The nickel layer acts as a barrier against copper diffusion and provides mechanical strength for solder joints.
The standard (with its latest amendments) defines specific plating thickness ranges for the three layers: Nickel (Ni): 3 to 6 μm [118.1 to 236.2 μin]. Palladium (Pd): 0.05 to 0.15 μm [2 to 6 μin]. Gold (Au): ipc-4556 pdf
Electroless Nickel / Electroless Palladium / Immersion Gold (ENEPIG) The nickel layer acts as a barrier against
). This protective layer prevents the nickel from oxidizing and helps mitigate the "Black Pad" defect seen in older ENIG finishes. Gold (Au): Gold (Au): Electroless Nickel / Electroless Palladium /
Most professional PCB fabricators assume Class 2 unless you specify Class 3. For high-reliability projects, always write: "Shall meet IPC-4556 Class 3 requirements for ENIG finish."
To understand the importance of IPC-4556, one must first understand the architectural anatomy of the ENEPIG finish it governs. ENEPIG is a tertiary (three-layer) metallic structure plated over the PCB's base copper. The base layer is electroless nickel, which acts as a barrier to prevent copper from diffusing into the solder. The middle layer is electroless palladium, which plays a unique and protective role by preventing the immersion gold from aggressively attacking and corroding the nickel beneath it. Finally, the top layer is a thin flash of immersion gold, which preserves solderability by preventing the oxidation of the palladium.
Do you need a deeper look at the of ENEPIG versus ENIG?
