Solder Paste Tack Test and Print Quality Control

A tack test measures how strongly a solder paste resists being pulled apart, and the property it describes decides whether components stay where the placement machine put them. Paste that has lost its tack will release the parts during the transfer to the reflow oven, and the resulting shifts are often blamed on the placement program.

Tack is also a sensitive indicator of paste condition. It changes with age, with humidity and with the time the paste has spent open on the stencil, so a routine measurement gives early warning of a material problem before print defects appear on the board.

What Tack Means in a Printing Process

Tack is the adhesive strength of the paste film, and it comes from the flux vehicle rather than from the alloy powder. It is what holds a chip component in place between printing and reflow, and it must be strong enough to survive the machine movement without being so strong that the paste is pulled off the pad.

Two quantities are usually reported: the peak force needed to separate a probe from the paste, and the energy absorbed during the separation. The peak force describes the initial grip, while the energy describes how the paste behaves as it stretches, which is what a component experiences when the board is moved.

Test Methods for Tack

The standard approach uses a probe that is lowered onto a printed deposit with a controlled load, held for a fixed dwell, and then withdrawn at a defined rate while the force is recorded. A tack curve is produced, and the peak of that curve is the reported value.

Repeatability depends on the method being fixed rather than on the value being high. Probe diameter, contact pressure, dwell time and withdrawal speed all change the result, so a specification without them cannot be compared between laboratories, let alone between suppliers.

Tack Force and Probe Geometry

Probe geometry sets the contact area and therefore the scale of the result. A 5 mm probe gives a larger force than a 2 mm probe on the same paste, and the reading is normally quoted per unit area so that the two can be compared.

The printed deposit also matters. A test performed on a large pad with a thick deposit reports a different value from one performed on a fine pitch aperture with a thin deposit, which is why the test should be run on a print that matches the production stencil and the aperture area ratio of the product.

Humidity and Temperature Effects

Flux vehicles are hygroscopic to different degrees, and a tack reading taken at 30 percent relative humidity is not the same as one taken at 65 percent. High humidity softens the paste and can raise the measured tack, which hides the loss of solvent that is actually occurring.

Temperature acts in the same direction. A stencil area at 30 °C will show a lower tack than the same paste at 22 °C, so the test area should be recorded with the result and the production floor kept inside a stated band rather than allowed to follow the weather.

Tack Loss on the Stencil

Paste on a stencil loses tack for the same reasons it loses print volume: the solvent evaporates, the vehicle thickens and the surface skins over. The tack curve flattens before the deposit height changes, which makes the test an early indicator rather than a confirmation.

The rate of loss depends on the exposed area, the room conditions and the paste chemistry, and it is fastest on a stencil with a large open print area. Where the tack reading falls more quickly than the open time limit suggests, the stencil is carrying a thinner film of paste than the operator believes, and the roll should be reworked or replaced. The measurement belongs with the print quality data for that stencil rather than in a separate file.

Tack and Component Retention

Placement machines accelerate the board and the head, and every acceleration applies a shear load to the paste beneath a placed component. Small, light parts are most at risk because they have the least mass holding them down and the largest relative effect from the head movement.

The defect appears as a component that has shifted, rotated or tombstoned after reflow, and it can be reproduced by running the placement program at a lower speed. Where the slower speed cures the problem, the paste tack rather than the machine calibration is the first thing to investigate.

Measuring Tack in Production

A tactile gauge is not a tack test, and pulling a component off the board by hand tells only whether the paste is sticky. A bench instrument with a controlled probe and a recorded curve is needed to produce a number that can be compared over time.

The practical frequency is once per lot on receipt and once per shift for paste that is already open, with the sample taken from the stencil rather than from the jar. Measuring at the same point in the roll gives comparable results, and the viscosity printing window should be recorded on the same sheet.

Acceptance Limits and Corrective Action

A tack specification is best written as a minimum, with the test method attached to it. When the reading falls below the limit, the paste is either too old, too warm, too dry or from a lot that does not match the qualification sample, and the four causes are separated by checking the age record first. The trend matters more than a single reading, because tack falls gradually with age rather than in steps.

Corrective action follows the material rather than the machine: replace the paste on the stencil, verify the room conditions, and re-test before restarting the run. Where several consecutive lots fail, the supplier should be asked for a tack curve rather than a certificate, because the curve shows whether the material being delivered still matches the slump and tack behaviour of the qualified paste.

Records and Supplier Communication

Records should carry the paste lot, the age at test, the room temperature and humidity, the probe geometry and the peak force. With those fields a change in tack can be attributed to the material, the environment or the measurement, and the argument about which one it was does not need to be repeated.

The same data is the strongest input to a supplier discussion. A tack trend that falls with each delivery is a material drift, while a trend that falls during a shift is a process issue, and the two call for entirely different conversations.

FAQ

What does a solder paste tack test measure? The peak force and the energy needed to separate a controlled probe from a printed deposit, which together describe how well the paste will hold a component before reflow.

How often should tack be tested? Once on receipt for each lot and once per shift for paste that is open on the stencil, with an additional test after any change of room conditions or paste batch.

Can humidity change the tack reading? Yes. A higher relative humidity softens the flux vehicle and raises the reading, which can mask the solvent loss that is really taking place in the paste.