Stencil Frame Tension And Its Effect On Printing

A stencil is a sheet of metal tensioned in a frame, and the tension is what keeps the foil flat against the board during printing. If the tension falls, the foil sags between the squeegee strokes, the board is not in full contact with the apertures, and the printed deposits become inconsistent.

Tension is measured in newtons per centimetre and it changes with use. A stencil that printed perfectly in its first month can be marginal after a year, and the change is gradual enough that it is rarely noticed until the defect rate rises.

Why Tension Matters

Printing depends on the foil sealing against the board and releasing cleanly after the squeegee passes. Tension provides the force that brings the foil back to flat, and it also determines how much the foil deflects under the squeegee.

A poorly tensioned stencil deflects more, which opens a gap between the foil and the board at the edges of the print area. Paste then squeezes into that gap and appears as a smear on the surface rather than as a deposit in the aperture.

Frame Types And Mesh Mounting

A mesh frame holds the stencil by bonding it to a stretched polyester mesh that is itself glued to an aluminium frame. The mesh allows the foil to be tensioned after mounting and to be retensioned later, which extends the life of the stencil considerably.

A direct frame bonds the foil straight to the aluminium, which gives a flatter and more precise result in the beginning but cannot be retensioned. Direct frames are common for very fine pitch work where the mesh would limit the achievable tension.

Measuring Tension

Tension is measured with a gauge that deflects the foil by a known amount and reads the restoring force. The measurement is taken at several points across the foil, because the tension varies between the centre and the corners, and the lowest reading is the one that matters.

A typical specification for a stainless steel stencil is 35 to 50 newtons per centimetre, with a minimum of about 30 for reliable printing. The figures differ for a mesh frame, which runs lower because the mesh itself stretches.

Effect On Paste Release

Paste release depends on the foil snapping back to flat as the board separates. A foil with good tension releases immediately and leaves a deposit with vertical walls and a flat top, while a slack foil follows the board upward and pulls the deposit apart.

The effect is worst on small apertures, where the paste has more contact with the aperture wall than with the pad. That is why a stencil that is acceptable for a coarse board can produce poor prints on a fine pitch area.

Squeegee Pressure And Its Relationship

Squeegee pressure and stencil tension work against each other. Higher tension requires more pressure to keep the foil in contact, and the correct pressure is the minimum that produces a clean wipe of the stencil surface.

When the tension falls, an operator often increases the pressure to compensate. That forces paste under the foil at the edges, increases wear on the squeegee, and does not restore the release that was lost. The correct response is to retension or replace the stencil.

Stencil Age And Tension Loss

Tension is lost through fatigue of the mesh, through the adhesive bond relaxing and through repeated thermal and mechanical cycles. Cleaning also plays a part, since some solvents soften the adhesive that holds the mesh to the frame.

The loss is not linear. A stencil holds its tension well for a long period and then falls quickly once the adhesive starts to creep, which is why periodic measurement rather than a fixed replacement interval is the better policy.

Storage And Handling

Stencils should be stored vertically in a rack that supports the frame and keeps the foil clear of everything else. Stacking stencils flat, or leaning them against a wall, distorts the frame and changes the tension across the foil.

Handling should avoid any pressure on the foil itself. A stencil is a thin sheet stretched under load, and a small dent from a dropped tool creates a permanent low tension area that will show up as a print defect.

When To Retension Or Replace

A mesh mounted stencil can be retensioned a few times before the mesh itself is exhausted. The decision is based on the measured tension and on whether the apertures are still within tolerance, since retensioning does not restore worn apertures.

A direct bonded stencil cannot be retensioned, so a falling reading means replacement. The same applies to a foil with a crease, a dent or a torn aperture wall. The print quality criteria involved are described under PCB design quality characteristics.

Print Quality Checks

The practical check is a print on a bare copper coupon, examined for completeness, shape and surface. Deposit volume measured by an inspection system gives a numerical trend that shows a slide in tension long before a defect appears.

When the volume trend falls while the paste and the printer settings are unchanged, the stencil is the suspect. Measuring tension confirms it, and the measurement should be recorded with the stencil so that the history is available. The stencil design rules are described under design guidelines for manufacturability.

Process Control and Verification

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

What tension should a new stencil have? Around 35 to 50 newtons per centimetre for a mesh mounted stainless stencil, and a minimum of about 30 before it should be retensioned or replaced.

How often should tension be measured? At least at every setup for a critical product, and on a periodic schedule for a stencil in regular use. A monthly check is a reasonable baseline.

Can a slack stencil be used on a coarse board? Sometimes, because large apertures release even when the tension is low. It should not be used on a fine pitch product, where the release problem appears first.