Soldering vs. Brazing

Soldering vs. Brazing
Aug 19,2020, 85
We receive many inquiries to silver solder, solder or braze components and many times there is confusion over this terminology and the various materials and processes used to bond metals, ceramic and/or glasses. This short article offers some clarification to the distinctions between soldering and brazing such that you can make informed decisions about your needs.

Brazing is a process where a molten metal is the joining agent (filler) between to materials where during the bonding process only the filler metal is melted. These molten fillers react with and adhere to the adjoining surfaces. AWS (American Welding Society) defines a material to be a braze filler when it melts over 450C°C (842°F). Example braze fillers include but are not exclusive to copper, copper-silver, Cu-P, and all copper alloys, Al-Si, NiCrBSi, Ni-P, FeCrBSi, gold, silver, palladium, etc.

Soldering is a related process to brazing and also employs molten metal fillers, with the exception (according to AWS definitions) that solder fillers melt below 450°C. Such fillers include lead-tin (Pb-Sn), Sn-Ag, Sn-Bi, Sn-Sb, Zinc, Zinc-Al, etc… these solders are used in electronics, plumbing, structural low temperature components, heat sinks and cold plates, sputter targets, etc.

Note there is some confusion over the term “silver solder”… in reality silver solder is a braze but this term has been adopted commercially since the fillers use copper-silver (Cu-Ag) alloys. These silver-solders are also associated with the term “hard solder” vs. soft solder… All “silver solders” are technically brazes since their melting temperatures are over 750°C (1382°F) and employ braze processes to make bonded components.

Is soldering or brazing more suitable? …. The answer is “it depends” on…

Strength requirements… brazed joints can be 3 – 10x the strength of soldered joints Corrosion resistance… solders are generally more susceptible to oxidation and degradation from chemicals and salt since the fillers are Sn, Zn or Pb based.

Temperature assemblies can be exposed to… solders melt from 100 – 250°C and are generally used in electronics and other temperature sensitive parts.

Thermal expansion… differing CTE assembly materials benefit from soldering since lower joining temperature lowers distortion upon cooling and “softer” filler metals permit CTE mismatch to be accommodated.

Cost… soldering is generally a lower cost process with the filler metals being less expensive and the lower temperatures processing reduced post joining clean-up lowers overall joining costs.

So, choose the processes and filler metals most suited to your assemblies and their expected service temperatures, remembering soldering is generally less expensive and less sensitive to thermal expansion mismatch.

When comparing soldering to brazing and their related filler alloys, one begins to see the processing changes made necessary by the significantly different processing temperatures (100 – 450°C) for solders and 450-1,600°C) for brazing. The higher temperatures needed to melt brazing fillers makes oxidation of the filler metals and the base materials (being joined) much more of a concern and problem. Since oxidation and the subsequent oxides formed interfere with wetting and adherence, oxidation must be minimized and a means to remove any formed oxides must be used. Chemical fluxes are commonly used and the most effective fluxes melt and flow just before the melting temperatures of the filler metal and are not thermally decomposed in the range of temperatures where the filler metals melt. For solder filler metals, fluxes are normally rosin based or low temperature acidic compounds that when melted can react with tin, lead, silver, copper or nickel oxides. For brazes, fluxes are normally organometallic salts, or higher temperature salts that when melted are acid and reduce the oxides forming on materials such as brass, steel, stainless steel and even aluminum. Normally to flux the more oxidation resistant materials (stainless steel and/or aluminum) very acidic and corrosive fluoride based acid are required. Soldering fluxes, as a result of their composition, are much less aggressive and generally less corrosive than the fluxes used in brazing.

Alternatively, brazing is also commercial done in furnaces… and those furnaces can either produce a “fluxing atmosphere” such as in cracked ammonia (reduces oxygen activity while reducing oxide scales formed in steel and copper based materials). Other furnaces exclude oxygen altogether by pumping the atmosphere out with vacuum pumps, then backfilling with inert or reducing gases (N2, Ar, or H2) pumping to high vacuum where the high vacuum really excludes oxygen and can in many metals reduce and/or evaporate the surface oxides on part. Thus “furnace brazing” many times is a preferred method over torch brazing due to temperature uniformity and part cleanliness after brazing.

Structural soldering (non-electronic) is generally not practiced in ovens since soldering temperatures are low. Soldering irons (hot metal tips), propane torches (in brazing MAP and even acetylene torches are used for their higher heating capacity), hot air guns and hot surface plates are used. In high volume soldering for electronics, batch or continuous flow (belt) furnaces are employed to re-flow solder pastes that consist of a decomposable organic carrier, a flux and the solder filler metal powder. When heated in a “reflow” oven, reducing gases or more inert gases such as N2 can also be used, however, if mildly acidic or no clean fluxes are uses, air solder reflow ovens can be used.

The bottom line, once you have the terminology down, is to choose the most compatible (technically and economically) filler metal (solder vs. braze) then select the most suitable process compatible with that filler metal in combination with the assemblies’ components.