Conformal Coating Coverage, Thickness and Cure

A board that works on a bench and fails in the field after a few months of humidity is usually the victim of an electrochemical process rather than a defective component. The damage is slow, invisible at first, and driven by the combination of moisture, voltage and contamination.

Surface Energy and Wetting

A coating wets a surface when the surface energy is higher than the surface tension of the liquid. The measurement that is used in production is the contact angle of a droplet or the result of a dyne test fluid.

The measurement should be taken on the production surface rather than on a clean coupon, because the surface the coating sees has been through the whole process. Our adhesion notes describe how the value is used.

Coverage at the Difficult Places

The places where a coating fails first are the edges of the board, the boundary with a connector, the sharp corners of a component and the area under a low clearance package.

Each of those is a place where the coating is thin or where it cannot reach. The design should avoid sharp transitions and should give the coating a path to the boundary.

Thickness and Its Measurement

Too thin a coating is a coating with pinholes; too thick a coating carries internal stress and it can crack at the corners as the assembly moves.

The thickness should be measured on a reference coupon that travels with the boards, or on a witness area, rather than estimated from the process parameters.

Cure and Its Verification

An incomplete cure leaves the coating soft, permeable and easily damaged. The cause is usually a mass that is too large for the oven time, or a formulation that was mixed approximately.

The cure should be verified by a measurement rather than by the schedule, using a hardness or a solvent resistance check on a sample. Our quality notes describe how the result is recorded.

Inspection Methods

The optical inspection uses ultraviolet light with a fluorescent coating, which shows the coverage as a bright layer and the gaps as dark areas. It is fast and it works on the assembly rather than on a sample.

The method shows coverage and it does not show thickness or adhesion, so it is used together with the coupon and the adhesion test.

Repair and Touch Up

A damaged coating can be repaired locally with the same material, applied by brush and cured. The repair is only equivalent if the surface is prepared and the thickness is correct.

A repair that is applied over a contaminated surface bonds to the contamination rather than to the board. Our repair notes describe how the repair is recorded.

Verification of the Requirement

The verification is the coverage inspection on every board, the thickness on a coupon and the adhesion test after cure and after the environmental exposure.

Where the coating is the protection that makes the product survive, the three checks together are the evidence, and one of them alone is not. Our moisture notes describe the exposure the protection is for.

Process Control and Verification

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

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. 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.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. 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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Does a thicker coating give better protection? It gives more material and more internal stress. The thickness should be within a window rather than above a minimum.

Can a UV inspection find a thin area? It finds a missing area. A thin area has the same appearance as a correct one, which is why the coupon measurement is also used.

What does gopcb provide for coating verification? We provide surface energy measurement on production surfaces, coverage inspection under UV on every board, thickness measurement on a coupon, cure verification by a measured property, adhesion testing after cure and after environmental exposure, and a defined repair method with its own preparation and cure.