FPC is a popular saying, and the correct term should be EN/IG. The formation of the electroless nickel layer does not need external current, and the nickel-phosphorus alloy layer can be continuously deposited on the activated metal surface at high temperature by the action of reducing agent in bath solution (such as sodium dihydrogen hypophosphite NaH2PO2, etc.). The electroless nickel layer can be used as a shielding layer for metal atom migration to prevent copper from diffusing into the gold coating.
As for gold immersion, it is a typical replacement reaction without reducing agent. When the chemical nickel surface enters the gold immersion bath, the nickel layer is dissolved and thrown, and then the gold layer is deposited on the nickel metal. Once the nickel surface is completely covered by the gold layer, the deposition reaction of the gold layer gradually stops, and it is difficult to increase to a considerable thickness. As for another series of thickened gold, a strong reducing agent is needed to gradually thicken the gold layer.
Nickel-plated gold is solderable and can provide the substrate for IC chip bonding (Au/Al wire), which has been widely used in PCB design.
In fact, the welding strength of the solder joints formed by plating Ni-Au on the surface of the plate is almost all built on the surface of the nickel layer. The purpose of gold plating is to protect the nickel surface from passivation or oxidation in the air and to maintain the minimum weldability. The gold layer itself is completely unsuitable for welding, and its solder joint strength is also very poor.
At the moment of high-temperature soldering of circuit boards, gold and samarium have already formed different forms of interface alloy eutectic (such as AuSn,AuSn2,AuSn4, etc.) and escaped. Therefore, the real solder joints are all on the nickel surface, and the strength of solder joints has nothing to do with gold. That is to say, samarium in the solder will form Ni3Sn4 interface alloy eutectic with pure nickel. The thin gold layer will be very short.
Therefore, if the nickel layer is too thin (less than 3um-5um) to provide a good solder base, SMT reflow solder will fall off or adhere poorly.