Soldering and brazing are the processes of joining similar or dissimilar metals. There are three processes of joining the metals i.e., soldering, brazing, and welding. The major difference between them is that a huge amount of heat is required in the case of welding, a low amount of heat is needed in the case of brazing and a very low amount of heat is consumed in the case of soldering.
The origin of soldering and brazing dates from about 3000 BC when metal workers in Mesopotamia melted lead to join pieces of copper. Since then jewelers, scientists, and trade people have adopted the techniques in pursuit of their crafts. Without these methods, our day-to-day life would be very different as most gadgets such as televisions, mobile phones, computers, etc., contain electronic circuit boards that rely on soldered joints to function.
This chapter will discuss soldering and brazing, their methods, and different types of fluxes and filler metal composition.
Soldering
Soldering is the process of joining two or more metal parts without changing their original structure. It is done with the help of a fusible alloy called solder. Solder is applied in a molten state on the metal and after solidification, it forms a joint.
Solder is a mixture of tin and lead whose melting point is about 235°C and 350° C, respectively. But when tin and lead are mixed, then the melting point of the mixture is reduced to 183°C. The ratio of the mixture of tin and lead is 63: 37. Some examples of solder are tin-lead for general purposes, tin-zinc for joining aluminum, lead-silver for strength higher than room temperature, zinc-aluminum for aluminum and corrosion resistance, tin-silver or tin-bismuth for electronics, etc.
Depending on the melting points, the soldiers are classified as
- Hard solder
- Soft solder
1. Hard Solder
The melting point of hard solder is high compared to all other solders. It is used to solder big metal pieces, thick wires, or cables of copper, brass, iron, etc. It is of two types.
(i) Spelter solderÂ
It is made in three compositions and used according to the requirement.
(a) Copper = 50%, Zinc = 50%
(b) Copper 67%, Zinc = 33%
(c) Copper = 50%, Zinc 37%, Tin = 13%
They are used for brazing in plumbing and sheet metal works.
(ii) Silver solderÂ
The melting point of silver solder is less than spelter solder. It is used to join precious and semi-precious metals such as gold, silver, German silver, etc. Its composition for joining different metals is
(a) Copper = 33%, Silver = 67% It is used to solder silver articles.
(b) Copper = 20%, Silver = 10%, Gold = 70% It is used to solder gold jewellery.
(c) Copper = 20%, Silver = 70%, Zinc = 10% It is used to solder ornamental jewellery.
(d) Copper = 35%, Zinc = 55%, Nickel = 10% It is used to solder German silver articles.
2. Soft Solder
The melting point of soft solder is less than silver solder. It is a combination of lead and tin. The melting point of soft solder is inversely proportional to the amount of tin in the solder, as shown in the table.
Melting Point and Composition of Lead and Tin of Soft Solder
| SL NO | Quantity of Tin | Quantity of Lead | Melting Points |
| 01 | 10% | 90% | 300℃ |
| 02 | 20% | 80% | 275℃ |
| 03 | 30% | 70% | 255℃ |
| 04 | 40% | 60% | 225℃ |
| 05 | 50% | 50% | 205℃ |
| 06 | 66% | 34% | 185℃ |
| 07 | 80% | 20% | 200℃ |
| 08 | 90% | 10% | 210℃ |
From the above table, we see that the solder having a composition of 50% lead and 50% tin is suited for electrical appliances.
TV, radio, and other electronic appliances can be soldered by using a solder having a composition of 80% of tin and 20% of lead.
To solder aluminum items ALCA-P solder or an alloy made up of 70% of tin, 25% of zinc, 3% of aluminum, and 2% of faster tin is used.
As we have discussed, a solder can join two metals together. But the quality of the soldering process is also a considerable concept because if the metals have some corrosion, dust, etc., then it will be difficult to solder those metals. So in this way, flux is to be introduced.
Flux
Flux is normally available in the form of powder, paste, and liquid. It protects the joining surface from oxidization and also cleans the surface. Some of its functions are given below.
(i) It is used to protect the joining surface from oxidation and helps to melt the solder.
(ii) Prevent the melted solder from forming the oxide.
(iii) It is in the molten state at soldering temperature.
(iv) It also helps to flow the solder easily to the joint surface.
(v) It removes carbon and other impurities formed during the soldering.
(vi) It helps in making strong joints.
The table given below shows the different compositions of filler metal for different metals.
