Emulsion Thickness: 5 Rules for Printing Screens

Emulsion thickness is the depth of the light sensitive layer that sits above the mesh on a printing screen, and it is the single biggest control over how much ink is deposited. The ink fills the opening, and the volume of that opening is the mesh thickness plus the emulsion above it, so the deposit follows the emulsion.

Emulsion thickness is also the parameter that drifts without anyone noticing. Every wipe of the squeegee wears a little material away, and a screen that has printed a thousand panels will deposit less ink than the same screen did on its first day unless the thickness is measured and maintained.

What Emulsion Thickness Sets on a Screen

The screen holds ink in the openings between the threads and in the emulsion that sits on top of them. When the squeegee passes, most of that ink is transferred to the panel, and the amount is proportional to the total depth of the opening. A deeper opening, in round terms, gives a thicker deposit.

Definition is the second effect. A thick emulsion layer improves the depth of the image but reduces the resolution, because the walls of the opening become taller and the ink has further to travel to release. A thin layer resolves fine detail well and deposits very little ink. The window between those two behaviours is the working range for the product.

Direct Emulsion Versus Capillary Film

Direct emulsion is coated onto the mesh with a trough and built up in passes. The thickness is controlled by the number of passes on each side, by the viscosity of the emulsion and by the drying between passes. It is flexible and inexpensive, and it depends completely on the operator.

Capillary film is a pre formed layer that is laminated to the mesh with water and a roller. It arrives with a known emulsion thickness, so the result is repeatable, and it produces fewer pinholes because the film is manufactured rather than coated. The trade is cost, and the need for a clean, even lamination so that the film bonds to every thread. Both routes are described in the process literature published by the SMTA.

Mesh Choice and Deposit Transfer

Mesh count and thread diameter set the size of the openings and the thickness of the mesh itself. A fine mesh with thin threads holds less ink, resolves finer detail and is the usual choice for a thin, precise coating. A coarse mesh holds more ink and is used where a heavy deposit is needed.

Transfer efficiency links the mesh to the result. Not all of the ink in the opening leaves the screen, and the proportion that does depends on the ink rheology, the print speed and the surface of the panel. That is why a screen that deposits correctly on one product may deposit too little on another, even at the same emulsion thickness. The mask thickness selection rules set the target that the screen has to deliver.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/High‑Speed_PCB_Manufacturing_Guide.jpg.webp" alt="Capillary film applied to a screen mesh before exposure” />

Measuring Emulsion Thickness

The measurement is taken as emulsion over mesh, which means the height from the top of the mesh to the top of the emulsion. A gauge with a small probe is used directly after coating and after drying, and the readings are taken at several points across the screen.

The reading should be recorded on the screen itself, and it should be rechecked after the screen has been used. A drop of a few micrometres across a production run is normal, while a drop of twenty per cent means the screen is worn and the deposit can no longer be predicted from the coating that was applied.

Exposure, Drying and Pinholes

Exposure works with the emulsion thickness rather than separately from it. Light has to penetrate the full layer to cross link it, so a thicker coating needs a longer exposure. Under exposure leaves the bottom of the layer soft, and the edge of every opening then breaks down during printing, which shows as a ragged print and a short screen life.

Drying matters for the same reason. A screen dried in a humid room keeps water in the emulsion, and the trapped moisture becomes a pinhole or a weak spot after exposure. Screens should be dried flat, in the dark, in an area with controlled humidity, and coated screens should be handled with clean gloves rather than fingers.

Wear Over the Life of a Screen

Every print stroke abrades the emulsion a little. The squeegee slides across the surface, the ink carries particles and the cleaning step adds another cycle of mechanical action. The loss is slow, and it appears as a deposit that becomes thinner while the printing parameters are unchanged.

Wear is not uniform either. It concentrates where the squeegee travels over a high point in the mesh, at the edges of the image and near the frame where the squeegee changes direction. A screen that shows a thin deposit at one end of a panel is often worn there rather than coated unevenly. The mask skip checks are the practical way to catch that pattern before the customer sees a board with exposed copper.

Symptoms of Too Much or Too Little

Too much emulsion thickness shows as a deposit that is thick and soft, with poor resolution at fine features, longer drying and a greater chance of sagging before the cure. The screen also releases ink poorly, so the image fills solid areas but loses small openings.

Too little shows the opposite. The deposit is thin, mesh marks are visible and coverage over a trace edge is marginal, which is where the coating fails first in service. Either symptom is a coating measurement waiting to be taken, and the printer should reach for the gauge before the pressure adjustment.

A Control Routine for the Screen Room

The routine is short. Measure the emulsion thickness after coating, record it on the screen, expose with the time that matches the coating, and measure again after the screen has been used. Keep the measurements with the screen so that the next user knows what it will deposit.

Then set a wear limit. When the measured value falls below the limit, the screen is either recoated or retired, and the decision is made from data rather than from a print that looks slightly pale. The legend printing work is a good place to start, because the deposit there is easy to see. The solder mask defects list should be worked back to the same measurement when a coating problem appears.

FAQ

How many coating passes does a solder mask screen need? The number comes from the target deposit and from the emulsion being used, and it is set once and then verified by measurement. A screen built by counting passes alone will vary with viscosity, temperature and drying time.

Can a worn screen be recoated? It can, if the mesh is undamaged and the old emulsion is fully stripped first. Recoating a screen with emulsion still in the openings builds an uneven layer, so the stripping step has to be complete before the new coating is applied.

Does emulsion thickness change the cure of the printed ink? It changes the thickness of the ink that has to cure, so a heavier deposit needs more cure energy for the same result. The cure schedule should be set with the measured deposit rather than with the nominal one.