De-wetting is where the conformal coating material is applied to a printed circuit board evenly but the coating refuse to stick to the surface that it has been applied to.
Factors that influence de-wetting usually involve non-ionic contamination such as:
- Residues from board manufacture including silicone surfactants from solder resist & HASL rinse contamination
- Component residues like mould release agents
- Silicone oil from adhesives in production.
- Soldering processes
- Cleaning bath contamination where rinsing has failed
- Operator handling adding contaminants
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When considering the long term reliability of Printed circuit boards (PCBs), it is important to understand that they can fail due to being exposed to harsh or extreme environments such as airborne contamination, salt spray, moisture, fungus, dust and corrosion.
By definition conformal coating is a protective non conductive dielectric layer that is applied onto the printed circuit board assembly to protect the electronic assembly from this damage. The coating when applied well gives added protection. So, if the conformal coating is poorly applied to the PCB then the circuit board is more likely to fail in the long-term.
Looking specifically at the application of the coating there are several defects that can occur during processing that can fundamentally impair the performance of the conformal coating. This can lead to reduced long-term reliability of the PCB. These defects can include Capillary Flow, De-wetting, De-lamination, Cracking, Orange Peel, Pin holes, Bubbles & Foam.
Logically, it is wise to avoid these effects in the process which can potentially improve the lifetime of the circuit board and limit problems in the field.
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How do I measure the surface energy of the printed circuit board to check if I have an adhesion problem with the conformal coating?
The surface energy of the laminate or solder resist can have a huge impact on the ability of the conformal coating to stick or adhere. To achieve the optimum adhesion, it is necessary to
ensure that the the surface energy of the substrate is above that of the conformal coating material to be applied.
Surface energy is measured in mN/m or Dynes. There are a few ways to determine surface energy and these include measuring the contact angle or by the use of Surface Energy Test Pens (Dyne level testing). The latter is a simple test and easy to do with the right set of Dyne Pens (which are also referred to as Corona Pens).
When the Dyne pen is applied to the surface, the conformal coating will either form a continuous film on the circuit board or de-wet (pull back) into small droplets of coating. According to Dyne Technology, a manufacturer of the Dyne Pens,
“if the Dyne test fluid remains as a film for 3 seconds, the substrate will have a m
inimum surface energy of that ink value, expressed in mN/m (Dynes). Should the Dyne test liquid reticulate or draw back into droplets in less than 1 second then the surface energy of the substrate is lower than that of the liquid itself. The exact surface energy (Dyne level) can be determined by applying a range of increasing or decreasing values of Dyne test pens.”
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De-wetting is the tendency of the conformal coating material to refuse to wet the surface of the printed circuit boards and components that it has been applied to evenly.
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