Composition of Solder and Flux for Various Metals
| SL NO | Metal | Flux | Sn | Pb | Al | p | Zn | Bs | Melting point |
| 01 | Brass | Zinc chloride resin or ammonium chloride | 66 | 34 | – | – | – | – | 185° |
| 02 | Gunmetal | Zinc chloride resin or ammonium chloride | 63 | 37 | – | – | – | – | 183°C |
| 03 | Copper | Zinc chloride resin or ammonium chloride | 60 | 40 | – | – | – | – | 180°C |
| 04 | Tinned steel | Zinc chloride resin or ammonium chloride | 64 | 36 | – | – | – | – | 180°C |
| 05 | Galvanized steel | Hydrochloric acid | 58 | 42 | – | – | – | – | 192°C |
| 06 | Zinc | Hydrochloric acid | 55 | 45 | – | – | – | – | 202°C |
| 07 | Iron and steel | Ammonium chloride | 50 | 50 | – | – | – | – | 205°C |
| 08 | Silver | Zinc chloride | 67 | 33 | – | – | – | – | 183° |
| 09 | Gold | Zinc chloride | 67 | 33 | – | – | – | – | 183°C |
| 10 | Aluminium | Stearin | 70 | – | 03 | 02 | 25 | – | – |
Besides the filler metal, fluxes used on the surface of the metal to be joined have great importance. It plays an important role in soldering.
The different flux composition and their applications are given in the table below.
Flux Composition and Its Applications
| Sl No | Composition | Application |
| 01 | Zinc chloride 10-13%, water 90-70% | Iron, steel, copper, and their alloys |
| 02 | Zinc chloride 25-30%, sal ammoniac 5-20%, water 50-70% | Same metals as above but more efficient flux |
| 03 | Zinc chloride 20%, sal ammoniac 5%, petroleum jelly 74%, water… 1% | Soldering paste for above or noted metals |
| 04 | Saturated solution of zinc chloride in hydrochloric acid | Soldering stainless steel |
| 05 | 30 g of powder resin dissolved in 100 cc of orthophosphoric acid (1.6 specific gravity) and 400 cc alcohol or ethylene glycol | Soldering of stainless steels with pure tin or tin soldiers at 290-300°C |
| 06 | Saturated aqueous solution of 34% zinc chloride, 33% methanol, and 33% glycerine. | Soldering paste for use with blow lamp. |
| 07 | Resin | Copper and its alloys |
| 08 | Zinc chloride 85%, sal ammoniac 10%, Sodium fluoride 5% | For aluminium soldering |
| 09 | Zinc chloride 90%, sal ammoniac 10% | For aluminium soldering |
| 10 | Zinc chloride 95%, sodium fluoride 5% | For aluminium soldering |
| 11 | Zinc chloride 90%, sal ammoniac 8%, sodium fluoride 2% | For aluminium soldering |
| 12 | Sodium fluoride 8-10%, barium chloride 15-20%, sodium chloride 15-20%, zinc chloride 30-40% and the remaining potassium chloride | For repairing cracks in aluminum cast alloys. |
Cleaning the Surfaces of Soldering
The parts to be joined are first cleaned to remove foreign matter such as oil, grease, scale, and dirt. So that, the joint formed after cleaning is more strong. The cleaning may be done by mechanical or chemical methods.
1. Mechanical Methods
Mechanically, the surface can be cleaned by scraping, filing, or with an emery cloth.
2. Chemical Methods
For cleaning a surface, most pickling chemicals are used, which are as follows
(i) For iron, steel, and brass Solution of 10% HCL
(ii) For copper Dilute sulphuric acid.
(iii) For stainless steel 3 parts sulphuric acid + 1 part nitric acid + 10 parts water.
Grease or oil can be removed from the surface by using a solvent of alcohol carbon tetrachloride or trichloro-ethylene.
Cleaning Soldering Equipment
Cleaning of soldering types of equipment before soldering is essential because it reduces the loss of energy, solder, etc. Cleaning of the soldering iron is done by heating, its bij and then rubbing it with the sand.
In this way, the oxides formed on the upper surface were removed. Now, a thick layer of resin or naushadar is coated on the bit. This whole process is known as the cleaning of soldering equipment.
Methods of Soldering
According to the method of heating, soldering is classified as.
(i) Soldering iron method
(ii) Soldering torch method
(iii) Dip and wave method
(iv) Induction method
(v) Resistance and hot plate method
(vi) Furnace and hot plate method
(vii) Spray method
(viii) Ultrasonic method
(ix) Condensation method
In the above methods, only the first two methods are mostly used. So, here we will discuss these two methods.
1. Soldering Iron Method
It is one of the oldest methods of soldering. It is a type of hand tool used for soldering. It supplies heat to melt the solder so that, it can flow into the joint between two workpieces. A soldering iron is composed of a heated metal tip, a stem,- and an insulated handle.
A soldering iron can be a fuse soldering iron, which is heated in a furnace to have sufficient temperature to melt the filler metal or it can be an electric solder iron, An Electric solder iron is heated by passing an electric current through it.
