UV Curing Solder Mask For Quick Prototypes

A photoimageable mask is applied as a liquid or a dry film, exposed through a artwork and developed to open the pads. The exposure step uses ultraviolet light to cross link the coating where it should remain, and UV curing is therefore the step that defines the pattern rather than simply drying the film. Without it the coating would remain soluble everywhere.

UV curing is not the whole story, because a mask that has only been exposed remains soft and chemically weak. A thermal post cure follows, and the combination of the two steps determines the final hardness, adhesion and solvent resistance of the coating.

What UV Curing Does

The mask contains photoinitiators that absorb ultraviolet energy and start a cross linking reaction. Where the light reaches the coating the film becomes insoluble; where it is blocked by a dark feature on the artwork the film stays soluble and is washed away during development. UV curing therefore creates the pattern, and its uniformity decides how clean the pattern edges are.

Because the reaction is driven by energy rather than by time alone, the important quantity is the exposure dose, measured in millijoules per square centimetre. The same dose can be delivered by a long exposure with a weak lamp or a short exposure with a strong one, which is why dose rather than seconds is the figure that gets specified.

Photoimageable Mask Chemistry

A photoimageable mask is an epoxy acrylate system with photoinitiators, fillers and pigments. Liquid versions are screen printed or sprayed and then dried, while dry film versions are laminated with heat and pressure. Both are exposed and developed in the same way, and both need a thermal cure to reach their final properties.

Pigment absorbs light, so a darker mask needs a higher exposure dose than a lighter one to cross link to the same depth. This is why a recipe cannot be copied unchanged between colours, and why a green and a black mask on the same order may need different exposures. The ink properties behind this are described under solder mask ink thixotropy.

Exposure Dose And Resolution

Resolution is limited by the way light spreads as it passes through the coating. A thicker film scatters more and needs a higher dose, but a higher dose also widens the cross linked region slightly, which closes up small openings. The practical limit for a conventional liquid mask is around 0.1 mm openings on a 20 micron film.

Underexposure leaves the film soft and the sidewalls ragged, and the mask can lift during development or later during soldering. Overexposure closes small openings and leaves a residue that is difficult to remove without damaging the pattern. The window between the two is narrower than most people expect.

Development And Rinsing

Development removes the unexposed mask using a mildly alkaline solution, usually sodium or potassium carbonate, in a spray developer. Time, temperature, concentration and spray pressure all affect the result, and the process must be controlled so that the unexposed film dissolves completely without attacking the cross linked regions.

Rinsing after development is as important as the development itself. Residual alkaline solution left in a fine opening will continue to attack the mask and can leave a conductive residue that appears later as a leakage path. A final rinse with deionised water and a proper dry complete the step. How the finished appearance is judged is described under PCB design quality characteristics.

Thermal Post Cure

The thermal post cure completes the cross linking and drives off remaining solvent. A typical schedule is 150 C for 60 minutes, though the exact values come from the mask supplier. Under-cured mask stays tacky, has poor solvent resistance and shrinks during assembly, all of which cause problems later at the assembly stage.

Over-curing is also a risk. Too much heat makes the coating brittle, reduces adhesion to the laminate and can discolour the mask, and on a board operating close to the glass transition temperature of the resin it can affect the laminate itself. The cure window should be verified by a solvent rub test on each batch.

Limits And Typical Uses

Photodefined mask is the standard for production boards because it holds fine geometry and can be inspected before the finish is applied. It is used on everything from a simple two layer board to a high density interconnect, and it works with every common surface finish.

Where it struggles is on very tall features, in deep cavities and on a board with an unusual shape, because the coating has to be applied and dried evenly. In those cases a sprayed or hand applied mask may be the only practical route. How mask selection fits into the overall build is described under PCB design and fabrication.

Process Control and Verification

On a design of this kind, UV curing 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.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Process Control and Verification

On a design of this kind, UV curing 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.

Process Control and Verification

On a design of this kind, UV curing 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can a UV cured mask be used without a thermal cure? For a prototype handled gently it can, but the coating stays soft and has poor chemical resistance. Any board that will be soldered more than once should receive the full thermal cure.

Why does exposure dose matter more than lamp power? The reaction depends on the total energy delivered, so a weak lamp with a long exposure can equal a strong lamp with a short one. Dose in millijoules per square centimetre is the transferable figure.

Does mask colour change the process? Yes. Darker and more heavily pigmented masks absorb more ultraviolet light and generally need a higher dose and a longer development to open the same feature size.