Heavy Copper PCBA Conformal Coating: How to Write Edge Coverage into the PO

Why heavy copper makes coating a PO problem

• Thick copper raises each conductor above the laminate. Liquid coating flows off the top and pulls back from the upper corners.
• Coverage that looks complete on a flat area can be thin or missing on conductor edges, corners and around tall power parts.
• Those edges sit next to the highest voltage gaps on a power supply board.
• "Conformal coat per IPC" on a PO does not say where to measure, what thin means or who pays for recoat. Write it down.

Five things the PO has to fix

Material: type, product and substitutes

• Name the material type and, if qualified, the product. Do not leave it to "supplier standard".
• State whether equivalents are allowed and who approves a switch. A substitute changes the rework method and may need retest.
• Ask for a UV tracer in the material if you plan UV inspection.
• Ask for the material's qualification reference to IPC-CC-830 and the lot record on each shipment.

Coating drawing and masking

• Mark three zones: coat, do not coat, and boundary tolerance.
• Typical keep-outs: connector contacts, test points, adjustable parts, thermal interface pads, mounting holes and press-fit areas.
• Say whether the supplier may decide masking for parts not on the drawing, or must ask first.
• Ask for a first-article photo set under UV showing every keep-out.

Coverage and thickness on edges and corners

• Write the coverage rule per surface: conductor top, conductor side wall, upper corner, and around component leads.
• Take the thickness range from your own design and environment analysis. The PO should state it; the supplier should not pick it.
• Name the measurement method: wet-film gauge during application, dry-film on a witness coupon, or cross-section on a sample board.
• Name the measurement points on the coating drawing. On heavy copper, include at least one point on a conductor edge or corner, not only flat laminate.
• Say whether more than one coat pass is allowed or required to build edge coverage, and whether bridging between fine-pitch leads is a defect.

Inspection: what, how often, and what fails

• Defects to list by name: bubbles, voids, dewetting, orange peel, bridging, coating in keep-out areas, missing coat on edges.
• State which defects are rejectable and which are cosmetic, by acceptance class.

Rework, recoat and who pays

• Agree when spot touch-up is allowed and when the board must be stripped and recoated.
• Set a limit on recoat cycles per board and require the rework to be logged against the serial number.
• Coating defects found at goods-in: the supplier reworks at its cost if the PO criteria were not met.
• Defects caused by a drawing error or late design change: the buyer pays. Write this split into the PO.
• Field repair: state whether the coating must be removable and which removal method your service team will use.

Records to ask for with each lot

• Material type, product name and lot number
• Coating drawing revision used
• Thickness readings at the named points
• UV inspection result and sample size
• Rework log by serial number

How this goes wrong

• PO says "coat per IPC" with no drawing. The supplier coats a connector and the lot is held.
• Thickness is checked only on a flat coupon. Conductor corners on the board are thin and fail hi-pot later.
• Material is swapped without notice. Your service team cannot strip it.
• No recoat limit. A board is reworked several times and the history is lost.

PO checklist

• [ ] Material type per IPC-CC-830, product, approved equivalents
• [ ] Coating drawing with coat, keep-out and boundary tolerance
• [ ] Coverage rule for conductor tops, edges and corners
• [ ] Thickness range, method and named measurement points
• [ ] UV inspection, sample plan, IPC-A-610 class
• [ ] Rework method, recoat limit, cost owner
• [ ] Lot records on the CoC

FAQ

Why does conformal coating run thin on heavy copper boards?

Thick conductors stand higher above the laminate. Liquid coating flows away from the top edges and corners as it levels and cures, so those points end up thinner than flat areas.

Is "full coverage" enough wording for the PO?

No. Say which surfaces must be covered, including conductor edges and corners, and name where thickness is measured.

Which standards should the PO cite?

IPC-CC-830 for the coating material qualification and IPC-A-610 for coating acceptance criteria on the assembly. IPC-HDBK-830 is a useful design and process guide.

Who should set the coating thickness?

The buyer, from its own design and operating environment. The supplier applies and measures to that range.

Who pays for recoat?

The supplier, when the lot misses the PO criteria. The buyer, when the cause is a drawing error or a design change. Put this split in the PO.

Related links

• Heavy copper acceptance loop diagram, used here to frame conformal coating coverage checks on thick conductor edges
• /en/products/heavy-copper-pcb/
• /en/products/pcb-assembly/
• /en/blog/heavy-copper-solder-mask-coverage-rfq-declaration/
• /en/blog/heavy-copper-power-supply-pcb-isolation-hipot-declaration/
• /en/quote/