The heating element is in the sink. The purpose of the bit is to carry heat from the shank down to the work and is usually made of copper, which conducts heat very easily. The use of electric solder iron is cost-effective and used in making very precise joints in electronic and electrical equipment.
2. Soldering Torch Method
The method of soldering in which heating is done by using heat energy provided by a gas torch is known as the soldering torch method. The different types of torch method soldering are shown in the diagram.
For soldering by blow lamp method, the following steps are involved.
Step I First the blow lamp is lit and by using the blow lamp the joint is heated.
Step II Now, apply the appropriate flux, and then the solder rod is melted at the joint, and the joint is filled.
Step III If the joint is overheated, then the blow lamp is taken away at a certain distance to avoid excess overheating.
Step IV After the joint is filled with solder remove the solder rod then blow the lamp and allow the joint to cool down.
Merits of Good Solder
Good solder should have the following merits.
(i) It can easily spread over the base metal surface.
(ii) It should have good wetting properties.
(iii) It should form a metallic bond with the base metal to form an alloy. It is also known as the alloying property of solder.
(iv) The melting point of solder must be 60° C less than the melting point of base metal.
(v) It should have the property of capillarity.
(vi) It should be capable of forming strong and clean joints.
Steps Involved in Soldering
The steps involved in the soldering process are
Step I Clean the surface of the base metal before soldering.
Step II Select the right flux and apply it on the surface of the base metal.
Step III Select the right type of soldering instrument for specific joints and heat it to the required temperature.
Step IV This heated instrument part made contact with the flux-applied surface.
Step V Now, move the tip of an instrument with solder on it, along the joint. The soldering speed is kept according to the depth of the joint.
Step VI Do not shake or move the joint during solidification of solder because it may result in fracture.
Step VII After soldering, flux is removed from the surface of the metal.
The process in which sweating base metal is heated, after that a layer of solder is applied on the surfaces and they are held together under pressure and heated from outside. The solder present in between the surface melts and forms the joint. This process is known as sweating.
Precautions while Soldering
There are certain precautions while soldering and if these precautions are not taken, then soldering may be not of good quality.
The precautions taken are as follows.
(i) The bit of soldering iron should be of copper.
(iI) The bit of soldering iron should be clean and if required stand paper should be used.
(iii) Excess heating of soldering iron is avoided.
(iv) A thin layer of solder should be placed on the soldering iron bit.
(v) Joint should not be oily, it should be cleaned.
(vi) The joints should be filled and shiny.
(vii) Solders other than joints should be removed.
(viii) One should not leave holes in joints.
(ix) Dry joints should not be made by soldering because current flow is resisted by dry joints.
(x) Excess flux is removed after soldering.
(xi) During the cooling/solidification period joint should not be disturbed. If they are disturbed their strength and conductivity are endangered. The result is called cold solder and the joint will be defective.
(xii) Cooling should not be accelerated, if it is accelerated then the solder will become crystalline which will lower the mechanical strength of the solder.
Some Important Safety Measures while Soldering
The following are the safety measures to be taken while soldering to avoid injuries.
(i) Inspect the iron regularly for physical damage, especially the cord.
(ii) Keep the iron in a stand when not in use.
(iii) Iron should not be treated roughly.
(iv) While using iron keep it away from the body and its power cord.
(v) Never flick the solder with an iron, it may cause a burn or damage the work.
(vi) When pouring solder over the joint, keep the ladle as low as possible to prevent splashing of molten solder over the side of the pot.
(vii) Beware of naked flames of blow lamp.
Besides soldering, brazing is also a metal joining process in which, the physical property of the parent metal does not change.
It is carried out at a temperature slightly above the temperature of soldering and the joint obtained by brazing is more stronger than soldering.
Brazing
Brazing is also a metal joining process where a filler metal is heated above the melting point and distributed between two or more close-fitting parts in capillary action. The filler metal is heated in a protected atmosphere, usually a flux, above its melting temperature.
It then flows over the base metal and then cooled to join the workpiece together. It is similar to soldering, except the temperature used to melt the filler metal is higher for brazing. For good joint strength, the joint clearance recommended for the best capillary action is 0.03 to 0.08 mm (0.0012 to 0.0031 inch).
Spelter or Brazing Alloy
The filler metal used for brazing is known as spelter brazing alloy or solid solder. A variety of alloys is used as filler metal for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with desired properties. Good brazing alloys should have the following properties.
(i) Its ability to wet the base metal.
(ii) Easily fill the joints, due to capillary action.
(ii) The melting point of the filler metal should be less than the melting point of the base metal so that the surface of the base metal does not get damaged.
(iv) Mechanical properties of the joint should be good.
