Discoloration of Immersion Silver Board Finish
Recently, a customer called the EMPF Helpline after observing discoloration of an immersion silver (IAg) finish on their printed circuit boards (PCB). The customer wanted to know the origin of the discoloration (Figure 2-1) and whether the discoloration would affect the reliability of the assembly.
Since the implementation of RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment) regulations by the European Parliament in 2006, immersion silver has become one of the choices for a lead free board finish. IAg is attractive because it minimizes copper oxidation from high temperature lead-free assembly processes better than organic solderability preservatives (OSPs). Also, the IAg finish does not form tin-copper intermetallic growth during storage as 100% tin finishes do. The consistent distribution of IAg plating thickness can meet the high density surface mount component requirements better than hot air solder leveling (HASL) finishes. The IAg plating process also does not attack solder mask and substrate and the IAg solder surface is compatible with lead-free and tin-lead solder temperatures.
IAg finishes have a 12-month shelf life in a controlled environment (<35ºC/<85 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 the IPC-4553 recommendations and lead-free, high silver content solders are used. IAg is not recommended for high reliability assemblies that must meet the IPC-6010 Qualification and Performance Series class 3 requirements, but is acceptable for class 1 and 2.
Fourier Transform Infrared (FTIR) spectroscopy was performed to confirm the presence of silver tarnish (Figure 2-2). 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 that the discolored area is a mixture containing silver tarnish (Ag2S) which can further oxidize to silver sulfate (Ag2SO4).
Excessive tarnish can inhibit the effectiveness of flux during manufacturing and may eventually lead to corrosion of the silver finish and the underlying copper conductors. Evaluation of the solderability of tarnished PCB lots can be determined by testing to J-STD-003B, Solderability Tests for Printed Circuit Boards.
Discoloration of IAg finishes may originate due to problems in the board manufacturing process or due to poor storage conditions. Incomplete, thin, or low silver content finishes will affect the ability of the IAg 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/or low silver content. Confirm that the cleaning process is removing solder mask residues from the PCB prior to plating.
When the IAg finish is applied correctly, silver tarnish may develop due to poor post cleaning or poor storage conditions. Exposure to sulfur, dirty rinse water, and improper drying can promote tarnishing. Sources of sulfur are air pollution, rubber bands, latex gloves, desiccant, and sulfur bearing paper used to separate parts. Use deionized (DI) water for the final rinse and thoroughly dry boards after rinsing. Check that the DI water filter system is functioning properly. Limit board exposure to atmosphere circulation by storing boards in sealed, dry bags or dry boxes. Use non sulfur content paper to separate boards. Do not use rubber bands and avoid desiccants containing sulfur. A common ingredient of commercially available laboratory grade desiccant is gypsum (calcium sulfate anhydrite, CaSO4) which contains sulfur. If a desiccant is to be used, consider choices that do not contain sulfur, such as silica gel (silicon dioxide, SiO2).
The EMPF offers a wide rang of analytical services that support PCB failure investigations. These services include FTIR spectroscopy, elemental analysis using XRF, sequential electrochemical reduction analysis (SERA), solderability tests, optical and scanning electron microscopy. The EMPF also offers lead-free manufacturing process development support such as board design reviews and independent vendor qualifications. More information about these services can be found on the EMPF website, www.empf.org or by calling the EMPF technical staff at 610.362.1320.