Stencil Tension Is Directly Related to Solder Paste Release Capability

The JJY 150-mesh laser-cut stencil we use daily can ensure complete solder paste release from the bottom of apertures only when the tension is maintained at 35–45 N/cm. If the tension falls below 30 N/cm, stencil sagging will cause solder paste to remain on aperture walls, resulting in insufficient solder volume on pads. Conversely, if the tension exceeds 50 N/cm, it will easily accelerate stencil fatigue deformation, especially after continuous printing of over 2000 boards, when the risk of sagging increases sharply. At this point, the tension meter must be recalibrated and the solder paste outline at pad edges must be inspected under a microscope to check whether it is uniform.

Blade Parameter Combination Determines the Stability of Solder Paste Transfer Rate

JJY production lines generally use 60° polyurethane blades with a 0.8 mm stroke depth. When the printing speed is set to 45 mm/s, the blade pressure must be adjusted synchronously to 5.2 kg; these three parameters must be adjusted in coordination. If the speed is increased to 55 mm/s without increasing the pressure, the solder paste fill rate will drop by more than 12%, leading to half-pad solder deficiency on 0201 component pads. Conversely, if the pressure is increased to 6.0 kg without reducing the speed synchronously, trailing and stringing will easily occur at stencil edges, increasing the probability of solder bridging between adjacent QFN pads by 3 times.

Batch Variations in Solder Paste Must Be Closed-Looped Through First-Article Verification

The thixotropic index fluctuation range between different batches of the same solder paste model often reaches 0.72–0.85. Before switching to a new batch, we must use the same stencil and the same printer to produce 5 first-article boards, then use SPI equipment to measure the solder paste volume CV value on pads. If CV > 12%, it indicates that this batch of solder paste is not compatible with the current blade angle; the blade angle needs to be fine-tuned from 60° to 58° and re-verified. Otherwise, the solder deficiency rate on IC component pads will exceed 8% after mass printing.

Environmental Temperature and Humidity Disturbances Cannot Be Ignored

When the workshop temperature rises from 23°C to 26°C and the relative humidity drops from 50%RH to 42%RH, the volatilization of solder paste solvent accelerates, and the apparent viscosity increases by 18%. If the solder paste warm-up time is not extended in advance from 2 hours to 2.5 hours, jagged burrs will appear at pad edges after printing, the AOI false alarm rate will increase by 23%, and the nozzle will easily cause component flying during subsequent placement due to sudden changes in Solder Paste Viscosity.

Cleaning Frequency Must Be Dynamically Set According to Production Volume

The back of the stencil must be wiped with a lint-free cloth dipped in 99.9% alcohol after every 300 boards printed. If cleaning is delayed until 450 boards, the thickness of the dried solder paste film accumulated on the inner walls of stencil apertures can reach 8 μm, directly causing the solder paste height on pads to decrease by 15% starting from the 451st board. Using JJY's dedicated nano-coated stencil can extend the cleaning interval to 550 boards, provided that the alcohol concentration is not lower than 99.5%; otherwise, the coating will be corroded and fail.