26.4 Case study 3: Joining Metals by Soldering and Brazing We might think it is always a good thing
We might think it is always a good thing to have a protective oxide film on the surface of a metal. But not always—if you want to join metals together by soldering, the protective film is a problem.
For example, we often need to “soft” solder copper together—copper wires to copper tracks in PCBs, copper tubes and fittings together in plumbing installations. Traditional soft solder is an alloy of lead and tin, with a melting point between 184 and 327°C, depending on the percentages of Pb and Sn (there is pressure to replace the Pb in soft solders because of possible health risks).
Even at room temperature, a copper surface has a very thin layer of oxide, and when the copper is heated to the melting temperature of the solder, the oxide grows. In order to solder copper parts together successfully, it is essential to have metal-to-metal contact between the solidified solder and the copper. The test is whether the solder—when molten—“wets” the copper surface. The solder will not wet the copper if there is oxide on the surface of the copper.
Figure 26.4 shows a piece of copper, which was heated to the melting point of soft solder, and then had soft solder melted on top of it. You can see that the solder has “balled-up” on the copper surface. It has not spread out over the surface, because the solder has not “wetted” the copper. After cooling the copper back down to room temperature, you will find that it is possible to lever the ball of solder off the copper—there has not been metal-to-metal contact for the solder to stick to the copper.
So the problem reduces to: “how do we get rid of the oxide layer on the copper before we apply the molten solder, and how do we make sure that the oxide cannot regrow while we are doing the soldering?”.
Figure 26.5 shows the same test done on a copper component which was cleaned with abrasive paper, and then coated with flux. The flux is a soft paste of natural resin extracted from pine trees, which melts when the copper is heated up, and spreads out over the surface. The flux contains resin acids, which “dissolve” the oxide film at the soldering temperature, and it also acts as a barrier to oxygen, preventing the oxide from regrowing. You can see that the solder has wetted the copper surface. The solder is firmly attached to the copper, and the joint has considerable strength.
The same issues arise when you try to soft solder other metals. Some, like copper alloys, mild steel, zinc-coated steel, and tin-coated steel, solder well—although different fluxes may be used to get the best results (for example zinc chloride solution—traditionally referred to as “tinner's fluid”). But many other metals—such as aluminum, chromium, stainless steel (chromium again), magnesium, titanium, and beryllium—are difficult (or impossible) to soft solder in air, because their oxide films are so stable (mechanically and chemically). Techniques include using aggressive fluxes, and disrupting the oxide film mechanically while soldering, using a high-frequency transducer.
For high-strength and/or high-temperature applications, metals are soldered with hard solders (often referred to as brazing). These typically melt in the range 610–950°C, depending on the alloy composition. Some are based on silver, and can contain exotic elements like palladium, in which case they are very expensive. At such high temperatures, oxidation is much faster. However copper, copper alloys, and carbon steel are easy to hard solder if aggressive fluxes, such as borax (sodium borate), are used.
Of course, a great way to stop oxides forming is not to have any oxygen present at all. This principle is used in vacuum brazing. The parts to be brazed are cleaned mechanically and assembled, and then preformed rings or foils of hard solder are put in place. The assembly is placed in the vacuum furnace, the pressure is reduced to zero, and the furnace is heated up to the temperature needed to melt the brazing alloy. When the alloy melts, it wets the surfaces of the parent metal, and is sucked into the joints by capillary action. This produces a very fine and strong joint line, with none of the defects (such as trapped air or flux) that can be present in air-brazed joints. For “difficult” metals, hydrogen gas can be fed into the vacuum chamber to actively reduce any oxide (MO + H2 = M + H2O).
Figure 26.6 shows an example of a component fabricated by vacuum brazing—the lid of a small pot containing an electronic device. This was press formed from thin Kovar sheet (a controlled expansion alloy with composition 54Fe29Ni17Co and melting point 1450°C). Kovar pins were brazed into holes in the lid as shown, using Pallabraze 950—an alloy with composition 54Ag25Pd21Cu and melting range 901–950°C. The furnace temperature was set at 1000°C. Figure 26.6(a) shows the assembly before heating, with the pin in position and a tiny preformed ring of Pallabraze around the pin. There is a small clearance between the pin and the hole (typically a few thou) so the brazing alloy can flow through the joint. When the furnace comes up to temperature, the ring melts, wets the oxide-free Kovar surfaces, and is sucked into the joint (Figure 26.6(b)).
The diagrams also show the fabrication of a glass-to-metal seal between the lid and some of the other pins (designed to electrically insulate the lid from the pins). Small beads of low-expansion borosilicate glass were placed between the pins and the lid, and when the furnace came up to temperature the glass flowed and wetted the oxide-free metal surfaces. The brazing alloy was chosen so that both the brazing and the glass sealing could be done in one single operation.