Applying a conformal coating to the PCB during PCB assembly process can enhance the PCB's resistance to moisture, salt spray, dust, and corrosive environments. However, it is essentially an insulating protective film. For this reason, not every area of a PCB is suitable for direct coating. Areas that require electrical contact, mechanical movement, heat dissipation, or optical transmission can actually develop new reliability issues if they are improperly coated.
Especially for the following components, applying conformal coating directly without proper masking can lead to a situation where the PCB has been coated for protection, but the product develops problems because critical areas were coated when they should not have been. So, let’s take a closer look at where PCB conformal coating needs special attention.
Connectors are usually one of the most important components to keep clear of conformal coating. The reason is simple: their contacts need to maintain reliable electrical contact. For example, if a connector contact normally has a contact resistance of only a few tens of mΩ, allowing conformal coating to enter the contact area essentially adds an insulating film between the mating surfaces.
The problem can be even more complicated because some connectors may still work after the first few mating cycles, as the coating can be scraped away from the contact area. But that does not mean the connection will remain reliable. Over time, vibration, temperature and humidity cycling, coating residue, contamination, and oxidation of the contact surface can all increase contact resistance or cause intermittent contact.
In actual PCB conformal coating applications, the electrical contact area, mating area, and other surfaces that must maintain reliable contact generally need to be masked. The exact masking area should be determined according to the connector manufacturer's coating requirements.
This is especially important in automotive electronics and industrial control equipment that operate under long-term vibration. Being able to conduct electricity once does not necessarily mean the connection will remain reliable.
（Connectors component）
Contacts and moving mechanisms also require special attention, making relays another type of component that needs careful consideration during conformal coating.
A relay typically contains contacts, an armature, springs, and other mechanical structures. If conformal coating enters the moving mechanism, it may affect the mechanical action of the relay. More importantly, the contacts themselves are critical because a relay relies on mechanical contacts to make and break an electrical circuit. If the contact surfaces are contaminated by coating, contact resistance, arcing characteristics, and long-term reliability may all be affected.
Therefore, it does not mean that the entire relay must always be left uncoated. For sealed relays that have been verified as compatible with conformal coating, the outside of the component and the surrounding PCB may still be coated for protection. The areas that require particular attention are the contacts, moving mechanisms, and any areas specifically designated as coating-prohibited by the manufacturer.
For high-voltage relays, additional considerations include creepage distance, clearance, and the effect of the coating on insulation performance. It should not simply be assumed that conformal coating automatically provides sufficient electrical isolation.
Components that contain moving or contacting mechanisms also require special attention. Mechanical buttons, potentiometers, DIP switches, and similar components can be affected if conformal coating enters their internal moving or contact areas.
For example, a potentiometer contains sliding contacts. If conformal coating penetrates into the mechanism, it can form a thin insulating contamination layer over the sliding contacts.
What can happen? Resistance jumps, poor contact, and increased noise are all possible. This can be particularly troublesome in analog circuits because the problem may not appear as an obvious component failure. Instead, it may show up as increased output noise, ADC reading drift, or other intermittent problems that are difficult to trace.
Therefore, for mechanical buttons, potentiometers, DIP switches, and similar components, the masking area should be determined based on the component structure and manufacturer's requirements rather than simply applying full-component coating.
（Buttons component）
MOSFETs, DC-DC converters, LDOs, and other power components often have large copper areas around them to help dissipate heat. The effect of conformal coating on thermal performance cannot simply be described as "coating always causes the temperature to increase by a certain amount." Instead, the entire thermal path needs to be considered.
For power components that rely on PCB copper, thermal vias, thermal pads, or external heat sinks for heat dissipation, the coating location, coating thickness, and overall mechanical design may all affect the final thermal performance.
For example, assume a power component dissipates 2 W and its junction-to-ambient thermal resistance is 30 °C/W. Under this simplified model, the temperature rise is approximately:
ΔT = P × θJA = 2 × 30 = 60 °C
If the overall thermal conditions cause the effective thermal resistance to increase to 40 °C/W:
ΔT = 2 × 40 = 80 °C
Under this assumption, the temperature rise increases by 20 °C.
It is important to note that this is only a simplified example showing how changes in thermal resistance affect temperature rise. It does not mean that conformal coating itself will necessarily increase θJA from 30 °C/W to 40 °C/W. The actual result depends on factors such as PCB copper area, thermal vias, component package, airflow, heat sink design, coating material, and coating thickness.
Therefore, for high-power MOSFETs, power inductors, rectifier diodes, DC-DC converters, and similar components, the key question is whether the overall thermal path becomes less effective after coating.
For thermal pads, thermal vias, exposed heat-dissipation surfaces, and areas that contact a heat sink, the need for masking should be determined based on the component and mechanical design rather than coating everything simply to achieve complete coverage.
Conformal coating can also directly affect the performance of optical components. LEDs, photoelectric sensors, infrared receivers, cameras, light-sensitive devices, and similar components require particular attention.
Conformal coating can change the transmittance, reflectivity, or surface condition of an optical window. For example, if a photoelectric sensor is calibrated for a specific optical window and an additional coating layer is applied to that surface, the optical path may change. As a result, sensors in the same production batch may produce inconsistent outputs.
For components involving an optical path, first confirm with the component manufacturer whether conformal coating is permitted and what materials, coating thicknesses, and coverage areas are allowed. Do not assume that a coating will have no optical impact simply because the coating itself is transparent.
One of the most common mistakes in PCB conformal coating is treating complete coverage as the ultimate goal.
A better approach is simple: protect the areas that need protection, and keep critical areas free of coating.
During the PCB design stage, the coating requirements for different areas should be defined in advance. In general, they can be considered in three categories:
The third category should not simply be classified as "coat" or "do not coat." The decision should be based on the component manufacturer's datasheet, coating compatibility, and the actual application environment.
Conformal coating should not be considered only after the PCB has been manufactured. The design stage should clearly define which areas require coating, which areas require masking, and what coating requirements need to be met.
Areas that require protection from moisture, contamination, corrosion, or other environmental factors.
Areas that must remain free of conformal coating, such as connector contacts, test points, and optical windows.
Areas that need to be protected during manufacturing using masking tape, fixtures, or other masking methods.
The target thickness of the conformal coating should be specified according to the application and material requirements.
Quality requirements should be defined for coating coverage, missed coating, runs, bubbles, and other appearance or process-related defects.
If you are sending your design for PCB production, it is recommended to clearly indicate the required coating and masking areas in the Gerber files, assembly drawings, or other production documentation.
Conclusion
Finally, do not rely only on the conformal coating manufacturer's documentation. The coating requirements of individual component manufacturers also need to be considered.
For example, some connectors may allow coating while others explicitly prohibit it. Likewise, some MOSFETs may tolerate full coating, while others have thermal structures that require special treatment.
Good PCB conformal coating design is not simply about maximizing coverage or increasing coating thickness. It is about clearly defining where coating should be applied, where it should not be applied, and why.
The purpose of conformal coating is to improve PCB reliability in harsh environments, not to make the entire board look completely covered. By defining the Coating Area, Keep-Out Area, and Masking Area during the PCB design stage, the coating process and subsequent quality control can be much clearer and more reliable.
