It seems you are looking for the IPC-4556 document, likely in PDF format. This is a specification for "Electroless Nickel / Immersion Gold (ENIG) Plating for Printed Circuit Boards."
The core function of the document is to provide standardized requirements that ensure ENEPIG performs reliably across various assembly technologies. ipc4556 pdf
Measurement Standards: IPC-4556 specifies X-ray Fluorescence (XRF) as the primary method for verifying thickness. It mandates the use of Solid State Detectors (SSD) for better resolution on tri-level coatings and requires calibration against national standards. It seems you are looking for the IPC-4556
The widespread adoption of ENEPIG, guided by IPC-4556, is driven by several key performance benefits over traditional finishes like ENIG (Electroless Nickel/Immersion Gold): Black Pad Mitigation: guided by IPC-4556