Paste Tack: Why Deposits Lose Hold Before Reflow

Paste tack is the adhesion that a printed deposit offers to a component lead or body before reflow. It is what holds a chip capacitor in place while the board travels to the oven, and it is the reason a placement machine can run at speed without gluing every part down. Tack is not the same as viscosity, and it is not the same as stickiness as an operator would judge it by touch.

A deposit loses tack as the flux vehicle absorbs moisture, loses solvent or simply sits exposed to air. When it falls below the force that placement and conveyor handling impose, parts shift, tombstone or fall off, and the defect is usually blamed on the placement machine. This note explains what tack is, how it is measured and how to set an open time limit.

What Paste Tack Actually Measures

Tack is the peak force needed to separate a probe from the paste surface, measured in grams or newtons. A standard test presses a probe of defined area into the deposit at a controlled speed, holds it briefly, then pulls it away while recording the force. The peak value is the tack force.

The result depends on the test conditions, so a number without them is meaningless. Probe geometry, contact pressure, dwell time and pull speed all change the reading, and so does the age of the deposit. When two pastes are compared, the same conditions must be used on the same day, or the difference that is seen may belong to the method.

Open Time and the Loss of Tack

Open time is the period between printing and reflow during which the deposit still behaves as intended. Part of that window is about tack and part is about slump and drying, and the tack portion is usually the shortest. Most pastes hold useful tack for a few hours, and some hold it for under an hour in a dry room.

The limit is not fixed by the supplier. It depends on the deposit size, because a small deposit dries through faster than a large one, and on the environment around the printer. Our notes on paste open time set out how to measure the window on your own line rather than taking a figure from a datasheet.

Humidity, Temperature and Tack

Humidity softens the flux vehicle and makes the deposit feel tacky while reducing its cohesive strength. In practice a humid room gives a paste that sticks to the probe but releases a component more easily, which is why operators and instruments sometimes disagree. A stable room between 40 and 60 percent relative humidity keeps the deposit consistent.

Temperature works in the other direction. Cold paste is stiff and gives a high tack force reading, but it also prints badly, so the reading is not useful. Tack should be measured at the working temperature of the stencil, which means at the printer and not in the lab cooler.

Tack, Stencil Printing and Component Placement

Stencil printing sets the shape of the deposit and the area of contact with the component. A deposit with a flat top and straight sides gives full contact and good tack. A deposit with a rounded top from over printing contacts the lead on a small area, and it releases the part at a fraction of the expected force.

Component placement adds its own loads. Placement force compresses the deposit, nozzle blow off pushes the part sideways, and conveyor transfer shakes the board. Each of these is small, but a part with 20 milligrams of mass and a contact area of a square millimetre has very little margin, and that is where tack decides the outcome.

Failure Modes When Tack Drops

The first sign is a rise in placement defects that appear after the oven rather than before it. Tombstoning on small chip parts, parts shifted off their pads, and components missing from the board are the classic outcomes. None of them look like a paste problem in the defect data, because the placement machine reports that it placed the part correctly.

The second sign is a deposit that picks up on the nozzle. Where tack is high at the surface but low inside the deposit, the top layer separates and climbs the nozzle, leaving a hollow deposit that reflows into a weak joint. Inspecting deposits after placement shows this, and the paste inspection system can be set to flag the volume change.

Recovering Tack: What Helps and What Does Not

Adding fresh paste to a dried deposit does not restore tack, because the new paste sits on a skin that blocks the solvent from moving. Stirring a jar that has been open too long does not restore tack either, and it adds air that makes the deposit more porous. Once the vehicle has begun to gel, the deposit is scrap.

What does work is prevention. Keep the queue between printer and oven short, return paste to the jar at the end of a run, and cover the stencil during breaks. Where the process needs a longer window, the answer is a paste formulated for it, and that is a change to be qualified on the line rather than assumed from a datasheet.

Setting an Open Time Limit from Data

Print a panel and measure tack on the same deposit at intervals, using the same probe and the same pull speed. Plot the fall in tack against time and mark the point at which the reading falls below the level at which placement defects begin. That point, with a margin, is the open time limit for the product.

The limit is product specific. A board covered with fine pitch chip parts needs more tack than a board with a few large components, and a board with a long conveyor run needs more than one that goes straight into the oven. Recording the limit on the handling and contamination record keeps the number visible to the operators who have to respect it. Volume measurements taken at the same time, using the method in our notes on paste volume measurement, confirm whether the deposit itself is still sound.

Checks Before Release

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

FAQ

Does a higher tack force always mean better placement? No. A paste can be very tacky and still release a part if the deposit is thick and rounded. Tack has to be matched to the deposit geometry and to the load that the board will see before reflow.

Can tack be restored by reflowing sooner? Only if the deposit has not yet gelled. Running a shorter queue preserves the tack that exists; it does not bring back tack that has gone. Once the deposit skins over, the placement risk remains even if the paste still looks wet.

How does nitrogen storage change tack? It slows the oxidation of the flux surface and can extend the window slightly. It does not stop solvent loss, which is the main cause of lost tack over a long open time, and the IPC test methods remain the reference for measuring it.