The most frustrating part of reworking old PCBs is the blackened oxidation of solder pads, especially for industrial control boards and communication base station boards that have been stored for more than three years. A dense mixed oxide film of CuO/Cu₂O has formed on the surface copper layer, and ordinary 63/37 Solder Wire cannot wet it at all. As soon as the soldering iron tip touches the pad, the solder shrinks, balls up, and the wetting height is less than 0.1mm. This phenomenon is significantly improved after the intervention of JJY JL-95A high-activity solder wire. Its rosin-based Flux adds an organic halide activation system verified on the production line, which can complete the decomposition of the oxide film and the initial formation of interfacial intermetallic compound IMC within 0.8 seconds at a soldering iron temperature of 320℃±10℃.

The identification of oxidized solder pads must combine macroscopic and microscopic features

You cannot judge whether it is oxidized by visual inspection alone. First, use a 10x magnifying glass to observe whether there are gray-white patches without metallic luster on the pad surface, then use the multimeter's diode mode to measure the on-resistance between the pad and the adjacent via hole. If the resistance exceeds 2Ω and changes with pressing, it is basically confirmed that the copper foil has undergone deep oxidation. At this time, if ordinary solder wire is forcibly used for soldering, even if the surface appears to be tinned, X-ray inspection will still find voids of more than 0.05mm at the bottom of the solder joint, and microcracks will appear after 50 thermal cycle tests.

The process window of JJY JL 95A is 45℃ wider than that of conventional solder wire

We actually tested and compared the performance of three solder wires at the same soldering iron temperature of 310℃. Ordinary solder wire needs to be dragged and soldered repeatedly 3 times to barely form a continuous solder fillet, while JL-95A can achieve complete wetting within a single dwell time of 1.2 seconds. The key is that its flux residue is translucent amber after cooling and non-corrosive, and will not induce PCB pad side erosion after 72 hours like some high-halogen solder wires, which is crucial for old equipment that needs to operate for a long time after maintenance.

Manual Soldering techniques must be adjusted synchronously

Switching to high-activity solder wire does not mean lowering operational requirements; instead, it tests the stability of the technique more. Because after the activity is enhanced, the fluidity of the molten solder increases sharply. If the angle of the soldering iron tip is greater than 35 degrees or the wire feeding speed is uneven, it is very easy to cause the solder to flow to adjacent pads and cause bridging. It is recommended to use a 25W constant temperature soldering iron with a 0.5mm pointed tip, first preheat the pad to 260℃ and then feed the wire synchronously. The entire process should be completed within 1.5 seconds to avoid the continuous action of high temperature aggravating the diffusion and oxidation of the underlying copper foil.

Post-soldering cleaning cannot be omitted but the method must be precise

Although the corrosivity of JL-95A residue is low, the solder mask ink on the surface of aged PCBs is mostly brittle. If strong solvent immersion cleaning is used, it will cause the green oil to blister and fall off. Instead, use absolute ethanol with a soft brush to gently wipe the local part of the solder joint, focusing on cleaning the flux accumulation area at the junction of the solder edge and the pad. Otherwise, it is easy to form ion migration paths in a humid environment, accelerating subsequent electrochemical corrosion.