Tin is cheaper than gold for PCB finishing. It has a limited shelf life of about 3-4 months due to corrosion and formation of "tin whiskers" which can short closely spaced features on the board.
Gold has a long shelf life and provides a very flat, uniform, surface for soldering of SMDs such as BGA. Care must be taken to control the thickness of the gold layer to avoid embrittlement of the soldered joint. There are two types of gold plating commonly used - ENIG and DIG.
There are dozens of articles available on specific finishes if you do a Google search using each of the types (i.e. immersion tin, immersion silver, ENIG, direct immersion gold, HASL, etc.)
Admire, house_cat, you are really a pcb manufacturing expert
Generally speaking, immersion tin is enough for most cases. Only some big and rich companies would like to ENIG for their most production.
In fact, the richest company I've been seen is Agilent, almost all their productions are gold-plating
aha, Agilent is not only used to edge or finger connector, but also the "whole" board.
That's really a crazy and luxurious style.
I guess the income in Agilent must be extremely high.