It consists of a thin immersion silver plating over the copper traces.
Advantages of IAg
- Excellent surface planarity (compared to e.g. HASL)
- Low High Frequency Signal loss due to skin effect
Disadvantages of IAg
- Rapid degradation of surface due to oxidation or contamination (with e.g. sulfur or chlorine)
- Silver "whiskers" form across electrical potentials and short out components 
IPC Standard: IPC-4553
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