Brazing and Soldering Alloys: When Welding Won't Work

## Brazing vs. Soldering vs. Welding The distinction is defined by temperature and base metal melting: - **Welding**: base metal melts and fuses with filler. Highest joint strength. - **Brazing**: filler melts above 450 °C but below the base metal solidus. Filler flows into the joint gap by capillary action. Joint strength can exceed the base metal strength in some configurations. - **Soldering**: filler melts below 450 °C. Joint strength is lower than brazing but sufficient for electrical connections, plumbing, and electronics assembly. Brazing and soldering do not melt the base metal, preserving its microstructure, temper, and dimensional precision. This makes them essential for joining thin-walled components, heat-sensitive assemblies, and dissimilar metal combinations that would form brittle intermetallics if fusion welded. ## Common Brazing Alloys ### Silver-Based (BAg Series) BAg alloys (AWS A5.8) are the most versatile brazing fillers. They wet and flow on steel, stainless steel, copper, brass, and nickel alloys. The silver content provides fluidity and a low liquidus temperature. | Alloy | Composition | Solidus/Liquidus (°C) | Typical Use | |-------|-------------|----------------------|-------------| | BAg-1 | 45Ag-15Cu-16Zn-24Cd | 607/618 | Carbide tool tips, HVAC | | BAg-5 | 45Ag-30Cu-25Zn | 663/743 | General purpose (Cd-free) | | BAg-7 | 56Ag-22Cu-17Zn-5Sn | 618/652 | Stainless steel, food equipment | | BAg-24 | 50Ag-20Cu-28Zn-2Ni | 660/705 | Carbide tools, wear parts | Cadmium-bearing alloys (BAg-1) offer the lowest brazing temperatures and best flow but are increasingly restricted due to cadmium fume toxicity. BAg-5 and BAg-7 are cadmium-free alternatives. ### Copper-Based (BCu Series) Pure copper (BCu-1) brazes at 1083–1100 °C and is used in hydrogen atmosphere furnace brazing of steel assemblies. It produces extremely strong joints (shear strengths above 250 MPa) and is inexpensive. Copper-phosphorus alloys (BCuP-5, 80Cu-15Ag-5P) are self-fluxing on copper and brass and are the standard filler for copper tube joints in refrigeration and plumbing. They must never be used on ferrous metals or nickel alloys because phosphorus forms brittle iron and nickel phosphides. ### Nickel-Based (BNi Series) BNi alloys braze at 980–1150 °C and are used for high-temperature service (jet engine components, heat exchangers). BNi-2 (Ni-7Cr-3B-4.5Si-3Fe) is the most common, used for Inconel, stainless, and cobalt alloy assemblies in vacuum or hydrogen atmosphere furnaces. The boron and silicon act as melting point depressants. ## Soldering Alloys ### Lead-Free Solders The RoHS directive effectively eliminated lead-based solders (Sn-37Pb, eutectic at 183 °C) from electronics. Major lead-free replacements: | Alloy | Melting Range (°C) | Use | |-------|-------------------|-----| | SAC305 (Sn-3Ag-0.5Cu) | 217–220 | General electronics | | Sn-0.7Cu | 227 | Wave soldering | | Sn-3.5Ag | 221 | High-reliability joints | | In-48Sn | 118 | Low-temperature, thermal interfaces | SAC305 has become the industry standard for surface-mount electronics. It has adequate wetting on copper pads with appropriate flux, though it requires reflow temperatures 30–40 °C higher than the old Sn-Pb eutectic. ### Plumbing Solders Lead-free plumbing solders (mandated for potable water systems since 1986 in the US) are typically Sn-95/Sb-5 or Sn-97/Cu-3, melting at 230–240 °C. They are used with acid flux (zinc chloride) or water-soluble flux for copper tube joints in domestic water systems. ## Joint Design for Brazing Brazed joint strength depends critically on the gap between the parts: - **Optimal clearance**: 0.025–0.125 mm (0.001–0.005 in.) for most silver brazing alloys. At this gap, capillary action draws filler through the entire joint, and the thin filler layer is constrained by the base metal, achieving shear strengths of 300–450 MPa. - **Too tight** (under 0.013 mm): filler cannot enter the joint; voids and unfilled areas result. - **Too wide** (over 0.25 mm): capillary force is insufficient; filler pools at the bottom of the joint; strength drops sharply. Lap joints are preferred over butt joints because the overlap area can be sized to achieve any desired load capacity. A lap length of 3–4 times the thinner member thickness is a common starting point. ## Flux and Atmosphere Brazing flux (typically borax-based pastes, AWS FB3-A or FB3-C) dissolves oxides and promotes wetting. Flux residue must be removed after brazing to prevent corrosion. Controlled-atmosphere brazing eliminates flux entirely: hydrogen, vacuum, or inert gas atmospheres prevent oxidation during the brazing cycle. Vacuum brazing at 10⁻⁴ to 10⁻⁵ mbar is standard for aerospace components where flux residue is unacceptable.