Tack Time: How Long Paste Holds Components in Place
Tack time is the period during which a printed paste deposit is sticky enough to hold a component after placement. It begins when the board leaves the printer and it ends when the paste can no longer resist the small forces that act on a placed part, whatever the clock says. On a fast line the interval is comfortable; on a line with a queue it is the setting that decides whether the product can be built at all.
The forces involved are small but real. A vacuum nozzle lifting away, a conveyor bump, a fan near the machine and the vibration of the next placement head all push a component sideways. Tack is what holds it in position, and once tack is lost the component slides on a paste that has become a solid film rather than an adhesive.
What Tack Time Measures
Tack is the adhesive strength of the paste surface, and it comes from the flux vehicle rather than from the alloy. The metal content affects the bulk of the deposit, but a part is held by the sticky flux film at the top and around the sides. That is why a paste can print perfectly and still lose a component.
Tack time, then, is not a fixed property of a paste. It is the useful life of that sticky surface under the conditions on the line, and it shortens as solvent leaves the film. A paste that holds for eight hours in a sealed jar may hold for twenty minutes on a board in a dry room, and the number that matters is the second one. What the line needs to know is the component hold available at the end of its queue, not the value measured on a fresh print.
The Tack Test and How to Run It
The measurement is made with a probe of defined diameter that is pressed onto a printed deposit and withdrawn at a defined rate, with the peak force recorded. The values are quoted in grams-force or newtons, and the paste supplier publishes a minimum figure for a fresh print.
A practical production version prints a row of deposits, tests one immediately, then tests the rest at intervals while they sit under production conditions. The result is a decay curve rather than a single number, and the point where it crosses the supplier minimum is the tack time for that line on that day. Our notes on the open time of a paste describe the same curve from the process side.
Why Tack Falls Away: Paste Drying
Paste drying is the mechanism behind the decay. Solvent leaves the surface film into the air above it, the film becomes stiffer and less compliant, and it stops flowing into the small gaps around a component lead or a chip end. The loss is fastest in the first minutes, because the surface area of a printed deposit is very large compared with its volume.
The rate depends on the deposit geometry as well as the paste. A tall, narrow deposit has less exposed area for its volume and keeps its tack longer than a thin smear. This is one more reason to control print volume, because a starved deposit loses both the volume the joint needs and the tack the placement needs.
Room Humidity and Its Effect
Room humidity pulls the decay in two directions. Dry air removes solvent quickly and shortens tack time, so a room at 25 percent relative humidity can halve the usable window compared with the same room at 45 percent. Humid air slows the loss but softens the flux vehicle, and a soft vehicle slumps and produces solder balls.
The practical range for an SMT area is somewhere between 40 and 60 percent relative humidity, held steady rather than drifting with the weather. Stability matters more than the exact value, because the line settings are tuned to the room the line runs in, and a seasonal swing moves the slump behaviour at the same time.
The Placement Window in Production
The placement window is the time between printing and the last placement on the board, and it must be shorter than the tack time with a margin. A board that takes four minutes to populate on a line whose paste holds for twenty minutes is comfortable; the same board on a line with a twenty minute queue is a defect generator.
Measuring the window means timing the actual board, not the process in the manual. Stamp or log the print time and the exit time from the mounter on a sample of boards, and use the longest observed interval rather than the average. The worst board sets the specification, because the defect will come from that one, and the component hold has to survive that longest interval with margin left over.
What Happens to Components That Shift
A component that slides before reflow does not always fall off. A chip capacitor that moves a fraction of a millimetre ends up on a deposit whose centre of mass has changed, and the unequal pull of the two fillets during reflow can stand it on end. The result is the classic tombstone, and the paste looked perfectly printed in the inspection image.
Larger parts move less but tilt more. A connector that settles into a drying film can sit at a slight angle, which changes the standoff and the coplanarity of every pin, and the result is a joint that is open at one end. Both cases are inspected as placement faults when the cause was the paste surface, and our notes on tombstoning follow the same logic.
Tack Time and Double-Sided Assembly
On a double-sided board the second side is printed after the first side has been reflowed, and the components on the first side are then held only by their joints. That makes the handling of the board between the second print and the second reflow the critical step, because adhesive failure is not an option for parts that are already soldered.
Where the process is double-sided with an adhesive rather than paste, the adhesive cure and the placement window have to be matched, and the same rule applies: measure the worst case rather than the nominal one. The working life of the material sets the limit for the whole flow.
Setting a Working Window for Your Line
Set the limit from a test rather than from the paste datasheet, then reduce it by a margin that reflects how variable the line is. If the tack curve crosses the minimum at twenty five minutes and the longest observed queue is twelve, the remaining margin is thin but acceptable; if the queue varies with product mix, the limit should be tightened.
Then control the two variables that move the curve. Hold the room humidity and temperature inside their window, and keep the queue short by design rather than by exhortation. Where the product cannot be built inside the window, the answer is a second printer, a smaller batch, or a paste with more component hold, not a higher tolerance for shifted components.
Recording and Reacting to Drift
The record that matters is the tack time measured at a fixed interval with the production paste, plotted over weeks. A downward trend means the paste is changing, the room is drying out or the deposits are getting thinner, and any of those is worth knowing before a customer reports a shift.
Where a single lot behaves differently from its predecessors, quarantining it while a tack test and a print test are run is far cheaper than sorting the assemblies it produced. Paste drying is a property the line can measure directly, and the measurement takes minutes, which makes it one of the cheapest checks available on an SMT line.
FAQ
Does tack time appear on the paste datasheet? A tack value usually does, and a working life may be quoted as well, but neither is the tack time for your line. The datasheet value is measured under controlled conditions on fresh paste, and the production figure depends on the room, the deposit and the queue.
Can tack be restored by reflowing sooner? Moving the reflow oven closer shortens the interval and is a legitimate fix, but it does not restore tack that has already been lost. If a component has shifted on a dried deposit, the joint it forms will show the offset regardless of how soon the board reaches the oven.
Is a longer tack always better? Not necessarily. Pastes with very long tack often have a stickier flux that is harder to clean and more prone to slump, and the extra open time may not be needed. Choose the tack the line requires, then control the room so the paste delivers it consistently.
