Soldered joints depend on capillary action drawing free-flowing molten solder into the gap between the fitting and the tube. Flux acts as a wetting agent and, when properly applied, permits uniform spreading of the molten solder over the surfaces to be joined.
The selection of a solder depends primarily on the operating pressure and temperature of the system. Consideration should also be given to the stresses on joints caused by thermal expansion and contraction. However, this may not be necessary when a tube length is short or when an expansion loop is used in a long tube run. In such cases, the stresses caused by a temperature change are usually insignificant.
Rated internal working pressures for solder joints made with copper tube using 50-50 tin-lead solder (ASTM B32 Alloy Sn50), 95-5 tin-antimony solder (ASTM B32 Alloy Sb5), and several lead-free solders (ASTM B32 Alloy E and Alloy HB) are listed in Tables 14.3a-e.
The 50-50 tin-lead solder is suitable for moderate pressures and temperatures. For higher pressures, or where greater joint strength is required, 95-5 tin-antimony solder and alloys E and HB can be used. For continuous operation at temperatures exceeding 250°F, or where the highest joint strength is required, brazing filler metals should be used (see Table 14.4a).
Solder alloys listed in Section 1 of Table 1 Solder Compositions in ASTM B32, Standard Specification for Solder Metal, can be used to join copper tube and fittings in potable water systems. Solders containing lead at concentrations of greater than 0.2% are banned for potable water systems by the 1986 amendment to the Federal Safe Drinking Water Act (SDWA). Some state and local jurisdictions may allow the use of 50-50 tin-lead solder in some HVAC, drainage and other piping system applications.