Dry Film Thickness Control: Lamination, Resolution and Exposure

Dry film photoresist is supplied as a sandwich of a polyester cover sheet, the resist itself and a polyethylene separator, and it is laminated onto the copper under heat and pressure. The thickness of the resist layer is chosen when the film is ordered, and it then constrains the resolution, the exposure energy and the development time for that layer. Dry film thickness control is therefore a decision made at the start of the process and verified at lamination.

What Dry Film Thickness Controls

The resist has to withstand the etching or plating chemistry that follows, and that sets a minimum thickness. It also has to resolve the finest feature on the layer, and that sets a maximum, because a thicker film requires more exposure energy and develops with a wider sidewall slope.

The two requirements meet in the middle of a small range. A standard film for inner layers is commonly 25 to 38 micrometres thick, thinner films are used for fine-line work, and thicker films are used for plating resists where the resist has to stand up to a long immersion in an aggressive bath.

Film Thickness Options and Selection

Films are quoted by their total thickness, which includes the two protective sheets, so the resist thickness is not the number on the label. The supplier’s data sheet gives the resist thickness, and that is the value the process has to be built around.

The selection follows the finest feature and the chemistry. Where lines are below about 75 micrometres, a thin film gives better resolution and a more vertical sidewall. Where the layer will be plated in a copper bath for an hour, a thick film is needed to prevent the resist from lifting at the edges of the pattern during the long immersion.

Lamination Parameters and Temperature

Lamination takes place in a hot roll laminator, and the three parameters are roll temperature, roll pressure and pass speed. The resist has to reach its softening temperature so that it conforms to the copper surface, including the topography around the traces already present.

Too cool and the film does not adhere, leaving pockets of trapped air that appear as open circuits after development. Too hot and the resist flows, becoming thinner in some areas and thicker in others, which changes both resolution and the development behaviour. The optimum is found on a test panel and then held, and the relevant controls are covered in the standard practice for dry film lamination.

Resolution and Aspect Ratio of the Resist

Resolution depends on the ratio of resist thickness to feature width, because a thick film is a long tunnel for the exposing light and for the developing solution. The thicker the film relative to the line width, the more the image spreads and the harder it is to develop cleanly to the bottom.

The practical consequence is that the same artwork resolution cannot be held on a thick film. Where a design needs fine lines and heavy plating, the answer is usually to split the operation rather than to force both requirements onto one resist layer, because the film cannot satisfy them at the same time.

Exposure Energy and Thickness

Exposure energy needed to cure the resist to the bottom of the film increases with thickness. A film that is 25 percent thicker requires roughly a proportional increase in energy, and the energy has to be sufficient to leave a fully cured layer at the interface with the copper, not merely a hard surface.

Where exposure is carried over from one product to another without adjustment for thickness, the result is under-cured resist at the bottom of the film. The symptom is a pattern that looks correct but lifts or undercuts during development and etching, and it is often attributed to the developer rather than to the exposure, which is why the exposure series should be repeated whenever the film batch or the thickness changes.

Development and Undercut

Development removes the unexposed resist, and a thicker film takes longer. The developer has to penetrate the full thickness of the channel and remove the material at the bottom, which is also where the resist is least exposed.

The relationship between film thickness and undercut is direct: a thicker film develops with a sloped sidewall, and the slope transfers to the etched copper profile. Where the finished line width is critical, the film thickness is one of the contributions to the overall tolerance, alongside the artwork dimension and the etch factor of the chemistry.

Defects Linked to Film Thickness

A film that is thinner than intended leaves pinholes and resists the plating chemistry poorly, so the defects are open circuits in etching and plating voids in pattern plating. A film that is thicker than intended produces lines that are narrower than the artwork after development and shorts where the developer failed to clear the bottom of a narrow channel.

Thickness variation across a panel is the third defect class. It usually comes from uneven roll pressure or temperature, and it shows as a pattern that develops at different rates in different areas of the panel. Measuring resist thickness at several points after lamination is the only way to see it before the boards are processed.

Verification and Records

Thickness is measured after lamination with a hand-held gauge on a sample of panels, using the same points each time, and the values are recorded against the lot. The measurement should be taken after the cover sheet is removed, because the sheets are not part of the resist.

The record should also carry the roll temperature, pressure and speed, the film batch and the exposure energy used. Together with the dimensional checks on the artwork film, this makes it possible to separate a film problem from a lamination problem when a pattern defect appears, and it is the record that a customer will ask for when a lot is questioned.

Comparing Dry Film With Liquid Resist

Liquid resist can be applied in controlled thickness with curtain coating or spraying, and it conforms better to heavy topography than a laminated film. Dry film is more consistent, easier to handle in a shop without a coating line and easier to verify, which is why it remains the standard for most rigid board imaging.

The choice affects the process around it. A switch from dry film to liquid resist changes exposure energy, development time and the film thickness tolerance, so it should be treated as a full process change with a qualification run rather than as a substitution of one material for another on the same parameters.

FAQ

What dry film thickness should be used for fine lines? Thinner films, commonly 20 to 25 micrometres of resist, resolve fine features better than thicker ones because the light and the developer travel a shorter distance through the film.

Does a thicker film need more exposure? Yes. The energy needed to cure the resist to the copper interface increases with thickness, and carrying the same exposure over from a thinner film leaves the bottom of the resist under-cured.

How is dry film thickness measured? With a hand-held gauge on a sample of panels after lamination and with the cover sheet removed, at the same set of points each time, so that the readings can be compared between lots.