Different Spelter or Brazing Alloys
| SL No | Base Metal | Spelter/ Filler metal | The major constituent of the flux | Temperature range of flux °C | Remark |
| 01 | Aluminium | Al-Si alloy | Fluorides and chlorides | 400-650 | Low-temperature melting alloy |
| 02 | Copper and its alloy | Cu-P | Borax, boric 560-890 acid | 560-890 | Free-flowing flux |
| 03 | Aluminium bronze and aluminum brass | Ag-Cu Cu-Zn Cu-P | Borax, boric acid | 550-860 | Corrosive resistance |
| 04 | Magnesium alloy | Mg | Fluorides, chlorides | 488-620 | Low melting. deoxidizing |
| 05 | Copper silver, phosphorous | Ni-Cu Ag-Cu | Borates, fluorides | 550-790 | Low melting. deoxidizing |
| 06 | Gold copper alloys | Co-Ni-base alloys | Borax, boric acid | 570-800Â | Corrosion resistance |
| 07 | Nickel chromium alloy | Cu, Co | Borates, fluorides, chlorides | 650-900 | Heat resistance at low-temperature |
| 08 | Magnesium chromium alloy | Al-Mg alloy | Flouride, chloride, borax | 500-720 | Heat and corrosive resistance |
| 09 | Ferrous alloys | Cu-Zn | Borates, chlorides boric acid | 700-930 | Free-flowing alloys |
Methods of Brazing
The different methods used for the brazing process are discussed below. These processes are almost the same as soldering.
1. Blow Pipe Brazing
This method is mainly used by the goldsmith in the brazing of ornaments. It is used for small-size job joints. The method of blowpipe brazing is the same as blowpipe soldering.
2. Furnace Brazing
In this method, the furnace is used to heat the workpiece to be joined by brazing operation. In medium production, usually in batches the parts and brazing metal are loaded into a furnace, heated to brazing temperature, and then cooled and removed, while for higher production rates all the parts and brazing material are loaded on a conveyer to pass into a furnace. The internal stress developed in a joint by this method is minimal.
3. Torch Brazing
In torch brazing, flux is applied to a surface, and a focusing flame torch is used against the work at the joint. A reducing flame is used to prevent the oxidation of the filler metal. A filler metal wire or rod is added to the joint. Torch uses a mixture of two gases, oxygen, and acetylene, as a fuel in gas welding.
4. Vacuum Brazing
It is a special type of furnace brazing, in which metal is prevented from oxidation, by creating a vacuum around the surrounding.
5. Induction Brazing
In this method, electricity is used for brazing. This method uses the electrical resistance of the workpiece to generate the heat. The parts to be joined are pre-loaded with filler metal and placed in a high-frequency AC field. Frequencies ranging from 5 to 5000 kHz are used. High-frequency power source provides surface heating, however low frequency causes deeper heating into the workpiece. Low frequency current By recommended for heavier and bigger workpieces. By this process, any production rate from low to high can be achieved.
6. Dip Brazing
In dip brazing, the parts that have to be joined are dipped inside molten filler metal after applying flux on it.
The filler metal present in the molten state, enters between parts to be jointed and on cooling or solidification forms the joint.
7. Resistance Brazing
In resistance welding, the workpieces are directly connected to an electrical current rather than the induction of an electric current line. Heat to melt the filler metal is generated by the resistance of the flow of electric current through the joint to be made. The equipment for resistance brazing is the same that is used for resistance welding, only lower power ratings are used in this case. Filler metal is placed between the gaps before passing current through them. It is recommended to make smaller joints.
Comparison of Soldering and Brazing, of Welding
All three processes soldering, brazing, and welding have similarities in that these are bonding processes. In all three processes filler metal, flute, and heat energy are used. But the strength of the bond and its durability and other features as different. So, let us compare these three, as shown below.
Comparison of Soldering, Brazing, and Welding
| SL No | Welding | Brazing | Soldering |
| 01 | These are the strongest joints used to bear the load. | These are stronger than soldering but weaker than welding. | These are the weakest joints out of the three. It is only used to make electrical contacts. |
| 02 | The temperature required is up to 3800° C of the welding zone. | It is 600° C. | The Temperature required is up to 350° C. |
| 03 | The mechanical property of the material changes at the joint. | It has almost negligible change. | No change in mechanical properties after joining. |
| 04 | The Workpiece is heated till its melting point. | The workpiece is heated but below its melting temperature. | No need to heat the workpiece. |
| 05 | Skill labor is required. | The skill required is between these two. | The skill of labor is very low. |
| 06 | Heat treatment is required after welding. | No heat treatment is required. | No heat treatment is required. |
| 07 | Dissimilar metals cannot be joined together by this method. | By brazing dissimilar metals can be joined together. | Dissimilar metals can be joined together. |
| 08 | No preheating of the workpiece is required before welding. | Preheating of the workpiece is required. | Preheating of the workpiece is required before soldering to obtain a good-quality joint. |