A sensor is designed to detect something, and a layer of resin is the last thing it needs on its sensing surface. Yet the rest of the circuit board around it often does need protection from moisture, dust, corrosion and vibration. That is exactly why conformal coating masking exists: it keeps the coating off the parts of a sensor board that must stay exposed, while the surrounding assembly still receives full protection.
Conformal coating masking is the practice of temporarily covering selected areas of a printed circuit board before the coating is applied, so those areas remain free of coating. On a sensor board, the masked areas are usually the sensing element itself, its openings, connectors, test points and any surface that must make electrical, optical or mechanical contact after assembly. The protective materials used for this job are called maskants, and they can be tapes, dots, plugs, caps, boots or peelable masks.
Three terms come up often in this context. No-coat areas are the board regions or component surfaces that must stay free of coating. Keep-out zones are the areas where coating should be kept away because of function, access or assembly fit. Maskants are the materials used to protect those areas during the coating step. Defining all three clearly before production begins is the difference between a smooth coating run and a board that needs rework.
Sensors work by converting a physical quantity into an electrical signal. If coating covers the sensing element, it can interfere with that conversion in ways that are easy to miss until the product is already in the field:
Beyond the sensing element itself, connectors, test points and programming pads on the same board also need to stay accessible. A coated connector can fail to mate, and a coated test pad can block the probes used in ICT or FCT testing. Masking protects all of these at once.
The exact no-coat areas depend on the product, but most sensor boards share the same list of features that need attention:
It is worth checking both sides of the board. Connectors, pads and contacts can appear on the bottom side as well, and marking only one side risks missing an interface that matters during final assembly.
The right masking method depends on the board geometry, the number of boards, and how the coating will be applied. Common options include:
For sensor boards with many sensitive interfaces, manual masking can take longer than the coating step itself. On repeat production, fixtures or reusable masking boots often reduce that labour because the same areas are protected on every run. On automated lines, selective spraying can also be programmed to skip defined keep-out zones, which reduces the amount of manual masking needed.
Each sensor type has its own masking needs, and the masking plan should be matched to the sensing principle:
The masking plan should also be coordinated with the test sequence. If testing happens before coating, confirm early that masking will not block probe access later. If the coated board goes into an enclosure, the masking plan needs to account for the final assembly fit as well.
Masking is not the only way to keep coating off a sensing surface. Selective coating machines can spray only the areas that need protection, which reduces manual masking work when the keep-out zones are well defined. Connectors, tight keep-outs, tall parts and sensitive interfaces may still need local protection even on a selective line.
For sensor boards that need stronger environmental protection, low pressure molding for sensitive electronics is an alternative worth considering. Low-pressure injection molding encapsulates the body of the assembly in a protective layer while the sensing surface can be kept exposed, and it is often used for medical and industrial sensors, connector harnesses and microswitches. Choosing between pcb conformal coating with masking and low-pressure molding depends on the level of protection the product needs and how the sensor will be mounted.
A clear masking plan helps a contract manufacturer quote accurately, plan the coating step and avoid rework. Before sending a sensor board for coating, it helps to provide the Gerber files, top and bottom views with all no-coat areas marked, notes on sensitive components, test and programming requirements, and the expected production quantity.
Farway Electronic, a Shenzhen-based PCB and PCBA manufacturer, runs an automated conformal-coating line that supports boards up to 550 mm by 470 mm, selective masking, double-sided spraying and baking, and fan and needle spraying. The company also offers low-pressure injection coating, SMT and DIP assembly, PCBA testing and finished-product assembly, and holds ISO 9001, ISO 13485 and IATF 16949 certifications. Its engineering team works with customers on sensor boards for transportation, medical, new energy, security and communication products, from prototype to volume production.
Which parts of a sensor board should stay free of coating?
The sensing element and its openings, connectors, test points, programming pads, grounding pads and mechanical mating surfaces. The exact list depends on the sensor type and how the board is assembled.
Does masking need to be done by hand?
Not always. Automated selective coating can skip defined keep-out zones, which reduces manual masking. Boards with many connectors and sensitive interfaces may still need local protection by hand or with fixtures.
Can a sensor be coated if the sensing surface is masked?
Yes. Masking keeps the sensing surface exposed while the rest of the board receives protection. The key is to mark the no-coat areas clearly before production so the masking plan is consistent on every board.
Is low-pressure molding better than coating for sensors?
It depends on the product. Low-pressure molding gives stronger protection for the body of the assembly and is often used for sensors that face harsh environments, while conformal coating with masking is lighter and suits boards where weight, serviceability or cost matters more.
What should I send before requesting a coating quote?
Gerber files, assembly drawings, top and bottom views with no-coat areas marked, notes on sensitive components, test and programming requirements, coating requirements and the expected production quantity.
Conformal coating masking is a small step that protects the part of a sensor that cannot be protected. Defining the no-coat areas clearly, choosing the right maskant and coordinating with the test and assembly plan keeps the coating where it belongs and the sensor working as designed.
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