Screen Printing Solder Mask: Mesh, Emulsion and Thickness

Screen printing applies solder mask by forcing ink through a woven mesh onto the panel. It is one of the older ways of coating a board and it remains in use for thick deposits, for certain ink systems and for shops whose imaging route does not require a photoimageable material. The deposit is set by the mesh, the emulsion and the printing parameters rather than by a roller gap.

The control problem is different from the one in roller coating. The deposit is less uniform by nature, because the mesh leaves a pattern in the surface, and it depends on a screen that wears with every print. The thickness that matters is still the one that survives drying and curing, and it is measured on the panel rather than inferred from the screen.

How Screen Printing Differs

In roller coating the film is formed by a gap between two rollers, while in screen printing the ink is pushed through a mesh by a squeegee. The mesh is stretched over a frame and coated with a photosensitive emulsion that defines the open areas, and the ink passes only through those areas. The deposit is therefore patterned by the screen rather than by exposure of the panel.

That difference means the screen printing step can define features as well as coat the surface. The same step that deposits the mask also leaves the pads open, provided the screen carries the pattern. Where a photoimageable material is used, the screen defines only the overall coverage and the pattern is created later by exposure and development.

Mesh Count and Ink Deposit

The mesh count is the number of threads per unit length, and it sets both the open area and the theoretical ink volume. A coarse mesh passes more ink and gives a thicker deposit, while a fine mesh gives a thinner one and better edge definition. Typical counts for solder mask screens range from about 60 to 120 threads per centimetre.

The theoretical volume is calculated from the mesh thickness and the open area and is quoted by the mesh supplier. The actual deposit is lower, because some ink remains on the screen and some is pulled back by the squeegee, and the difference is the reason the deposit is measured on the panel rather than calculated from the mesh data.

Emulsion Thickness and Edge Definition

The emulsion is the layer that defines the stencil on the screen, and its thickness adds to the mesh to set the deposit. A thicker emulsion gives a thicker print and lower resolution, and a thinner one gives a sharper edge and a thinner deposit. The two requirements pull against each other exactly as they do with a stencil.

Edge definition determines whether the mask stops cleanly at a pad or bleeds onto it. A ragged edge leaves a sliver of mask on the pad which interferes with soldering, and the defect looks like a mask bleed but originates in the screen. The controls that keep the deposit inside its window are described in the notes on solder mask thickness control.

Squeegee and Stroke

The squeegee pushes the ink through the mesh and then releases it as the screen snaps back. Its hardness, its angle and the pressure it applies all change the deposit: a harder squeegee with a steeper angle leaves less ink, and a softer one with a heavy pressure leaves more. The speed of the stroke changes the time available for the ink to fill the mesh openings.

The stroke is a setting that is easy to adjust and hard to relate to the result, because it interacts with the ink viscosity and the screen tension. The useful practice is to fix the stroke and the pressure for a product and to adjust the ink and the screen rather than the machine, so that the process remains comparable between shifts.

Snap Off and Screen Tension

Snap off is the distance the screen lifts from the panel after the squeegee passes, and it determines how cleanly the ink transfers. Too little and the screen stays in contact, leaving a smeared deposit; too much and the ink is stretched as it leaves the mesh, which produces a rough surface and poor definition.

Screen tension sets how well the screen behaves. A screen that has lost tension deforms under the squeegee, and the deposit becomes thicker in the middle of the panel and thinner at the edges. Tension is measured on the screen with a tensiometer on a regular schedule, and screens are retired when they fall below a limit rather than when they are visibly damaged.

Ink Rheology and Replenishment

Screen printing inks are thixotropic: they shear thin under the squeegee and recover after it passes. The recovery rate sets how the deposit levels, and it depends on the ink temperature and on the solvent content. An ink that has thickened in the screen does not fill the mesh properly, and one that has been thinned too far slumps after printing.

Replenishment keeps the ink on the screen constant in volume and in viscosity. Adding a fixed amount of fresh ink on a schedule is more predictable than adding ink when the print looks thin, and the addition is recorded with the material lot. The relationship between viscosity and the printed result is described in the roller and curtain coating notes, which face the same material behaviour with a different applicator.

Drying and Imaging

The printed panel is dried before the next step, and the drying removes the solvent and sets the film. Printing a thicker deposit than intended makes the drying harder, because the solvent has further to travel, and a panel that is not fully dried will stick to the artwork or will develop poorly.

Where the material is photoimageable, the exposure and development follow the print, and both are set with the deposit thickness as an input. A thicker deposit needs more exposure energy and more development to clear the same opening, which is one reason a print thickness change is not an isolated event. The development side is described in the notes on solder mask development.

Measuring the Deposit

The deposit is measured on the dried film with an eddy current gauge over a copper area, and the measurement is taken at several positions across the panel. The mesh pattern makes the surface slightly uneven, so a single reading can be high or low depending on whether it falls on a thread, and the average of several readings is more meaningful than one.

The measurement should be recorded with the screen identification and the mesh count, because the deposit is a property of the screen as much as of the ink. Where a screen is replaced mid-order, the deposit changes and the exposure may need to follow it, and the record is what shows that the two are connected. The alternative methods are compared in the notes on curtain coating control.

Records and Change Control

The records are the screen identification with its mesh and emulsion thickness, the tension, the ink lot and its viscosity, the squeegee and stroke settings, the deposit measurements and the drying schedule. With those fields, a print problem is traced to a screen or to an ink lot in one review.

Change control applies to the screen as a consumable. A new screen batch, a re-stretched frame or a change of emulsion thickness changes the deposit without any machine setting moving, and the verification that follows should be a thickness measurement on the first panel of the order rather than a visual check. The adhesion of the finished mask, which depends on the same preparation, is described in the notes on solder mask adhesion preparation.

FAQ

Is screen printing still used for solder mask? Yes, for thick deposits and for certain ink systems. Photoimageable materials applied by roller or curtain coating have taken much of the market for fine features.

Why does the deposit vary across the panel? Usually a screen that has lost tension or a snap off that is too small. Both are measurable and both are corrected on the screen rather than on the panel.

Does a thicker print help coverage? It improves coverage over tall features and makes drying and development harder. The specified range is the answer rather than a maximum.