If you just need the technical requirements (not the full PDF), here are the critical points from the standard:
Prior to IPC-4556, many manufacturers used generic or proprietary ENIG processes, leading to issues like "black pad" (hyper-corrosion of nickel) and inconsistent solder wetting. This standard provides a rigorous set of controls to eliminate those failures. ipc4556 pdf
Typically 100–150 µin (approx. 3–6 µm). It serves as the primary barrier against copper diffusion. If you just need the technical requirements (not
The palladium layer acts as a barrier that prevents the aggressive immersion gold process from corroding the underlying nickel, a common failure point in standard ENIG finishes. you are likely dealing with:
If you are downloading the IPC4556 PDF to solve a specific design challenge, you are likely dealing with: