What exactly does ENEPIG protect against? Why isn't ordinary immersion gold plating used on optical module PCBs?

Those familiar with optical modules friends know that the surface treatment process used for the PCB of optical modules is ENEPIG

This is not because it is "superior" to ENEPIG, but because it faces a different connection method.

ENIG's positioning

The standard immersion gold process (ENIG) mainly corresponds to soldering.

During solder paste reflow, the thin gold layer on the surface dissolves into the solder. At this point, the nickel layer and the solder are the ones that actually participate in the interfacial reaction, forming a stable intermetallic compound (IMC).

However, on the PCB of the optical module, you will find that in addition to solder paste, some places require gold wire bonding.

Gold wires are pressed onto the solder pads, and a connection is formed using heat, pressure, and ultrasonic energy.

The gold layer does not melt into the solder as in reflow soldering.

In this case, the gold is left in place, rather than being incorporated into the solder.

Immersion gold can be soldered, but that doesn't mean it's suitable for bonding.

Copper / Chemical Nickel / Immersion Gold

This structure works fine during SMT reflow soldering. The gold layer acts as a pre-soldering shield, and during reflow, the gold enters the solder, while the nickel layer absorbs the subsequent interface reaction