Solder Paste Printing and SPI Quality Control

Solder paste printing is the operation that decides the ceiling on the quality of an assembly, and it is also the operation that receives the least attention when a product is being planned. Placement and reflow are visible; printing produces a deposit that is measured in fractions of a millimetre and disappears under the components a few minutes later. The consequence is that a printing problem is usually discovered as a soldering problem, at which point the cause is no longer on the board. The control described here is built around measuring the deposit while it still exists.

The Parameters That Decide the Deposit

The stencil sets the geometry. The aperture defines where the paste goes and how much of it is needed; the foil thickness defines the volume that a given aperture will deliver; the wall finish and the tension affect how cleanly the paste releases from the opening. Apertures are designed against the area ratio of the opening so that release is consistent across the whole pattern, since an aperture that releases poorly leaves paste behind in the stencil and robs the pad.

The printer sets the transfer. The squeegee speed, the pressure and the angle control how the paste is pushed into the aperture and how much of it is left on the stencil surface. The separation speed controls whether the paste stays in the pad or is drawn back out with the stencil. The cleaning interval controls how the paste that accumulates under the stencil is removed before it prints onto the board as a smeared deposit.

The paste itself is a consumable with a state. Its viscosity changes with age, with temperature and with repeated working, and a paste that has been left on the stencil too long behaves differently from one that has just been dispensed. The storage condition, the warming time and the open time on the machine are recorded, because a paste that is within its useful life behaves predictably and one that is not does not.

Measuring the Deposit

SPI measures the deposit immediately after printing, and the measurement is a volume rather than a picture. For every aperture on the board it reports the area, the height and the volume of the paste, together with its offset from the pad. Those four values allow a deviation to be attributed to a cause rather than discussed as an impression.

A uniform print offset across the board usually means the alignment between the stencil and the board, or the positioning of the panel. An offset that grows progressively across the panel points to a thermal or mechanical change during the print rather than to a setup error. A paste volume that is consistently low in one region points to the aperture geometry or to a support problem beneath that area. A volume that is high suggests that the aperture is too large for the pad, or that the paste is being drawn out of the stencil rather than released from it.

The value of the measurement is that it converts an opinion into a number attached to a position. When the same parameter fails at the same place on successive boards, the printing process is adjusting to a condition it cannot correct, and the cause is upstream of the printer.

Where the Anomalies Come From

An excess of paste in one area of the board is most often a support problem. The board flexes where nothing holds it, the stencil contact changes and the deposit grows. The same symptom can come from a stencil that is not sitting flat, from a foil that has lost tension, or from a paste that has become too thin.

A shortfall is more often geometric. An aperture whose area ratio is unfavourable releases only part of its paste, and the shortfall is concentrated at the pads that have the smallest openings, which is why the fine pitch devices show it first. It can also come from a partially blocked aperture, which is a progressive effect as the paste dries on the walls.

Contamination appears as a deposit outside the pad rather than as a deviation in the pad itself. Paste that has collected under the stencil is squeezed onto the board surface on the following print, and the result is a scatter of stray deposits that are attributed to handling until the stencil is examined.

Closing the Loop

When the paste inspection reports an anomaly, the first step is to decide whether it is isolated or repeated. An isolated deviation on one board is noted and the board is reworked or depanelised. A repeated deviation in the same place is a process condition and is investigated immediately, because the printing stage is the last point at which the problem can be corrected without touching a soldered joint.

The sequence of investigation runs from the stencil to the support to the printing state. The stencil is examined for aperture condition, for contamination and for tension. The support is checked for gaps beneath the board. The printing state covers the squeegee condition, the paste and the cleaning interval. Only after those three are cleared is the placement or the reflow considered, because a print defect is present before either of them.

The record of the parameters and of the corrections is held with the order, so that a repeat batch starts from a known setup rather than reproducing the state in which the previous batch happened to be running. Our SMT assembly lines run that control, the prototype builds that prove a stencil are handled under rapid PCBA prototyping, the measurements are held with quality management, and the electrical confirmation follows under PCBA testing.

Why the Print Is the Deciding Operation

The reason printing carries more weight than any other stage is arithmetic. A reflow profile can be adjusted, a placement offset can be corrected and an inspection programme can be rewritten, but the quantity of alloy that forms the joint is fixed the moment the paste is deposited. Everything after the print either preserves that quantity or loses it; nothing adds it back. A pad that received eighty percent of the paste it needed can be placed and reflowed perfectly and will still produce a joint that is smaller than the design intended.

That is also why a print problem tends to be diagnosed late. The paste is invisible under the component, the resulting joint often looks acceptable from the outside, and the true condition is only revealed by an X-ray image or by a functional failure. By then the board has been through the reflow oven and, in the worst case, through a test fixture, so the correction arrives after the value has been added rather than before.

Keeping the Process in a Known State

A printing process is not set once. The paste changes as the batch progresses, the stencil accumulates residue beneath it, the squeegee wears, and the ambient conditions in the room move with the season. A process that is in control is one in which those drifts are detected by the measurement rather than discovered through the yield.

The practical approach is to hold the parameters as a revision attached to the product rather than as a machine setting. The stencil identification, the foil thickness, the squeegee speed and pressure, the separation speed, the cleaning interval and the paste specification are recorded for each product, and a change to any of them is a change to the process that is confirmed on the paste measurement before the batch continues. When a repeat order arrives, it starts from those recorded values instead of from whatever the machine happens to be doing that morning.

The same record makes the investigation of an anomaly a comparison rather than a search. When the deposit volume in one region begins to fall, the recorded values show whether anything in the setup changed; if nothing did, the cause lies in the material or in the environment rather than in the parameters, and the investigation moves to the right place immediately.

FAQ

What does SPI measure? The area, the height and the volume of each paste deposit, together with its offset from the pad, measured immediately after printing.

Why is a repeated anomaly investigated immediately? Because printing is the last stage at which the cause can be corrected without disturbing a soldered joint.

What is the most common cause of too much paste? Inadequate support beneath the board, which allows it to flex and changes the contact between the stencil and the pad.