Pin Dip Soldering is not a remedial measure but a pre-assurance step. It targets the surface oxidation and blackening phenomenon of incoming pins caused by thin plating, long storage time, and high environmental humidity. Such problems are particularly prominent on 0.4mm pitch QFP and SOIC devices. If they are directly fed into the placement machine without treatment, consecutive defects such as tombstoning, offset, and excessive solder joint void rate often occur after the first batch passes through the reflow oven.

Dip soldering temperature and time must match the Solder Composition

When using JJY SN63PB37 leaded solder, the dip soldering bath temperature is set at 245±2℃, and the pin immersion time is controlled at 3.5 seconds. Too short a time results in discontinuous plating, while too long a time easily forms solder nodules at the pin root. After switching to SAC305 lead-free solder, the bath temperature must be raised to 255±2℃ and the time compressed to 2.8 seconds; otherwise, Flux residue will aggravate ionic contamination on the PCB surface.

Flux selection directly affects the adhesion of the dip soldering layer

Water-washable RMA flux has moderate activity and low residue, making it suitable for military single boards that require subsequent cleaning, but it has insufficient penetration for severely oxidized pins. No-clean RA flux has stronger activity and can effectively strip the sulfide film on the Ni/Au plating surface. However, its halogen content is relatively high, so the post-reflow ionic contamination value must be strictly controlled below 1.56μg/cm² NaCl equivalent.

There are clear production line standards for appearance judgment after pin dip soldering

Pins after dip soldering must present a mirror-like silver bright luster, with no exposed copper, no dewetting, and no solder spikes. Observed under a 10x magnifying glass, there must be no discontinuous plating or foggy oxidation areas. The sampling frequency is implemented as first article of each batch + one inspection every 2 hours. If abnormalities are found, the line must be stopped immediately to retest the bath specific gravity and wave height.

Equipment maintenance details determine long-term stability

The titanium alloy nozzle of the JJY dip soldering machine must be manually polished with 320-mesh silicon carbide sandpaper every 8 hours of continuous operation; otherwise, solder spatter will accumulate on the nozzle edge to form burrs, which in turn leads to uneven tin adhesion on pins. Every week, the inner wall of the solder bath must also be cyclically cleaned with a 5% citric acid solution to prevent Cu6Sn5 intermetallic compound precipitation from blocking the heat dissipation channel of the heating tube.

Linkage verification must be performed with the SMT placement process

Devices after dip soldering must be placed within 24 hours. If the time limit is exceeded, re-baking for dehumidification and secondary dip soldering are required; otherwise, during the reflow phase after placement, a wetting angle greater than 90° easily occurs due to re-oxidation of the pin surface. This defect appears as gray-white solder joints without metallic reflection in AOI inspection, and the NG rate will suddenly rise from the normal 0.02% to above 0.8%.