Solder Mask Viscosity Control: Mixing, Thinner and Pot Life

Solder mask viscosity decides how the ink behaves in the screen printer, and it changes from the moment the can is opened. It affects the thickness of the printed film, the definition of the openings and how the ink behaves when it is cured.

Viscosity is not a fixed property of a product; it is a property of a batch, a mixing routine and a pot life. Solder mask viscosity control is therefore about the ink store and the print room rather than about a single measurement taken on arrival.

<img src="https://www.gopcba.com/wp-content/uploads/2020/04/ptt_service-1.jpg" alt="Solder mask viscosity measurement before screen printing” />

What Viscosity Means for Solder Mask

Solder mask is a thixotropic ink, which means its viscosity depends on how fast it is being sheared. It is thick when it stands still and thinner when the squeegee moves across it, and it recovers after the stroke has finished.

That behaviour is what makes screen printing possible, because the ink has to flow through the mesh and then stay where it was put. A mask that recovers too slowly will slump and close a fine opening, and one that recovers too quickly will not fill the mesh. The balance between the two is what the ink supplier formulates and what the printer setting has to respect.

Mixing Before Use

Ink settles in the can, and the pigment and the fillers do not settle at the same rate. Mixing brings the batch back to a uniform state, and it should be done to a written procedure with a defined time and a defined tool.

Under mixing leaves a soft layer at the top and a stiff layer at the bottom, so the first boards printed from a can are different from the last. The same principle that governs paste, described in solder paste expiry and mixing work, applies to mask ink. A can that is mixed on a bench scale is easier to control than one mixed by eye.

Thinner and Its Limits

Thinner is added to bring a batch back to its working viscosity, and it should be added in small measured amounts with mixing and a measurement between each. Adding thinner by eye is the fastest way to lose control of a batch.

Every addition also changes the solids content, so the cured thickness for a given print will be lower. The film thickness target described in solder mask spray coating control work applies to screen printed films as well, and it is set with the solids in mind. Thinner that is added and then allowed to evaporate leaves the solids where they started but the batch disturbed.

Measuring Viscosity

A rotational viscometer gives a number, and the number only means something if the spindle, the speed, the temperature and the sample history are all the same. A reading taken at a different temperature is not comparable with the standard.

The measurement should be taken on a sample that has been allowed to rest for a defined time, because the ink is still recovering from the mixing. A reading taken immediately after mixing will always be low. The rest time should be identical between readings or the numbers cannot be compared.

Pot Life and Working Time

Mixed ink has a pot life, and its viscosity rises through it as solvent evaporates from the surface of the screen. The rise is faster in a warm room and faster still when the ink is left spread over a large screen area. A screen that is left loaded over a break loses solvent faster than a closed can does.

The pot life should be recorded and the ink should be discarded at the end of it. Ink that has been topped up with thinner repeatedly is no longer the batch that was qualified, and its behaviour will not match the process window. Ink that has been thinned repeatedly should be quarantined rather than run to the end of the can.

Printer Settings and Viscosity

The printer settings are chosen for a viscosity, so a change in ink requires a change in setting. Squeegee pressure, speed and flood stroke interact with the ink, and adjusting one to compensate for the other usually hides the real problem.

Where the settings are adjusted, they should be recorded against the ink batch so that the pair can be reproduced. The principle that a setting is only valid for the material it was tuned on is the same one behind stencil thickness selection. A setting that is not written down beside the batch cannot be repeated. The setting and the batch should travel together through the shift handover.

Defects That Point at Viscosity

An ink that is too thin slumps, closes fine openings and produces a film that is thinner than the target. One that is too thick does not release from the screen cleanly, leaving a ragged edge or a pitted surface.

The appearance faults that follow are described in solder mask orange peel and in solder mask blistering work, and both can have a viscosity cause. Checking the ink before adjusting the printer is the cheaper route. Most print defects that look mechanical begin in the ink cabinet. The ink record is the cheaper half of the investigation and should be read first.

Storage and Batch Records

Ink should be stored in a sealed container within its shelf life, and the storage conditions that apply to the product are set out in solder mask shelf life control work. A can that has been opened and resealed repeatedly will not behave like a fresh one. The oldest can should be used first, and the date should be read rather than assumed. A can with no date should be treated as suspect rather than used to clear the shelf.

The record should carry the batch, the mixing time, the thinner added, the measured viscosity, the pot life and the printer settings. Where the shop works to a published standard, such as the coating documents from IPC, the acceptance criteria should come from that source. The film thickness and the resolution should both be measured on the cured board, not judged by eye. A cured board kept as a reference makes the comparison between batches far easier.

Defects and Corrective Action

Where a print defect appears, the sequence should start with the ink and only then move to the printer. Viscosity, pot life, batch and mixing are all cheaper to check and all more likely to have changed than a machine setting. The ink record should be on the same sheet as the print settings for that reason.

Corrective action should be recorded with the result, so that the next occurrence can be handled the same way. A shop that changes a printer setting to compensate for an ink problem will need that setting again on the next batch, and the process will have moved without anyone deciding to move it. Keeping the ink and the settings together is what keeps the process where it was qualified. Nothing in the ink store should be changed without a note against the batch and the date.

FAQ

Should thinner be added to every can? Only to bring the batch back to its working viscosity, in small measured amounts. Regular additions without measurement change the solids content of the ink.

Why does a viscosity reading change between operators? Because spindle, speed, temperature and rest time all affect it. The method has to be identical for the numbers to be comparable.

What happens when the pot life is exceeded? The ink thickens, releases poorly from the screen and prints a thinner film. Ink past its pot life should be discarded rather than thinned back into use.