Material Selection and Process Development: Key Consideration in Solder Joining

It has been more than 20 years since the passage of the Federal Clean Drinking Water Act, which limited the amount of lead (Pb) in solder to less than 0.2% when the solder is used for potable water systems. Shortly after that legislation was passed, a new solder alloy was introduced to the plumbing industry for the joining of 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 previously in such applications. Use of the non-Pb-bearing alloys has grown significantly over the years 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 the water or juice product, such as the heat exchanger in refrigeration units.

Regardless of the particular alloy composition, the integrity of the joint is directly related to proper soldering processes. Process development must address thermal management, flux application, as well as pre- and post- assemblies cleaning processes. However, the best assembly practices are only effective on properly designed joints. In solder joint design, 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 so as to promote a minimum amount of porosity (unbonded areas) in the joint clearance between the two faying surfaces.

The optimum joint clearance present at the time that the solder turns 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 if the heating is uniform, then the solder is drawn to the closest fitting surfaces. If the joint clearance is uniform, then the solder is hottest surfaces. In automatic soldering, fixture design for a part is very important toward control of the joint clearance because of nonintervention by the operator during the actual soldering process. The design of the fixturing 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 when it comes to the manufacture of a quality, soldered assembly. Cleaning will only briefly be touched upon in this article; nevertheless, its importance cannot be overstated. The mating surfaces to be joined must be cleaned of organic residues, paints and oxide scales in order for solder to wet and spread over them. The need for pre-assembly cleaning can be best understood by recognizing that solder fluxes are not detergents, solvent or degreasing agents. In the absence of a proper pre-cleaning procedure, the risk is extremely high for a solder joint of marginal-to-poor quality, because the flux cannot penetrate heavy contaminate layers in order to react with the underlying faying surfaces.

Types of Fluxes

Commercial soldering fluxes are normally designated by one of the following types: 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 such compounds 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. such so-called "fluxed core wire" allows ease of application with a wire feeder and eliminates the need for a separate fluxing operation.

Selecting a Flux

The selection of a flux is primarily driven by the base material and, specifically, by the type and thickness of surface oxide that is to be removed. In the case of joining Cu piping, relatively thin tarnishes are effectively eliminated by the use of a rosin-based flux. On the other hand, oxides on the surfaces of stainless steel and aluminum, although tending to be thin, are very tenacious (i.e., resistant to chemical attack), thus they require a very aggressive flux such as an inorganic acid. It must also be understood that the potential exists for latent corrosion of the base metal with fluxes of greater aggressiveness in the event that the flux residues are not completely removed from the part manufactured.

The role of the flux toward support of wetting by the solder stems not only from the reduction of surface oxides on the base metal faying surfaces and molten solder surface, but also from two other functions. Those functions are - 1) its capacity to prevent further oxidation of the base metal surfaces during the soldering operation 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 the proper selection of a flux, is an important step that, along with the correct solder processing steps, will minimize void formation and the strength loss due to unbounded joint surfaces.

Improper heating is a major contributor to solder joints having marginal or poor quality. This point is true, irrespective of whether the joints are made by hand or automated processes. The primary problem appears to be the inability to realize a uniform temperature at all surfaces that are sufficiently high for the solder to flow freely for completion of the joint. There are many factors that affect heat distribution in the joint region. By and large, the most overlooked factor is that of heat sinking. Heat sinking arises from differences in the physical mass of the two structures that are to be joined together. 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 base metals comprising the joint; this situation occurs when two parts of similar nominal physical dimensions, but having vastly different thermal conductivities, are to be joined together. The material with the greater thermal conductivity will draw heat away from the joint area much faster than the material of lower thermal conductivity.

The Importance of Uniform Cooling

Cooling of the joint after soldering is very important. This point is especially pertinent 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 (which may be compounded by elevated mechanical loads) may cause the solder to crack. A 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 93-121°C (200-250°F). Fixture design should be scrutinized so that cooling of the part while it is still attached to the fixture should not generate residual strsses that may cause deformation or cracking of the solder in the joint.

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 always 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 the adhesion of subsequent surface coatings such as paints or electroplated finishes.

In summary, whether a product is soldered by hand or by 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 (heating rate, re-flow spike and cooling rate) are the critical factors for complete flow of the solder onto surfaces or into joint clearances for a void-free solder joint. The post-assembly cleaning procedure must ensure that the product is free of flux residues to prevent unwanted corrosion while it is in service.