Effect of FPC Electroless nickel immersion gold on SMT welding
FPC chemical nickel-gold is a common term, and the correct term should be called Electroless nickel immersion gold (EN / IG).

Electroless nickel immersion gold is a type of surface plating used for printed circuit boards. It consists of an electroless nickel plating covered with a thin layer of immersion gold, which protects the nickel from oxidation. ENIG has several advantages over more conventional (and cheaper) surface platings such as HASL (solder), including excellent surface planarity (particularly helpful for PCBs with large BGA packages), good oxidation resistance, and usability for untreated contact surfaces such as membrane switches and contact points.
The formation of the chemical nickel layer does not require an external current. Etc.), at high temperatures, the nickel-phosphorus alloy layer can be continuously deposited on the activated metal surface to be plated. The chemical nickel layer can be used as a shielding layer for metal atom migration to prevent copper from diffusing into the gold plating layer.

As for immersion gold plating, it is a typical replacement reaction without reducing agent. When the surface of chemical nickel enters the immersion gold bath, the gold layer is deposited on the nickel metal at the same time as the nickel layer is dissolved and thrown. Once the nickel surface After it is completely covered by the gold layer, the deposition reaction of the gold layer gradually stops, and it is difficult to increase it to a considerable thickness. As for the other series of thickened gold, a strong reducing agent is required to gradually thicken the gold layer.

Electroless nickel immersion gold has solderability, and it can also provide the substrate for IC chip wiring (Au / Al wire), which has been widely used in PCB design.In fact, the welding point formed by plated nickel-gold is almost all built on the surface of the nickel layer. The purpose of gold plating is to protect the nickel surface in the air from passivation or oxidation, and maintain a minimum of welding. It's just that. The gold layer itself is completely unsuitable for welding, and its solder joint strength is also very bad.

At the moment of high-temperature soldering of circuit boards, gold has already formed with different forms of interface alloy compounds (such as AuSn, AuSn2, AuSn4, etc.) and escaped, so the real solder joint foundation is on the nickel surface, and the strength of the solder joint is strong. Weakness has nothing to do with gold. That is to say, the plutonium in the solder will form a Ni3Sn4 interface alloy with pure nickel. The thin gold layer will quickly dissipate in a short time, slipping into a large number of solders, and the gold layer It is impossible to form a reliable solder joint.

Therefore, if the nickel layer is too thin (less than 3um-5um), it cannot provide a good solder substrate, which will cause the SMT reflow solder to fall off or have poor adhesion.