Solder Paste Stringing: 7 Causes at Stencil Lift

Solder paste stringing is the fine thread of paste left between the stencil and the deposit when the two separate. The thread stretches, breaks and falls, sometimes onto the mask beside the pad, sometimes back across the deposit as a ridge. On a fine pitch board a single thread is enough to bridge two pads after reflow.

Stringing is a release problem rather than a filling problem, so it appears after the print has otherwise succeeded. The deposit looks full and well formed when the stencil is still down, and the defect appears in the moment of separation. This note explains the mechanism and the sequence that removes it.

What Solder Paste Stringing Looks Like

A string is a visible filament of paste, usually thinner than a human hair and a few tenths of a millimetre long, standing between the aperture wall and the deposit. It is distinct from a smear, which is a flattened trail of paste on the stencil underside, and from a satellite, which is a separate drop of paste sitting on the mask.

The defect is easy to miss in a visual check because the thread is fine and often falls before the board reaches the next station. Looking at the deposit immediately after separation with a low angle light shows it clearly, and the same inspection under a microscope will show whether the thread came from the corner or the side wall of the aperture.

Why Tails Form at Stencil Lift

At separation, paste in the aperture is held by three forces: adhesion to the pad, cohesion within the paste and friction against the aperture wall. If the paste is stuck to the wall more strongly than it is to itself, a filament is drawn out as the stencil rises. The thread breaks where the paste is weakest.

The balance changes with wall roughness, with the bond between paste and foil, and with the speed of separation. A rough or damaged wall grips, a clean laser cut wall releases, and a slow separation gives the paste time to relax and pull back into the deposit instead of forming a thread.

Paste Viscosity and Recovery Rate

Thin paste strings more easily than stiff paste, because it has less cohesion to resist being drawn. The effect is not linear in viscosity alone, however, because the recovery rate matters as much. Paste that builds viscosity quickly after shear holds its shape at separation and breaks cleanly.

This is why two pastes with the same jar viscosity can behave differently on the same stencil. Measuring both at several shear rates, at the working temperature, separates the two properties. The tests are described in our notes on solder paste viscosity measurement, and the reading at the separation shear rate is the one that predicts stringing.

Print Speed and Separation Speed

Separation speed is set by the printer, and it is often left at a default that was never evaluated. A fast separation pulls the paste away before it can relax, which favours threads. A slow separation lets the paste flow back into the deposit, which costs cycle time but removes the defect without changing any material.

Print speed acts in the same direction during the stroke, because a fast stroke leaves the paste under higher residual shear at the moment the board drops. Where stringing appears at one end of the panel only, the cause is usually uneven separation rather than the paste. The full set of settings is covered in our snap-off guide.

Aperture Geometry and Wall Finish

Aperture geometry decides how much wall the paste has to slide against. A thick foil with a small opening presents a long wall and a high wall area to volume ratio, which invites both poor release and stringing. Laser cut walls with a slight taper release better than straight walls, and a coated wall releases better still.

Damaged walls are worse than any design flaw. A nick from handling or a burr from a worn laser cutter grips the paste and produces a repeatable thread at the same aperture on every panel. Inspecting the stencil at the start of a run under magnification, and rejecting any aperture with a visible burr, removes a whole class of the defect. The limits that apply are set out in our notes on stencil aperture area ratio.

Stencil Underside Contamination

Paste that has been left on the stencil underside changes the separation geometry. The foil no longer lifts cleanly from the deposit, because the soiled area holds the stencil down and the paste is stretched as the two part. The result is a field of threads across the region that was printed last.

Underside paste builds through a run, and the rate depends on the print gap, on the paste and on the frequency of the wipe. A dry wipe every few cycles and a solvent wipe at intervals keeps the surface clean, and the method that suits each paste class is described in our notes on under stencil cleaning.

Fixing Stringing in Order

Work from the machine to the material. Confirm the stencil is clean and undamaged, check the separation speed and the print gap, then review the paste. Changing paste first is expensive, slow to verify and often unnecessary, because the majority of stringing cases come from separation settings and dirty foil.

Where a paste change is needed, keep the stencil, the settings and the board the same so the comparison means something. Measure paste release on the same apertures before and after, and record the counts of threads seen per panel rather than a subjective note. The evidence decides whether the change stays.

Checks That Keep It Away

Put four checks in the setup sheet: stencil underside clean, separation speed set and recorded, print gap within band and paste within its open time. Each is quick, each is objective, and together they cover the causes that account for most stringing. The checks belong at the start of a run, not at the end.

During the run, watch the first panels after every wipe and after every break. Stringing often reappears in the first panel after a pause, because the paste surface has changed while the machine stood still. Recording those two moments alone will show whether the process is drifting, and the accepted test methods for paste behaviour are published by IPC.

FAQ

Does a longer hold before separation help? A short controlled dwell can let the paste relax before the stencil lifts, which reduces threads on some pastes. Too long a dwell lets the paste dry onto the wall and makes release worse, so the value has to be found by test rather than set from a rule.

Can stringing be caused by a worn squeegee? Yes, indirectly. A worn or nicked blade leaves an uneven paste load and a smear at the trailing edge, and that smear adds to the paste on the underside of the stencil. Replacing the blade is a cheap test when other causes have been excluded.

Is nitrogen printing the answer? Nitrogen slows oxidation of the paste surface and can reduce the build up of a skin on the roll, which helps release slightly. It does not change separation speed or wall condition, and those are the primary controls for stringing.