Designing PCBs for Harsh Environments: Conformal Coating and Protection

A product that has to survive outdoors, on a factory floor or inside a vehicle is subjected to conditions that a laboratory prototype never sees, and those conditions attack the board through several routes at once. Designing for them is a matter of closing each route rather than of adding a single protective measure.

Temperature Range and Its Consequences

An industrial product may be required to operate from minus forty to plus eighty-five degrees Celsius, and a board that works at both extremes is not simply a board that works at room temperature. The extremes change the material properties, the mechanical stress and the behaviour of every component.

The laminate is the first casualty. A standard material becomes brittle at low temperature and softens at high temperature, so a stack that is flat and rigid in a laboratory becomes compliant and then fragile in service. Conformal coating also changes, becoming stiff when cold and more permeable when hot.

Conformal Coating and What It Actually Does

A conformal coating seals the surface against moisture, condensation and airborne contamination, and it is the single most effective measure available for a harsh environment. What it does not do is seal the assembly completely, because coatings have pinholes and the materials expand at different rates.

The coating therefore works as a barrier that slows the ingress of contaminants rather than as a hermetic seal, and the design has to remain tolerant of the small amount of moisture that will reach the surface. Our industrial systems notes describe the environmental requirements.

Cleanliness Before Coating

Coating over a contaminated surface traps the contamination against the board, and the result is worse than no coating at all because the contaminant is held in place and cannot be removed by the environment. Ionic contamination from flux residue is the usual culprit.

The cleaning requirement therefore becomes part of the specification rather than an optional step, and the coating supplier’s requirement for surface energy should be verified rather than assumed.

Condensation and Drainage

Condensation forms when a surface falls below the dew point, which happens whenever a warm humid day follows a cold night. Water that collects on a board can bridge adjacent conductors unless the design gives it somewhere else to go.

The remedies are mechanical as much as electrical: avoid traps where water can pool, provide drainage paths, and keep the spacing between conductors large enough that a film of water cannot bridge them at the working voltage.

Vibration and Mechanical Fatigue

Vibration damages a board through fatigue, and fatigue depends on the amplitude of the deflection rather than on the force. A board that flexes in the middle of a span will crack its solder joints long before a stiffer board of the same design does.

The design levers are the mounting points, the board thickness and the mass of the components. Moving a mounting point or adding a stiffener changes the resonant frequency, which can move the assembly away from the frequency the environment excites.

Corrosion and Dissimilar Metals

Where two different metals are in contact in the presence of moisture, the more active one corrodes. The effect is familiar from the connectors on a marine product, but it also applies on the board wherever a finish meets a solder of a different composition.

The design response is to avoid unnecessary combinations and to keep the surfaces dry. Where a combination is unavoidable, the joint should be coated so that the electrolyte cannot reach it.

Ingress Protection and Enclosure Interface

The board is only part of the product, and the enclosure is what actually provides the ingress protection. Sealing a board inside a housing moves the problem to the gasket, the cable entry and the pressure equalisation of the enclosure itself.

A sealed enclosure without pressure equalisation will pull water in through the seals as it cools, which is why a breather membrane is used in products that experience temperature cycling. Ignoring that mechanism is a common cause of water ingress into an enclosure that was tested as watertight when new.

Testing the Design Against the Environment

The tests that matter are the ones that reproduce the mechanism rather than the ones that are easy to run. Thermal cycling, damp heat and vibration are the three that find the majority of problems, and they should be applied to a production-representative assembly rather than to a hand-built prototype.

The failure data from those tests is what allows the design to be corrected while it is still a drawing. Once the product is in service, the same failures cost far more to correct. Our design release checklist lists where the environmental review belongs.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Is conformal coating a substitute for cleaning? No, and applying it over a dirty board makes matters worse by trapping the contamination. The surface has to meet the coating supplier’s cleanliness requirement, which is verified by an ionic contamination measurement rather than by appearance.

Does a coated board need larger conductor spacing? It does not need larger spacing for the coating itself, but the design should remain safe if the coating is damaged. Sizing the spacing for the uncoated condition gives a margin that survives a scratch or a void in the film.

What does gopcb need for a harsh environment design? We need the temperature range, the humidity and contamination exposure, the vibration profile, the required cleanliness before coating and the coating type. With those we can confirm the laminate, the surface finish, the spacing and the mechanical features before the panel is built.