Alkaline Resist Stripping Control in PCB Fabrication

Resist stripping removes the plating resist and the etch resist from a panel after the copper has been defined, and the step has to do it without attacking the copper, the solder mask or the laminate. The chemistry is alkaline, the reaction is a swelling and dissolution of the polymer, and the control is a balance between a strip that is complete and one that is gentle enough to leave the surface ready for the next step.

What the Strip Step Has to Do

The resist must be removed completely, including from the walls of a plated hole and from a fine line where the film is thin. Any residue left behind becomes a plating defect, a solderability problem or a cosmetic reject that appears after the next process.

The step also has to leave the copper in a defined state. A strip that is too aggressive etches copper at the line edges and dulls the surface, while one that is too gentle leaves fragments that the following rinse cannot remove. Residue that survives to the next step is almost always cheaper to remove at the strip tank than later.

Chemistry of Alkaline Stripping

Alkaline strippers are based on hydroxides or on organic amines and rely on swelling the resist until it lifts from the copper. The reaction rate depends on concentration, temperature and the degree of cure of the resist itself.

Because the mechanism is swelling rather than dissolution, mechanical action matters. Spray pressure, the angle of the nozzles and the dwell time in the spray zone all contribute, and a bath that is chemically correct can still fail on a panel if the spray pattern is blocked. A dry film resist behaves differently from a liquid one, and the same bath rarely suits both.

Concentration and Temperature

Typical conditions are 2 to 5 percent of the active alkali by weight with a temperature of 45 to 60 degrees Celsius. Concentration is measured by titration rather than by conductivity, because the alkaline content is consumed by the resist and by carbon dioxide from the air.

Temperature is the parameter that changes fastest when a line is busy. A bath that runs hot strips quickly and attacks the copper and the mask, while one that is cool leaves resist at the edges of the panel where the spray loses energy. Temperature is checked at the spray header rather than in the tank, because the two differ while the line is running.

Strip Rate and Residence Time

Rate is measured by running a coupon through the line at a known speed and inspecting it for completeness, and it is normally expressed as the conveyor speed at which the strip is just complete, with a margin of 20 to 30 percent.

The margin exists because the load changes through a shift. A heavy load consumes the chemistry faster and lowers the temperature of the spray, so a line that is set at the edge of completeness will leave residue on the last panels of a batch. That margin is the only protection the line has against a load heavier than the one used to qualify it.

Rinsing and Copper Protection

The strip is followed by a rinse that has to remove both the dissolved polymer and the alkali. A poorly rinsed panel carries alkali into the next bath, which raises its pH and changes its behaviour, and it leaves a surface that oxidises quickly.

Because the copper is bare and freshly exposed at this point, the hold time before the next step matters. A panel that waits in a rack for an hour before the next process has a different surface from one that moves directly, where the panels are usually held wet rather than allowed to dry, which is a common source of variation in the bare copper control that follows.

Resist Residue and Scum

Residue left after stripping takes two forms. A film that remains on the copper is a lifting failure, while a scum that redeposits from the bath onto the panel is a contamination failure, and the two look similar at the exit of the line.

Scum forms when dissolved polymer reaches its solubility limit in the bath or when the resist is a dry film that breaks into fine particles rather than lifting in sheets. It appears as a haze on the copper that is visible in the rinse and as a streak along the direction of travel, and it becomes a plating defect in the next step if it is not removed. A scum that returns after a bath change points to the resist rather than to the chemistry.

Effect on Solder Mask and Finish

The stripper works on the resist, but it also meets the solder mask, which is itself an organic coating. A hot, strong bath attacks the edges of the mask and softens it, and a mask that is under-cured will show lifting or a change of colour after stripping.

The lesson is that strip performance cannot be assessed on bare test panels alone. A coupon printed and cured with the production mask shows the real effect, and a change of strip chemistry should always be qualified in that way rather than on a sample of laminate. A mask that is correctly cured survives a well controlled bath with no measurable change in adhesion.

Common Faults

A bath that is weak or cold leaves resist on the panel. One that is hot and strong etches the copper at the line edges and attacks the mask. A blocked spray nozzle leaves a stripe of resist that follows the nozzle pattern, and a rinse that is under-running leaves alkali that changes the next bath.

Each fault has a positional signature. Nozzle faults follow the machine, bath faults affect the whole panel, and faults caused by the resist itself affect the areas where the film is thickest, which is usually where it was applied over a step or a track edge.

Process Window and Records

The window is described by concentration, temperature, spray pressure, conveyor speed and rinse quality. Strip completeness is confirmed on a coupon at each shift change, and the copper condition is checked at the same time with a visual standard. The titration interval shortens as the load rises, and a heavy day is the reason a bath is analysed twice.

Records should carry the titration, the temperature, the conveyor speed and the rinse conductivity, together with the coupon result and the hold time before the next step. A bath control discipline applied to the strip tank is what keeps the step from becoming a variable that is blamed on the resist supplier.

FAQ

Why does resist residue appear only on the last panels of a batch? Because the load consumes the chemistry and cools the spray, so a line set at the edge of completeness runs out of margin before the batch ends.

Can alkaline stripping damage the solder mask? It can when the bath is hot or strong and the mask is under-cured. Coupons cured with the production mask are the only reliable qualification.

What causes scum on the panel after stripping? Dissolved polymer that has reached its solubility limit, or resist that breaks into fine particles instead of lifting in sheets, both of which are bath condition problems.