Squeegee Blade Selection: Durometer, Edge Condition and Paste Release
The blade that pushes solder paste across a stencil decides how much of the aperture is filled and how cleanly the stencil is wiped afterwards. Its hardness, its edge condition and the pressure behind it form one system, and changing any of the three changes the deposit on the board.
What the Blade Does
The squeegee has two jobs that pull in opposite directions: it has to push paste into the aperture under pressure, and it has to wipe the stencil surface clean as it passes. A blade that fills well but leaves a film behind produces smeared deposits, while one that wipes perfectly may not fill a small aperture at all.
Both jobs are done in the same stroke, so the blade is chosen for the balance the product needs. Fine pitch work pushes the choice towards paste release and clean wiping, while a board with large apertures and heavy deposits tolerates a softer blade and a lower pressure. A squeegee is therefore specified as a set of properties rather than as a part number, because two blades of the same material and hardness can print differently once their edges have seen different numbers of strokes.
Metal and Polyurethane Blades
Metal blades are usually stainless steel, from 0.15 to 0.20 mm thick, and the edge is machined square. They hold their shape against the stencil, so the deposit is very consistent and the stencil is wiped clean, and they are the default for fine pitch work on a flat stencil.
Polyurethane blades are available in a range of hardnesses and conform better to a stencil that is not perfectly flat or to a board that is not perfectly supported. That conformity is also their weakness, because a soft blade rolls into the aperture and scoops paste out of it rather than leaving it in.
Durometer and How It Changes the Print
The durometer of a polyurethane blade is quoted on the Shore A scale, and printing blades are usually between 70 and 90. A blade at the soft end conforms to a warped board and leaves more paste around the apertures, while one at the hard end behaves more like metal and cleans the stencil better.
The practical effect of the choice is seen in the deposit height rather than in the deposited area, because a soft blade tends to scoop. Where a board has both fine pitch and large apertures, the harder blade is normally chosen and the missing conformity is bought back with a better support of the board or with a flatter stencil.
Edge Geometry and Condition
A new blade has a sharp, square blade edge, and the edge is where all the work happens. A nick of a tenth of a millimetre leaves a line of paste across the board at every stroke, and a rounded edge from wear leaves a film that the next aperture picks up.
The edge is inspected under magnification before the blade is fitted and at intervals afterwards, and the check looks for nicks, for rounding and for a change in the angle. A blade that has been re-ground is measured after grinding, since the angle and the flatness can both change in the process.
Print Pressure and Its Interaction With the Blade
The print pressure is set so that the blade touches the stencil across its whole length without bowing, and it is the lowest pressure that produces a clean wipe. More pressure wears the blade, distorts the stencil and drives paste under the stencil onto the board surface. The pressure is read from a gauge on the printer rather than estimated from the appearance of the print, and it is re-checked after any change of blade.
Because the pressure and the blade work together, a change of blade is followed by a print study rather than by a copy of the previous settings. A harder blade usually needs less pressure for the same wipe, and the difference is visible in the deposit volume from inspection; our SPI notes describe how the result is measured.
Blade Length, Holder and Clamping
The blade is clamped in a holder that has to keep it straight along its full length, and a holder that is distorted produces a pressure gradient from one end to the other. The gradient shows as a difference in deposit volume between the two sides of the board, which is easy to mistake for a stencil problem.
The blade length is matched to the widest board that runs on the line, and a blade that is much longer than the board wastes pressure and wears unevenly. Where several products share a printer, the blade length is chosen for the largest and accepted for the rest. A holder that has been dropped is checked for straightness before it is used again, because a bend of a tenth of a millimetre along its length is enough to produce a measurable difference between the two sides of a wide board.
Paste Release and Aperture Filling
Filling happens when the paste is pushed ahead of the blade and rolls into the aperture, and release happens when the stencil lifts away. A blade that skips over an aperture leaves it partly filled, and the deposit is short even though nothing about the stencil has changed.
The two effects are separated by inspecting the stencil after a stroke on a test setup, where a partly filled aperture is visible as a depression in the paste. That check is done when a print problem is being diagnosed, and it takes less time than changing the paste and the profile to find the same answer.
Maintenance, Inspection and Replacement
The maintenance routine is a visual check of the edge, a check of the holder for distortion and a check of the clamping bolts for tightness. The blade is replaced on condition rather than on a calendar, because the number of strokes that wears out an edge depends on the paste and on the pressure used.
A spare blade is kept for each printer so that a damaged edge can be replaced within a shift, and the replacement is recorded with the print parameters that were re-established afterwards. The used blade is kept with the printer record until the first article printed after the change has been accepted, so that a later problem can be traced rather than assumed. Our paste volume notes describe the print study that follows any change of blade or stencil.
Records and Process Control
The record for a printer carries the blade material and durometer, the blade age in strokes, the pressure and the speed. Where a deposit problem appears, the first question is which of those four has changed, and the answer is usually in the maintenance log rather than in the process data.
A change of blade is a process change, and it is treated as one, with a first article printed and measured before the line runs at production speed. Our AOI notes describe what the inspection station will and will not see afterwards, which is a useful reminder that a short deposit is easier to see before reflow than after it.
FAQ
Which is better, metal or polyurethane? Metal gives a more consistent deposit on a flat stencil and a flat board, and polyurethane conforms better to a board that is not held flat. The choice follows from the board support and from the smallest aperture on the product.
How long does a blade last? It is counted in strokes and it depends on the paste and the pressure, so the useful measure is the inspection result rather than a number. Many shops replace a blade when the edge shows a nick or visible rounding at 10 times magnification.
Can a worn blade cause a bridging defect? It can, because a rounded edge leaves a film of paste that the next aperture picks up, and the extra volume appears as a bridge on fine pitch. That is why the edge is inspected rather than only the deposits.
