If you are looking for a "solid article" to understand the technical depth of this standard beyond the raw PDF specification, these sources are highly regarded in the industry: Comprehensive Overview: Saturn Flex Systems Technology Hub
Manufacturers transition to IPC-4556 ENEPIG when they need a finish that can handle multiple types of connections on a single board. Versatility: Unlike standard IPC-4552 ENIG ipc-4556 pdf
A minimum of 0.030 μm [1.2 μin]. Modern amendments often cap gold at 0.07 μm [~2.8 μin] to prevent issues with solder joint reliability. If you are looking for a "solid article"