The stencil aperture dimensions and component pads must be designed strictly at a 1:1 ratio, while accounting for the spreading characteristics of Solder Paste under shear force. If the aperture wall roughness exceeds 0.8μm, the solder paste release rate will drop below 75%, leading to an increased rate of cold solder joints after placement and reflow, which is particularly pronounced on 0201 and smaller package devices.

During printing, the squeegee angle should be set between 45° and 60°, the pressure controlled within the range of 3.5kg to 5.5kg, and the printing speed maintained between 25mm/s and 45mm/s. These three parameters must be adjusted synchronously; otherwise, the solder paste will not form a complete fill within the stencil apertures, and solder paste stringing or collapse will occur during demolding. Such anomalies manifest as pad coverage below 80% in AOI inspection, while the actual solder joint height after reflow is often less than 120μm.

Solder paste must be opened and used in an environment with a constant temperature of 22℃±2℃ and humidity of 50%RH±5%. After being removed from the refrigerator, it must be allowed to warm up for more than 4 hours before being loaded onto the machine. Otherwise, condensation mixed into the solder paste will cause Flux splattering and spattering during the reflow phase, resulting in tiny solder balls remaining at the bottom of the BGA. These defects appear as irregular diffuse distribution under X-ray and cannot be removed by cleaning.

During the placement process, the nozzle vacuum value must be stable above ?85kPa, and the Z-axis descent height error must not exceed ±0.05mm. Otherwise, it will cause solder paste extrusion offset or nozzle tinning. Especially for QFN-type devices with no bottom pads, once the solder paste is pressed into the gaps of the thermal pad, it is extremely easy to form an excessive void rate after reflow. The measured void area proportion often reaches more than 35%, far exceeding the 25% upper limit specified in IPC-A-610E.

The Reflow Soldering temperature profile must be dynamically adjusted according to the Tg value of the solder paste used. The liquidus temperature of JIAJINYUAN SN63/PB37 solder paste is 183℃, the peak temperature must be controlled at 235℃±5℃, and the duration maintained between 45 seconds and 65 seconds. If the temperature in the soaking zone is below 150℃ for more than 90 seconds, the rosin component in the solder paste will carbonize prematurely, causing the solder joint to turn black and the wetting angle to be greater than 30°. If the heating ramp exceeds 3℃/s, it is easy to cause microcracks in MLCCs. Such cracks do not appear in ICT testing, but will cause open-circuit failure after high-temperature and high-humidity aging tests.

The first piece of each shift must undergo solder paste thickness measurement. Use SPI equipment to sample 5 different areas, with a thickness standard of 120μm±15μm. If the tolerance is exceeded, stop the line to adjust the stencil tension or squeegee parameters. Never rely on visual judgment, because the human eye has no ability to distinguish thickness changes at the 10μm level, and a thickness deviation of 0.03mm is sufficient to cause the tombstoning rate of 0402 resistors to soar to more than 12% after reflow.