types of pcb finish
HASL - cheapest but with environment harmful, basiclly forbidden in US and Euro markets.
OSP - cheaper one with short deposit time
Immersion Tin, Silver - Cheap with reasonable deposit time
Immersion Gold/ENIG - Not cheap but with very good hardness and quality, adequate for most cases
Electrolytic Gold - high price with best quality, merely used for gold finger basically
To save money, some buys choose OSP + selective ENIG, for one side osp save lots of money, for the other side, selective ENIG can protect some import and unwearable parts