Manual Soldering of wire harness terminals is not simply melting Solder Wire onto copper wires; it requires simultaneous control over the contact area between the soldering tip and the terminal body, heating time, solder wire feed timing, and Flux activity matching. For example, using a KJY-SP160 soldering tip customized by JiajinYuan paired with 1.6mm silver-containing solder wire, applying stable hot pressing to the tinned copper terminal at 350°C for 1.2 seconds, then feeding the solder wire at a constant speed at a 45-degree angle, ensures that the solder fully wets all capillary gaps at the terminal root and the wire strand area.

We have found that when many production lines use 0.8mm Solder Wire to weld 6AWG wire gauge terminals, the solder joint surface appears bright but has internal micro-voids. This is because the melting volume of small-sized solder wire is insufficient to fill the gap between the bottom of the terminal's U-shaped groove and the wire bundle. After switching to 1.6mm solder wire, a single wire feed provides approximately 2.3 times the solder volume. Combined with a slight dragging motion of the soldering tip along the terminal's length, this forces the liquid solder to naturally flow toward the low-temperature zone and complete self-filling.

On-site measured data from JiajinYuan shows that the manual soldering yield using large-sized solder wire increased from 92.7% to 99.1%, with the proportion of cold solder joints dropping by 6.8 percentage points. This is mainly due to the continuous metal bridging structure formed by the solder at the junction of the terminal crimping area and the exposed wire section. This structure showed no cracking during temperature cycle tests from -40°C to 125°C, and maintained a contact resistance below 0.8mΩ after withstanding 50G acceleration and 10~2000Hz swept frequency vibration for 30 hours.

Operators must wipe the terminal surface with a small amount of rosin-based flux using a lint-free cloth before the soldering tip contacts the terminal. Otherwise, the oxide film will hinder solder spreading, and even with large solder wire, it will cause solder joint buildup and a wetting angle greater than 60 degrees. However, excessive flux application easily causes post-soldering residual corrosion. Therefore, it is recommended to apply 0.02ml per square centimeter of terminal surface area, which is the optimal range cross-validated by JiajinYuan across 12 production lines.

During soldering, the soldering tip must not remain in the same position for too long. Otherwise, the terminal plating will blister and peel due to overheating, preventing the solder from metallurgically bonding with the base material. In this case, even if the solder joint appears full, it is a false solder joint, with an actual thrust test below 8N, far below the minimum requirement of 12N specified in IEC 60352-2. Therefore, the single-point heating time must be strictly controlled to no more than 1.5 seconds, and any external force disturbance must be avoided before the solder joint solidifies.

When training technicians, JiajinYuan emphasizes that solder joint judgment cannot rely solely on whether the surface reflection is uniform. It is necessary to use a 30x magnifying glass to observe whether there are unwetted dark bands on the terminal side wall. If present, it indicates that the solder wire was fed too late or the soldering iron temperature was too low, requiring immediate parameter adjustment. This type of defect is difficult to detect during factory inspection but can cause intermittent power failure failures during the customer's vehicle road test phase.