The immersion silver finish provides a ROHS compliant finish for printed circuit boards (PCB) but is thinner than ENIG or traditional HASL finishes. Immersion silver is a newer PCB finish that offers better performance and superior finishes for final finishes on PCBs.
This finish is well established in the industry; the learning curve regarding handling, packaging, and storage has been the biggest challenge in introducing immersion silver to the market.
Immersion silver is deposited directly on the copper surface by a chemical displacement reaction and is available in the industry all co-deposit and organic anti tarnish with the immersion silver. This reaction is fast and does not require the relatively high temperatures of ENIG.
Immersion silver can be measured using XRF equipment. The proper setup of the equipment is critical for reproductive results.
The primary use of IAg is as a solderability preservative. During assembly the immersion silver dissipates into the solder and allows the formation of a Cu/Sn intermetallic.
Immersion silver is an active surface and readily combines with sulfur from the environment. Silver sulfide tarnishes the surface and creates doubt about the integrity of the finish. You can add an anti-tarnish post deposition step to protect the surface from the environment.
The proper packaging of immersion silver finished boards are critical to control sulfurization. The key is to minimize contact of the surface with the environment and to ensure all materials used in the packaging and during storage are sulfur free.
All indications are that IAg will transition readily into lead free assembly because the SAC alloy contains a relatively high percentage of silver.
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