People often come to EWI and want to know how to solder certain parts. As we ask questions about the end application, we learn that the ‘soldered’ assembly will be exposed to temperatures up to 400°C (752°F), and will be subjected to stresses as high as 10 ksi at room temperature. This is not a soldering application. It may be suitable for brazing, which by definition includes the use of a filler metal which melts above 450°C (842°F); soldering occurs below this temperature. In both processes, the base metals do not melt – only the filler metal melts. Typical strengths of solder joints are below 10 ksi, while braze joint strength, in a properly-designed and fabricated joint, is equal to base material strength.
Some alloy systems bridge the solder/braze temperature limit. For example, alloys in the Zn-Al system can be used to join a number of ferrous and non-ferrous alloys; the eutectic alloy (94Zn-6Al) has a liquidus of 381°C (718°F), putting it clearly in the ‘solder’ camp. Another popular alloy, 78Zn-22Al, is a brazing alloy, with a liquidus of 475°C (887°F). Means of applying heat to the joint for both soldering and brazing can be similar: torch; induction; furnace; resistance, laser,etc.
Recent work at EWI has included use of ultrasonic soldering for a process known as transient liquid phase bonding (TLPB), a diffusion-based process. In TLPB, or its close cousin partial transient liquid phase bonding (PTLPB), a series of interlayers is deployed between two substrates. Included in the interlayers is some form of melting point depressant, either as a distinct layer or as a constituent of the primary bonding layer. Interdiffusion or solid solution formation during thermal cycling followed by an isothermal hold enables formation of a joint which exhibits a remelting temperature well above the initial joining temperature, thus potentially enabling ‘brazing properties’ with a ‘soldering thermal cycle’.
The most common base materials joined by soldering, brazing or TLPB are metals, but these techniques and PTLPB are increasingly being applied to ceramics, composites, and semiconductors, and dissimilar joints among these material classes. Selection of a suitable process and filler metal includes asking questions such as “What will the service temperature and stress conditions be?”, “What temperature can the base material experience without unacceptable changes to its properties?”, “What filler metal will bond suitably to my base materials?” We even recently discussed the potential of soldering a metal to a polymer, very doable using ultrasonic soldering with the proper selection of materials.
Workhorse alloys for soldering include the tin-based alloys (historically Sn-Pb has been used extensively, but toxicity issues associated with Pb have brought about a focus on Pb-free solder formulations, which often include other low-melting constituents (e.g., Bi, In, Sb). Silver (Ag) and copper (Cu) are added to increase conductivity and improve flow properties of solders. Many brazing alloys historically contained Cd, now known as a toxin; braze alloys with melting-point depressants such as Zn and Sn have replaced many of the Cd-containing braze alloys. Silver- and nickel-base braze alloys are very widely used in applications ranging from small medical devices to jet engine components.
So back to the original question, what’s the difference between soldering and brazing? The filler metal melting point and application temperature are the difference. Practically, however, they are very similar processes, the selection of which depends on the materials and service temperatures involved, the required assembly strength, and economic factors.
If you would like assistance in selecting a suitable brazing or soldering process for your application, please contact, Kirk Cooper, by clicking here.