Soldering and brazing of copper and copper alloys Contents 1. Introduction 4 5. Quality assurance 47 2. Material engineering fundamentals 9 6. Case studies 48 2.1. Fundamentals of copper and copper alloys 9 6.1 Hot-air solder levelling of printed circuit boards 48 2.2 Filler materials 10 6.2 Strip tinning 49 2.2.1 Soft solder 11 6.3 Fabricating heat exchangers from copper 49 2.2.2 Brazing filler metals 13 6.4 Manufacture of compact high-performance 2.3 Soldering or brazing pure copper 16 radiators from copper 49 2.4 Soldering / brazing copper alloys 18 2.4.1 Low-alloyed copper alloys 18 7. Terminology 50 2.4.2. High-alloyed copper alloys 22 8. Appendix 51 3. Design suitability for soldering/brazing 26 References 57 4. Soldering and brazing methods 29 Index of figures 58 4.1 The soldering/brazing principle 29 4.2 Surface preparation 30 Index of tables 59 4.3 Surface activation 32 4.3.1 Fluxes 33 4.3.2 Protective atmosphere / Shielding gases 35 4.4 Applying the solder or brazing filler metal 36 4.5. Soldering and brazing techniques 37 4.5.1 Soldering with soldering iron 38 4.5.2 Dip bath soldering or brazing 38 4.5.3 Flame soldering or brazing 40 4.5.4 Furnace soldering or brazing 40 4.5.5 Electric resistance soldering or brazing 43 4.5.6 Induction soldering or brazing 44 4.5.7 Electron beam brazing 45 4.5.8 Arc brazing 45 4.5.9 Laser beam soldering or brazing 46 2 | KUPFERINSTITUT.DE List of abbreviations Abbreviations Nd:YAG laser Neodymium-doped yttrium aluminium garnet laser SMD Surface-mounted device PVD Physical vapour deposition RoHS Restriction of (the use of certain) Hazardous Substances EG Europäische Gemeinschaft EC European Community MIG Metal inert gas process TIG Tungsten inert gas process DVGW German Technical and Scientific Association for Gas and Water [Deutsche Vereinigung des Gas- und Wasserfaches] Chemical elements and compounds Ag Silver Al Aluminium Ar Argon Be Beryllium C Carbon CO2 Carbon dioxide Cr Chromium Cu Copper H2 Hydrogen H2O Water HF Hydrofluoric acid Mn Manganese Ni Nickel O2 Oxygen P Phosphorus Pb Lead S Sulphur Sb Antimony Si Silicon Sn Tin Te Tellurium Zn Zinc Zr Zirconium KUPFERINSTITUT.DE | 3 1. Introduction Copper is a material that has been used by man for thousands of years because of its special properties. As a native metal, i.e. one that is also found naturally in its pure me- tallic form, copper was used early in human history because of its good malleability and formability and because of its colour. Copper thus became man’s first working metal. With increasing industrialisation, other technique to be used, the choice of filler This booklet aims to reflect the current properties of copper became important, material and any preparative or state of soldering and brazing copper and such as its excellent electrical and thermal after-treatment procedures, need to be copper alloys in industrial applications, but conductivity and its resistance to carefully selected on the basis of the does not claim to be complete. As research atmospheric corrosion, and its generally materials to be joined. The factors that and development work in this field is high resistance to chemical attack. influence the solderability or brazeability of continuing, enquiries should be directed to Copper can form alloys with many a material are shown in figure 1 and need the German Copper Institute or corre- different metals and a large numbers of to be taken into account both individually sponding organisations. alloy systems are now available that enable and in combination. mechanical and technological properties, A component is considered solderable or such as hardness, tensile strength, yield brazeable if the parent material is suitable strength, chemical resistance, resistance to for soldering or brazing, and one or more wear, to be modified in a controlled way. soldering or brazing techniques can be If their particular physical and mechanical applied, and if the parts to be joined are characteristics are taken into account, designed so as to facilitate the soldering/ copper and the majority of copper alloys brazing process and to ensure that the show a high degree of solderability or soldered/brazed part is safe under the brazeability. Fabrication process variables, conditions in which it is to be used [1]. such as the particular soldering or brazing 4 | KUPFERINSTITUT.DE Chemical and metallurgical Physical properties properties Mechanical properties · Chemical composition · Wettability · Strength and formability · Oxidation behaviour · Solidus temperature · Residual stresses · Corrosion behaviour · Thermal expansion · Diffusion and solubility characteristics · Thermal conductivity, · Ability to undergo precipitation heat treatment specific heat capacity · Microstructure Material suitability for soldering/brazing Solderability or brazeability of a workpiece Manufacturing suitability Design suitability for for soldering/brazing