- Orange peel - This looks exactly what the term implies – an uneven, mottled appearance. Again, there can be multiple causes. If using a spray system, air pressure that is too low can cause uneven atomization and ultimately result in this effect. If using a thinner in the spray system to reduce viscosity, incorrect choice of thinner can sometimes cause it to evaporate too quickly. This does not allow the coating adequate time to level out properly. Another cause of the orange peel is applying too many heavy wet coats. If you identify this as the issue, reset the sprayer parameters and adjust the spray technique accordingly.
- Cracking/crazing - Highlighted by long cracks or areas of smaller cracks (crazing). This is usually caused by excessive film thickness or insufficient time between overcoats. If curing with heat acceleration, exposing the wet film to excessively high temperatures can also cause this to occur. A staged evaporation rate is always desired, so a lower temperature for a longer time may be in order. Some coatings recommend a two-step accelerated cure. This process requires a lower temperature for a short amount of time, followed by a time of increased temperature. The two-step accelerated curing process allows for the more volatile solvent to evaporate at a slower rate, while slower solvents such as the tailing solvent are flashed off at the higher temperature.
These are only a few of the more common defects, and most others are variations of the same themes. For more information on selecting the best conformal coating for your application, check out our conformal coating guide. If you need any expert advice about your specific application, contact us.
