Photoresist Selection for Inner Layer Imaging Guide

On an inner layer the photoresist is the stencil for the etch, and its properties decide the finest line the shop can hold and the defect rate it can live with. Choosing a resist is therefore a matter of matching its resolution, its adhesion and its stripping behaviour to the copper thickness and the feature size of the product. The resist is a consumable, but it determines the capability of the line, so the choice should be reviewed when the product mix changes.

What the Resist Has to Do

The resist has to accept an image from the exposure tool, survive the development step with the pattern intact, protect the copper during etching, and then strip completely without leaving residue on the finished traces.

Each of those steps places a different demand on the material. A resist that resolves very fine lines may be thin and fragile in the etcher, while one that is thick and tough may not resolve the fine lines at all. The two requirements have to be balanced for the specific job rather than optimised for one of them.

Dry Film Versus Liquid Resist

Dry film is laminated onto the copper as a solid layer, which gives a uniform thickness and a very clean surface. Liquid resist is coated and dried, which allows a thinner layer and better resolution but depends on the coating step for its uniformity.

Dry film dominates inner layer production because the lamination step is simple and repeatable. The film also protects the copper from handling damage between the imaging and the etching steps, which is a benefit beyond the pattern itself. Liquid resist appears where the resolution requirement exceeds what a film can deliver or where the surface is not flat enough for lamination. Our etching process guide covers the step the resist has to survive. Where a shop runs both dry film and liquid resist, the two processes should be kept separate so that the parameters do not become confused.

Resolution and Thickness

Resolution and thickness pull in opposite directions. A thick film requires a longer exposure and develops with a wider sidewall, which limits the finest gap it can hold, while a thin film resolves better but has less resistance to the etchant.

The practical rule is to use the thinnest resist that will survive the etch for the copper thickness in question. For a one ounce inner layer that is a relatively thin film, and for heavy copper it is a considerably thicker one. The resist thickness is normally quoted in micrometres, and the shop should work from the measured value rather than the nominal one.

Adhesion to Copper

The resist has to bond to the copper well enough that the etchant cannot creep underneath the edge. Adhesion depends on the surface preparation, the lamination temperature and pressure, and the chemistry of the resist itself.

A surface that carries oxide, oil or a heavy micro etch profile will produce a resist that lifts at the edges during etching, which appears as an over etched or ragged trace. The preparation step and the resist selection are therefore decided together. Where the copper has been given a heavy micro etch for adhesion, the resist may struggle to cover the profile, which is a common compromise.

Exposure and Development Latitude

Latitude is the range of exposure and development conditions over which the resist still produces the correct pattern. A wide latitude resist is easier to run in production because a small drift in the lamp or the developer does not change the result.

A narrow latitude resist may give better resolution but demands tighter control of the exposure energy and the developer concentration. The trade off should be made against the capability of the shop rather than against the data sheet alone. A resist with a narrow latitude will produce a defect rate that reflects the stability of the process rather than the skill of the operator.

Stripping After Etching

After the etch the resist has to be removed completely, usually in an alkaline solution. A resist that strips cleanly leaves bare copper ready for the next step, while one that leaves a film causes adhesion problems later in the process.

Stripping is affected by the exposure and the cure history of the resist as well as by its chemistry. A film that has been over exposed and over baked may cross link enough to become difficult to remove. The stripping step should be checked whenever the exposure or the bake conditions change. Our quality documentation describes how these conditions are classified at gopcb.

Defects and Their Causes

The common defects are poor resolution of fine gaps, resist lifting at the trace edge, residue after stripping and pinholes that allow the etchant through to the copper beneath. Each of them points at a different part of the process. Recording which defect appeared, and where, is usually enough to identify the step responsible for it.

Pinholes usually come from contamination on the copper or from particles trapped during lamination, while edge lifting usually comes from insufficient lamination temperature or an over active surface. Our multilayer guide explains how the inner layer sequence is organised around these steps. A defect that appears on one side of the panel only is more likely to be a lamination problem than a chemistry problem.

Selection Criteria

The criteria are the minimum line and gap to be held, the copper thickness, the surface finish of the copper, the exposure tool, the developer and the stripping chemistry. All of them together define the resist that will work.

Where a shop runs a range of products, it is usually better to standardise on one or two resists that cover the range than to change material for every job. Standardising also makes it easier to train operators and to compare results between shifts. Standardising makes the process parameters familiar and the defects predictable. Our tooling guide covers the alignment that the resist image depends on. The resist and the exposure tool are a pair, and changing one without reviewing the other is a common source of a capability loss.

Process Control Points

The controls are the resist lot and its shelf life, the lamination temperature, pressure and speed, the exposure energy, the developer concentration and temperature, the strip chemistry and the inspection of the developed image. Each of those is recorded per lot, and the inspection result is the evidence that the pattern was correct before the copper was removed.

A step tablet or a resolution target on every panel confirms the exposure is still correct without a separate test. That single addition turns the resist from an assumption into a measured process. Where a step tablet is used, the exposure can be corrected before a whole lot is affected rather than after.

FAQ

Should I use the thinnest resist available? Only as thin as the etch will allow. A thin resist resolves better, but it will not survive a long etch on heavy copper, so the choice follows the copper thickness.

Why does the resist lift at the trace edge? Usually because of insufficient lamination temperature or pressure, or because the copper surface was contaminated or over etched before lamination.

What causes residue after stripping? Over exposure or over baking can cross link the film enough to resist the stripper. The strip chemistry and the thermal history should both be checked.