Learn Brazing, Soldering and Lower Temperature Joining Explained – Brazing (Filler Metals, Fluxes, Joint Strength, Methods), Soldering (Solder Types, Fluxes, Solderability, Techniques), Applications and Design Guidelines

Introduction:

Most joining processes rely on heating the mating surfaces of components to elevated temperatures to cause fusion and bonding at the joint. However, this approach presents significant challenges when working with materials that cannot withstand high temperatures such as electronic components. Similarly, parts that are fragile, intricate or made of two or more materials with very different characteristics, properties, sizes, thicknesses and cross sections require alternative methods. Brazing and soldering involve placing filler metals in or supplying them to the joint and melting them with an external heat source. Upon solidification, a strong joint is obtained. The distinction between these processes is based on temperature with soldering requiring lower temperatures than brazing and producing joints with much lower strength. Adhesive bonding an ancient method of joining parts with animal derived glues typically employed in bookbinding, labeling and packaging has developed into an important technology for metallic and nonmetallic materials. This process has wide application in numerous consumer and industrial products, as well as in the aircraft and aerospace industries. Bonding materials such as thermoplastics, thermosets, ceramics, and glasses either to each other or to other materials presents various challenges. While all the joining methods described are of a permanent nature, many applications require components to be taken apart for replacement, maintenance, repair or adjustment. When joints must be non permanent yet as strong as welded joints, mechanical fastening with bolts, screws, nuts or a variety of other fasteners provides the obvious solution. Brazing and soldering provide effective solutions for joining materials that cannot withstand the high temperatures of fusion welding. Both processes rely on capillary action to draw filler metals into joints producing strong bonds without melting the base materials. Brazing with its higher temperature range offers greater joint strength while soldering operates at lower temperatures and is particularly suited for electronics.

Brazing:

Brazing is a joining process in which a filler metal is placed between the faying surfaces to be joined or at their periphery, and the temperature is raised sufficiently to melt the filler metal but not the components, as would be the case in fusion welding. This makes brazing a liquid solid state bonding process. Upon cooling and solidification of the filler metal, a strong joint is obtained. Filler metals used for brazing typically melt above 450°C which is below the melting point of the metals to be joined. The term derives from the word brass, an archaic word meaning to harden and the process was first used as far back as 3000 to 2000 B.C. In a typical operation, a filler metal wire is placed along the periphery of the components. Heat is then applied by various external means, melting the braze metal and by capillary action filling the closely fitting space at the interfaces. In braze welding, filler metal, typically brass is deposited at the joint by a technique similar to oxyfuel gas welding but the base metal does not melt. The main application of braze welding is in repair work typically on parts made of cast steels and irons. Because of the wider gaps between the components more braze metal is used than in conventional brazing. In general dissimilar metals can be assembled with good joint strength. Intricate, lightweight shapes can be joined rapidly and with little distortion.

Filler Metals:

Several filler metals are available with a range of brazing temperatures. Unlike those for other welding operations, filler metals for brazing generally have a composition significantly different from those of the metals to be joined. They are available in a variety of shapes such as wire, rod, ring, shim stock and filings. The selection of the type and composition is important to avoid embrittlement by grain boundary penetration of liquid metal, the formation of brittle intermetallic compounds and galvanic corrosion in the joint. Because of diffusion between the filler metal and the base metal, the mechanical and metallurgical properties of a joint can change as a result of subsequent processing or during service life. For example when titanium is brazed with pure tin as the filler metal, it is possible for the tin to diffuse completely into the titanium base metal when subjected to subsequent aging or heat treatment causing the joint to no longer exist.

Fluxes and Joint Strength:

The use of a flux is essential in brazing as it prevents oxidation and removes oxide films. Brazing fluxes generally are made of borax, boric acid, borates, fluorides and chlorides. Wetting agents may be added to improve both wetting characteristics and capillary action. Surfaces must be clean and free from rust, oil and other contaminants to ensure proper wetting and maximum bond strength. Sandblasting may also be used. Because they are corrosive, fluxes must be removed after brazing, usually by washing with hot water. The strength of a brazed joint depends on joint clearance, joint area and the nature of the bond at the interfaces. Joint clearances typically range from 0.025 to 0.2 mm. The smaller the gap, the higher the shear strength which can reach 800 MPa by using brazing alloys containing silver. There is an optimum gap for achieving maximum tensile strength. Because clearances are very small, surface roughness becomes important and surfaces must be cleaned chemically or mechanically to ensure full capillary action.

Brazing Methods:

The heating methods used in brazing identify the various processes.

Torch Brazing uses oxyfuel gas with a carburizing flame. Brazing is performed by heating the joint with the torch and depositing the rod or wire. Suitable part thicknesses are typically in the range from 0.25 to 6 mm. This method is difficult to control and requires skilled labor though it can be automated with multiple torches and is also used for repair work.

Furnace Brazing involves cleaning and preloading parts with brazing metal then placing the assembly in a furnace for uniform heating. Furnaces may be batch type for complex shapes or continuous type for high production runs of small parts. Vacuum furnaces or neutral atmospheres are used for reactive metals. Skilled labor is not required and complex shapes can be handled.

Induction Brazing uses induction heating by high frequency AC current. Parts are preloaded with filler metal and placed near induction coils for rapid heating. Unless a protective atmosphere is used, fluxes are generally required. Part thicknesses usually are less than 3 mm. This method is suitable for continuous brazing.

