Is your conformal coating failing to properly wet the PCB surface?
Are you experiencing pitting, fisheyes, poor flow, or coating delamination during or after coating?
In many cases, these defects are not caused by the coating material itself, but by insufficient PCB surface energy. When the surface energy of the PCB substrate is lower than the surface tension of the conformal coating, proper wetting and adhesion become impossible.
At KKPCB, we frequently receive customer inquiries related to conformal coating defects. One of the first questions our engineers ask is:“What is the surface energy of your PCB substrate?”
This article explains:
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The relationship between surface energy and conformal coating wetting
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How to quickly measure surface energy in production
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Practical methods to improve and maintain optimal surface energy for reliable coating performance
1. Surface Energy vs. Surface Tension — Why It Matters
Surface energy is a key parameter that directly affects:
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Conformal coating wetting behavior
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Coating uniformity
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Adhesion strength between coating and PCB
In simple terms:
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High surface energy → good wetting and strong adhesion
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Low surface energy → poor wetting, defects, and delamination
For conformal coatings to flow smoothly and bond effectively, the surface energy of the PCB must be equal to or higher than the coating’s surface tension.
2. How to Quickly Measure Surface Energy in Production
2.1 Dyne (Dean) Test – The Industry Standard
The Dyne test is the most commonly used method for measuring surface energy in PCB manufacturing and assembly environments.
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Unit: dynes/cm
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Tool: Dyne test pen
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Method: Comparative wetting test
Each Dyne pen contains a liquid with a known surface tension. When applied to the PCB surface:
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If the liquid spreads evenly, the surface energy is equal to or higher than the pen value
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If it beads up or retracts, surface energy is lower than required
Most commercial Dyne pen kits cover a range of 30–44 dynes/cm, making this method:
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Fast
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Low-cost
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Suitable for inline inspection
Always follow the Dyne pen manufacturer’s usage instructions for accurate results.
Other measurement methods include:
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Contact angle measurement
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Capillary rise method
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Rotating droplet method
However, these are typically used in laboratory settings rather than mass production.
3. Recommended Surface Energy for Conformal Coating
For optimal wetting and adhesion, KKPCB recommends a minimum PCB surface energy of 38 dynes/cm before conformal coating.
Important considerations:
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Soldering processes can significantly reduce surface energy
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Flux residues, oils, and contamination lower surface energy
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Once assembly is completed, it is often difficult to increase surface energy without cleaning or treatment
Therefore:
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Surface energy requirements should be clearly defined in PCB specifications
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Incoming PCBs should be tested during IQC
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Surface energy should be verified after key assembly steps
4. Common Causes of Low Surface Energy on PCBs
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Flux residues after soldering
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Grease, oils, fingerprints
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Silicone contamination (lubricants, sealants, coatings)
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Improper handling during assembly
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Insufficient PCB cleaning after manufacturing
Even small amounts of contamination can cause:
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Fisheyes
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Pinholes
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Poor coating flow
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Film delamination
5. Precautions to Maintain Optimal Surface Energy
To ensure stable conformal coating performance, KKPCB recommends the following best practices:
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Specify ≥38 dynes/cm surface energy in PCB purchase requirements
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Perform surface energy testing after each major process step
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Use appropriate ESD and protective gloves during handling
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Prohibit hand creams, perfumes, or surface modifiers in coating areas
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Eliminate silicone-containing materials near conformal coating lines
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Optimize soldering parameters to ensure full flux activation and minimal residue
6. Effective Methods to Improve PCB Surface Energy
Depending on contamination type and severity, the following methods can be applied:
6.1 Deionized Water Rinse
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Removes water-soluble residues and dust
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Low cost, simple process
6.2 Solvent Cleaning
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Effective for localized, water-insoluble residues
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Must be followed by proper drying
6.3 Saponification Cleaning
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Removes stubborn flux, grease, and organic contamination
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Common in high-reliability PCB assembly lines
6.4 Plasma Treatment
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Removes hard-to-clean contamination
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Activates PCB surfaces to extremely high surface energy levels
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Ideal for demanding conformal coating applications
Many conformal coating defects—including pits, fisheyes, and delamination—are directly linked to insufficient PCB surface energy rather than coating material issues.
By:
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Measuring surface energy correctly
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Maintaining ≥38 dynes/cm throughout production
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Implementing proper cleaning and handling controls
Manufacturers can significantly improve coating appearance, adhesion, and long-term reliability.
At KKPCB, we support customers with:
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PCB surface condition control
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Conformal coating process optimization
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Manufacturing-oriented design and DFM guidance
Contact KKPCB for professional PCB manufacturing and coating-ready solutions tailored to high-reliability electronics.
FAQ
Q1: Why does my conformal coating fail to wet the PCB surface?
The most common cause is low PCB surface energy. When the surface energy of the PCB substrate is lower than the surface tension of the conformal coating, the coating cannot spread evenly. This results in:
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Poor wetting
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Fisheyes
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Pitting
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Uneven coating thickness
Key point: Conformal coating performance depends more on surface condition than on coating material selection.
Q2: What PCB surface energy level is required for reliable conformal coating?
For stable wetting and strong adhesion, KKPCB recommends a minimum surface energy of 38 dynes/cm before conformal coating.
If surface energy is:
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≥38 dynes/cm → Good wetting and adhesion
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<38 dynes/cm → High risk of coating defects
This requirement should be:
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Specified in PCB procurement documents
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Verified during incoming inspection (IQC)
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Monitored after key assembly steps
Q3: How can surface energy be measured quickly on the production line?The most practical method is the Dyne (Dean) Test using Dyne test pens.
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Measurement unit: dynes/cm
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Typical test range: 30–44 dynes/cm
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Test result: Visual spreading or beading of the test liquid
Dyne testing is:
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Fast
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Low-cost
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Suitable for mass production environments
Other laboratory methods (contact angle, rotating drop) are less suitable for inline production control.
Q4: Can soldering and assembly reduce PCB surface energy?Yes. Assembly processes are one of the main contributors to surface energy degradation, including:
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Flux residues after soldering
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Oils and greases from handling
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Fingerprints and cosmetic contaminants
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Silicone vapors from nearby materials
Even small residues can significantly reduce surface energy and cause coating delamination or fisheyes.
Q5: Why do coating defects still appear even when using high-quality conformal coating materials?Because coating quality cannot compensate for poor surface conditions.
If the PCB surface energy is insufficient, even premium coatings will show defects such as:
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Poor flow
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Pinholes
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Film separation
Surface preparation is always the first priority in conformal coating reliability.
