This is perhaps the most critical yet invisible metric in IPC-4552. The electroless nickel layer is actually a nickel-phosphorus alloy.
Do not rely on random forum posts or outdated screenshots. Purchase the official PDF from the IPC store today, and use the guidelines outlined in this article to enforce quality control across your supply chain. ipc-4552 pdf
The IPC-4552 standard, "Specification for Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Circuit Boards," includes this paper for free in its appendix to provide additional technical context for the industry. Primary Focus: This is perhaps the most critical yet invisible
The technical paper titled Standard Developments Efforts of Electroless Nickel Immersion Gold ipc-4552 pdf