Gold coated pads
The main purpose of the coating on the pads is to prevent corrosion.
Copper oxidises very easily in a normal atmosphere - it goes green. By adding a coating on the top you are sealing the copper away from the atmosphere and protecting it.
You have a choice of coating, and coating methods, and each has its own benefits.
There are basically 3 methods:
- Hot-Air Solder Levelling (HASL)
- Electroplating
- Chemical plating
The most common "silver" one you see is HASL. This basically involves covering the pads with a thin layer of solder and using hot air to reflow it and level it off.
Chemical plating - dipping the board in a tin solution - is the most common "home brew" method since it is easy to do and the chemicals are easily available.
Electroplating is usually used with gold. Similar to silver and gold plated jewellery. It's very very expensive.
The gold chemical plating (ENIG) is the most common way of depositing gold. It is not as expensive as electroplating, but it does still give very good results. Not only does it give a good clean low resistance contact, but the perfect flat surface is also good for pick and place machines - especially with fine pitched leaded surface mount components (such as TQFP) where the slightly raised pads of the HASL can cause misalignments to happen.
HASL is cheap though, and still gives "good enough" connections.
Also the gold pads just look so much cooler.
That looks like ENIG to me, thin gold over nickel and it's main purpose is to protect the copper below from oxidation. It actually gets absorbed up into the solder, or solder ball during soldering and allows the solder to make a direct connection to the copper underneath. Too much gold can actually lead to brittle solder joints.
So gold enig vs silver immersion, gold enig is going to have a longer shelf life before your silver immersion boards start to oxidize and become unusable (or shouldn't be useable :).
VS HASL in addition to even better shelf life they're going to be much flatter, so easier for assembly with BGA's QFNs etc. Silver immersion also has nice flatness.
Plus they look prettier over time since the gold stays shiny.
Here's some nice links on surface finish differences:
http://www.epectec.com/articles/pcb-surface-finish-advantages-and-disadvantages.html http://www.multicircuits.com/pcb/tech/surface_finishes.html