Copper Foil Wrinkle and Dimple Defects in Lamination

A wrinkle in the copper of a multilayer board is a small fold that forms during lamination and stays in the finished product. It may look harmless on the surface, but it changes the thickness of the copper, the width of the trace after etching and the flatness of the layer stack. Because it is created inside a pressed stack, it cannot be corrected afterwards, and the panels it affects are usually scrapped rather than reworked.

How Wrinkles Form

During lamination the stack is heated and pressed, and the resin flows to fill the space between the copper features. If a sheet of copper foil is not held flat while that flow happens, it can fold in on itself. The fold may be only a few micrometres high, but the copper in that area is doubled and the surface above it is no longer planar.

The root causes are mechanical rather than chemical. Foil that is too thin for the panel size, handling that introduces a crease before pressing, damage to the separator plates and a press cycle that moves the stack while it is hot can all produce a wrinkle, and the pattern of the wrinkles on the panel usually points to which of them was responsible.

Foil Types and Surface Profile

Copper foil comes in two broad types: rolled annealed, which is made by rolling and has a smooth, ductile structure, and electrodeposited, which is grown on a drum and has a treated surface that bonds to resin. The two behave differently under pressure, and the choice is made from electrical and mechanical requirements as well as from availability.

The surface profile of the foil affects both adhesion and the way the resin flows around it. A very low profile foil adheres less strongly to the laminate and can slip during lamination, while a high profile foil grips the resin and resists movement. The profile is also what determines how fine a line can be etched, as discussed in the guide to the PCB etching process.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Production-line-with-robotic-electronics-assembly-960×585-2.webp" alt="Copper foil wrinkle visible on the surface of a laminated panel” />

Lamination Pressure and Resin Flow

The press cycle has to move enough resin to fill the spaces between traces while keeping the stack stable. Pressure that is applied too early, before the resin has softened, forces the copper to move rather than allowing the resin to flow. Pressure that is too low leaves voids and does not bond the layers properly.

The heating rate and the point at which full pressure is applied are the parameters that control this behaviour. They depend on the laminate, on the number of layers and on the copper distribution, which is why a stack with an unbalanced copper pattern is more likely to wrinkle. Copper balance is discussed in the guide to copper balance and thieving.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/91.png" alt="Dimple defect measured on copper foil after lamination” />

Tooling, Separator Plates and Flatness

The flatness of everything in the stack matters as much as the press settings. Separator plates that are bowed or scratched, press pads that have lost their resilience and tooling pins that do not locate the layers accurately all introduce a mechanical distortion that the foil has to absorb.

Plates and pads are consumables and should be inspected on a schedule. A worn pad leaves a pattern in the surface that repeats in the same place on every panel, which makes the defect easy to recognise once the possibility is considered. Tooling hole clearance also matters, because a tight pin can stretch one corner of a layer and create a wrinkle diagonally across the panel.

Dimples, Pits and Dents

A dimple is a local depression in the copper surface rather than a fold. It is usually caused by a particle that was trapped in the stack, by a mark in the separator plate or by a feature in the layer below telegraphing through the resin. Dimples are measured by their depth and their diameter, and the acceptable limit depends on the layer and on the line width being etched there.

Because a dimple is a depression rather than a doubling of material, it affects the process differently: it reduces the local copper thickness and can cause an undercut or an open after etching. It is also harder to see with the eye, so it is usually detected by an automated optical system or by a surface profiler rather than by a visual check.

Effects on Etching and Line Width

A wrinkle is thicker copper, so it etches more slowly than the surrounding area. Traces running across a wrinkle come out wider, and the spacing between them comes out narrower, which changes the impedance of the line and can create a short if the spacing closes completely. On a controlled impedance design that variation is a functional defect rather than a cosmetic one.

A dimple has the opposite effect and etches more quickly, thinning the trace and potentially opening it. Both conditions are worse on fine line designs, where the tolerance on line width is small and where a variation of a few micrometres is significant.

Detection Methods

Detection starts with a visual check of the surface under angled light, which reveals the shadow cast by a fold. Automated optical inspection is more consistent for small features and can be programmed to look for the change in contrast that a wrinkle produces, but it must be taught the defect or it will pass it.

Where the requirement is tight, surface profilometry or a dedicated copper thickness measurement gives a quantitative result. These methods are slower, so they are used on samples and on first articles rather than on every panel, and they are what establish whether a visual limit is being met.

Rework and Disposition

Rework is rarely possible. A wrinkle is inside the finished laminate, and the only way to remove it is to separate the layers, which destroys the board. Panels are therefore dispositioned as pass, use as is with a deviation or scrap, and the decision depends on where the defect falls and on the function of the layer.

A wrinkle in a non critical area, away from controlled impedance traces and away from fine pitch pads, may be acceptable under a written deviation. The same defect over a dense area is not, and the decision should be made by engineering rather than on the shop floor.

Prevention and Process Control

Prevention comes down to three things: flat material, a flat stack and a press cycle that lets the resin move before the pressure is applied. Foil and laminate should be stored flat and handled with both hands, plates and pads should be inspected, and the press recipe should be verified with a recorded cycle rather than trusted to the panel.

The process window is narrow on thin laminates and on large panels, which is why the parameters are usually recorded for each build. Where wrinkles appear repeatedly, the data that identifies the cause is the pattern on the panel: a fold near an edge points to tooling, a fold in the middle points to foil handling, and a repeating fold in the same position points to the press tooling itself.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can a copper foil wrinkle be repaired? No. The fold is bonded inside the laminate, so removing it would mean separating the layers, which is not a viable repair. Affected panels are either accepted under a deviation for a non critical area or scrapped.

Why does the defect repeat in the same position? A repeating pattern points to tooling rather than to material. A bowed separator plate, a worn press pad or a misaligned tooling pin will produce a mark in the same place on every panel until the tooling is replaced or corrected.

Does a wrinkle always cause a functional failure? Not always. A fold in a solid copper area away from fine features may have no effect on the circuit. A fold on a trace, especially a controlled impedance line, changes the width and the thickness and is treated as a functional defect.