Flex PCB Coverlay Openings: Budget for Registration and Squeeze-Out

UPDATED October 9, 2026

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A coverlay opening can look comfortably larger than a copper pad in CAD and still leave too little exposed area after fabrication. The drawing must account for where the opening lands, how large it finishes and how far adhesive reaches into it. These effects should be reviewed together without counting the same supplier allowance twice.

This article develops an illustrative edge-clearance budget. It complements the flex PCB materials guide; it does not establish a universal coverlay expansion rule.

Define the acceptance edge first

Identify which area must remain exposed and why. A solder land, a connector contact and a probing location may have different usable-area requirements. Draw the protected area and the required exposed area as separate boundaries. State whether acceptance concerns the metal edge, the adhesive edge or both. Otherwise two reviewers can agree on “opening clearance” while measuring different things.

Also decide whether the pad dimension used in the budget is nominal or a worst-case finished dimension. If copper dimensional variation has already been included in a maximum pad width, do not add it again as a separate allowance.

Ask for definitions before combining tolerances

Request three clearly defined quantities: maximum relative displacement between the pad and opening, minimum finished opening size, and the adhesive intrusion allowance applicable to the proposed process. A supplier may instead offer a directly controlled exposed-area requirement that already combines these effects. Use that combined acceptance requirement as such; do not add the individual effects again.

The manufacturer data sheet for Pyralux LF coverlay identifies a polyimide film with B-staged modified acrylic adhesive. Its typical-property table for LF0110 lists adhesive flow of 0.05–0.10 mm under the stated test method. That is product test data, not a guaranteed pad-opening allowance for every coverlay construction or fabrication process. Pyralux LF coverlay data sheet, Table 2.

Work one edge through an explicit budget

The following example is entirely hypothetical. Assume a centered square pad with maximum finished width P = 0.80 mm, a nominal square opening O = 1.30 mm, maximum total opening undersize U = 0.04 mm, maximum relative offset R = 0.10 mm toward the edge being checked, and an adhesive intrusion allowance A = 0.08 mm at that edge. None of these assumed allowances is a claimed manufacturing capability.

Under these definitions, the minimum remaining clearance at that edge is C = (O − U − P) / 2 − R − A. The model assumes the opening remains square and its size reduction is symmetric. Irregular shapes, rounded corners or a supplier’s different measurement definition require a different geometric check.

If the project requires at least 0.08 mm of remaining clearance, this assumed construction misses that requirement by 0.03 mm. Solving the same expression for opening width gives O ≥ P + U + 2(R + A + C_required), or 1.36 mm. This is a result of the example assumptions, not a recommended default opening size.

An enlarged opening creates another check

Before changing the opening, inspect the adjacent conductor, nearby openings and the remaining coverlay web. A larger opening may satisfy exposed-pad clearance while exposing copper that should remain covered or leaving a protection feature the fabricator cannot support. Review these boundaries on the same drawing instead of approving the opening in isolation.

Near reinforcement, show the coverlay and stiffener edges together. The stiffener design guide helps locate these regional transitions. Use the stackup builder to record which protection and bonding layers are present, then put the actual opening geometry in the fabrication data.

Check the opposite edge and the adjacent protected feature

The one-edge model is intentionally local. A shift of the opening toward one side reduces clearance on that side of the pad while increasing it on the opposite side, assuming rigid translation of the same opening. Do not subtract the same directional displacement from both sides when reconstructing a single physical position. For worst-case acceptance, however, each side still needs its own adverse position check. Clearly distinguish a set of bounding cases from one claimed geometry.

Now add a nearby trace that must remain covered. Increasing the opening width moves its nominal boundary toward that trace. The change may improve the exposed pad area and reduce the protection margin at the same time. Mark both controlled boundaries on the drawing, then evaluate the same opening displacement against both. A design decision that satisfies only the pad clearance can miss the competing requirement on the adjacent conductor.

For an original, simplified example, suppose two nominal square openings have centers 2.00 mm apart and each is 1.30 mm wide along the line between centers. Their nominal web is 2.00 minus 0.65 minus 0.65, or 0.70 mm. Enlarging both openings to the earlier calculated 1.36 mm reduces that nominal web to 0.64 mm. These values are hypothetical and omit variation. They illustrate that a change to exposed-area clearance also changes the remaining coverlay geometry.

