Peelable Mask for Temporary Protection on PCB Panels

A peelable mask is a temporary coating that is applied over areas that must not be soldered, plated or coated, and then removed by hand after the operation is complete. It is a form of temporary protection that is used to protect gold fingers during wave soldering, to keep a connector free of conformal coating and to mask holes that must stay open during a plating step. It is a consumable that adds labour to the process, so it is used where a permanent mask cannot do the job and where the protected feature really matters.

What Peelable Mask Provides

The material has to do two contradictory things: adhere well enough to survive the process it is protecting against, and peel cleanly enough to be removed without leaving residue or damaging the surface beneath it. The balance between those two is what makes the product difficult to use well. A coating that is optimised for adhesion will leave residue, and one that is optimised for peeling will lift during the process it was meant to protect.

Typical applications are wave soldering, conformal coating, plating and the cleaning processes that follow them. In each case the protected area has a requirement that the surrounding process would violate. The requirement should be written down before the material is chosen, because it determines the peel strength and the thickness that will work.

How It Is Applied

The material is normally applied by screen printing, by dispensing or by a manual brush for small areas. Screen printing gives the best edge definition and the most repeatable thickness, which matters because the peel behaviour depends on the film thickness as much as on the chemistry. A film that is too thin tears, and one that is too thick traps solvent and blisters during the soldering process.

Coverage has to extend beyond the area being protected by a margin, because the coating can shrink slightly during cure and because a mask edge that sits exactly on the boundary may leave a sliver of exposed surface. The margin should be stated in the work instruction so that the coverage does not depend on the operator’s judgement.

Cure and Peel Strength

The coating is cured to a defined state that is not fully cross linked, because a fully cured film tears rather than peeling. The cure schedule is therefore a balance between a film that is tough enough to survive the process and soft enough to remove in one piece. A cure that is verified with a travelling thermocouple is more reliable than one taken from the oven display.

Peel strength is measured by pulling a strip of the cured film at a defined angle and speed, in the same way a tape adhesion test is performed. The result is compared with a range rather than a single value, because both too little and too much adhesion cause problems. Our quality documentation describes how these results are classified at gopcb.

Compatibility with Soldering

During wave soldering the coating is exposed to flux, to preheat and to a solder wave at around two hundred and fifty degrees. It has to remain intact through all three without blistering, cracking or lifting at the edge.

Compatibility with the flux is often the limiting factor, because a flux that attacks the coating will soften it and make it impossible to peel in one piece. The pairing should be tested on a real assembly rather than assumed from the data sheets. A coating that survives the preheat can still fail in the wave, so the test has to include the whole process rather than only the thermal profile.

Masking Gold Fingers and Connectors

Gold fingers are the classic application, because solder on a gold contact finger destroys the contact surface. The coating is applied over the finger area before wave soldering and removed afterwards, and the removal must not scratch the gold. The peel direction should be chosen so that the film lifts away from the finger rather than across it, which is a small detail with a large effect.

Connectors and press fit holes are masked for the same reason, and in those cases the coating also has to keep the hole clear so that the pin can be inserted later. A coating that flows into a hole and cures there is difficult to remove without damaging the plating. Where a hole has to stay clear, a plug rather than a coating is often the better answer.

Removal and Residue

Removal is normally done by hand, starting at a tab that was left deliberately for that purpose. A tab that is too small is difficult to grip, and one that is too large wastes material and may interfere with the process it was meant to protect against. The tab should be placed where the operator can reach it after the board has been through the machine.

Residue is the most common complaint. A thin film of the coating left on the surface can prevent soldering, interfere with a connector and cause a cosmetic rejection. Our surface finish guide describes how a contaminated surface behaves during soldering. A visual check under ultra violet light where the material fluoresces makes residue easy to find.

Common Problems

The typical problems are a film that tears during removal, a film that lifts at the edge during soldering, a film that leaves residue and a film that pulls solder mask or legend off with it. Each of them points at a different variable. Recording which of the four appeared, and where on the board, usually identifies the cause without any further testing.

Tearing usually means the cure was too complete or the film too thin, while edge lifting usually means the adhesion was too low or the surface was contaminated before application. Removing mask from the board along with the coating indicates that the mask beneath was not fully cured. That particular failure is worth checking at the source, because it also means the permanent mask is at risk in normal service.

Alternatives to Peelable Mask

Where the geometry allows it, a mechanical fixture, a high temperature tape or a purpose made silicone plug can replace a peelable coating. These alternatives avoid the residue question entirely but they require labour and they can be dislodged. The choice between them is usually made on the number of units and on how well the fixture fits the board.

For conformal coating, a dispensed mask or a gel that is cured in place can be used instead. The choice depends on the number of units, the geometry and the process being protected against. Our production flow guide places these choices in the wider sequence. Where the volume is high and the geometry is stable, a fixture usually wins; where it is low and variable, a peelable coating is more flexible.

Process Control Points

The controls are the material lot and its shelf life, the applied thickness and coverage margin, the cure profile, the compatibility with the flux or coating being used, and the peel test result on a sample. Those five items are enough to make the process repeatable, and each of them can be checked in a few minutes.

Because the material is removed by hand, the operator technique matters. A written instruction that defines where to start the peel, at what angle and at what speed removes most of the variation between operators and between shifts. Labour is the largest part of the cost of this process, so consistency is where the saving is.

FAQ

Why does the peelable mask tear when removed? Usually because it was cured too completely or applied too thinly. Both make the film brittle, so it breaks instead of peeling in one piece.

Can peelable mask be used in a reflow oven? Some products are formulated for that, but the exposure is much longer than in a wave solder machine and the compatibility has to be confirmed. The data sheet should state the maximum exposure.

What causes residue after peeling? An under cured film leaves a sticky layer, and a contaminated surface can leave patches of coating behind. The cure schedule and the surface preparation should both be checked.