Solder Mask Ink Viscosity: 4 Checks Before Screen Printing

Ink viscosity is the property that decides how a solder mask behaves as it passes through the screen. Too thick and the ink will not flow into the openings; too thin and it bleeds under the emulsion, closing the fine clearances that the design depends on.

The number is not fixed for a given product. It moves with the formulation, with the temperature of the workshop, with the thinner that has been added and with the shear that the squeegee applies, so the value on the pot is only a starting point. Two operators measuring the same pot can report different numbers if one stirs the sample and the other does not.

Why Ink Viscosity Decides the Print

Solder mask is a thixotropic material, which means its viscosity falls while it is being sheared and recovers when the shear stops. The squeegee stroke is what thins the ink enough to fill the screen, and the recovery afterwards is what keeps the printed pattern from slumping. The resting viscosity is what stops the printed pattern from spreading before the tack dry.

That behaviour is the reason a single reading is not enough. What matters is the balance between the viscosity measured at rest and the way the ink flows under the squeegee, and both are affected by the screen mesh and the print speed.

Measuring Viscosity on the Line

Viscosity is normally measured with a rotational viscometer or a flow cup, and the two instruments do not give the same number. The procedure has to state which instrument is used, at which spindle speed and at which temperature, or the readings cannot be compared. A spindle that is too large for the ink gives an unstable reading that changes with the depth of immersion.

A reading taken at rest and a reading taken immediately after stirring will also differ, which is why the sample should be conditioned and measured in the same way every time. The absolute value matters less than the consistency of the measurement. Thinner should be added in small increments with a full mix in between, so that the ink is not driven past the working window in one step.

Thinner, Mixing and Rest Time

Thinner is added to bring the ink back to its working viscosity, and adding it changes the solids content at the same time. An ink that has been thinned repeatedly prints a thinner mask, because the resin that remains after curing has been diluted.

Mixing should be complete before the ink is used, and the mixture should be left to rest so that air entrained during stirring can escape. Ink that is used immediately after mixing carries bubbles that show as pinholes in the cured film.

Temperature and Workshop Conditions

Viscosity falls as temperature rises, and a screening room that warms through the day will show a falling reading even when nothing has been added to the ink. Over a long shift that change can be large enough to alter the print.

Air conditioning, or at least a stable temperature with the ink stored in the same conditions as the press, removes most of the variation. Ink taken from a cool store and used at once will print differently from ink that has reached room temperature. Ink left in an open pot on the press also loses solvent faster than ink in a sealed container, so the pot should be covered between prints.

Screen, Squeegee and Print Parameters

The screen mesh, the emulsion thickness, the squeegee hardness and the print speed all interact with viscosity. A harder squeegee and a faster stroke shear the ink more, so a pair of settings that works with one ink may not work with another.

Because the interactions are many, the practical approach is to fix the screen and the print parameters for a product and then adjust only the ink to suit them. Changing both at once makes the cause of a print defect impossible to identify. The squeegee should be checked for wear at the same time, because a rounded edge changes the pressure it applies to the ink.

Viscosity Drift Through a Shift

Ink sitting on a screen loses solvent to the air, and its viscosity rises as the shift continues. The drift is slow at first and then faster, and the point at which it starts to affect the print arrives without warning unless the reading is taken at intervals. The interval should follow the length of the run and the volatility of the ink being used.

A defined check interval, with a small addition of thinner and a full mix when the reading leaves the window, keeps the print within the process. The addition should be recorded, because repeated top-ups change the solids content of the ink in the pot.

Tack Dry and the Window After Printing

After printing, the mask is tack dried so that it can be handled and exposed. The tack dry has to be matched to the printed thickness, because a thick print holds more solvent and needs more time than a thin one. A print that is too thick also takes longer to cure, which extends the window between printing and exposure.

Under-dried mask sticks to the artwork during exposure and damages the pattern; over-dried mask loses photospeed and develops poorly. The window between printing and exposure belongs in the specification, and the general rules are described in hold time between process steps.

Symptoms of the Wrong Viscosity

Ink that is too thick leaves skips, mesh marks and a rough surface, and it may fail to fill small openings entirely. Ink that is too thin bleeds under the emulsion, prints a thinner coat and can bridge the clearance between two fine features.

Skips and mesh marks that appear late in a shift usually point at a viscosity that has drifted upward. The defect is often first seen after imaging. Bridging that survives development appears as mask residue where it should have washed away, which is described in solder mask developing control.

Records and Setup Verification

The record should carry the ink identity and batch, the viscosity reading with its instrument and temperature, the thinner added and the print parameters used. That combination is what allows a print defect to be traced to the ink or to the machine. Readings should be taken from the same point in the pot and at the same depth so that the trend from one shift to the next is comparable.

Setup verification follows the same logic. The first panel of a run is checked for print thickness and definition before the exposure energy is confirmed, as described in exposure energy control, and the marking that follows the mask is covered in legend ink curing. Reference methods are published by IPC.

FAQ

Should viscosity be measured before or after adding thinner? Both, and the readings should be recorded. The before value shows how far the ink had drifted, and the after value confirms that the addition brought it back into the window.

Why does the same ink print differently in the afternoon? Because viscosity tracks temperature. A warmer room lowers the reading, so a stable screening temperature is as important as the ink itself.

Can old ink be brought back with more thinner? Only within limits. Each addition lowers the solids content, so an ink that has been thinned repeatedly will print a thinner and weaker mask even if the viscosity reads correctly.