Hard gold for Rigid flex PCB is a gold alloy with complexes of cobalt, nickel or iron. A low-stress nickel is used between gold over-plating and copper. Hard gold is not suitable for wire bonding.

We recommend using the hard gold surface finish for components and applications with heavy usage and a high likelihood of wear and tear, like:

• Edge Connector Fingers
• Interconnected Carrier Boards
• Keypads
• Contacts

The thickness of the hard gold surface finish will vary depending on the applications. When hard gold is used for compliance in military applications, the minimum thickness shall be 50 -100 micro inch. 

Nonmilitary applications require 25 to 50 micro inches. Here are the recommended minimum and maximum thickness values:

• 17.8 μin is recommended for IPC maximum solderable thickness
• 25 μin gold over 100 μin nickel for IPC Class 1 & Class 2 applications
• 50 μin gold over 100 μin nickel for IPC Class 3 applications