Solder Mask Ink Deaeration: 4 Checks for a Bubble-Free Print

Solder mask ink deaeration is the step that lets trapped air escape from the ink before it is printed. Air enters whenever the ink is stirred, poured or pumped, and it leaves slowly, which is why the rest period after mixing matters as much as the mixing itself.

A bubble that reaches the screen becomes a defect. If it bursts during printing it leaves a pinhole; if it survives into the cured film it becomes a void that appears at development or, worse, at assembly.

Where the Air Comes From

The first source is mixing. Every stroke of a spatula folds air into the ink, and the amount depends on the speed and on how much of the ink surface is disturbed at once.

The second is transfer. Pouring ink from a can into a pot, or scooping it onto the screen, drags air along with the liquid and creates a foam at the surface.

Pumping and recirculation add air at the seals and at any point where the flow falls and the pressure drops. Each of those sources can be reduced, and the cheapest reduction is usually made at the mixing stage. An ink circuit that runs continuously can hold a surprising amount of entrained air.

What Bubbles Do to the Print

A bubble at the surface of the screen interrupts the transfer of ink to the panel. The result is a pinhole in the printed coating, which becomes a bare spot on the laminate or on a trace.

A bubble that stays in the film becomes a void. During cure the void expands slightly, and during development it can open into a pinhole or a thin patch.

Because the mask is an insulator and a solder dam, a pinhole is not only a cosmetic fault, and its position matters as much as its size. It can expose copper to flux, allow solder to bridge a clearance, or create an insulation failure at electrical test.

Rest Time and Stand Time

Rest time allows bubbles to rise to the surface, where they can be removed or simply left behind as foam. Its length depends on the viscosity of the ink and on the size of the bubbles.

The rest should be measured from the end of mixing and recorded, because it is easy to shorten under production pressure. An ink that was mixed five minutes before use has not had time to release its air.

Covering the pot during the rest keeps the surface from drying, which would otherwise trap the bubbles under a skin. A skin that forms and is stirred back in defeats the purpose of the rest. A skin that forms and is then stirred back into the ink defeats the purpose of the rest.

Mixing Technique and Speed

Mixing should be deliberate rather than vigorous. Folding the ink with a flat tool introduces less air than whipping it, and the same result is achieved in a little more time.

Mechanical mixers should be run at the lowest speed that achieves homogeneity, and the blade should be kept below the surface so that it does not draw air into the body of the ink.

Where a mixed batch is large enough to last a shift, the ink can be left to stand while it is not needed and stirred gently before use. The viscosity behaviour that governs how quickly the air escapes is described in ink viscosity control.

Viscosity, Thinner and Air Release

Air escapes more slowly from a thick ink than from a thin one, so a high-viscosity batch needs a longer rest. Thinner lowers the viscosity and speeds the release, but it also changes the print.

Adding thinner immediately before printing mixes air back into the ink and undoes the rest period. Where an addition is necessary, the ink should be mixed, rested and then used.

Temperature has the same effect as thinner, because a warm ink is thinner, so the temperature of the printing area belongs in the record. Ink that is warmed by a nearby oven will release air faster than the same ink in a cool room, which is one reason the printing area should be temperature controlled.

Pumping, Recirculation and Screen Loading

Where ink is supplied to the screen by a pump or a closed circuit, the circuit itself becomes a source of air. A pump that is oversized, or a return line that discharges above the liquid level, will aerate the ink continuously.

Screen loading should be done with a smooth pour against the side of the screen rather than dropped onto the mesh. The ink then reaches the screen without a foam layer on top of it.

Ink left on the screen at the end of a run should be returned carefully. Scraping it back into the pot drags air in with it, so the pot should then be rested before the next use.

Vacuum and Centrifugal Deaeration

Where the print quality demands it, ink can be deaerated under vacuum. The reduced pressure makes the bubbles expand and rise quickly, and the process takes minutes rather than hours.

Centrifugal deaerators achieve a similar result by spinning the ink so that the denser liquid moves outward and the air collects at the centre, where it can be drawn off.

Both methods need equipment that is clean and compatible with the ink chemistry, and both should be followed by a measurement of viscosity, because the treatment can change it. Where ink has been left standing for a long period, the treatment should be repeated before it is printed.

Symptoms of Poor Deaeration

Pinholes in the printed mask are the most direct symptom, and they often appear in a pattern that follows the squeegee stroke rather than the panel.

Foam on the screen and a print that looks grainy or thin in patches are earlier signs, visible to the operator before the panel reaches development.

A defect count that rises after a break or at the start of a shift points at ink that was mixed and used too quickly. Counting pinholes over a defined area of the first panel is a practical way to confirm that the deaeration worked. The same pattern appears when a fresh pot is opened and used without a rest.

Records and Verification

The record should carry the ink batch, the mixing time, the rest time, the viscosity reading and the printing parameters for the run.

Verification is visual and should be done on the wet print as well as after cure. Inspecting the screen for foam and the first panels for pinholes catches most deaeration faults before a lot is completed.

The batch control that supports this is described in ink shelf life control, the development step that reveals the rest in solder mask developing control, and reference methods are published by IPC.

FAQ

How long should ink rest before printing? Long enough for the visible foam to collapse and for bubbles to clear, which depends on the ink and the temperature. The time should be specified, not estimated.

Can vacuum deaeration replace a rest period? It reduces the time required, but a short rest afterwards is still useful because the treatment can leave fine bubbles that rise slowly.

Why do pinholes appear only at the start of a shift? Because fresh ink has just been mixed and poured. The air it contains has not had time to escape before the first panels are printed.