Solder Mask Ink Mixing: Ratio, Pot Life and Quality

Ink mixing is the first step of the solder mask process and the one that is least often measured. A two-part epoxy ink is supplied as a resin and a hardener, and the ratio in which they are combined decides how the coating cures, how long it can be worked and how it behaves on the panel. The step takes a few minutes and the tools cost very little, which makes it the cheapest control point in the whole mask process.

Mixing errors do not appear immediately. A batch that is slightly off ratio will print and develop normally and then fail in the cure oven, or pass the cure and fail in service, which is why the step deserves a scale rather than a guess. A batch that is mixed and then left standing for an hour has already spent part of its working life before it reaches the screen.

Why Ink Mixing Is Critical

The hardener reacts with the resin, and that reaction goes to completion only when the two are present in the intended proportion. Excess resin leaves an under-cured film that stays soft and can be lifted during later processing. The specification normally allows a small tolerance on the ratio, but the tolerance is not a target and the aim should be the stated figure.

Excess hardener produces a film that is hard and brittle, with poor adhesion and a tendency to crack at the edges of the mask openings. Both errors are invisible in the mixed ink itself, which is exactly why they survive to the board. Automatic mixing and dispensing equipment removes the weighing step, and it still has to be checked against a manual weight.

Two-Part Ratio and Measuring

The ratio is specified by weight rather than by volume, because the two components have different densities and a volumetric mix will be wrong in a way that still looks plausible. A scale of suitable resolution is the only reliable tool for the job. Paddle speed matters less than coverage, so the goal is to move the whole volume rather than to create a fast current in the middle.

The components should be brought to the same temperature before weighing, since a cold hardener pours slowly and a warm one pours fast, and both lead to a rushed measurement at the bench. Mixed ink should be covered while it stands, because the surface skins over and the skin does not re-disperse into the batch.

Mixing Method and Shear

Mixing has to be thorough without whipping air into the ink. A slow paddle that sweeps the whole container is better than a fast one that only moves the middle, and the sides and base of the pot should be scraped down during the mix. Pot life should be stamped on the container at the moment of mixing, since the mix time is the only reference that matters.

What is left on the wall of the container is unmixed material, and unmixed material prints as a streak of soft or glossy ink that shows up after cure. Viscosity behaviour of the mixed product is covered in solder mask viscosity control notes. A batch that has reached the end of its window should be discarded even if it still looks workable, because the cure has already started.

Pot Life and Working Time

Once mixed, the ink begins to cure at room temperature, and pot life is the window in which it can still be printed. The window shortens with temperature, so a warm room can halve the time the specification allows for. Viscosity standards are usually quoted at a fixed temperature, so a reading taken in a cold room will look higher than it really is.

Ink at the end of its pot life thickens and starts to gel, and printing with it produces skips and poor release from the screen. The pot should be re-mixed gently during use and discarded at the end of the window rather than stretched. Shelf life before mixing is covered in solder mask shelf life work. A falling reading across a print run points to solvent loss, while a rising one points to a reaction that has begun in the pot.

Viscosity Checks Before Use

Viscosity is checked on the mixed ink before printing, and checked again during long runs. A falling viscosity in a screen printing ink usually means the solvent is flashing off or the ink is warming in the pot. Thinners should be measured as well, because an addition made by eye changes the ratio that was so carefully weighed out.

The measurement should use the same instrument and the same conditions every time, because a reading taken at a different temperature is not comparable. Thinners should only be added to the grade the manufacturer allows, and never to correct an off-ratio batch. Standing time is the simplest deaeration method, and it costs nothing except the part of the pot life that it uses up.

Deaeration and Voids

Air introduced during mixing becomes voids in the cured film. A void in the mask over a trace is a cosmetic defect, but a void at the edge of a pad opening is a path for moisture and flux residue. Voids are also caused by a rough screen or a poor print stroke, so the source should be confirmed before the ink is changed.

Deaeration by standing, by a vacuum cycle or by careful mixing all reduce the problem. Where voids appear repeatedly the mixing method should be examined before the ink supplier is blamed. Every print run should be able to name the ink batch that produced it, and the batch record should be kept with the lot.

Batch Records

The record should carry the ink part numbers, the lot numbers, the weights of each component, the mix time and the pot life expiry. That set is what allows a mask defect to be traced to a batch rather than to a whole shift. Mask adhesion is tested after cure on a coupon, because an off-ratio batch can pass a visual inspection and fail a tape test.

Where the ink is used across several print runs the pot should be labelled with the mix time, so that an operator arriving later knows how much window is left. Cracking at mask openings after reflow is one of the clearest signs that the hardener proportion in the batch was too high.

Consequences on the Board

An off-ratio mask shows up as poor adhesion, as lifting during reflow or as cracking at the mask edges. All of them are expensive, because they are usually found after the board has been assembled.

Development behaviour also changes, since an under-cured film develops faster and loses fine detail. Mask development is described in solder mask development notes, and acceptance follows the published IPC documents.

FAQ

Why mix solder mask ink by weight? Because the two components have different densities, so a volumetric mix gives the wrong ratio in a way that still looks correct.

What happens if the pot life is exceeded? The ink thickens and begins to gel, so it prints with skips and releases poorly from the screen.

Can thinner fix an off-ratio batch? No. Thinner changes how the ink prints, but it cannot correct the chemistry that the ratio error has already set.