If the surface of wire terminals and chip pins is not treated before storage, transfer and SMT mounting, poor wetting during Reflow Soldering is highly likely to occur due to the formation of an oxide film. Our three-month continuous tracking at an automotive-grade factory in Dongguan found that the cold solder joint rate of 0.4mm-pitch QFN pins without tin dipping was as high as 3.7%, while the figure dropped to below 0.12% after adopting JiajinYuan's tin dipping process synchronously. This does not simply rely on enhanced Solder Paste activity, but because the tin dipping layer forms a dense Sn-Cu or Sn-Ag alloy transition phase under nitrogen protection, which fundamentally blocks the contact path between the substrate and oxygen and sulfur in the air.

In actual production lines, tin dipping process parameters must be strongly coupled with the substrate status. For example, bare copper wires need to stay in a 195℃±2℃ tin bath for 0.6 to 0.9 seconds, while silver-plated terminals require the temperature to be lowered to 188℃ and the time shortened to 0.4 to 0.6 seconds. Otherwise, the silver layer will excessively dissolve into the tin liquid, causing local copper exposure. We once caused a batch of BGA substrate rework on a customer's production line in Suzhou due to this issue. The root cause was that after the silver content in the tin liquid exceeded 0.35wt%, the surface tension changed abruptly, leading to tailing and stringing when pins exited the tin. JiajinYuan's equipment controls this risk within the threshold by real-time monitoring the silver concentration in the tin liquid and linking with the tin replenishment module.

Terminals after tin dipping cannot be directly mounted on the machine; the SMT process must be completed within 24 hours. Otherwise, even with a tin layer covering, the copper substrate will still slowly precipitate oxides from the grain boundaries. We tested at a medical equipment factory in Wuxi that for tin-dipped terminals stored for 36 hours, the wetting angle deteriorated from 18° to 52° at a reflow peak temperature of 235℃, and the Flux residue increased by 40% synchronously, indicating that oxidation has begun to erode the bonding force at the tin-copper interface. At this point, even enlarging the stencil aperture cannot improve the solder joint void rate.

The centrifugal drying step of JiajinYuan's tin dipping equipment is set at a rotation speed of 280rpm for 1.2 seconds. This parameter is the balance point verified through 27 groups of wires with different wire diameters (0.15 to 0.5mm). A rotation speed below 260rpm will lead to uneven flux film thickness, while a speed above 300rpm will cause micro-deformation of thin-walled pins due to excessive centrifugal force, which in turn affects the accuracy of subsequent SPI inspection in determining solder paste volume. All parameters are solidified in the equipment's PLC and cannot be modified on site to avoid misadjustment by production line personnel.

The tin dipping process is not a panacea; it cannot salvage terminals that originally have burrs, scratches or coating peeling. We disassembled a batch of NG products at a customer's site in Hefei, and found under a microscope that there were mechanical scratches at the root of the pins. After tin dipping, the tin layer accumulated into nodular protrusions here, which not only failed to melt during reflow but also hindered the spreading of surrounding solder. Such problems must be controlled in the incoming inspection stage. JiajinYuan recommends using a 50x stereomicroscope with measurement function for AQL sampling, rather than relying on the tin dipping process to cover up defects.