Attaining ‘zero defect’ production requires a thorough knowledge of soldering technology. Here is a lowdown on the different kinds of techniques, material and equipment used in soldering
As defined in Digital Electronics by Tokheim, soldering is the process of joining two metals together by the use of a solder alloy to form a reliable electrical path. This technique was first developed in ancient Egypt. Soldering skills are needed to assemble any electronic circuit. Faulty solder joints may cause failure of major equipment. It is therefore necessary to have high standards of workmanship in soldering.
A soldering iron consists of three main blocks: a resistance-heating unit, a heater block that acts as a heat reservoir and a soldering tip (also known as a bit). Soldering irons come in a variety of sizes, shapes and wattage. The wattage varies from 15W to 100W. The advantage of using a high-wattage iron is that heat can flow quickly into a joint and soldering can be done rapidly.
The temperature of the soldering bit depends on the mains input voltage. When using a soldering iron, some points to remember are:
1. Before using a soldering iron, clean the bit by wiping it on a wet sponge.
2. The surface of the bit should be constantly tinned to ensure proper heat transfer and to prevent impurities from entering the solder joint.
3. When the iron is not in use, keep it in a holder, with the bit clean and coated with solder.
In temperature-controlled soldering irons, a small magnet, which forms a part of the tip, activates a temperature-controlled magnetic switch. The magnet is designed to lose its magnetic properties at a predetermined temperature and the switch turns power to the heater ‘on’ or ‘off,’ controlling the temperature of the tip.
Another version of this soldering iron has an inbuilt temperature sensor. The power to the heater is turned on or off automatically to maintain the bit at the set temperature. Most soldering irons work off 220-240V AC mains. Soldering irons with low voltage operation (12V DC or 24V DC) generally form part of a soldering station, and are designed to be used with a special controller.
Bit. Soldering iron bits are available in different diameters and shapes. These can be changed depending on the type of work. Often, bits are either bright-plated or made from unplated copper to prolong their life. The bit should be cleaned periodically to prevent a layer of oxide from accumulating between the bit and the heating element. A bright tinned surface must be maintained at the working surface of the bit. Proper use and care of the soldering iron bit prolongs the life of the bit and prevents solder-joint failure.
Solder is an alloy of tin and lead in different proportions. It is usually available in the form of wires of different sizes and grades. The grades of solder refer to the percentage of tin to lead as 60/40 (indicating 60 per cent tin content and 40 per cent lead), 50/50 and 40/60. The most commonly used solder for hand soldering in electronics circuit assembly is 60/40 with non-corrosive flux cores.
Like electric wires, soldering wires are specified in gauges. Solder of 18 Standard Wire Gauge (SWG) is used for general purposes. Thinner soldering wire of 22 SWG or 24 SWG is used for printed circuit assembly.
Perfect solder joints can be accomplished only with properly cleaned soldering surfaces. The flux present in multicore solders is a chemical for removing the oxide film deposited on the surfaces to be joined. The corrosive action of flux at melting temperatures removes metal oxides very fast. It also prevents reformation of new oxides, which allows the solder to form a rigid bond. Flux helps molten solder flow more easily over the joint.
Flux melts at a temperature lower than the solder temperature. Thus it performs the task of cleaning the surfaces before soldering takes place. A variety of flux are available for many purposes and applications. The most common types specified by rosin content are ‘no clean,’ rosin mildly activated, rosin activated and water soluble. Flux consists of natural or synthetic rosins. Flux used in hand soldering are either pure rosins or rosins combined with mild activators.
Key to good soldering
Cleaning of the surface, soldering at the right temperature and for the right duration, and adequate solder coverage are the key factors affecting the quality of solder joints.
It is absolutely necessary to ensure that the leads of components are free from grease, oxidation and other contamination. The oxide layer repels molten solder. It is very difficult to solder old components or tracks on the PCB because of the layer of oxidation that builds up on the surface of the component leads. Even surfaces that look clean may have a thin, invisible film of oxide on them. So for appropriate solder bonding, surfaces must be cleaned using flux.
It is important that the surfaces to be soldered together are brought to the same temperature. When both are heated at a high temperature, solder flows evenly to make a good mechanical and electrical joint.
The parts to be joined should be heated with a soldering bit for the right time length. Excessive heating may damage the components or the printed circuit track. The heating period depends on the size of the joint and the temperature of the soldering bit. Semiconductor components such as diodes, transistors and ICs are sensitive to heat and should not be overheated.
