One of the most frequently asked questions of Technical Services is, “How do I solder to
stainless steel?” Before discussing the “how to” aspect we need to first discuss the relation
between electronics and acid fluxes.
The use of acid fluxes for any electrical or electronic applications is not recommended!
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 can
not 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.
What can assemblers do if they need to make an electrical connection to stainless steel? There
are one of two ways to make the connection. First settle for a mechanical connection. Using a
screw or rivet is perfectly adequate for most applications where stainless steel is involved. The
second way involves plating the stainless plated with a more solderable material such as copper
or nickel. The assembler can then solder to the newly plated stainless steel with standard
electronic solder and fluxes.
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 fluxs in order to acheive 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. This concludes the discussion of soldering to stainless steel.
Source: David Scheiner – Technical Services Kester Solder Co.