Pumice Scrub Surface Preparation Before Coating
A pumice scrub is a mechanical surface preparation in which a slurry of fine abrasive is brushed across the copper to remove oxide, contamination and a small amount of metal before a coating step. It is used ahead of solder mask, before dry film lamination and before some surface finishes, and its purpose is to present a surface that the next process can bond to.
What the Scrub Does
The scrub removes three things at once. It takes off the oxide that has formed since the last rinse, it lifts organic contamination from handling, and it abrades the copper surface into a controlled micro-roughness that gives the coating a mechanical key.
The amount of copper removed is small but not zero. A typical pumice line removes 0.5 to 2 micrometres per pass, and that figure has to be known, because it adds directly to the reduction in copper thickness that the board sees before it is finished. The same step is used ahead of some surface finishes where a mechanical key is required, and the removal figure is checked on a coupon rather than on production panels.
How a Pumice Line Works
The panel passes between rotating brushes while slurry is sprayed onto both surfaces. The brushes oscillate as well as rotate, so the pattern is not a set of parallel lines, and the panel is then rinsed in stages to remove the abrasive. The brushes are arranged in pairs so that both faces are prepared at once, which means a fault on one roller shows as a difference between the two sides of the board.
The abrasive itself is a fine powder suspended in water, and its concentration is controlled as carefully as the brush pressure. A slurry that is too dilute polishes rather than cuts, while one that is too concentrated packs in the brush and leaves a pattern of scratches.
Brush Pressure and Rotation
Brush pressure is set by the gap between the roller and the backing roller, not by a load cell, so the figure is a machine dimension that has to be gauged. A change of pressure changes the material removal rate and the heat generated at the surface.
Rotation speed sets the cutting speed between the brush and the copper. Both too low and too high reduce uniformity: a slow brush leaves a directional pattern and slips, while a fast one generates heat and drives the slurry away from the contact point. Brush diameter falls with use, so a gap that gave the correct pressure when the brush was new produces a lighter contact after a few days.
Water Quality and Rinsing
The water in the slurry and in the rinse stages has to be free of hardness and of particulates, because both are carried into the brush contact and pressed into the copper. Rinse water is normally deionised, and its quality is verified on the same schedule as the slurry. Hard water leaves scale on the copper that the brush then presses into the surface.
Rinsing after the scrub is the step that decides whether the preparation is useful. Abrasive left on the surface becomes a particle defect under the mask, and slurry residue left in a hole cannot be removed after lamination, so the rinse is usually followed by an inspection and a dry film step within a defined time.
Surface Roughness and Oxide Removal
Roughness is specified as an average deviation, typically 0.2 to 0.5 micrometres measured across the surface. The figure is a compromise: too smooth and the coating has no mechanical key, too rough and the mask or film cannot wet the valleys between the peaks. The measurement is taken on a coupon that has been through the line rather than on a sample of raw laminate.
Oxide removal is confirmed by a water break test, in which a clean copper surface holds a continuous film of water rather than beading. A surface that beads has not been brought to the required condition and will not give the <a href="https://www.gopcba.com/solder-mask-adhesion-test/” title=”adhesion”>adhesion the next step depends on. A surface that beads after the rinse has either been polished by an over-long scrub or not prepared at all.
Hold Time Before the Next Step
Copper that has just been scrubbed is chemically active and begins to oxidise within minutes in a normal shop environment. The hold time between the scrub and the next process therefore belongs in the process document as a limit rather than as a note.
A typical limit is two to four hours in a controlled area, with a shorter figure where humidity is high or where the following step is a coating that is sensitive to oxide. Panels that exceed the limit are normally re-scrubbed rather than used as they are. A re-scrub removes the oxide but also removes copper, so repeated rework is authorised rather than assumed.
Defects from a Bad Scrub
A scrub that is too light leaves oxide and contamination in place, which shows as poor adhesion at a mask edge, as blistering after cure and as lifting after thermal stress. The failure appears one or two steps downstream from its cause.
A scrub that is too heavy removes more copper than intended and leaves a surface that is too rough for the coating to wet. It also produces rolled abrasive particles that are pressed into the copper and appear later as pitting in the plating or as roughness under the mask. Rolled particles are the hardest of these to find, because they only become visible after the coating has been applied and cured.
Maintenance and Consumables
Brushes wear and their diameter falls, so the gap that produced the correct pressure when they were new produces a lighter one after a few weeks. The gap is gauged on a schedule and the brushes are replaced at a measured diameter rather than when the finish looks wrong.
Slurry concentration, nozzle condition and rinse quality are checked on the same round. A blocked nozzle leaves a dry stripe on the panel that is scrubbed unevenly, which appears at the mask stage as a band of poor adhesion across the board.
Verification and Records
Verification is a water break test on a coupon, a roughness measurement, and a copper thickness check taken before and after the scrub so that material removal is quantified. An adhesion test on a coated coupon completes the picture, and it is repeated whenever the brush or the slurry is changed.
Records should carry the brush gap and diameter, the slurry concentration, the water quality, the material removal figure and the hold time before the next step. A solder mask defect that appears as poor adhesion can then be traced to the mechanical preparation instead of to the mask material.
FAQ
How much copper does a pumice scrub remove? About 0.5 to 2 micrometres per pass, which has to be included in the copper budget for the finished board.
What roughness should the scrub produce? Typically 0.2 to 0.5 micrometres average deviation, fine enough to be covered completely and rough enough to give the coating a mechanical key.
Why is a water break test used? Because a properly prepared copper surface holds a continuous film of water, while oxide or contamination causes the water to bead and shows that the surface is not ready.
