Solder Paste Drying on the Stencil: Tack, Open Time and Print Quality

Solder paste drying on the stencil is one of the quiet causes of poor printing. The paste is a suspension of alloy powder in flux, and the flux is designed to stay active for a defined period after the jar is opened. Left on the stencil, the solvent fraction evaporates from the surface of the roll and from the paste trapped in the apertures, so the material that is printed later in the shift is drier, tackier and less able to release cleanly onto the pad.

What Happens as Paste Dries on the Stencil

Two processes run at once. Solvent leaves the exposed surface of the paste roll, and the paste inside the apertures loses solvent to the air gap between the stencil and the board. The second is faster because the volume involved is small and the surface-to-volume ratio is high, which is why the apertures are often the first place the problem shows.

As the paste dries its viscosity rises, its tack falls, and the powder particles become less mobile. A dry paste does not flow into the aperture corners when the squeegee passes, so the deposit is short and rounded rather than flat and slightly over-filled.

Stencil Life, Open Time and Paste Tack

Stencil life is the time the paste can be left on the stencil and still print to specification; open time is the time the paste can sit in an open jar before use. Both come from the paste specification, and both are measured under defined conditions of temperature and humidity, typically 23 C and 50 percent relative humidity.

Paste tack is the property that holds the component in place between placement and reflow. It falls as the paste dries, so a board printed with paste at the end of its stencil life can lose components during handling even though the print looked acceptable. This is why open time and stencil life are recorded against the lot rather than treated as housekeeping figures.

Humidity and Temperature Around the Printer

Humidity controls how fast the solvent leaves. In a dry room the paste skins over in minutes; in a humid room it lasts longer but may absorb moisture, which shows up as solder balling or as spatter during reflow. A printing area held between 40 and 60 percent relative humidity and between 22 C and 25 C gives a workable balance.

Temperature matters almost as much. Warm paste is thinner and prints well until it dries, then fails suddenly; cold paste taken straight from a refrigerator behaves unpredictably until it reaches room temperature, and the condensation it collects adds water to the print. Paste should be allowed to stand sealed until it reaches room temperature before the jar is opened.

Pause Length and Recovery After a Stop

Every stoppage changes the state of the paste on the stencil. A five-minute adjustment is harmless on most pastes; a thirty-minute break in a dry room may be enough to make the roll unusable. The practical control is a defined pause limit for the line, after which the paste roll is removed and the stencil is cleaned.

Recovery matters as much as the limit itself. After a pause the first prints should be inspected before production resumes, and where the pause has been long the roll is discarded rather than stirred back into condition. Adding fresh paste to a dried roll produces a mixture with unpredictable rheology, which is harder to control than a clean restart.

Print Quality Symptoms of Dry Paste

The signature of drying paste is a gradual change in the deposit rather than a sudden failure. Deposits become shorter and their edges round off; the paste stops peeling cleanly from the aperture, leaving a residue that builds up on the stencil underside; and the printed volume falls by 10 to 20 percent before anyone notices.

Later in the line the same cause appears as missing paste in small apertures, as bridged deposits where paste smeared across the webs, and as solder balling after reflow. All three are commonly blamed on the stencil or on the profile, while the record shows that the print was made four hours into the shift.

Stencil Cleaning Interval and Method

Cleaning interval is set from the paste specification and from the pitch of the product. A board with 0.4 mm pitch apertures needs an underside wipe every few prints; a board with coarse apertures may run for much longer. Where the interval is fixed by habit rather than by measurement, the fine-pitch product is usually the one that suffers.

The cleaning method has to match as well. Solvent wipes restore the aperture openings but leave the stencil wet unless it is dried before printing, and a wet stencil contaminates the next print. Vacuum-assisted wiping, followed by a short air dry, is the more controllable approach on a high-mix line.

Paste Roll Management on the Squeegee

The roll should be a rolling cylinder of paste that turns over as the squeegee advances, not a static puddle. A roll of roughly 12 mm to 18 mm diameter behaves consistently; a thin ribbon spread across the stencil has a large exposed surface and dries quickly. So does a roll that is too large, because the paste at the centre of it sits unused for hours.

Roll length should cover the print stroke and no more. Paste left at the ends of the stroke, outside the printed area, dries first and is then carried back into the roll when the direction reverses. Keeping the roll to the working width reduces the volume that is exposed to air at any moment.

Recovery Rules and Rejection Criteria

A written rule is worth more than judgement. Paste that has been on the stencil beyond the stated stencil life is discarded, paste that has been in an open jar beyond its open time is discarded, and paste that has been left through a scheduled break is discarded. Discarded paste should not be returned to the jar, because mixing dried material with fresh paste changes the rheology of the whole jar.

Where a defect is traced to drying, the corrective action should address the cause rather than the symptom. Increasing squeegee pressure to compensate for a drying roll thickens the deposit and drives paste through the webs, which turns a volume problem into a bridging defect.

Verification by SPI and First Article

Print quality is verified with a solder paste inspection system that reports volume, area and height per deposit against the stencil aperture. Trending the volume across a shift shows the drying effect as a slope, which is a far better warning than a single out-of-tolerance reading at the end. The relevant SPI measurement practices apply directly here.

The first article confirms the setup and the aperture design, and it should be repeated after any pause, paste change or stencil change. Where deposits at the limit of the process are involved, the area ratio of those apertures explains much of the sensitivity, because a low area ratio leaves less margin when the paste begins to dry.

FAQ

How long can solder paste stay on the stencil? As long as the stencil life stated for that paste, typically four to eight hours under controlled conditions, with shorter limits in a dry room. The limit applies from the moment the paste is placed on the stencil, not from the start of the shift.

Can dried paste be revived by adding fresh paste? No. Mixing dried material with fresh paste produces a roll with inconsistent rheology that prints unpredictably. Remove the roll, clean the stencil and start again.

Does humidity help paste last longer? Up to a point. Higher humidity slows solvent loss but allows the paste to absorb moisture, which appears later as solder balling or spatter. A range of 40 to 60 percent relative humidity is the usual compromise.