Hard gold PCBs are commonly used in applications that require high reliability, such as aerospace, military, and medical devices. These industries often involve frequent mating and unmating of connectors, which can cause wear and tear on the PCB contacts. The hard gold finish ensures that the PCB can withstand these repeated cycles without degradation.

It's important to note that hard gold plating is costlier compared to other surface finishes, such as HASL (Hot Air Solder Leveling) or ENIG (Electroless Nickel Immersion Gold). The choice of surface finish depends on factors such as the specific requirements of the PCB design, the intended application, and cost considerations.