Coverlay Adhesive Squeeze-Out: Diagnose Where Pad Area Was Lost

UPDATED October 22, 2026

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When a finished flex PCB has less exposed pad area than expected, adhesive squeeze-out is one possible explanation. It is not the only explanation. A smaller cut opening, displacement between the opening and copper, or a different pad geometry can produce a similar photograph. Effective diagnosis separates these contributions before changing the lamination process.

This article presents an original investigation worksheet for a specific question: where was the intended usable pad area lost? It focuses on evidence from an actual build, rather than selecting a nominal opening allowance. Use the flex PCB materials guide to identify the construction, then keep the investigation tied to that material and revision.

Preserve the evidence before proposing a correction

Start with the affected part identifier, panel position, lot and drawing revision. Photograph the area with a scale and a clear orientation reference. Retain an unmodified specimen when possible. A cleaned or reworked surface may no longer show the original boundary that the investigation needs to measure.

Record the actual complaint in functional terms. A solder land may have insufficient exposed area for the agreed assembly requirement. A contact may have material inside a specified mating region. A test location may no longer offer the required access. These are different acceptance questions, even when the photograph looks similar.

Separate the observation from the proposed cause in the report. Write that the visible usable boundary intrudes into a required region, then list adhesive intrusion as a hypothesis if it has not been confirmed. This small distinction prevents later reviewers from treating an early label as an established root cause.

Identify three different boundaries

Mark the copper perimeter, the coverlay film opening and the innermost boundary of material obscuring the pad. Use distinct colors or labels in the inspection image. If one boundary cannot be resolved reliably, record that limitation and obtain a suitable inspection method before assigning a dimension.

The material identity matters here. The manufacturer describes Pyralux LF coverlay as polyimide film coated with a B-staged modified acrylic adhesive. Its data sheet reports adhesive flow as a typical property and states that performance depends on construction and processing. It also provides material traceability information and directs users to a processing guide. See the Pyralux LF coverlay data sheet, EI-10118, October 2022.

Those statements support identifying both film and adhesive in the investigation. They do not make a typical material flow result a universal acceptance limit for a finished pad. Use the agreed drawing requirement to judge the part, and use applicable material and process evidence to investigate how the observed boundary developed.

Reconstruct the intended and actual geometry

Obtain the released copper and coverlay data, including approved manufacturing changes. Compare the intended pad with the actual copper dimensions, then compare the intended film opening with its measured boundary. Establish the common datum used to calculate relative position. Measuring each feature against an unrelated origin can create an apparent displacement that is only a coordinate mismatch.

Next, determine where the obscuring material begins relative to the film edge. This is a different measurement from the offset between copper and film. Keep both measurements, even when the immediate acceptance criterion concerns only the final exposed area. The separate values help distinguish which step needs attention.

If the features are irregular, retain the boundary map rather than reducing the result to one opening width. A rounded corner or a narrow neck can be the critical location even when the overall width appears acceptable. Define whether the report uses minimum local clearance, exposed area, or an inscribed usable region. Do not interchange these quantities.

Use a measured example to separate displacement from intrusion

The following hypothetical example uses one horizontal section through a rectangular pad. All coordinates are invented millimetres measured from the same datum. Let the copper pad occupy x = 0.00 to 0.80. The intended film opening occupies x = -0.15 to 0.95. Suppose inspection finds the actual film edges at -0.07 and 1.03, and the final clear boundary at 0.03 and 0.98.

The intended and actual film widths are both 1.10 mm, but the actual opening has shifted 0.08 mm to the right. Relative to the actual film edges, the final clear boundary has moved inward by 0.10 mm on the left and 0.05 mm on the right. These are separate geometric observations. Neither one, by itself, identifies a pressure, temperature or material cause.

The clear interval is wider than the remaining clear copper width because part of that interval extends beyond the pad. The usable width must be calculated from the intersection of the two intervals. Reporting the clear opening width alone would miss the loss at the left pad edge.

