Solder Paste Slump Causes and Control

Slump is the change in shape of a paste deposit between printing and reflow, and it is measured as the spread of the deposit beyond the aperture footprint. A paste that slumps bridges two pads before the alloy ever melts, and the defect appears at reflow rather than at print.

What Causes Slump

The paste is a suspension, and its shape after printing is held by the yield stress of the vehicle. When the yield stress falls, the deposit flows under its own weight and the printed walls round over.

The fall is caused by time, by temperature and by humidity. A paste that is left on the board for an hour at a warm room temperature slumps more than one that is reflowed within ten minutes. Our <a href="https://www.gopcba.com/solder-paste-volume-stencil/” title=”paste volume”>paste volume notes describe the deposit that is at risk.

Points to Confirm at First Article

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.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

Documented Process Control

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

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.

Inspection and Traceability Notes

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.

Viscosity and Yield Stress

Viscosity is the resistance to flow once the paste is moving, and yield stress is the resistance to starting. A paste can have a high viscosity and a low yield stress, which prints well and slumps afterwards.

The two are therefore specified together, and the specification is a range rather than a minimum. A paste that is too stiff does not release from the aperture cleanly. Our SPI notes describe how the deposit is measured against the target.

Humidity and Its Surprising Effect

Humidity affects the paste through the vehicle, and the direction of the effect depends on the chemistry. A vehicle that absorbs water becomes thinner and slumps, and a vehicle that loses solvent to a dry room becomes thicker and stops releasing.

The room condition therefore belongs in the process record rather than in the facilities report. A room that is controlled for the operator and not for the paste produces a print that changes with the season.

Stencil and Aperture Geometry

A deposit that is too tall slumps because its own weight is acting on a shape that cannot support it. The stencil thickness and the aperture area together set the height, so a thick stencil on a small aperture produces a tall deposit.

The area ratio is the figure that keeps both within range. Where a design needs volume, the aperture is enlarged rather than the stencil thickened, because enlargement increases the area without increasing the height. Our profile notes describe the reflow stage where the slump becomes a defect.

Time Between Print and Reflow

The time between printing and reflow is the exposure that the deposit has to survive, and it is set by the line rather than by the paste. A line with a buffer before the oven gives the paste an hour to slump.

Reducing that buffer is a process change that costs nothing and removes a defect class. Where the buffer cannot be reduced, the paste is chosen for its resistance to slump rather than for its print quality alone. Our board quality notes describe how the resulting joints are judged.

Measuring Slump

Slump is measured by printing a pattern with defined gaps, holding it for a defined time and inspecting the bridges. It is also measured by comparing the deposit area at print with the area after the hold, which gives a figure rather than a count.

The test is worth running once for a new paste and once after any change to the room or to the line timing. A paste that passes the test on a cool morning and fails in the afternoon is a paste at the edge of its window.

Practical Controls

The controls that work are a short time between print and reflow, a room condition that is recorded, a paste that is at ambient temperature before use, and an aperture geometry that keeps the deposit height moderate.

Those four address the cause rather than the symptom. Running the printer more slowly to compensate for slump does not reduce the slump.

FAQ

Does a stiffer paste always slump less? It resists slump and it releases from the aperture less cleanly, so the choice is a balance rather than a maximum.

Can slump be seen before reflow? It can be measured by deposit area, and a visible bridge before reflow is already a defect the printer created.

What does gopcb provide for slump control? We provide paste selection with yield stress and viscosity stated as a range, aperture geometry sized to keep the deposit height moderate, room conditions recorded against the print, time between print and reflow controlled, and slump tested on a defined pattern rather than judged by eye.