soldering/brazing · Dimensional stability of the parts to be joined · Location of soldering/brazing seam or zone · Condition of surfaces · Cross-sectional ratios · Surface coating · Notch effect · Shape and size of the assembly gap or · Seam irregularities the gap/joint to soldered or brazed · Static or dynamic loads · Stresses in the region of the soldered/brazed joint · Location of inserted solder/filler metal and air vents · Loading rate · Fastening of parts to be joined · Loading temperature · Solder or brazing filler metal · Load-transfer medium · Materials and methods of oxide removal · Soldering or brazing cycle · Soldering/brazing rate · Post annealing treatment · Joint clean-up · Joint testing Figure 1 – Definition of solderability/brazeability (see [2]) KUPFERINSTITUT.DE | 5 Like welding, soldering and brazing are Soldering and brazing do not involve any The following points should, however, be important methods of thermally joining melting of the parent material, i.e. of the noted: the strength of a soldered or brazed materials, typically metals. As it is the surfaces to be joined. Instead, the joint is typically not as great as that of the resulting joint – irrespective of the method workpieces are joined by introducing an parent material; the parent metal and the used to make it – that ultimately deter- additional molten metal, the ‘filler metal’, solder/braze metal have different chemical mines the properties of the part being possibly in combination with a flux and/or potentials; there is a risk of chemical fabricated, the two methods are classified in a protective gas atmosphere [4]. corrosion due to the presence of flux (see e.g. [3]) in terms of the chemical residues; design restrictions may be nature of the joint, the chemical composi- Some of the advantages of soldering or relevant because of factors such as narrow tion of the parent metal (or metals) and brazing compared to other joining soldering/brazing gaps and tight dimen- the type of filler material used, if any. Both methods are: [5] sional tolerances at the joint. Extensive welding and soldering/brazing lead to the · soldering/brazing enables dissimilar preparatory and after-treatment proce- formation of a metallic joint, however the materials to be joined; dures are often required, such as degreas- chemical composition of these joints differ. ing, etching, removal of flux residues, etc. [6]. Whereas a welded joint has the same · as less heat is applied in the joining The related joining techniques of soldering chemical composition as that of the two process, soldered or brazed parts tend and brazing are distinguished in the identical parent metals being joined, the to exhibit greater dimensional accuracy DIN ISO 857-2 standard by the liquidus use of a filler alloy in a soldering or brazing and less distortion; temperature of the filler metal used. In procedure means that the soldered or · multiple soldered/brazed joints can soldering, the liquidus temperature of the brazed joint has a different chemical be created on a single workpiece in a filler metal is below 450 °C; in brazing it is composition to that of the parent single operation; above 450 °C. Up until February 2007, materials. A soldered or brazed joint · intricate assemblies can be soldered/ high-temperature brazing (at temperatures comprises the heat-affected parent brazed without damage; above 900 °C) was defined in the earlier materials, the diffusion/transition phase and now withdrawn DIN 8505 standard. · soldered/brazed joints exhibit good and the solder/braze metal. The solder Today, high-temperature brazing is thermal and electrical conductivity; and metal or braze metal can be formed by the classified simply as brazing. action of heat either with or without a · as soldering/brazing directs less heat filler material. into the joint than welding, there is less residual stress and distortion in the component. Brazed joint Brazing seam Ag72Cu28 brazed metal Brazed metal zone Cu Ag Diffusion zone Heat-affected zone Figure 2 – Example of a brazed copper-silver joint. (Note that no filler metal was used; the alloy Ag72Cu28 is formed by diffusion during the brazing process.) 6 | KUPFERINSTITUT.DE Both physical and chemical processes are Heat is applied to melt the filler metal and σ involved in soldering/brazing. Soldering/ any flux being used. The method used to 1,2 1 brazing joints are created through surface heat the parts to be joined will depend on chemical reactions and diffusional processes the type of join to be created (see ϑ σ1,3 σ2,3 2 of the liquid filler metal and the solid parent Section 4.3). Fluxes serve to activate the material. The soldering/brazing mechanism surfaces to be joined. The molten filler comprises the following steps: [7] metal will only be able to wet the surfaces 3 1. Heating of the parts to be joined to be joined if they are clean and free from oil, grease and other surface deposits.