This one-dimensional example does not determine exposed pad area or assembly acceptance. Repeat the analysis where the functional region requires it, including the other axis and important corners. Confirm that the obscuring boundary is adhesive before describing the calculated intrusion as adhesive flow. The calculation organizes measurements; it does not replace material identification.

Keep measurement uncertainty with the coordinates. If the apparent difference between two locations is comparable to the uncertainty of locating their edges, the evidence may not distinguish those locations. Improve the measurement or report the result as unresolved. Do not rank candidate causes using differences that the inspection method cannot reliably resolve.

Map the pattern across the panel

Inspect selected affected and apparently unaffected locations using the same method. Keep panel coordinates, feature orientation and neighboring construction with each result. A distribution can help choose the next investigation, but an apparent pattern is not proof of a process mechanism.

For example, a consistent film displacement across many openings suggests that registration deserves attention. Different clear-boundary intrusions with similar film alignment suggest a different set of questions. A problem concentrated near a local thickness transition makes the regional build and tooling relevant. Treat each as a hypothesis to check against records and additional measurements.

Show any nearby reinforcement boundaries on the evidence map. The stiffener design guide helps distinguish supported and unsupported regions. Record whether the reinforcement was present during the operation being investigated; a feature added later cannot directly explain the earlier geometry without an additional mechanism.

Request records that discriminate between hypotheses

Collect the material code, adhesive and film thickness designation, lot label and applicable storage records. Ask for the approved process revision and the records associated with the actual panel. A nominal recipe and a production record are different documents. Identify which one is being compared with the observed result.

Ask the process owner which recorded temperatures represent the part and which represent the equipment. Similarly, identify the meaning of the recorded pressure and the timing reference. The goal is to understand the actual evidence, not to prescribe a replacement cycle from a photograph. Use the material supplier’s applicable guidance and the fabricator’s qualified process when assessing a proposed change.

Include the local copper pattern, lamination stack and relevant release or support arrangement in the discussion. The stackup planning tool can organize the nominal layers, while the manufacturing record must describe the actual build. Do not present the planning output as proof of what was pressed.

Design a correction trial around a clear question

Choose a hypothesis that the available evidence can test. If the film opening is displaced, define a trial and measurement plan that evaluates registration. If material is identified inside an otherwise correctly located opening, define a trial that evaluates that observation while preserving other required properties. Do not change several unrelated inputs and then claim to know which one caused an improvement.

A practical trial record states the baseline, approved change, controlled conditions, sample identification and response measures. Include the smallest usable region and relevant bonding or protection requirements among those responses. A larger visible opening is not a complete success if the change creates another unacceptable feature.

Use representative geometry in the trial. A large isolated window may not answer a question about a dense contact array. Retain the actual layout context or explain why the chosen specimen represents it. Agree the evidence needed for release before running the trial so that the decision does not depend on selecting a favorable photograph afterward.

Keep disposition separate from root-cause closure

The immediate decision about affected parts and the longer investigation can proceed on different tracks. Record whether parts are held, accepted under an authorized disposition, or subject to an approved rework process. Keep the acceptance criterion and decision authority visible. A root-cause hypothesis does not itself authorize a deviation from the drawing.

Avoid prescribing solvent cleaning or scraping from appearance alone. Any proposed rework needs review for the actual materials, surfaces and downstream requirements, followed by appropriate verification. Preserve original specimens and before-and-after records if rework is evaluated. Otherwise the investigation may lose the evidence needed to understand the initial condition.

Close the investigation only when the explanation accounts for the measured boundaries and the corrective action has relevant evidence. Retain the original observations, geometry comparison, process records, trial results and any changes to controlled documentation. If a cause remains unresolved, say so explicitly rather than relabeling a temporary improvement as proof.

For a focused coverlay construction and manufacturability review, provide annotated images, released copper and opening data, the required usable region and the regional stack. That package turns a vague squeeze-out complaint into measurable questions about registration, opening geometry and the final material boundary.

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