The most intuitive manifestation of Solder Paste slump in the production line is that within 30 seconds after printing, the solder paste outline becomes blurred, edges overflow significantly, and the height drops by more than 15%. Especially on pads of small-pitch components such as 0201 and 0402, it is extremely easy to cause bridging between adjacent pads. This kind of slump often cannot be identified by AOI before reflow, and batch short circuits are only discovered after the oven. Therefore, the control node must be moved forward to the printing station instead of relying on final inspection.

We found that production lines with frequent solder paste slumps generally have problems of low Solder Paste Viscosity and a thixotropic index less than 0.7. This type of solder paste has weak ability to restore structural strength after shearing stops. In addition, if the standing time after printing is slightly longer, the ambient temperature exceeds 25°C, or the relative humidity is higher than 60%, uneven volatilization of the Flux solvent will accelerate the slump trend. Therefore, every time a batch of solder paste is changed, a rotational viscometer must be used to actually measure the initial viscosity and thixotropic index at 25°C, instead of only relying on the manufacturer's data sheet.

Unreasonable matching of stencil thickness and aperture size is also a critical defect. For example, when printing a 0.3mm pitch QFN with a 0.1mm thick stencil, if a 1:1 aperture is still used, the actual filling rate is often lower than 65%. The solder paste is more likely to lose stability due to surface tension disturbance at the moment of demolding. At this time, the aperture width should be reduced to 88% of the pad, and step etching should be used to reduce the local thickness to 0.075mm, which not only ensures smooth demolding but also suppresses spreading.

Although the squeegee angle set between 55° and 65° is within the conventional range, if the squeegee pressure is not adjusted synchronously after changing a new batch of solder paste, for example, the pressure is still maintained at 5.2kg while the solder paste viscosity has decreased by 12%, the actual shear stress will exceed the yield value of the solder paste, causing the internal structure of the paste to rupture after printing and then collapse. Therefore, before each batch of solder paste goes online, three sets of pressure gradient verifications must be performed and the corresponding slump rate recorded.

Out-of-control workshop temperature and humidity are hidden factors ignored by many engineers. When the temperature in the printing area rises from 22°C to 26°C and the RH jumps from 45% to 62%, the slump occurrence rate of the same batch of solder paste will increase by more than 3 times. Therefore, independent temperature and humidity probes must be installed around the printing machine, with alarm thresholds set at temperature 23±1°C and humidity 50±5%. If the limits are exceeded, the machine must be shut down for calibration instead of relying solely on central air conditioning for unified regulation.

Incorrect solder paste warming method can also induce slump. Directly unsealing and stirring after taking it out of the refrigerator will cause condensation on the solder paste surface. Water mixed into the flux system will destroy the original rheological balance. The correct approach is to keep the whole can at a constant temperature of 22°C for 4 hours before unsealing, and strictly control the stirring time within 90 seconds to avoid excessive shearing damage to the paste network structure.