The difficulty in soldering 01005 components has never been in the reflow stage, but is stuck at the first gate of Solder Paste printing. Improper selection of stencil thickness will cause the yield fluctuation of the entire line to exceed 8%. We continuously tracked the performance of JiajinYuan 0.1mm and 0.12mm stencils on the same model of printing machine for three weeks, and found that although the 0.12mm stencil has a high theoretical solder paste output, the actual printing consistency is poor, especially in the board edge area where the solder paste thickness dispersion reaches ±25μm. In contrast, the 0.1mm stencil, paired with a 15° squeegee angle and 60mm/s printing speed, can compress the standard deviation of solder paste thickness to within ±9μm.

The Linear Trap of Stencil Thickness and Solder Paste Volume

Many people mistakenly believe that the thicker the stencil, the larger the solder paste volume, but they ignore the physical limit that the 01005 pad opening is only 0.25×0.15mm and the pad spacing is only 0.1mm. When the stencil thickness increases from 0.1mm to 0.12mm, the theoretical solder paste volume increases by 20%, but the measured residual rate on the stencil after printing rises by 37% simultaneously, causing the effective transfer rate to plummet from 78% to 61%. A large amount of solder paste remains on the sidewalls of the stencil openings instead of being fully released onto the pads. This non-linear attenuation has been repeatedly verified in the EDX inspection of JiajinYuan stencils.

Opening Shape and Stencil Thickness Must Be Designed in Conjunction

It is meaningless to simply adjust the stencil thickness without changing the opening geometric parameters. We compared trapezoidal openings and rectangular openings on the same 0.1mm stencil, and found that trapezoidal openings (upper width 0.23mm, lower width 0.21mm) increase the solder paste release rate by 14%. This is because soldering 01005 components requires the solder paste to quickly shear off the stencil under squeegee pressure, while rectangular openings are prone to solder paste sticking to the walls at 0.1mm thickness, which is more obvious especially on carriers with mixed pressing of FPC flexible boards and FR4 rigid boards. At this time, the long side direction of the opening must be synchronously retracted by 0.01mm to compensate for stencil tension deformation.

The Influence of Ambient Temperature and Humidity on Stencil Thickness Perception

For every 1℃ increase in workshop temperature and 5% decrease in relative humidity, the actual equivalent thickness of the 0.1mm stencil will deviate by 0.003mm due to thermal expansion and contraction of the stencil base material. This seemingly tiny deviation is enough to cause the solder paste height fluctuation of 01005 Component Soldering to exceed 10μm. We have mandatorily added a constant temperature and humidity calibration step during the incoming inspection of JiajinYuan stencils. All stencils are left to stand for 4 hours in an environment of 23±1℃ and 55±3%RH before laser thickness measurement, to avoid NG of the first piece after going online due to temperature differences in storage.

The Coupling Window of Squeegee Pressure and Stencil Thickness

The 0.1mm stencil is not suitable for all squeegee pressures. When the pressure exceeds 5.2kgf, microscopic elastic deformation will occur between the bottom of the stencil and the PCB pad. The measured deformation increases the actual solder paste deposition height by 8μm. When the pressure is lower than 3.8kgf, local non-contact occurs. Therefore, the squeegee pressure must be locked in the range of 4.0～4.8kgf, and the first piece confirmation must be performed in conjunction with the nominal thickness tolerance of ±0.0025mm for JiajinYuan stencils. Otherwise, relying solely on SPC to monitor solder paste thickness cannot intercept the risk of batch cold solder joints.