Solder Mask Ink Mixing Ratio: Two-Part Control

The mixing ratio of a two-part solder mask ink decides how the chemistry cures, and it is one of the few settings that cannot be corrected later. Part A carries the resin and the pigment, and part B carries the hardener. Getting the proportion wrong changes the cure, the adhesion and the print behaviour at the same time. The two parts are supplied separately because the reaction starts the moment they meet.

Mixing is often treated as a kitchen task rather than a process step, which is how ratio errors reach production. The ratio should be measured by weight, recorded and repeated the same way on every shift. A batch mixed by eye will vary even when the operator is careful. A batch sheet signed by the operator is worth more than a verbal handover.

Why the Mixing Ratio Matters

The ratio sets the stoichiometry of the cure reaction, so it decides how completely the mask cross-links. Too little hardener leaves the film soft and soluble, while too much leaves unreacted chemistry that can affect adhesion. Both faults appear after imaging or after the thermal cure, which is why the solder mask ink shelf life control rules begin with the mix. A soft film also shows up later as poor adhesion under thermal stress.

The supplier datasheet states the ratio and the tolerance around it, and those numbers should be followed exactly. Where a shop adjusts the ratio to change viscosity, it is changing the cure at the same time. Viscosity should be adjusted with the correct solvent instead. The tolerance is usually narrow, so a small weighing error matters more than it looks.

Two-Part Ink Chemistry

Two-part inks are supplied as separate components because the reaction starts as soon as they are combined. Once mixed, the clock is running and the batch has a working window. That window is the reason mixing is done in small batches throughout the shift. Mixers should prepare only the quantity that will be printed within the pot life.

Temperature affects the reaction rate strongly, so a batch mixed in a warm room will gel faster than the same batch in a cool one. The ink and the room should both be within their stated range. Cold ink also takes far longer to reach a workable viscosity, which tempts operators to over-thin it. Ink that is cold should be allowed to reach room temperature before mixing.

Measuring and Dispensing the Ratio

The ratio is measured by weight, using a scale that is accurate at the batch size being mixed. Dispensing pumps and metering systems remove operator variation, but they need their own calibration. The simplest reliable method remains a calibrated scale and a written batch sheet. Two operators using different scales will produce two different inks.

Where the two parts are very different in density, mixing by volume introduces an error that is easy to underestimate. That is why the datasheet gives a ratio by weight and why the batch sheet should follow it. Scales should be checked with a test mass at the start of a shift. A scale that drifts silently is worse than one that fails outright.

Induction Time Before Use

After mixing, the ink is usually left to stand so that air escapes and the components reach a consistent state. This induction period is specified by the supplier, and it is measured from the end of mixing. Using the ink immediately gives a different viscosity and a different print behaviour.

Induction also allows the bubbles introduced by mixing to rise out of the ink. Printing a fresh batch straight away puts those bubbles onto the panel as pinholes. A covered container and a timer at the station make the step easy to follow. A timer at the station removes the guesswork about when mixing finished.

Pot Life and Working Window

Pot life is the period after mixing during which the ink can still be printed properly. It shrinks as the temperature rises and as the batch sits on the bench. A batch that is past its pot life will print unevenly and will cure to a weaker film. Batch size should be matched to the run length rather than to the container size.

The rule should define a maximum batch size so that pot life is never exceeded at the end of a run. Leftover ink from an expired batch should be discarded rather than stretched, because the cost of the ink is far less than the cost of the boards. Disposal should follow the supplier safety data.

Viscosity and Print Quality

Viscosity follows the ratio and the solvent content, and it decides how the ink flows through the screen and levels on the panel. Too thick and the ink will not release cleanly from the mesh, leaving a stippled surface. Too thin and the ink sags into the openings or bleeds over the pads. Viscosity measured at one temperature cannot be compared with a reading at another.

Viscosity should be measured at a defined temperature with a defined instrument, otherwise readings cannot be compared between shifts. The checks covered in solder mask viscosity control apply to the same measurement. A batch outside the window should be adjusted or rejected before printing begins. A stippled print is usually a release problem rather than an exposure problem.

Mixing Equipment and Cleanliness

Mixers, containers and tools must be clean and dry, because moisture and cured ink residue both disturb the mix. A container with dried ink from a previous batch introduces particles and changes the effective ratio of the new batch. Cleaning should be written into the mixing instruction.

Mixing speed and time should also be controlled, since fast mixing whips air into the ink while slow mixing may not disperse the components fully. A defined programme on the mixer removes that variation, and the same programme should be used for every batch of that ink. Mixed ink should never be returned to the component containers.

Records and Batch Traceability

The record should carry the ink part numbers, the lot numbers, the measured weights, the mixing time and the operator. With that, a cure or adhesion problem can be traced back to a specific batch. A record that names only the ink type cannot settle any question.

Traceability also supports the customer requirement that material lots be identifiable on demand, and the expectations in the documents published by IPC give the general reference. Where a batch fails a test, the record shows which panels it was used on, which limits the containment to the affected boards. The same discipline applies to the solder mask printing process that follows. Lot numbers are what make a containment decision possible later.

FAQ

Can the mixing ratio be adjusted to change viscosity? No. Adjusting the ratio changes the cure chemistry, so viscosity should be changed with solvent or with temperature instead.

What is induction time? The rest period after mixing, specified by the supplier, during which bubbles escape and the components reach a consistent state before printing.

What happens if the pot life is exceeded? The ink gels and prints unevenly, and the cured film is weaker, so an expired batch should be discarded rather than stretched. Ink that has started to gel cannot be saved by adding solvent.