The dense oxide film on the nickel sheet surface is extremely difficult to remove by the weakly active Flux released by ordinary Solder Wire during melting, so we must select low-temperature Active Solder Wire with a silver content between 0.3% and 3%. For example, SN63PB37 Eutectic Solder wire can complete wetting at 183°C, while SAC305 has a slightly higher melting point, but the silver element can significantly improve the spreading ability on nickel substrates. If pure tin or low-silver lead-free solder wire is forcibly used, not only must the soldering iron temperature be pushed above 380°C, but also brittle cracking of solder joints will be caused due to the excessively fast growth of the interface IMC layer (Ni?Sn?).

Our production line tests have found that for 0.8mm diameter rosin-core SN63PB37 solder wire at a soldering iron temperature of 320°C, the wetting time for 0.15mm thick nickel sheets must be controlled within 2.5 seconds to ensure a solder joint tensile force >25N. Once it exceeds 3 seconds, obvious solder ball splashing and solder joint blackening will occur, which indicates that the flux activity of ordinary solder wire is fundamentally insufficient to continuously maintain the reduced state of the nickel surface. Especially during continuous operation, the accumulation of solder dross will further deteriorate the wetting effect.

When purchasing, it should be noted that the outer diameter tolerance of the solder wire must be controlled within ±0.02mm, because the pin spacing of nickel sheets is generally less than 0.5mm. If the solder wire diameter deviation is too large, it will easily cause wire feeding jams or unstable solder output. We once had a batch of 0.85mm outer diameter solder wire mixed in due to lax supplier inspection, which resulted in the automatic soldering machine having an excessive solder joint void rate of 23% for three consecutive days. During rework, microscopic observation revealed that all defective solder joints had unfused exposed nickel substrate areas at the bottom.

The flux type is more critical than the solder wire model. RMA-type rosin-based flux must be selected instead of no-clean water-soluble flux, because the organic acids in the latter's residues will form a passivation layer on the nickel surface. In our comparative tests, the average shear force of the same batch of nickel sheets soldered with RMA solder wire was 28.6N, while it directly dropped to 14.2N after switching to water-soluble solder wire, and obvious green rust corrosion traces appeared on all samples after 72 hours of high-temperature and high-humidity aging.

Before soldering nickel sheets, they must be unidirectionally polished with 320-grit silicon carbide sandpaper for 15 seconds and then wiped with anhydrous ethanol. Steel wire brushes or ultrasonic cleaning cannot be used, otherwise nickel grain boundary oxidation will be induced. Once we mistakenly used ultrasonic cleaning on phosphorus-containing nickel sheets, resulting in millimeter-level microcracks appearing at all solder joint edges after reflow. X-ray inspection showed that the crack orientation was completely consistent with the grain orientation.