To understand why soldering to stainless steel should be limited to mechanical purposes only rather than electrical applications, and why acid fluxes or acid-based solders are unsuitable for electrical applications, it is essential to first discuss the relationship between electronics and acid fluxes.
Kester does not recommend the use of acid fluxes for any electrical or electronic applications. This precaution is known throughout the industry. The clerk at the local hardware store will tell you this, teachers in every level of electronic teaching will tell you this, and yet, many people harbor a feeling that it is OK to use acid fluxes in electronics if you know the right secrets. There are no secrets. During soldering, acid fluxes deposit zinc chloride in the solder and this salt cannot be removed. Exposure of the chloride to carbon dioxide and moisture initiates a corrosion cycle. The chloride reacts with the lead in the solder, converting it to lead carbonate. After the lead carbonate is formed, the chloride is free to attack more lead. The corrosion continues until the solder joint dissolves.
For making electrical connections to stainless steel, assemblers have two main options. The first is to use a mechanical connection, such as a screw or rivet, which is sufficient for most applications involving stainless steel. The second option is to plate the stainless steel with a more solderable material like copper or nickel. This allows the use of standard electronic solder and fluxes for the connection.
Now that we have eliminated any thought of soldering to stainless steel in an electronic application, we will look at how to solder to stainless steel for mechanical applications. Stainless steel requires the use of special fluxes in order to achieve good adhesion of the solder to the stainless steel. Typical acid-core fluxes will not work on stainless. Kester has 817, which is specially formulated for applications of soldering to stainless. Kester 817 must be used with solid wire or it can be used in addition to acid-core solder. Kester 817 flux is typically brushed on the stainless and then the solder is reflowed using standard reflow procedures with an iron or a torch.
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Related Topics
Source - Kester: Soldering to Stainless Steel
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