Pin Dip Soldering is not simply a matter of feeding the PCB into the solder bath and calling it a day; it requires repeatedly balancing four variables: immersion depth, dwell time, solder temperature fluctuation, and Flux coating uniformity. For example, when the pin pitch is less than 0.8 mm, if the conventional 4.5-second dip soldering is still applied, excessive capillary action will cause Solder Paste to climb excessively between adjacent pins, ultimately forming bridges after cooling. JJY found on-site that for the same model of relay, the difference in solder thickness between 3.2-second and 3.8-second dip soldering at a solder temperature of 255°C can reach 0.025 mm, and this tiny change falls exactly at the critical point of bridging.

When adjusting pin dip soldering parameters, we never look at a single value in isolation. Instead, while setting the solder pot temperature to 252±2°C, we must simultaneously stabilize the conveyor speed at 1.1 m/min, control the flux specific gravity at 0.815±0.003 g/cm³, and maintain the board surface temperature at the preheating zone outlet at 95±5°C. This is because once preheating is insufficient, the oxide film on the pin surface is not fully reduced, and subsequent dip soldering will result in coexistence of cold solder joints and solder beads. If the flux specific gravity is too high, it will aggravate the accumulation of solder at the pin root, causing the dip solder thickness to increase sharply by 0.018 mm at a distance of 0.3 mm from the root.

In actual production lines, it has been found that when using JJY JY-703 flux, if the spray pressure is increased from 0.18 MPa to 0.22 MPa, although wettability is significantly improved, the solder thickness at the pin tip instead increases by 0.012 mm. This requires us to simultaneously reduce the dip soldering time from 3.5 seconds to 3.1 seconds to hold the 0.06 mm process upper limit. Moreover, the solder pot liquid level must be recalibrated every time a Solder Bar is replaced, because a 2 mm deviation in the liquid level will change the actual immersion depth of the pins by 0.35 mm, thereby affecting the capillary filling rate and the final solder thickness distribution.

For dual-row through-hole IC devices, we mandate an inclined solder entry method, where the PCB enters the solder surface at a 5° angle. This can significantly weaken the vertical solder adsorption force of the entire pin group. Combined with a uniform sinking action of 0.4 mm/s, the probability of solder bridge formation between adjacent pins can be reduced by 42%. At the same time, this angle also makes it easier for the pin sides to be wrapped by solder, avoiding secondary bridging caused by rework after unilateral poor wetting. In addition, the root cross-sections of pins on the first 30 boards after each solder change must be sampled under a microscope to confirm that there are no solder bump protrusions and that the solder thickness gradient is gentle; otherwise, the line must be immediately stopped to investigate whether the solder pump flow rate is abnormal or whether the nitrogen protection flow has fallen outside the 12 L/min standard range.

Data accumulated over three years of operation of the JJY DIP line shows that when the solder thickness after pin dip soldering exceeds 0.075 mm, the AOI false bridge alarm rate increases by 3 times, and 83% of actual bridge defects are concentrated in the 3rd to 7th rows of pins. This is because this area is shielded by the front row of pins, resulting in uneven flux volatilization. Therefore, we reserved a 0.15 mm pin misalignment margin during the fixture design stage, and added a local hot air pre-blow step before dip soldering to drive residual solvent vapor out of the gaps. This detail reduced bridge rework labor by 1.7 hours per day.