Conformal Coating Adhesion and Surface Preparation

A conformal coating that lifts from a surface has failed at the interface, and the interface is decided before the coating is applied. Adhesion is a surface property rather than a property of the coating.

What Adhesion Requires

The coating must wet the surface, and wetting requires the surface energy to be higher than the surface tension of the liquid. A clean polymer surface usually meets that condition and a contaminated one does not.

The surface must also be rough enough at a microscopic scale for the coating to key into it. A very smooth surface can be wetted and still fail, because there is no mechanical interlock. Our cleanliness notes describe the contamination that reduces wetting.

Contamination Sources

The common sources are flux residue, silicone from a release agent or a lubricant, and the oils transferred by handling. Silicone is the most damaging because a very small amount changes the surface energy over a large area.

Silicone can enter a process from a spray, from a mould release, or from a tool that was used elsewhere in the plant. Where a coating adhesion problem appears without a process change, silicone is the first item to rule out. Our coating notes describe the process that precedes application.

Surface Preparation

The preparation is cleaning, and where the surface energy is still low, an activation step such as a plasma treatment. The treatment raises the energy and it also removes a thin organic layer.

The effect of the treatment decays with time and with exposure to the air, so the delay between preparation and coating should be defined and controlled. Our fabrication notes notes list the process attributes that should be stated.

Measuring the Surface

The surface energy is measured by the contact angle of a droplet or by a dyne test fluid. Both are quick and both give a number that can be compared with a limit.

The measurement should be taken on the actual production surface, not on a clean coupon, because the surface the coating sees is the one that has been through the process.

Testing the Adhesion

The test is a cross hatch or a tape pull on a sample, and it should be performed on the production assemblies rather than on a specially prepared sample.

The test should be repeated after the environmental exposure the product will see, because a coating that passes when new can fail after thermal cycling as the coating and the surface move differently. Our thermal cycling notes describe the exposure.

Where Adhesion Fails in Service

The failure appears at the edges of the coating, at the sharp corners of a component, and at the boundary with a connector or a mask dam. Those are the places where the coating is thin and the stress concentrates.

The design should avoid sharp transitions and should ensure the coating can reach the boundary. A coating that stops short of a connector edge leaves a path for moisture. Our coating inspection notes describe the coverage that is required.

Verification

The verification is the surface energy measurement before coating, the adhesion test after cure, and the same test after the environmental exposure.

Where adhesion fails, the surface preparation and the environment between preparation and coating should be examined before the coating material is replaced. Our quality notes describe how the result is recorded.

Process Control and Verification

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

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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Process Control and Verification

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

Process Control and Verification

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

Process Control and Verification

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

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

Can a primer fix a low energy surface? It can, and it adds a process step and a chemistry. Raising the surface energy by cleaning or activating is usually the better first move.

Does a thicker coating adhere better? No. Adhesion is an interface property, and a thicker layer carries more internal stress and tends to fail at the edges.

What does gopcb provide for coating adhesion? We provide surface energy measurement on production surfaces, cleaning and plasma activation with a controlled delay before coating, silicone exclusion and contamination control, coating at edges and transitions, adhesion testing after cure and after environmental exposure, and documented process attributes.