Squeegee Pressure and Angle in Solder Paste Printing

The squeegee is the part of the printing process that turns a stencil into a measured deposit, and it is usually the last thing to be adjusted and the first thing to be blamed. Pressure, blade angle, print speed and separation speed act together on the same event, which is the transfer of paste from the aperture to the pad. This article explains what each parameter controls and how to set them from a deposit measurement rather than from habit.

What the Squeegee Actually Does

The blade rolls the paste ahead of itself and presses it into the apertures. Two variables describe the action: the force applied per unit length of blade, and the angle between the blade and the stencil. Both change how much paste enters an aperture and how much stays on the stencil face.

Pressure is expressed in kilograms per centimetre of blade length rather than as a total, so a setting can move between printers of different widths. A starting point of 0.3 to 0.5 kilograms per centimetre for a metal blade is refined by measuring the deposit. The aperture side of the same problem is covered in our paste volume guide.

Blade Type, Angle and Wear

Metal blades run at 45 to 60 degrees and hold an edge, while polyurethane blades run at a shallower angle and conform better to an uneven stencil. On most printers the angle is fixed by the holder geometry, so the blade material is the practical choice the process engineer makes.

Wear changes the effective angle. A blade that has rounded develops a contact patch that behaves like a shallower blade, and the paste is pushed rather than rolled. Edge condition belongs in the maintenance schedule, with replacement based on print count rather than on visible damage.

A blade that has been replaced at the correct interval looks almost identical to a worn one, which is why the interval is counted rather than inspected.

How Pressure Affects the Deposit

Too little print pressure leaves paste standing on the stencil face and produces deposits short of the aperture volume. Too much drives the stencil into contact with the board, deflects it between the support pins, and squeezes paste underneath where it appears as bridging and as beads beside the pads.

The correct setting is found by printing a test pattern at several pressures and measuring the deposit, by weight or with a paste inspection system. The setting that produces the most stable volume across the pattern is the one to keep, even if another setting gives a slightly higher average.

Print Speed and Paste Rheology

Paste is thixotropic: its viscosity falls while it is sheared and recovers when the shear stops. A high print speed shears the paste less and can leave a deposit that never flowed into the corners of the aperture, while a very low speed lets it slump after release.

Typical print speeds are 20 to 50 millimetres per second, and the window narrows as apertures get smaller. Speed and pressure are set together, because changing one alters the effect of the other on the deposit.

Separation Speed and Paste Release

After the print the stencil lifts away from the board, and the paste has to shear off the aperture wall and stay on the pad. A separation that is too fast tears the deposit and leaves material on the wall, which the next stroke pushes onto the mask.

Slower separation, or a two stage separation that lifts slowly at first, improves release on fine pitch apertures. The gain is largest where the area ratio is marginal, which is exactly where the deposit is most sensitive.

Stencil Support and Gasketing

The stencil has to seal against the board around each aperture so paste cannot escape sideways. Support pins underneath define that seal, and a board that sags between pins opens a gap that paste can travel through. The layout of the support is discussed in our capability notes.

Support pin patterns belong in the machine programme for each product rather than in a general setting, because the positions depend on the component layout. A missing pin under a large connector is a common cause of a print defect that appears on one product only.

Support pins that are slightly different in height produce a seal that varies across the board, and the variation shows up as a print that is good in one area and marginal in another.

Deposit Measurement and Feedback

Deposit is measured by weighing a coupon or with an inspection system that reports height and area for each aperture. Volume is the figure that correlates with the joint, because a deposit of the right height but too small an area still leaves a joint short of alloy. The inspection methods are described in our SPI guide.

Closed loop control uses that measurement to adjust pressure automatically, and it works well while the measurement itself is stable. Where the measurement includes the mask surface as well as the paste, the loop corrects against the wrong signal and the deposit drifts.

Defects Attributed to the Squeegee

Insufficient paste, bridging, beads beside a pad and a slump along one edge are the classic squeegee symptoms. The first check is whether the defect follows the blade direction, because a defect confined to one side of the board points to a blade or pressure problem.

A defect that follows a single aperture points to the stencil or the board, and one that follows a component points to placement. Separating those three patterns is most of the diagnosis and it costs nothing, so it belongs before any adjustment of the machine. The yield effect of getting it wrong is visible in the first pass yield record.

Setting and Recording the Print Recipe

The recipe should record blade type and condition, pressure per centimetre, angle, print speed, separation speed and the support pin pattern, together with the deposit measurement that justified them. A recipe without that measurement cannot be transferred to another line with any confidence.

Recipes should also be re-verified after a stencil change, because a new stencil with a different thickness or finish changes the deposit even when nothing else has moved. That verification is one print and one measurement, not a full qualification.

The pressure that is needed rises with the contact area of the blade and with the stiffness of the stencil, so a 0.12 mm stencil on a long board needs a different setting from a 0.15 mm stencil on a short one.

A quick check that the pressure is in range is the paste roll itself: a smooth continuous roll ahead of the blade and a clean stencil face behind it show that the paste is being rolled rather than pushed.

The angle between blade and stencil is measured on the holder when the blade is new and re-checked when the holder is rebuilt, because a bent holder changes the angle on one side of the blade only.

FAQ

Is more pressure always better for filling apertures? No. Beyond the point where the stencil seals against the board, additional pressure deflects the stencil and pushes paste underneath, which produces bridging rather than a fuller deposit.

Why does the deposit vary across the board? Usually a support problem rather than a squeegee problem, because the seal between stencil and board depends on the board being held flat against the pins.

How often should the blade be replaced? On a print count that has been correlated with deposit measurement, not on visible damage, because a worn blade still looks serviceable.