Solder Mask Adhesion: Surface Preparation and Cure

Solder mask that lifts, blisters or flakes is almost never caused by the mask itself. It is caused by what was on the copper before the mask was applied, or by the conditions under which it was cured.

How the Mask Bonds

The mask is an epoxy based material applied as a liquid or a dry film and bonded to the copper and to the laminate. Its adhesion depends on mechanical keying to a slightly roughened surface and on chemical bonding to a clean one.

A surface that has been roughened beyond a certain point has a weak boundary layer of loosely attached particles, and a surface that is too smooth offers little keying. The process window lies between the two.

The bond also depends on the state of the copper. An oxide layer that is too thick is cohesive but weak, and one that is chemically different from the expected oxide will not bond to the mask chemistry.

Surface Preparation Steps

The copper is prepared by a mechanical or chemical treatment. Mechanical preparation uses a brush or a pumice slurry to remove the surface and to create a texture, and chemical preparation uses a micro etch to remove the outermost layer.

A brown or black oxide treatment is used where the mask must bond to an inner layer or where a rougher surface is needed. The thickness and the chemistry of the oxide determine whether the mask adheres to it or to a layer that will later separate.

The preparation is followed by a rinse and a dry, and both matter. Residue from the preparation chemistry is a releasing agent, and water left in a hole will cause a blister during cure. Our etching process notes describe the surface state that the preparation produces.

Cure and Its Effect

The mask is cured in stages, with a pre cure to dry it and a final cure to complete the reaction. An undercured mask is soft and will not resist the soldering process or the cleaning chemistry.

An overcured mask becomes brittle and can crack under thermal cycling, and the crack becomes a path for moisture and contamination. The cure windows are narrow and they are established by the material supplier.

Where the board is cured with a component already fitted, the temperature is limited and the cure may be incomplete. This is the case for a mask applied as a repair, and the resulting adhesion is usually poorer than a production mask. Our coating notes describe the related cure problem for materials applied in shadowed areas.

Contamination and Its Sources

Oils from skin, flux residue from a previous process, silicone from a mould release or a lubricant, and residues from the plating chemistry all interfere with adhesion.

Silicone is the worst of these because it spreads and because it is difficult to remove completely. A single source of silicone in the area can affect every board for weeks.

The remedy is a cleaning step before the mask is applied, with a chemistry chosen for the contaminant, and with a verification that the surface is clean. Our quality notes describe the surface checks that precede the mask.

Design Features That Stress the Mask

A mask that crosses a step in the copper, such as the edge between a plane and a trace, is under stress at the step. A thick copper layer creates a taller step and a larger stress concentration.

A mask over a via that is not plugged will crack or tent and open. A tented via with a thin mask over a large hole will fail during reflow when the air inside the via expands.

A mask over a sharp copper corner has a stress raiser, and rounding the corner in the artwork reduces it. These are small design changes that make a large difference to the mask’s life. Our solder defects notes describe how a lifted mask appears after assembly.

Testing Adhesion

The standard test is a tape pull, in which an adhesive tape is pressed onto a crosshatched area of the mask and then pulled away. The amount of mask that comes with the tape is assessed against a reference.

The test is repeated after a thermal excursion, since a mask that passes at room temperature can fail after reflow. The relevant test is therefore the one performed on a board that has been through the assembly process.

A humidity test is also informative, because a mask that has absorbed moisture has lower adhesion and will fail more readily. Both tests are simple and both give a number rather than an opinion.

Field Failures and Their Diagnosis

A mask that lifts around a pad creates a path for solder to wick under it, producing a bridge that appears intermittently and that cannot be seen from above.

A mask that blisters in the field indicates that moisture entered through a crack or a void and then expanded. The blister is a symptom, and the crack is the cause.

The section through a lifted mask shows whether the failure was at the mask to copper interface or within the mask itself, and the two indicate different root causes. Adhesion at the interface points at preparation, and failure within the mask points at cure.

Process Control and Verification

On a design of this kind, solder mask is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Process Control and Verification

On a design of this kind, solder mask is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

Can a lifted mask be repaired? It can be touched up with a repair material, and the repair will not have the adhesion of the original. Where the lifted area is at a pad, the repair is better done after rework and before coating.

Does a thicker mask adhere better? No. Thickness adds stress and makes cracking more likely, while adhesion is a property of the interface.

What does gopcb provide for solder mask quality? We provide surface preparation matched to the copper and the mask chemistry, cure records with the material supplier’s window, adhesion testing before and after thermal excursion, and sectioning of any field failure to establish which interface failed.