Controller board conformal coating: what to coat, what to mask, and how thick, for boards with relays, connectors and a large processor

01

Coat the board, but keep coating off contacts, moving parts and test points

PCBWay groups the areas of a board into three kinds: those to keep free of coating or mask, which include connector contact areas, moving or contacting mechanisms in buttons and potentiometers, test points and optical windows; components needing evaluation against their datasheets, including power components, relays, transformers, sensors, crystals and oscillators; and the rest. It explains that coating in a connector's contact area adds an insulating film to a contact of a few tens of milliohms, which may still work for the first matings as the coating scrapes off but becomes unreliable under vibration, and that coating entering a relay's contacts, armature and springs can affect its action. [4]

Doug Pauls, presenting IPC committee changes reported by I-Connect007, stressed that designated coverage areas, keep-out zones and their tolerances come from the assembly drawing and documentation, that connector mating surfaces must not be contaminated, and that coating fillets on BGAs should be kept to a minimum to avoid failures from CTE and modulus mismatch in thermal cycling. [1] For power parts PCBWay notes that coating location and thickness can change the thermal path through copper, thermal vias and heat sinks. [4]

Give the board assembler the assembly drawing with coating and keep-out areas and tolerances, the coating material and thickness range, the components needing datasheet checks (relays, power parts, crystals, sensors), the masking method, the inspection method, and the operating environment of the finished board.

02

Thickness: inside the tested range, measured on coupons, inspected under UV

HumiSeal explains that coating makers give a thickness range because their datasheet properties are measured on specimens coated to thicknesses set by qualification standards such as IPC-CC-830, MIL-I-46058C or UL 746E; below 25 µm (1 mil) dielectric strength and other properties were not tested by those methods, and above the upper limit coatings rarely add protection and can crack, suffer CTE mismatch and wrinkle, with more bubbles. [2] Its 1B73 acrylic lists a recommended thickness of 25–75 µm, fluoresces under UV light for inspection, is MIL-I-46058C qualified and IPC-CC-830 compliant, and passes 50 thermal-shock cycles from −65 °C to 125 °C. [3]

The IPC committee guidance reported by I-Connect007 recommends representative process-control coupons for thickness, allows wet-film measurement where it is documented to correlate with dry thickness, gives default thicknesses by coating type in J-STD-001F Table 10.1 when the drawing is silent, and makes the drawing govern when it does specify; cure can only be confirmed by instruments such as DSC or TGA, while coverage is judged visually at no more than ×4 or under UV black light for coatings with fluorescent tracers. [1]

Editorial calculation: 25–75 µm is 0.025–0.075 mm, or about 1–3 mil; a drawing that gives the range in one unit should state the other for the coating line.

03

Send the drawing, material and inspection plan together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Drawing: [coat areas; keep-out areas; tolerances]
Components: [connectors; relays; test points; power parts]
Coating: [type; product; thickness range]
Masking: [method; parts]
Inspection: [coupons; UV; magnification]
Environment: [humidity; condensation; temperature]

See manufacturer records Send an enquiry through CNFX