Conformal Coating for Medical Device PCBs

A medical device that runs a large current through a small board produces heat, and the surfaces it presents have to survive repeated cleaning and disinfection. Conformal coating is the usual answer, but the material only works when the board underneath it is clean and the film is applied at the right thickness.

Why a Medical Board Is Coated

The board in a monitor, a pump or an imaging unit works in an environment that is cleaned frequently, sometimes with aggressive agents, and that may include humidity, condensation or a corrosive atmosphere. Bare copper and exposed solder joints will not survive that exposure indefinitely.

The coating also protects the circuit from the user and the user from the circuit. It insulates the conductors, prevents accidental contact and reduces the risk that a stray conductive path forms across a dense part of the pattern when the surface is wet.

What the Coating Protects Against

The film blocks moisture, salts from cleaning agents, dust and the fine particles that settle on a board in service. It also reduces the growth of fungus and the electrochemical migration that carries metal between adjacent conductors when a surface is damp and a voltage is present.

What it cannot do is compensate for a design that runs too hot or for a joint that was already marginal. Coating is a protective layer, not a cure, and a board that fails before coating will fail after it as well.

Materials and Their Trade-offs

Acrylic coatings are easy to apply and easy to remove, which suits boards that may need repair, but they are the least resistant to solvents. Silicone withstands high temperature and stays flexible, while polyurethane gives the best chemical resistance with a longer cure.

Parylene is applied by vapour deposition and covers every surface evenly, including edges and fine gaps, at a cost and a cycle time that most products cannot justify. The choice is made on the environment, the repair policy and the thermal range, not on the datasheet alone.

Thickness and Coverage

The protection is a function of thickness, and thickness has to be specified rather than assumed. A very thin film dries quickly but leaves the sharp edges of a conductor only partly covered, while an excessively thick film adds mass and can bridge the gap between fine pitch pads.

A typical target is in the range of tens of micrometres over the flat surfaces, verified on a coupon or a witness plate rather than on the product. The coverage of edges, component sides and the shadow behind a tall part matters more than the average thickness of the flat areas.

Cleaning Before Coating

Coating a dirty board seals the contamination underneath the film. Flux residue, handling oils and dust all reduce adhesion, and a film that lifts at the edge of a pad lets moisture travel along the interface into the joint it was meant to protect.

The cleaning step therefore has to be verified. An ionic contamination test or a simple water break test on a sample tells the operator whether the surface is ready, and the result should be recorded with the batch rather than assumed from the fact that a cleaning machine was used.

Application Methods

Spraying gives the most even film over a board with tall components and is easy to automate, but it wastes material and requires masking of connectors, switches and any surface that has to remain solderable or conductive.

Dispensing places a controlled bead of material and is slower but more economical for small batches, while brushing and dipping suit simple boards. Whichever method is used, the mask has to be applied and removed without leaving adhesive residue on the contact areas.

Cure and Inspection

The cure is part of the process specification. A material that has not fully cured stays soft, adheres poorly and can release solvents later; one that is cured too quickly can form a skin while the material underneath is still fluid, which traps solvent in the film.

Most coatings carry a fluorescent tracer so that coverage can be checked under ultraviolet light. That inspection catches the areas that receive no coating: the shadow behind a connector, the underside of a leaded part and the region where the spray was blocked by a tall component.

Rework After Coating

A board that has been coated and then needs a repair is more difficult to rework. The film has to be removed locally without damaging the solder mask or the neighbouring parts, and the repaired area must be cleaned and recoated or the protection is lost at the very point that was touched.

The repair procedure belongs in the process documentation, together with the material used to re-coat the area. A repair that leaves the joint exposed undoes the protection for the whole assembly, because moisture will find the one path that was left open.

Verification and Testing

Verification covers adhesion, thickness and insulation. A tape test on a coupon or on a scrap board shows whether the film is bonded, a thickness gauge confirms the film on a witness plate, and an insulation resistance measurement confirms that the coating has not bridged a connector or a test point.

Functional testing follows the same principle as the coating process used for other products: the test has to be performed in the condition the product will see, including the cleaning cycle and the humidity, if the result is to mean anything.

Documentation and Process Control

Every material, thickness, cure and mask has to be written down, because the coating is invisible in the finished product and its absence is only discovered when a unit fails in the field. A batch record that ties the coating to a set of parameters is what makes that failure traceable.

Where the board also carries a potting or staking compound, the two processes have to be planned together so that the materials are compatible. The dispensing and adhesion requirements and the coating specification should be written in one document rather than two.

Process Control and Verification

On a design of this kind, medical device is the item that decides how the rest of the board is arranged. 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. 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.

Process Control and Verification

On a design of this kind, medical device is the item that decides how the rest of the board is arranged. 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. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

How thick should a conformal coating be? Thickness is specified for the environment, typically tens of micrometres on flat surfaces, and verified on a witness plate rather than estimated.

Can a coated board be repaired? Yes, but the coating must be removed locally, the area cleaned, and the repair re-coated, or moisture will enter at the disturbed point.

Does coating allow the board to be cleaned with disinfectant? It helps, but the resistance depends on the material. A silicone or polyurethane film tolerates more aggressive agents than an acrylic one. See also medical board design requirements.