Resistance Brazing uses the electrical resistance of the components as the heat source with electrodes as in resistance welding. Parts typically with thicknesses of 0.1 to 12 mm are either preloaded with filler metal or supplied externally. The process is rapid heating zones can be confined to small areas, and it can be automated for uniform quality.

Dip Brazing is carried out by dipping assemblies into either a molten filler metal bath or a molten salt bath at a temperature just above the melting point of the filler metal. All component surfaces become coated. Metal bath dip brazing is used for small parts such as sheet, wire and fittings usually less than 5 mm in thickness or diameter. Molten salt baths which act as fluxes are used for complex assemblies of various thicknesses. Depending on part and bath size as many as 1000 joints can be made at one time.

Infrared Brazing uses a high intensity quartz lamp as the heat source. It is suitable for very thin components, usually less than 1 mm thick including honeycomb structures. Radiant energy is focused on the joint and brazing can be carried out in a vacuum. Microwave heating can also be used.

Diffusion Brazing is performed in a furnace where with proper control of temperature and time, the filler metal diffuses into the faying surfaces. Brazing time may range from 30 minutes to 24 hours. This process is used for strong lap or butt joints and difficult operations. Because diffusion rate does not depend on thickness, part thicknesses may range from foil to as much as 50 mm.

High energy Beams such as electron beam or laser beam heating may be used for specialized, high precision applications with high temperature metals and alloys.

Braze Welding uses an oxyacetylene torch with an oxidizing flame and filler metal is deposited at the joint rather than drawn in by capillary action resulting in considerably more filler metal used. Temperatures are lower than in fusion welding so distortion is minimal. Flux is essential and the process is used mainly for maintenance and repair though it can be automated for mass production.

Soldering:

In soldering, the filler metal melts at a relatively low temperature. As in brazing, the solder fills the joint by capillary action between closely fitting components. Two important characteristics of solders are low surface tension and high wetting capability. Heat sources are usually soldering irons, torches or ovens. The word solder derives from the Latin solidare meaning to make solid. Soldering with copper gold and tin-lead alloys was first practiced as far back as 4000 to 3000 B.C.

Types of Solders and Fluxes:

Solders melt at a temperature that is the eutectic point of the solder alloy. Traditionally, solders have been tin lead alloys in various proportions. For example, a solder of 61.9% Sn-38.1% Pb composition melts at 188°C whereas tin melts at 232°C and lead at 327°C. For special applications and higher joint strength, especially at elevated temperatures, other compositions include tin zinc, lead silver, cadmium silver and zinc aluminum alloys. Due to the toxicity of lead, lead free solders are being developed with candidates including silver, indium and bismuth eutectic alloys in combination with tin. Three typical compositions are 96.5% Sn-3.5% Ag, 42% Sn-58% Bi and 48% Sn-52% In. However none are suitable for every application. Fluxes are used in soldering for the same purposes as in brazing and are generally of two types: inorganic acids or salts such as zinc ammonium chloride solutions, which clean rapidly but require thorough water washing to avoid corrosion and noncorrosive resin based fluxes used typically in electrical applications.

Solderability and Techniques:

Solderability may be defined similarly to weldability. Special fluxes have been developed to improve solderability. As a guide, copper, silver and gold are easy to solder. Iron and nickel are more difficult. Aluminum and stainless steels are difficult because of their thin, strong oxide films. Steels, cast irons, titanium, magnesium, ceramics and graphite can be soldered by first plating them with suitable metallic elements to induce interfacial bonding. A common example is tinplate, steel sheet coated with tin used for food cans. Soldering techniques somewhat similar to brazing methods include torch, furnace, iron soldering, induction, resistance, dip and infrared soldering.

Ultrasonic Soldering uses a transducer to subject molten solder to ultrasonic cavitation removing oxide films and eliminating the need for flux.

Reflow Soldering uses solder pastes, which are solder metal particles held together by flux, binding and wetting agents. The semisolid paste is placed directly onto the joint often via stenciling for circuit boards. Surface tension helps keep packages aligned. In reflow, the product is heated in a controlled sequence: solvents evaporate, flux activates and acts, components preheat, solder particles melt and wet the joint and the assembly cools slowly to prevent thermal shock and fracture. There are several process variables at each stage requiring careful control.

Wave Soldering is a common technique for attaching circuit components. A standing laminar wave of molten solder is generated by a pump. Preheated and prefluxed circuit boards are conveyed over the wave. The solder wets exposed metal surfaces but not polymer packages or coated boards. An air knife blows excess solder away to prevent bridging. For surface mount packages, they must be adhesively bonded to the board before soldering usually by screening epoxy placing components, curing, inverting the board and then performing wave soldering.

Applications and Design Guidelines:

Soldering is used extensively in the electronics industry. However, because soldering temperatures are relatively low, a soldered joint has very limited utility at elevated temperatures. Since solders generally do not have much strength, the process cannot be used for load-bearing structural members. Joint strength can be improved significantly by mechanical interlocking. Soldering can join various metals of different thicknesses. While copper and precious metals are easy, aluminum and stainless steels are difficult due to oxide films though special fluxes can help. Manual operations require skill and are time consuming but automated equipment enables high speeds. Design guidelines for soldering are similar to brazing. Due to the low strength of solders, large contact surfaces are important for developing sufficient joint strength. Since faying surfaces generally would be small, solders are rarely used to make butt joints.