After Solder Paste printing, obvious burrs appear at the edge of the pad, and tin powder accumulation is visible under a microscope. The root cause is that the tin powder particle size distribution is too wide and the D50 is too small, resulting in an excessively high proportion of fine powder. Under the shear force of the squeegee, the fluidity is abnormally enhanced, and it is pulled out during demolding to form a tailing. The practical solution is to immediately stop using the current batch of solder paste and send it to a third party for testing of D10/D50/D90 data. Simultaneously, reduce the squeegee pressure from 0.45MPa to 0.32MPa and the speed from 40mm/s to 25mm/s. The key preventive measure is that before each batch of solder paste is put into storage, the laser particle size analysis report provided by the supplier must be verified. The D50 must fall within the range of 20±2μm, and the D90 must not exceed 38μm.

The solder paste volume consistency at the stencil opening is poor, and the solder shortage ratio exceeds 12% on three consecutive boards on the same PAD. The symptom is that the SPI detection map shows a standard deviation SD＞0.08mg. The reason is that the median particle size D50 of the tin powder exceeds the allowable upper limit of the stencil thickness-to-aperture ratio. When the stencil thickness is 0.12mm and the opening width is 0.25mm, the theoretically suitable D50 should be ≤25μm, while the measured D50 of the currently used solder paste reaches 28.6μm. The practical solution is to replace it with solder paste with D50=22.3μm and redo the first-article SPI verification. The key preventive measure is that when introducing a new stencil, the formula (maximum allowable D50=opening width×stencil thickness×1000÷15) must be used to back-calculate the particle size boundary value. For example, the corresponding upper limit for a 0.25mm×0.12mm stencil is 20μm.

After reflow, micro-bridging frequently occurs between the pins of QFP devices, and the AOI false alarm rate is as high as 35%. The symptom is that the bridging is concentrated in areas with a pin pitch of 0.4mm or less and mostly occurs at the edge of the board. The reason is that the tin powder particle size is too fine, causing the Flux viscosity to decrease, and the molten solder paste climbs along the sidewall of the pins under the action of surface tension. The practical solution is to reduce the wind speed in the cooling zone of the reflow oven from 1.8m/s to 1.2m/s to prolong the solidification time, and mix 5% by weight of coarse powder with D50=35μm into the solder paste for gradient compounding. The key preventive measure is that for devices with a pitch of 0.4mm or less, solder paste with D50≥25μm must be selected, and Type 5 and above fine powder grades are prohibited.

SPI of five consecutive boards shows that the void rate at the center of the pad is＞28%, and X-ray confirms that the voids are concentrated at the bottom of the solder joint. The symptom is that the voids are regular circles with diameters between 80 and 120μm. The reason is that the tin powder particle size is too concentrated, and the difference between D10 and D90 is＜15μm, resulting in insufficient thixotropy of the solder paste, which cannot maintain its shape and collapses the moment the squeegee stops. The practical solution is to extend the solder paste mixing time from 60 seconds to 90 seconds and switch to a dual-planetary vacuum mixer. The key preventive measure is that when accepting solder paste, the supplier must be required to provide the particle size distribution peak width value (Span=(D90-D10)/D50), which should be controlled between 1.4 and 1.8.

Residual solder paste at the bottom of the stencil after printing is difficult to clean, and stubborn adhesion remains after 30 minutes of ultrasonic cleaning. The symptom is that the residue is gray-black lumpy with metallic luster. The reason is that the tin powder particle size D90 exceeds the standard, causing some particles to get stuck in the micropores of the stencil nickel layer. The practical solution is to pre-rinse with pure water at 60℃ for 5 minutes before entering the ultrasonic tank, and simultaneously increase the cleaning agent concentration from 3.5% to 4.2%. The key preventive measure is that all stencils must undergo SEM scanning before first use to confirm that the hole wall roughness Ra＜0.2μm. If it exceeds the standard, it is prohibited to use solder paste with D90＞36μm.