Soldering: the process
When the soldering iron bit is adequately hot, apply a little solder on the flattened tip and wipe it off with a piece of damp cloth or sponge. The solder will form a thin layer on the bit. This process is known as ‘tinning’ the bit.
Heat up the joint with the bit and continue heating while applying solder. The molten solder flows quickly from the bit onto both parts to be joined. It is important to use the right amount of solder. Remove the iron and allow the joint to cool. During the cooling period, if the joint is disturbed, it may become dry and create serious problems in the working of the circuit. Sometimes it becomes very difficult to trace out this defect.
Post-solder flux cleaning
After the soldering process is complete, remove the residual flux from the PCB. Clean it by dissolving it in a solvent and wiping the area dry with a tissue paper. The complete assembly should be totally free from flux and residual cleaning agents. The cleaning method and solvent solution used should not have any adverse effect on the components and the connections.
While soldering, if molten solder comes into contact with a copper surface, the solder dissolves and penetrates the copper surface. Solder and copper form a new alloy. This solvent action is called ‘wetting’ and forms an inter-metallic bond between the two parts.
Wetting can occur only if the surface of copper is free of contamination and oxide film, and the solder and the work surface have reached the proper temperature.
Resoldering and desoldering
Resoldering. It is very difficult to correct poorly made joints. So make good joints in the first place. Try to prevent re-soldering of joints as much as possible. A dry or disturbed solder joint usually requires reheating and re-flow of solder by applying a suitable quantity of flux.
Desoldering. Desoldering is required when components in the circuit need to be removed or replaced. To remove a damaged or defective component, either a desoldering pump or a desolder braid can be used.
Using a desoldering pump. The desoldering pump consists of a spring-loaded plunger. During the desoldering operation, the plunger is released at the push of a button and the molten solder then drawn up into the pump. Sometimes, it may take more than one attempt to clean the soldered point. You may add more solder to the joint and then desolder the whole quantity with the pump. Ensure that the components and PCBs are not damaged by overheating.
Using a desoldering braid or wick. The desoldering braid or solder wick is especially effective for desoldering a difficult joint. It is a simple and excellent alternative to a desoldering pump.
The desoldering braid, made from fine copper, draws molten solder up into the braid. To desolder using it, place a short length of the wick on the point to be desoldered. Place the hot bit of the soldering iron over the wick and press. The bit subsequently melts the solder and the molten lead is drawn up into the braid. Take extreme care to ensure no damage happens to the printed circuit track when the braid is pulled off the solder point.
The wave soldering technique is used in the mass production of electronic circuits. It is an automated technique in which a large number of solder joints are made simultaneously. It reduces cost and increases the speed and reliability of production.
Wave soldering has three main requirements: solder, flux for wetting the soldering surface, and heat to activate the flux and melt the solder.
The PCB loaded onto a linear conveyor undergoes different operations, namely, fluxing, preheating and wave soldering. The conveyor is usually set at an angle of about 7º to 10º to the horizontal to enable excess molten solder to drain out from the board into the solder tray. For fluxing, a foamed standing wave or spraying is commonly used.
The next stage is preheating the PCB. Preheating is done to evaporate the flux solvent, activate the flux, and reduce thermal shock to the board when it touches the solder wave. Hot-air blowers or infrared heaters are commonly used.
Solder is heated in the solder bath bed in the wave soldering stage. A solder wave is generated by continuously pumping molten solder in the reservoir. The wave thus formed is a standing wave with a dross-free, clean surface. The wave soldering machine can be set to adjust the shape and height of the wave. The assembled PCB, after fluxing and preheating, is passed over the solder wave. All solder points, which are in direct contact with the solder wave, establish perfect soldering.
The PCB is the heart of an electronics assembly. The quest for reliability and high efficiency in minimal space has led to the introduction of surface-mounted devices. Smaller components and higher packaging densities, with marked savings in size, weight and high-speed automated assembly, are the main advantages of surface-mount technology. This technology is widely used in pocket calculators, computers, communication field (especially in satellite communication), consumer electronics and automotive industry.
In this type of assembly, all the components are assembled on the same side. Solder paste is applied on the pads. The surface-mount components are then placed on the paste. Heat is applied to make the paste melt and flow, forming joints. Solder paste plays an important role in the assembly of surface-mount devices. The flux content of solder pastes makes the track surface clean, allowing perfect joints.
The author is ex-Manager (R&D), UMS Radio Factory, Coimbatore and former general manager, Sulax Corporation, Bengaluru