The tin dipping process plays a decisive role in the electrical contact reliability of wire harness connector terminals, especially in the mass production of automotive electronics and new energy battery pack wire harnesses. Jia Jinyuan SAC305 lead-free solder must be used with a precisely temperature-controlled solder bath system to avoid defects such as coating segregation, oxide inclusions, and uneven solder wetting. Our actual tests found that when the solder temperature fluctuation exceeds ±2℃, the tin dipping coverage rate of 0.5mm thick phosphor bronze terminals will drop by more than 17%, and when the Sn content in the solder is lower than 96.5%, the solder joint wetting angle after reflow will directly exceed 45°.

The surface activation treatment before tin dipping of wire harness terminals cannot be omitted, and a Flux pre-coating compounded with zinc chloride and organic amine must be used. Otherwise, even if the 250℃±3℃ tin dipping temperature and 3～5 seconds residence time recommended by Jia Jinyuan are used, the copper substrate will still show local copper exposure or whitening of the tin layer. This problem is particularly prominent on terminals with a nickel plating thickness lower than 0.8μm, because an excessively thin nickel layer cannot effectively block the diffusion of copper-tin intermetallic compounds into the substrate.

The cycle time matching of the tin dipping process directly affects the overall line OEE of wire harness factories. Jia Jinyuan SAC305 solder needs to be supplemented with 0.3%～0.5% pure tin after 8 hours of continuous operation to maintain activity. If the material is not replenished in time, the tin slag generation rate will increase by 2.3 times, and at the same time, the bendable times of terminal pins after tin dipping will drop sharply from the standard 12 times to less than 7 times, which is closely related to the increased brittleness caused by excessive growth of the IMC layer.

Differences in terminal wire diameter bring obvious heat capacity changes. 0.3mm thin wire terminals are prone to tin layer accumulation at the root and insufficient plating at the tip under the same tin dipping parameters. At this time, the conveyor belt speed must be reduced by 15% and the solder disturbance intensity must be reduced synchronously, otherwise there will be a 23% probability of tin layer peeling during the subsequent crimping process. For 1.2mm thick wire terminals, the tin dipping time needs to be extended to 6 seconds and the solder circulation flow rate needs to be increased, otherwise void defects of more than 0.05mm are likely to form on the back of the terminal.

The halogen residue of Jia Jinyuan lead-free solder must be controlled below 800ppm, otherwise the leakage current between terminals will exceed the standard in the high temperature and high humidity aging test of the wire harness (85℃/85%RH/1000h). We once caused batch rework of the vehicle ECU wire harness because the measured halogen content of a certain batch of solder reached 920ppm, and the rework cost accounted for 3.7% of the single-unit BOM cost. Therefore, each batch of incoming materials must be spot-checked with an ion chromatograph, and it is strictly prohibited to put them into production before the results are issued.