Stencil Tension and Its Effect on Print Quality

A stencil is a sheet of metal held in tension across a frame, and the tension is what keeps it flat when the squeegee passes over it. A stencil that has lost tension prints a deposit that varies with the position on the board.

Why Tension Matters

The squeegee pushes the foil down onto the board and the foil has to return to flat behind it. A foil that is slack stays deflected, and the next aperture prints with a gap between the foil and the pad.

The gap allows paste to escape under the foil, which produces a smeared deposit and a dirty board surface. Our paste volume notes describe the deposit that is at risk.

How Tension Is Measured

Tension is measured with a gauge that deflects the foil and reports a force per unit of width, and the reading is taken at several points across the sheet. The four corners are where the loss begins.

The figure is compared with the range for the mesh type and the foil thickness. A stencil that reads low in one corner has a mesh that has begun to fail. Our board quality notes describe how the resulting print is judged.

What Causes the Loss

The mesh relaxes with cycles and it also relaxes with cleaning, because the solvent and the pressure of the wash change the adhesive and the weave. A stencil that is cleaned in a machine loses tension faster than one that is wiped by hand.

The storage matters as well. A stencil that is hung by one corner carries its weight on that corner, and the frame distorts over time. Our first pass yield notes describe how a print problem appears in the data.

A slack stencil produces a deposit that is short in volume where the foil stayed deflected and correct where it did not. The pattern of the defect follows the squeegee direction rather than the board layout, which is the fingerprint of the problem.

The second effect is a messy board surface, because the paste that escapes under the foil ends up beside the aperture rather than in it. That paste is then re-deposited on the next board.

The stencil is stored flat or hanging on its frame with the load distributed, and it is not stacked under anything. The frames are checked for distortion at the same time as the tension.

The identification matters because a stencil that is used beyond its life looks identical to one that is new. The tension reading and the cycle count are recorded together. Our SPI notes describe how the deposit is measured through the shift.

A stencil can be retensioned where the frame allows it, and a mesh that has failed cannot. The decision is made from the reading and the age rather than from the appearance of the print.

A stencil that is retensioned is measured again before it is returned to the line, because the operation itself can leave a corner low.

Acceptance and Its Evidence

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel.

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Checks Before Release

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

Does a slack stencil always show a defect? It shows a volume variation that may stay inside the tolerance until the tension falls further.

Is hand cleaning better for the stencil? It is gentler on the tension and it is less thorough, so the two are usually combined.

What does gopcba provide for stencils? We provide tension measured at several points against a range for the mesh, storage that distributes the load, cleaning and cycle history recorded with the reading, retensioning verified before the stencil returns to the line, and the print monitored against the deposit target.

Points to Confirm at First Article

The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.