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