Watch for silver embrittlement and corrosion.

A customer recently observed discoloration of an immersion silver (ImAg) finish on their printed circuit boards. The customer wanted to know the origin of the discoloration (Figure 1) and whether it would affect assembly reliability.

Immersion silver is one of the choices for a Pb-free board finish. ImAg is attractive because it minimizes copper oxidation from high-temperature Pb-free assembly processes better than organic solderability preservatives (OSPs). Also, ImAg does not form SnCu intermetallic growth during storage as 100%-Sn finishes do. The consistent distribution of ImAG plating thickness can meet the high-density surface mount component requirements better than hot air solder leveling (HASL) finishes. The ImAg plating process does not attack solder mask and substrates, and the ImAG solder surface is compatible with Pb-free and SnPb solder temperatures.

ImAG finishes have a 12-month shelf life in a controlled environment (<35ËšC/<85Ëš RH), which can be significantly shortened when exposed to air pollution with high sulfur and chlorine content. Silver embrittlement (Ag₃Sn intermetallics) of solder joints may occur when the finish thickness exceeds IPC-4553 recommendations and Pb-free, high silver content solders are used. ImAG is not recommended for high-reliability assemblies that must meet IPC-6010 Class 3 requirements, but is acceptable for Classes 1 and 2.

Fourier transform infrared (FTIR) spectroscopy was performed to confirm the presence of silver tarnish (Figure 2). The appearance of a broad peak around 3500 wavenumber (cm-1) is typical of O-H bonding and is indicative of waters of hydration. From the FTIR spectrometric analysis, it is likely the discolored area is a mixture containing silver tarnish (Ag₂S), which can further oxidize to silver sulfate (Ag₂SO₄).

Excessive tarnish can inhibit flux effectiveness during manufacturing and may eventually lead to corrosion of the silver finish and underlying copper conductors. Evaluation of the solderability of tarnished PCB lots can be determined by testing to J-STD-003B.1

Discoloration of ImAG finishes may originate due to problems in the board manufacturing process or poor storage conditions. Incomplete, thin, or low silver content finishes affect the ability of the ImAG finish to protect the underlying copper pad from oxidation. X-ray fluorescence (XRF) is a quick testing technique for measuring plating thickness and silver content. Plating issues typically have their origin in the plating bath. Check the bath for low temperature, excessive copper content and low silver content. Confirm the cleaning process is removing solder mask residues from the PCB prior to plating.

References

1. J-STD-003B, “Solderability Tests for Printed Boards,” March 2007.