Squeegee Speed Control: Aperture Fill and Paste Release

Squeegee speed is the rate at which the blade travels across the stencil, and it sets the time each aperture has to fill with solder paste. It is the parameter most often changed to improve a marginal print and the one least often measured, because its effect on deposit volume is gradual rather than abrupt. Speed, angle and pressure together define how the paste is pushed into the apertures and how much of it stays there when the stencil separates.

What Squeegee Speed Does

As the blade moves, it rolls the paste ahead of it and drives it into the apertures under a combination of pressure and shear. The paste has to fill the aperture completely and then release cleanly when the stencil lifts. Both of those events take time, and the speed sets how much time is available.

Too fast and the paste does not have time to fill the corners of small apertures, so the deposit is short and rounded. Too slow and the paste is left in contact with the stencil longer, which increases its tendency to stick to the walls and can also raise the temperature of the paste through shear.

Aperture Fill and Shear Rate

The paste behaves as a shear-thinning fluid: its viscosity falls when it is sheared and recovers when the shear stops. A faster squeegee shears the paste more quickly, which lowers its viscosity, so the paste flows more readily but also recovers more slowly, and the recovery has to happen before the stencil separates or the paste will slump out of the aperture.

For a given aperture the fill time depends on the aperture volume and the flow the paste can achieve. Small apertures with a low area ratio fill slowly because the wall area is large relative to the opening, which is why fine-pitch printing generally runs at a lower squeegee speed than coarse printing.

Squeegee Angle, Pressure and Blade Type

Angle determines how much of the blade’s motion is converted into downward pressure. A blade at 60 degrees delivers more pressure at the aperture than one at 45 degrees for the same downward load, and the standard range for metal blades is between 45 and 60 degrees. A smaller angle rolls the paste more and pushes it less.

Blade type matters because metal and polyurethane behave differently. A metal blade holds its angle and gives a consistent pressure but wears the stencil faster; a polyurethane blade conforms to the stencil surface and seals better on an uneven surface, but its angle changes with pressure. The choice is usually made on aperture pitch and on stencil condition.

Paste Release and Deposit Volume

Release is the last stage and the one that decides the deposit. As the stencil lifts, the paste has to detach from the aperture walls and stay on the pad. If the paste is still viscous and tacky, some of it stays on the wall; if it has recovered its viscosity fully and the walls are smooth, it transfers almost completely.

Deposit volume is the measurable result of the whole sequence, and it is reported as a percentage of the aperture volume. Values between 80 and 100 percent are normal, and the variation between deposits of the same aperture is often a better indicator of print health than the mean, because a wide spread shows that the process is marginal and that some apertures are filling while others are not.

Speed and Fine-Pitch Apertures

Fine-pitch apertures need the paste to fill a small, deep opening whose walls dominate its volume. Running at a lower speed allows the paste to reach the bottom of the aperture and gives it a moment at the end of the stroke to settle before separation. Typical fine-pitch printing runs between 20 mm/s and 50 mm/s.

The limit is throughput rather than quality. A line printing at 20 mm/s needs more time per board, and where the product allows it a faster speed with a higher separation delay may give the same deposit volume. The two parameters should be tuned together rather than one at a time.

Speed and Coarse Apertures

Large apertures fill quickly and have a high area ratio, so they tolerate much higher speeds. Some processes run at 100 mm/s or more on coarse work without loss of volume. The problem that appears at high speed on coarse apertures is not fill but scooping, where the front edge of the blade pushes paste out of an already filled aperture.

Scooping is recognised by deposits that are consistently short on the leading row of a component and correct elsewhere. It is controlled by reducing speed, by reducing pressure, or by adjusting the blade angle so that it shears the paste rather than pushing it.

Interaction With Separation Speed

Separation is the stage that follows the print stroke, and it interacts with speed because both affect the paste at the moment it detaches. A slow separation allows the paste to peel away from the aperture wall gradually; a fast one tears it away, leaving material behind.

The two parameters are often traded against each other, and the trade is not symmetric. A higher squeegee speed with a slow separation gives a better deposit than a slow squeegee speed with a fast separation, because the fill stage tolerates speed better than the release stage does. Where a print is marginal, the separation setting is usually the more valuable of the two to adjust.

Measuring the Result and Setting the Window

The result is measured with a solder paste inspection system that reports volume and area for every deposit. The relevant SPI measurement practice is to look at the distribution, not just the mean, and to compare it across the speed range being considered.

The window is established by printing a series of boards at increasing speed and measuring each one. The upper limit is the speed at which the volume or its spread falls outside the target for the smallest aperture; the lower limit is set by throughput and by the point at which the paste begins to stick in the apertures. Where apertures are clogged by paste that has been worked repeatedly, the speed has been pushed below that lower limit.

Records and Process Limits

The record for each product should carry the squeegee speed, angle and pressure, the separation speed and delay, the blade type, the aperture fill measured on the smallest and largest apertures and the deposit volume for each. Those fields describe the print well enough for another operator to reproduce it.

Where a bridging defect appears with no change in the stencil, the print parameters are the first place to look, since excess solder paste bridging between adjacent pads is often the result of pressure that is too high or of a paste that has been pushed sideways at speed rather than sheared.

FAQ

What squeegee speed should be used? Between about 20 mm/s and 50 mm/s for fine pitch, and higher for coarse apertures. The correct value is the fastest one that still fills the smallest aperture completely and releases it cleanly.

Does a faster squeegee reduce deposit volume? It can, because the paste has less time to fill the aperture and less time to recover its viscosity before separation. The effect is largest on apertures with a low area ratio.

Should speed or separation be adjusted first? Separation, in most cases. The fill stage tolerates a higher speed better than the release stage tolerates a fast separation, so the release setting usually gives the larger improvement.