The advantage of using a gold contact is at least 3-fold. They gold plating makes them more resistant to corrosion, so this means they are better for harsh environments. They can also withstand more mating cycles without wearing out so gold is better in a situation where they have to be mated and unmated often. The third advantage is they are a lower resistance contact. This factor gives them better continuity than a tin contact, and is definitely preferred in a low voltage circuit (5V and under).
Besides these basic rules there are some other limitations where gold (or palladium nickel) must be used:
For any type of analogue signals, gold is a must because of line noise created by tin plated contacts
For any connector being exposed un-mated in the field in an industrial atmosphere
For any connector with a contact spacing of less than 1mm because of whisker growth of pure tin-plated non annealed surfaces
Gold and tin in one mated connector system is a no go. The behavior of these two platings is not well researched, however, the standard electrode potentials tell you that the contact point will corrode (gold = +1,5V tin = +0,15V – look at your rainwater gutter when copper meets zinc) and the reliability is gone.
Read More From The Connector by Molex: http://www.connector.com/gold-or-tin-versus-gold-and-tin/#ixzz4irNbDrRT