Nano Coating: Design Rules and Process Limits
A nano coating is a thin chemical film applied to the walls of a stencil to change how paste behaves at the aperture. The claim is simple: less paste sticks to the wall, so the deposit is more complete and the underside of the stencil needs wiping less often. Proving that claim on a specific line takes a little more work than reading the datasheet.
What the Coating Changes
The coating lowers the surface energy of the aperture wall. Paste that would otherwise adhere to the wall and be pulled out of the aperture on separation instead stays with the deposit.
Two effects follow. The deposit area is more complete at fine apertures, where the wall area is large relative to the aperture volume, and the stencil underside stays cleaner for longer, which extends the wiping interval.
Where the Benefit Is Largest
The gain scales with the wall area to aperture volume ratio, so it is largest for small, deep apertures. A 0.4 millimetre pitch device sees more benefit than a 1.2 millimetre pitch part on the same stencil.
Where the apertures are large and the paste releases easily, the coating changes little and the cost is hard to justify. Our article on electroformed versus laser stencils covers the alternative route to better release.
Coating Types
The common chemistries are fluoropolymer based, and they differ in how they are applied, how thick the film is and how long they last. Some are applied by the stencil supplier and some are applied on site.
A thick film can reduce the aperture slightly, which matters at fine pitch, so the coating thickness should be specified. A film that is too thick changes the deposit volume it was meant to improve.
Lifetime and Wear
The coating wears with every print stroke and with every wipe. The useful life depends on the wiping method, the solvent and the squeegee pressure, so a claimed life in strokes is only a starting point.
The practical approach is to measure the deposit weight and the bridging rate over a run and to see when the benefit disappears. Our article on stencil cleaning and storage describes the handling that protects the coating.
Wiping and Cleaning Interactions
A coating is often paired with a solvent wipe rather than a dry wipe, because the solvent removes paste that the coating has repelled. The combination should be evaluated together, since a coating with the wrong wipe can perform worse than an uncoated stencil.
Aggressive cleaning, including some inline cleaners, can strip the film. The cleaning method is therefore part of the coating decision rather than a separate choice. Our article on inline cleaning machines covers the equipment side.
Evaluating the Benefit
A fair evaluation uses the same paste, the same printer settings and the same product for the coated and the uncoated stencil, and it measures deposit volume, bridging rate and wiping frequency. Anything less compares two variables at once.
The run should be long enough to include the point at which wiping becomes necessary, because that is where the coating earns its cost. Our article on paste volume and stencil design describes the measurement.
Cost and Decision
A coated stencil costs more and has a shorter life than an uncoated one, so the benefit has to be measured in yield or in throughput rather than assumed. For a fine pitch product with a difficult paste, the yield gain often pays for it.
The decision should be recorded with the evidence, so that the next order does not have to repeat the trial. Our article on control plan development explains where such decisions belong in the process documentation.
Checks Before Release
A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. 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 an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.
The sequence of operations is part of the specification, because a different order produces a different result from the same steps. 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.
Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.
Points to Confirm at First Article
A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. 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.
Verification and Records
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.
A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
FAQ
Does a nano coating eliminate bridging? No. It reduces paste left on the stencil underside, which reduces one cause of bridging, but bridging from excess volume or slump is not addressed by the coating.
Can a coated stencil be re coated? Some suppliers offer it, and the result depends on how the original film was removed. The decision should be based on a measured deposit result rather than on the promise.
Is the coating compatible with all pastes? Not all. The interaction between the coating and the flux chemistry should be checked, because a paste that wets the coating defeats the purpose.
