After Solder Paste printing, the solder joint thickness is too thin and has poor consistency, manifested as frequent AOI alarms for insufficient solder joint volume and an increased rate of poor ICT test contact after reflow. The root cause is that the metal content of the solder paste is lower than the nominal lower limit of 88.5%, which reduces the density of solderable metal particles per unit volume. In addition, the stencil aperture size has not been synchronously compensated, and the squeegee pressure setting still uses the parameters for solder paste with 90% metal content. The practical solution is to immediately stop using the current batch of solder paste and send it to a third party for metal content testing. Synchronously, fine-tune the squeegee pressure from 0.45MPa to 0.52MPa, reduce the printing speed from 40mm/s to 32mm/s, and lock in the SPC system that the stencil aperture area corresponding to this batch of solder paste is enlarged by 1.8%. The preventive measure is that XRF rapid testing of metal content must be completed before each batch of solder paste is put into storage, and the test results must be bound with the stencil design parameters and entered into the MES system to avoid the disconnection between process parameters and material characteristics.

The solder joint thickness is locally too thick, accompanied by edge burrs or slight collapse. Under a microscope, it can be seen that the solder extends more than 0.12mm outward of the pad. The symptoms are concentrated at the pin ends of QFP devices and the central area of BGA. The reason is that the measured metal content of the solder paste reaches 91.3%, exceeding the recommended upper limit of 90.5%. The high metal ratio increases the viscosity of the solder paste and decreases its thixotropy, causing the solder paste to be dragged and elongated by the stencil hole wall during demolding. The practical solution is to forcibly reduce the demolding speed from 2.8mm/s to 1.6mm/s, increase the stencil support height from 0.15mm to 0.18mm to reduce peeling stress, and enable a nitrogen-protected printing environment to reduce the volatilization rate of the Flux. The preventive measure is that the procurement agreement must clearly specify the metal content tolerance as 89.5%±0.5%, and require the supplier to provide the original ICP-OES test report corresponding to the independent number of each barrel of solder paste.

The dispersion of solder joint thickness on the same PCB exceeds ±12μm, especially significant in mixed boards with 0201 and SOIC components. AOI statistics show that the CV value reaches 18.7%. The reason is that the batch-to-batch fluctuation of the solder paste metal content reaches 88.2%～90.1%, while the stencil tension has not been periodically calibrated, and the clamping force of the frame has attenuated by 23%, leading to a more drastic decrease in the printing transfer rate in the fine-pitch area. The practical solution is to use a tension meter to monitor the four corners of the stencil in real time and recalibrate it to 38N/cm. Synchronously, locally increase the stencil thickness in the area corresponding to 0201 devices from 120μm to 135μm. The preventive measure is to establish a three-dimensional linkage table of solder paste metal content - stencil tension - printing parameters. When the metal content changes by 0.3 percentage points, the tension must be adjusted by ±1.2N/cm, and the squeegee angle must be synchronously corrected by ±0.7°.

After reflow, the overall solder joint thickness is low but there is no cold solder joint. X-ray tomography shows that the solder accumulation height is uniformly lower than the target value by 7.3μm, and the symptoms are accompanied by a pad wetting angle >35°. The reason is that although the metal content of the solder paste meets the range of 88.5%～90.5%, the measured D90 of the Sn particle size distribution is 28.6μm, exceeding the upper limit of the process window of 25μm. The coarse particles have high rolling resistance in the stencil aperture and a reduced transfer rate. The practical solution is to replace it with solder paste of the same metal content with D90≤24.5μm, and increase the stencil aperture ratio from 0.68 to 0.73. The preventive measure is that the original particle size distribution data spectrum must be requested when accepting the solder paste, and release based solely on the supplier's declaration is prohibited.

The solder joint thickness shows a stepwise decrease after 2 hours of continuous production, dropping by about 2.1μm every 50 pieces, and obvious stratification can be seen in the solder paste jar. The symptoms are synchronously accompanied by serrated defects at the printing edge. The reason is that the metal content of the solder paste is 89.2%, but the storage temperature has been higher than 28℃ for a long time, leading to the aging of the flux system and accelerated sedimentation of metal particles. The practical solution is to immediately shut down the machine to clean the squeegee and stencil, place the solder paste jar in a constant temperature cabinet for 4 hours, and then stir it at a constant speed for 30 minutes. The preventive measure is that the solder paste must be stored in an environment of 22±3℃ after thawing, and the first opening time and cumulative exposure duration must be marked after each can is opened. If it is not used up after more than 8 hours, it will be returned to the warehouse.