Do not interpret the calculated 0.64 mm as a permitted manufacturing web. Ask the fabricator what finished geometry it can support for the selected material and process, then apply the relevant dimensional allowances. Also distinguish a common translation of both openings from a change in their relative separation. A common shift does not alter the web between identical translated openings, while relative size or position variation can. The tolerance model needs to represent the actual quantity the supplier controls.

Extend the budget to rectangular pads carefully

For a rectangular pad and opening, apply the one-dimensional edge relationship separately in the two principal directions only if the geometry and tolerance definitions justify it. Record the maximum pad dimension, minimum opening dimension and adverse displacement for each direction. A larger clearance in the long direction does not compensate for insufficient exposure at a short edge when that short edge is functionally required.

Rotation requires an additional geometric check. An opening rotated relative to a pad can approach a corner even when the center offset is small. Likewise, a rounded opening corner cannot be evaluated as though it were an ideal square corner. Use the actual finished shape proposed by the fabricator and inspect the controlling local boundary. The simple formula is a useful screening model, but its assumptions should remain visible beside the result.

Separate the electrical pad artwork from the coverlay tooling definition. If the supplier applies a manufacturing adjustment to the submitted opening, record who owns that adjustment and which drawing defines acceptance. Otherwise, the designer may enlarge the CAD opening while the supplier independently applies another enlargement. The same problem can occur when an older release note remains attached to new artwork. Resolve the relationship before approving tooling.

Review usable exposure with the downstream operation

The required exposed region should come from the operation that uses it. For a soldered land, show the region that the assembly team needs accessible and the neighboring areas that must remain protected. For a probing location, identify the intended contact footprint and positional allowance of the probe. For a connector finger, use the exact mating-interface requirements. These are different functional definitions even if the same coverlay material is used on all three regions.

Do not use the amount of visible copper in a photograph as the only acceptance description. A photograph may not show the critical edge clearly, and lighting can make an adhesive boundary difficult to interpret. Agree an inspection view and boundary definition with the supplier. Where an edge is irregular, state whether the minimum local exposure or another explicitly defined measure controls. The inspection method should answer the same question as the tolerance calculation.

If an assembly trial finds a problem, retain the measured opening and adhesive condition with the trial result. A general statement that soldering was difficult does not identify whether the cause was limited exposure, another geometry issue or an unrelated process factor. Compare the observed condition with the release requirements before revising the coverlay. This preserves a connection between the proposed change and the evidence that motivated it.

Manage a change without silently broadening the allowance

A coverlay construction change should trigger a review of the assumptions it can affect, rather than an automatic reuse of the old opening expansion. Compare the named film and adhesive construction, the proposed process and the supplier’s acceptance commitment. Identify whether the opening dimensions are controlled before or after the relevant processing step. Keep the finished requirement distinct from any incoming material or tooling specification.

Suppose the supplier offers a combined guarantee for minimum exposed area instead of separate registration and adhesive-flow allowances. That can be a useful agreement if its measurement basis and scope are clear. Record that the combined requirement supersedes the individual design-budget assumptions for acceptance. Retaining both as independent requirements without explaining their relationship can create a contradiction or count the same effect twice.

For each revision, retain the old and new artwork identifiers, the changed dimensions and the reason for the change. Include the associated supplier response and assembly review when applicable. A small numerical opening change can affect multiple neighboring features, so list the region checked rather than describing the modification as globally safe. The objective is a local design decision supported by defined geometry and evidence.

Before release, have a reviewer reconstruct one controlling edge calculation directly from the drawing and stated allowances. If the reviewer must guess whether an undersize value is total or per side, the definition is incomplete. If the minimum exposed region cannot be located unambiguously, improve the acceptance drawing. This short reconstruction check is more informative than repeating the nominal opening dimension in several documents.

Make the inspection match the budget

Include a marked example showing the pad, finished opening edge and maximum permitted adhesive boundary. Agree how corners, partial pad coverage and irregular adhesive edges will be judged. A single centerline dimension may not detect the most restricted local edge. Ask the supplier how its proposed inspection samples the relevant features and records deviations.

For release, retain the assumptions, the supplier’s allowance definitions, the final artwork revision and the acceptance drawing together. If process selection or coverlay construction changes, reopen the affected allowances rather than assuming the old arithmetic still applies. Request a coverlay opening review with the marked geometry and required exposed area so the proposed process can be checked against a measurable result.

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