Skipped Deposits: 7 Causes in Stencil Printing

Skipped deposits are apertures that receive no paste at all, leaving a bare pad after the print. The defect is different from a deposit that is short or thin, because the failure is complete rather than partial, and the cause is usually a single blockage or a release event rather than a general loss of volume. Finding the pattern is what identifies it.

A single skipped deposit on every panel usually means a blocked aperture. Skips that appear in a line across the panel follow a squeegee defect, and skips that appear at the end of the stroke follow the paste roll. Reading the pattern before touching the machine saves the time spent changing settings that were never the problem.

What a Skipped Deposit Looks Like

A skip shows a clean pad with no paste and no residue of a deposit that was once there. Under the paste inspection system it appears as zero volume with an aperture footprint that matches the stencil, which distinguishes it from a printing offset or from a mask problem. The mask and the pad both look normal afterwards.

Where the pad shows a faint grey film, the aperture did deliver paste but the paste remained on the stencil wall or on the pad as a smear. That is a release failure rather than a skip, and the correction is different, which is why the inspection image should be checked before the diagnosis is written.

Filling Failures Inside the Aperture

An aperture that never fills produces a skip. The causes are a low area ratio, insufficient pressure at that point on the panel, a paste that is too stiff to flow, or a stencil that is not sitting flat on the board. Each leaves a different distribution across the panel, which is the clue to which one is present.

Pressure related skips appear where the blade is not making contact, typically at the ends of a wide panel or over a low area of the board. Paste related skips appear across the whole panel and worsen through the shift as the paste stiffens. The aperture geometry limits are covered in our notes on stencil aperture area ratio.

Release Failures at Stencil Lift

Paste that sticks to the aperture wall instead of the pad leaves the aperture full and the pad bare, and the next stroke either pushes the old paste through or compounds the blockage. The usual causes are a rough wall, a low area ratio or a separation speed that is too fast for the paste to relax. The defect often repeats at the same aperture position from panel to panel.

A single repeating skip is the signature of a blocked aperture. Push a fine probe through the opening and inspect the wall under magnification, because a paste lump that is wedged at the bottom of the aperture is invisible from above and will not be removed by a normal wipe. Wall quality and its effect on release are described in our notes on under stencil cleaning, and the same repair applies to a damaged or deformed opening.

Squeegee and Blade Condition

A nick in the blade leaves a line of missing paste, because the paste passes under the damaged edge instead of being pushed into the apertures. The pattern is a row of skips that follows the direction of the stroke and repeats at the same position on every panel. Rotating or replacing the blade corrects it immediately, which is a useful confirmation.

Blade hardness and pressure also affect fill, though they tend to reduce deposit volume before they cause a complete skip. Where a hard blade is paired with low pressure, the corners of a wide print area are the first to suffer. Selection and edge condition are covered in our notes on squeegee blade selection.

Stencil Blockage and Underside Paste

Paste left on the underside of the stencil changes the seal on the next stroke. The foil no longer sits flat, so paste is squeezed sideways rather than into the aperture, and the affected area produces skips or smears. The region that suffers is the part of the panel that printed last, which is the clue that links the defect to the underside rather than to the stencil itself.

Wiping frequency and wipe type both matter. A dry wipe removes paste but leaves the solvent film, and a solvent wipe without a dry pass leaves liquid that thins the next deposit. The combination that works is a dry wipe at short intervals with a solvent wipe at longer ones, adjusted to the paste and to the panel size. The paste volume measurement needed to confirm the effect is described in our notes on the paste inspection system.

Paste Condition and Open Time

Stiff paste fills apertures poorly, and paste that has begun to dry on the stencil leaves skips that increase through the shift. The paste volume on a test panel taken at the start and end of a run shows the trend clearly, and the difference is usually larger than an operator expects.

Stencil printing depends on the paste being inside its working window, so the first check on a run of skips is the age of the paste and the time it has been on the stencil. Signs that the paste has passed its useful working period are described in our notes on paste drying, and the remedy is to change the paste rather than to raise the print pressure.

Finding the Cause in Order

Work from the pattern to the part. A single repeating skip takes you to the aperture, a line of skips to the blade, skips at the end of the stroke to the roll and skips across the panel to the paste or to the board support. Each step takes a minute and eliminates a group of causes before any setting is changed.

Only then adjust the machine. Pressure and speed changes are easy to make and hard to attribute, and a run of skips is often cured by cleaning rather than by tuning. Where the cause is not obvious, print a test panel with a known good paste and a clean stencil, which separates the material from the hardware in one step.

Keeping Skips Out of Later Operations

A skipped deposit that reaches placement becomes a missing joint, and a missing joint that reaches the customer becomes a field failure. Inspection after print is the control that stops the defect travelling, whether it is a full paste inspection system or a trained visual check with a low angle light.

Where skips are found, the panel should be cleaned and reprinted rather than touched up by hand, because a hand filled deposit has a different volume and a different surface. Recording the count and the position closes the loop with the stencil supplier and with the paste supplier, and the acceptance criteria for the printed deposits are defined in the IPC standards for solder paste printing.

FAQ

Can a skipped deposit be seen without inspection equipment? A large skip is obvious under a low angle light, and a small one on a fine pitch pad is easy to miss. A trained operator finds most of them, and an inspection system finds almost all, which is the difference that justifies the equipment on dense boards.

Does raising squeegee pressure always fix skips? It fixes skips caused by incomplete fill and it makes release related skips worse, because the paste is pressed harder into the wall. Test the change on a single panel and measure the result before committing the whole lot.

How often should the stencil be checked for blocked apertures? At the start of every run and after every pause. A quick check with a lamp behind the stencil shows a blocked opening immediately, and the cost is a fraction of the cost of a panel that has to be reprinted.