Resist Strip: 6 Rules for Clean Copper

Resist strip is the wet process that removes the plating resist after the copper and the finish have been deposited. It looks like the simplest step in the line, and it is the one that most often leaves a film behind, which changes how the next chemistry behaves on the surface.

The target is clean copper with no residue and no attack on the metal that was just plated. Those two requirements pull in opposite directions, because a chemistry strong enough to remove a cured resist quickly is also strong enough to etch the copper beneath it.

What Resist Strip Has to Remove

The resist is a polymer film that has been imaged, developed and then exposed to plating baths. By the time it reaches the strip tank it has been in acid, in copper sulphate and in hot water, so it is not the material that was laminated onto the panel.

Plating also grows over the edges of the resist, forming a lip of copper that the strip chemistry has to undercut. Where the lip is thick, the resist tears and leaves a fringe of film along the trace edge, which is one of the most common sources of residue.

Chemistry Selection and Concentration

Most shops use an alkaline solution for the common dry films, with a concentration and a temperature that match the material. A bath that is too weak lifts the resist in sheets without dissolving it, and the sheets then re deposit elsewhere on the panel or block a spray nozzle.

Concentration is measured by titration rather than by appearance. The bath also loads with dissolved resist, so it is the combination of active strength and dissolved solids that decides whether it still works, and both are part of the analysis.

Time, Temperature and the Strip Window

The window is the range of time and temperature in which the resist is fully removed without damage to the copper. It is narrower than most operators expect, and it moves as the bath ages, so a setting that worked at the start of a campaign may be outside the window a week later.

Where the line runs a conveyor, the speed is the variable that sets the dwell. The speed should be adjusted from an analysis and a trial panel rather than from the previous shift, because the two shifts may see a very different bath.

Plating Resist Types and Their Behaviour

Dry film, liquid photoimageable resist and the resist used for solder mask all strip differently. A film designed for fine lines is thin and strips quickly, while a thick film used for a heavy copper pattern needs more time and a warmer bath to clear the same panel.

Mixing materials within a campaign makes the control harder, because the same bath has to serve both. Where the product mix includes a heavy copper pattern and a fine line board, the strip parameters should be set for the heavier of the two, and the fine line board should be checked for over exposure to the chemistry.

Rinse and the Film That Stays Behind

The strip tank dissolves the resist, and the rinse that follows has to carry the solution away before it dries. A rinse that is too short leaves a thin organic film that is invisible but changes the wetting of the next chemistry, which is how a strip problem becomes an etch problem.

Spray pressure and nozzle condition matter as much as the water quality. A blocked nozzle leaves a streak of residue down one part of the panel, and that streak is a recurring map of where the defect will appear. Our etching notes explain how the next step responds to a surface like that.

Clean Copper and the Next Step

The requirement is not only that the copper is free of resist but that it is chemically clean, meaning no organic film and no oxide that formed while the panel waited. The judgement is usually made by a water break test, where a sheet of water holds on a clean surface and pulls back on a contaminated one.

A water break test is quick, free and visual, and it should be used on every campaign rather than only when a defect appears. Where the surface fails it, the panel should be re cleaned rather than passed forward, because the next process will not correct it.

Dry Film Versus Liquid Resist

Liquid resist is applied thin and conforms to the topography, which makes it useful on a panel with heavy plating, and it tends to strip in finer particles. Dry film is thicker, more robust through the plating line and more likely to leave a fringe where the copper has grown over it.

The choice belongs to the process rather than to the shop preference. Where a heavy copper pattern is plated, a thin liquid resist may not survive the plating time, while a thick dry film may leave residue that has to be removed mechanically.

Equipment, Spray and Bath Loading

The strip module is a machine as well as a chemistry. Spray bars, nozzles, the conveyor and the pump all have a maintenance interval, and a pump that has lost pressure produces exactly the same defect as a bath that has lost strength.

Loading matters too: a tank that is fed panels faster than its chemistry can work will fail on the last panels of the run. The panel pitch and the dwell should be set together with the analysis, and our copper plating notes describe the loading limits of the line that precedes the strip.

Faults That Appear Later in the Process

Residue left by a poor strip shows up as a skip in the etch, a poor bond under the solder mask or a plating void in a later step. By then the panel has moved through several machines, and the strip is rarely the first suspect.

When the defect is a repeating streak or a pattern that follows a nozzle, the strip module deserves a look before the chemistry is changed. Checking the spray pattern with a glass plate is faster than a bath analysis and finds the fault in minutes. Our plating uniformity notes describe how that fault would present itself if it reached the plating step.

Records and Bath Maintenance

The record should show the analysis, the dosing, the temperature, the conveyor speed and any panel that was reworked. That combination is what makes a residue complaint answerable, and it also shows when a bath is close to its dump point.

Dumping and remaking a bath on a schedule rather than on failure keeps the process inside its window, at the cost of a planned stop. The unplanned alternative is a shift of scrap, which is always more expensive. The IPC process documentation gives the parameters that should be recorded.

FAQ

Can residue be removed after the mask is applied? Only by damaging the mask. The residue sits at the interface, so removing it means removing the coating above it. The strip step is the place to correct the problem, not a later rework station.

Why does the bath stop working suddenly? It usually does not. The analysis shows a slow decline in active strength and a rise in dissolved solids, and the sudden failure is simply the point at which the panel stops being clean. Trending the analysis finds that point in advance.

Is a stronger bath always faster? It is faster but it also attacks the copper and the finish, and it produces a more violent reaction that can redeposit film on the panel. The correct setting is the lowest concentration that clears the resist inside the dwell.