Automated Conformal Coating Program Setup
An automated coating machine applies a film along a programmed path, and the quality of the result depends on the pattern, the material condition and the path as much as on the material itself. The program is what holds all three together.
Pattern and Overlap
A spray pattern has a width, and consecutive passes have to overlap enough to give a uniform film without building a ridge at the edge of each pass. A heavy overlap gives a thick film in one place and a thin one elsewhere.
The pattern width should be measured rather than taken from the data sheet. Our coating notes describe the process.
Viscosity and Its Drift
The material viscosity changes as solvent evaporates in an open reservoir, which changes the flow rate and the film thickness. The reservoir level and the mix time both affect it.
The viscosity should be checked at an interval rather than assumed. Our fabrication notes list the points to confirm.
Film Build and Measurement
The film build is a wet thickness that cures to a thinner dry film, so the target should be stated in the cured state. Measuring wet film is useful for control and not for acceptance.
The dry thickness is measured on a coupon or on a witness piece. Our coating inspection notes describe where to measure.
Protecting the Keep Out
The path has to slow or stop at the keep out boundaries, and the overspray has to be accounted for. A program that runs to the edge of a connector will coat it.
The keep out should be drawn rather than described. Our masking notes describe the arrangement.
Curing and Handling
The coated board has to be handled between coating and curing without touching the film, and the cure schedule has to be reached before the board is packed. A partially cured film behaves differently from a cured one.
The cure should be recorded as a profile rather than as a time. Our thermal measurement notes describe the recording.
Changeover and First Article
A change of product, material or nozzle requires a new first article, because each of them changes the film. The first article should be measured rather than inspected visually.
Our first pass yield notes describe how the record is used.
Verification
The verification is a measured pattern width, a viscosity check at a stated interval, a dry film thickness measured on a witness piece, and an ultraviolet inspection that shows thin areas and pinholes.
Our quality notes describe how the records are kept.
Process Control and Verification
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. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
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.
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.
FAQ
Is a thicker film better? Up to a point. A film that is too thick can crack at a component edge and it traps solvent that is slow to leave.
Can the same program be reused for another product? The path can be copied, but the pattern, the film build and the keep out all have to be re established for the new geometry.
What does gopcb provide for coating programs? We provide a pattern width measured rather than quoted, a viscosity check at a stated interval, film thickness stated as cured and measured on a witness piece, keep out boundaries drawn and respected by the path, and a first article measured after every change.
