The control standard for Solder Paste printing thickness is set at 120±20μm on the vast majority of mass production lines. This value is not an ideal laboratory figure, but an engineering convergence result repeatedly verified on the production lines of customers such as Samsung, Huawei, and BYD over the past two decades. It not only meets the pad filling requirements of QFN0.4pitch devices, but also avoids the risk of solder balls at the bottom of BGA. When the stencil thickness is 100μm and the aperture area ratio is greater than 0.65, if the measured thickness continues to be lower than 100μm, it is most likely caused by squeegee wear leading to incomplete filling of the stencil apertures with solder paste, or increased Solder Paste Viscosity due to fluctuations in workshop temperature and humidity.

We found that when the printing thickness dispersion is greater than 15μm, the AOI missed detection rate will sharply increase to 37%. Behind this is not only the accuracy issue of SPI equipment, but more often caused by deformation of the stencil support frame leading to poor adhesion between the stencil and PCB, or micro-displacement caused by track transmission vibration. Especially when the panel size exceeds 250×350mm, the thickness of the edge area is generally 8 to 12μm lower than that of the center. At this time, a four-point support fixture must be used in conjunction with vacuum adsorption to stabilize the CPK above 1.33.

Three commonly used methods on the production line are used to cross-verify thickness data. First, SPI automatically samples 10 points per hour and generates an SPC chart. Second, cross-sectional slices are used with a metallographic microscope to measure the real solder paste volume. Third, a step block of known height is used for offline calibration. However, it should be noted that after the same can of solder paste is continuously printed for 200 pieces, the viscosity increases due to Flux volatilization, and its printing thickness decreases by an average of 6.3μm. Therefore, the solder paste warm-up time, stirring speed, and service life must be included in the thickness control parameter table.

When encountering high thickness in the dense area of 0201 components, the squeegee pressure cannot be simply lowered, because a pressure below 4.5kg will cause the stencil to bounce and create a gap, which instead causes solder paste trailing. The correct approach is to simultaneously reduce the printing speed to 25mm/s and increase the demolding speed to 3.2mm/s, which can balance the integrity of solder paste release and edge clarity. A certain automotive electronics factory once reduced the tombstoning rate from 1.8% to 0.27% because of this.

A special reminder to new production line engineers: do not blindly trust the theoretical thickness values provided by stencil suppliers. Once, when we accepted a 120μm thick laser-cut stencil, we found that the measured average thickness was only 109μm. The root cause was that the manufacturer used nitrogen shielded welding, which led to local thermal deformation. Later, it was corrected to meet the standard by using electropolishing + EDM secondary trimming. Therefore, the first article confirmation must be based on the actual SPI measurement. Any paper standard that deviates from the actual production line is ineffective control.