Solders for Potable Water
The U.S. Federal Clean Drinking Water Act limited the amount of lead (Pb) in solder to less than 0.2% for potable water systems. Shortly afterward, a new solder alloy was introduced to the plumbing industry for joining copper (Cu) potable-water lines. The alloy, which is tin (Sn) rich, has Cu in the 3.0 - 5.0% range and silver (Ag) in the 0.3 - 0.7% range.
The Sn-Ag-Cu alloy has joined the ranks of the Sn-Ag alloys, Sn94 and Sn96 as listed in ASTM B-32, and as a replacement alloy for the 50Sn - 50Pb solder. Use of the non-Pb-bearing alloys has grown beyond simply joining water conduits to include the construction of ice-making machines, refrigerator ice dispensers, drinking fountains and juice-dispensing machines. Many manufacturers have discontinued use of Pb-bearing solders, or those solders containing antimony (Sb) and bismuth (Bi), even on soldered assemblies that do not come in direct contact with water products, such as the heat exchanger in refrigeration units.
Regardless of the alloy, joint integrity is directly related to proper soldering. Process development must address thermal management and flux application, as well as pre- and post-assembly cleaning processes. Whether the joint must satisfy leak tightness, thermal conductivity or strength requirements, an objective of the soldering process is to optimize solder wetting and spreading, to promote a minimum amount of porosity (unbonded areas) in the joint clearance.
The optimum joint clearance present at the time that the solder turns to liquid must compensate for the thermal expansion coefficients of the two mating materials, the required depth of solder flow (travel) and the uniformity of heat application in order to realize the above objective. A simple rule of thumb is: at soldering temperature, the joint clearance should be approximately 0.003 - 0.005" (0.076 - 0.127mm). The joint clearance may be modified from this general rule depending on solder chemistry, joint length, base material or other factors which may influence design criteria. In automatic soldering, fixture design is very important to control the joint clearance because of nonintervention by the operator. The fixture design is particularly significant toward the success of automatic processes, since their purpose is to produce a large number of solder joints with a minimum of scrap and rework.
Pre-assembly cleaning and fluxing methods are important for the manufacture of a quality, soldered assembly. Surfaces must be cleaned of organic residues, paints and oxide scales in order for the solder to wet and spread over them. Note that solder fluxes are not detergents, solvent or degreasing agents, and the flux cannot penetrate heavy contaminate layers to react with the underlying faying surfaces.
Commercial soldering fluxes are normally designated as: rosin-based, organic acid fluxes and inorganic acid fluxes. The principle ingredient in rosin-based fluxes is white-water rosin (a derivative of pine tree sap). Organic acid fluxes are composed of compounds such as lactic acid or one of the citric acids. Inorganic acid fluxes contain zinc chloride, ammonium chloride, hydrochloric acid, sulfuric acid or nitric acid. The standard flux forms are liquid solutions, pastes and dry salts. For most wire forms of the Sn-Ag and Sn-Ag-Cu solders, the wire has a core of the suitable flux to allow ease of application with a wire feeder and eliminate the need for separate fluxing.
The selection of a flux is driven by the base material and the type and thickness of surface oxide to be removed. For joining Cu piping, relatively thin tarnishes are eliminated by a rosin-based flux. On the other hand, oxides on the surfaces of stainless steel and aluminum, although thin, are very tenacious (i.e., resistant to chemical attack), thus requiring an aggressive flux such as an inorganic acid. Note: The potential exists for latent corrosion of the base metal with fluxes of greater aggressiveness if the flux residues are not completely removed.
The role of flux in supporting wetting by the solder stems from the reduction of surface oxides on the base metal faying surfaces and molten solder surface, and also from two other functions: 1) its capacity to prevent further oxidation of the base metal surfaces, and 2) its ability to reduce the surface tension of the liquid solder. A lower-solder surface tension will improve the capillary-flow properties of the solder. Optimizing surface wetting and/or capillary flow by the solder, through proper selection of a flux, will help minimize void formation and the loss of strength due to unbounded joint surfaces.
Improper heating is a major contributor to solder joints having marginal or poor quality, whether the joints are made by hand or automated processes. The primary problem is inability to realize a uniform temperature at all surfaces that are sufficiently high for the solder to flow freely for completion of the joint. The most-overlooked factor affecting heat distribution is heat sinking. This arises from differences in the physical mass of the two structures that are to be joined. The larger mass will draw heat from the joint area faster than will the smaller part, thus lagging in temperature rise. A second source of heat sinking is the thermal conductivity of the base metals, when two parts of similar nominal physical dimensions, but having vastly different thermal conductivities, are to be joined. The material with the greater thermal conductivity will draw heat away from the joint area much faster than the material of lower thermal conductivity.
Uniform cooling after soldering is very important, especially to automatic processes, in which the assembly is heated and cooled rapidly to meet the demands of higher-volume production rates. Thermal mismatch stresses due to non-uniform cooling (compounded by elevated mechanical loads) may cause the solder to crack. The rule of thumb is to achieve uniform cooling as much as possible until the joint temperature has dropped to at least 50% of the solder solidus or melting temperature, then more rapid cooling can be applied to the joint. For the Sn-Ag and Sn-Ag-Cu alloys, this temperature limit would be 200 - 250°F (93 - 121°C). Fixture design should be scrutinized so that cooling of the part while it is still attached to the fixture should not generate residual stresses that may cause deformation or cracking of the solder.
Post-assembly cleaning procedures target the removal of flux residues to prevent latent corrosion of the product while it is in service. Corrosion is a particular concern with Al and Mg parts, but should be addressed for all substrate materials. Most flux residues are hygroscopic, which means that, over a period of time, the flux will absorb moisture that supports potential corrosion activity. Also, flux residues that remain on the assembly after soldering may be detrimental to adhesion of subsequent surface coatings, such as paints or electroplated finishes.
Whether joining copper (Cu) potable-water lines by hand or automated processes, optimal solder joints begin with properly designed piece-parts and fixturing. Pre-assembly cleaning, solder alloy and flux selections, as well as the proper heating schedules, are critical factors for complete flow of the solder onto surfaces or into joint clearances for a void-free joint. Post-assembly cleaning procedures ensure that the product is free of flux residues to prevent corrosion.
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