Gold plating is valued for its physical properties and since it’s a noble metal it does not oxidize or chemically react under normal conditions. Pure gold has excellent electrical and thermal conductivity characteristics and is a primary choice when electrical conductivity, soldering, and corrosion resistance are design requirements.
Pure soft gold is 99.9% pure with a Knoop hardness of less than 90. Typical uses are for wire bonding, die attach, and high reliability soldering applications. Specification callouts are generally ASTM B488, Type III, Code A and Mil-G-45204, Type III, Grade A.
Hard gold is typically 99.0-99.7% pure with an alloying agent of either nickel or cobalt and has a Knoop hardness of 130-200. Hard golds are generally used where electrical conductivity (especially low current) is necessary and if repeated sliding or connection wear are of concern. Spec callouts for Hard Gold are often ASTM B488, Type II, Code C or Mil-G-45204, Type II, Grade C.