During the soldering of high-frequency boards, Solder Paste printing must be synchronously matched to the difference in substrate thermal expansion coefficient. For example, Rogers 4350B has a CTE of 60ppm/℃ in the Z-axis direction. If the stencil aperture is not compensated with a 0.015mm unilateral trapezoid, post-reflow solder joint void rate exceeding the standard and BGA solder ball offset are highly likely to occur. We have measured on the JJY production line that when the squeegee angle is set to 58°, the printing speed is controlled at 32mm/s, and the demolding rate is 1.2mm/s, the solder paste volume variation coefficient of a 0.12mm thick laser-etched stencil for 0.15×0.25mm CHIP pads can be suppressed to 7.3%, which is far better than the 12% upper limit specified by IPC-7527.

The biggest concern in 5G high-frequency PCB SMT Soldering is local thermal stress imbalance during the reflow process. Especially when 0201 RF capacitors and 25×25mm large PA modules coexist on the same PCB, if the slope in the peak zone of the furnace temperature profile is not set according to the device heat capacity classification, the 0201 component terminals are prone to being pulled apart while cold soldering occurs at the bottom of the PA. JJY adopts a dual-track independent temperature-controlled reflow oven, setting the heating slope in stages as 1.8℃/s, 1.2℃/s, and 0.9℃/s within the 180～230℃ range, combined with controlling the oxygen content under a nitrogen atmosphere to within 150ppm, which compresses the S21 parameter fluctuation after high-frequency board soldering to within ±0.15dB.

Insufficient placement accuracy will directly amplify the phase error of high-frequency signals. We require that the solder paste position offset in SPI inspection must be <8μm; otherwise, even if the placement accuracy reaches ±25μm, it cannot meet the impedance matching requirements in the 28GHz frequency band. The high-rigidity nozzle used by JJY has a pickup repeatability of ±3.7μm for 0.3mm pitch QFN devices under a vacuum level maintained at 82kPa, and the visual system's light source brightness and lens distortion coefficient must be calibrated once per shift using a standard block to avoid XY coordinate misjudgment caused by image grayscale drift.

The cleaning step is often overlooked, but rosin-based Flux residue will cause an increase in the dielectric loss tangent tanδ at high frequencies. We measured that the insertion loss at the 10GHz frequency point of a batch of Rogers 4003C boards deteriorated by 0.8dB when not thoroughly cleaned compared to after cleaning. JJY enforces a two-stage ultrasonic + DI water spray process: the first stage uses 45℃ isopropanol ultrasonic for 120 seconds, and the second stage uses 65℃ deionized water spray at a pressure of 0.28MPa. After cleaning, the ionic contamination level is controlled at 0.38μg/cm² NaCl equivalent, ensuring that the dielectric properties of the high-frequency board do not undergo migratory degradation after soldering.

All high-frequency board soldering operations must be carried out in a constant temperature and humidity room. A temperature of 23±1℃ and humidity of 45±3%RH are hard thresholds, because every 5% increase in humidity will cause the FR4 substrate Dk value to drift by 0.02, which in turn causes the phase offset of the 5G millimeter-wave antenna feed path to reach 3.2°. The air conditioning system in the JJY workshop is equipped with a dew point closed-loop control module, which can correct the return air humidity to the target window within 90 seconds when the outdoor humidity load suddenly increases, avoiding rework of the entire batch of PCBs due to environmental fluctuations.