Solder Paste printing collapse and poor release often occur simultaneously. The root cause is that the solder paste fails to generate sufficient shear stress within the stencil aperture to maintain its shape, and lacks adhesion to the pad at the moment of separation. This is closely related to the viscosity, thixotropic index, and metal content of the solder paste itself, and is also directly affected by stencil thickness, aperture aspect ratio, and support rib layout. For example, if a 0.1mm thick stencil is used for 0.3mm×0.15mm CHIP components, the risk of collapse increases sharply when the aperture area ratio is below 0.65.

The squeegee angle setting must be dynamically matched to the rheological properties of the solder paste. When the thixotropic recovery time of the solder paste is greater than 2 seconds, a squeegee angle below 55 degrees cannot establish effective rolling shear during the printing stroke, leading to insufficient solder paste filling and edge trailing. Conversely, an angle exceeding 65 degrees will exacerbate residual paste at the stencil bottom, causing release stringing. We once fine-tuned the squeegee angle from 58 degrees to 62 degrees on a customer's production line, and the solder paste release defect rate for 0201 components dropped from 12.7% to 2.3%, provided that the squeegee pressure was simultaneously reduced by 0.3kgf to avoid stencil deformation.

The frequency and method of stencil cleaning directly affect release quality. If ultrasonic cleaning uses an alkaline cleaner with a pH value higher than 8.5, a micron-level saponification film will form on the surface of the nickel-alloy stencil, increasing the interfacial tension between the solder paste and the stencil. Even with new solder paste, obvious release lag will occur. It is recommended to switch to a neutral hydrocarbon solvent with 40℃ constant-temperature soaking, and perform mandatory cleaning after every 25 prints. At the same time, use a 100x microscope to spot-check whether there is residual accumulation on the sidewalls of the stencil apertures.

Uncontrolled workshop temperature and humidity are hidden contributing factors. When the temperature exceeds 26℃ and the relative humidity is below 40%, the rosin-based carrier in the solder paste volatilizes rapidly, and the thixotropic structure disintegrates prematurely. Slow edge collapse can be observed within 30 seconds after printing. At this point, simply adjusting the squeegee parameters is ineffective. It is necessary to activate the air conditioner to precisely control the temperature to 23±1℃ and humidity to 55±5%, and ensure that the solder paste is allowed to warm up for no less than 4 hours before being put into use.

Abnormal collapse and release are most easily induced when switching solder paste batches. The rheological curves of solder pastes of the same model from different manufacturers can differ by up to 35%, especially with obvious shifts in the viscosity inflection point within the -2℃~10℃ range. Therefore, after material change, three sets of actual tests must be conducted using the same stencil and the same printing machine. SPI data from the 1st, 10th, and 25th prints are compared to evaluate the volume change rate. If the fluctuation exceeds ±7%, parameter adaptability must be re-evaluated.

Finally, beware of levelness deviations in the template support platform. Even a local settlement of 0.03mm will create a tiny wedge-shaped gap between the stencil and the PCB, causing the solder paste to be forcibly torn rather than naturally separated at the initial stage of release. This type of defect is particularly typical in QFP pin areas. It is recommended to use a 0.02mm feeler gauge weekly to inspect the height difference between the four corners and the center point of the support platform across the entire board. If the difference exceeds 0.025mm, calibration is mandatory.