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April 10, 2017

Silver Solder for Audio Circuits

We manufacture an audio product. We have been told to use silver solder for all solder connections. Can silver solder possibly have any impact on the quality of our product, or will standard, lower-cost eutectic solders work just as well?

S.L.

Experts Comments

Compared to conventional tin& lead alloys, a much greater corrosion favoring effect exists with silver-based solders due to the high solubility rate of silver hydroxide in moisture films and the low critical relative humidity of 60%. In particular, carboxylic acid-based activators in solder pastes can result in a steady increase of the adsorbed moisture film. It is therefore possible that the value of the critical humidity level could drop below levels that are typically found in air-conditioned rooms. The removal of hygroscopic residues (carboxylic acid, salts) through assembly cleaning can contribute to a significant improvement of resistance to elevated humidity levels and reduce susceptibility to migration.  

The phenomenon can be explained by the uniform distribution of the silver with a maximum of 4% in the soldered joint. The electro-chemical migration mechanism activates only the silver that is close to the surface of the soldered joint. This means that only low concentrations of silver hydroxides are available. Accordingly, only filigree dendrites with a very low current-carrying capacity can be formed. These dendrites lower the surface resistance primarily in the final stage of their growth, but are then burnt off by a short-circuit effect so that the original surface resistance is re-established.  

At normal (ambient) temperatures, the rate of renewed silver supply to the surface of the solder joint is slower than the attack by electrochemical migration. This is the reason why short-life bridges are repeatedly formed that have never transformed into constant short-circuits. This phenomenon results in inexplicable defect patterns in electronic assemblies that cannot be systematically repeated and not be discovered by discontinuous resistance measurements.
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Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
All elements of the conductive path for audio signals have the potential for adding "noise" (aka: hiss, buzz, hum, etc) to the audio signal. Resistors are one of the most common sources of noise in audio circuits, carbon film being one of the worse, so most will use a lower noise generating resistors such as: metal file, wire wound, carbon comp, or foil type resistors, each having a trade-off between noise generation, size, and cost.  

It stands to reason that solder joints can also add noise, since the audio signal may pass through hundreds of solder joints on its way to your ears. Audio "purists" recommend a silver alloy solder, usually around 4% Ag. The use of a silver solder in audio equipment can be one the many hidden differences between lower priced audio products and "high end" professional audio equipment.
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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
I would venture to say following.
  1. If you are an OEM then I don't see that Silver solder is going to make that much of a difference.
  2. If your product is high end & directed to professional usage or to audiophiles (sic) then it's a selling point for your brochure.
  3. If you are a sub contract assembler & your customer is asking for silver solder, then use it & pass along the costs related
  4. Experience & research tells me that it makes little or no difference in audible -- & that's the issue -- quality. How many millions of car radios are out there? Silver solder? I don't think so.
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Jerry Karp
President
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
What is the composition of the silver solder? What are the reliability requirements? There are lower-temp eutectic alloys (cheaper or more expensive) that can be used based on the reliability requirements, solder process, equipment used.
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Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.
I agree with Jerry K's points (2) and (3). In addition, let me offer some insight into what may be driving the advice to use a silver-bearing solder.

Silver is the best-available electrical conductor, and has a long history in electronics. Audiophiles are a notoriously superstitious bunch, and have shown a shocking tendency to believe technical claims that have not been demonstrated to have an *audible* effect on equipment performance.

So, can a silver solder really improve the sound of the resulting equipment? Perhaps, but I am somewhat dubious. We could talk all day about skin effects, thermal (Johnson-Nyquist) noise, etc., but the bottom line is, if there is an audible effect, it should show up as detectable signal degradation at the frequencies of interest.

The whole question may be somewhat moot, however. If the assemblies you are producing are Pb-free, then it's easy to select a Sn-Ag-Cu solder that contains approx. 4% Ag. The cost difference between that and lower-Ag alloys is not that great.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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