SMT: Hot Gas Soldering & Resistance Soldering
2.1 Hot gas soldering
Here, the heat transfer between gas and the material to be heated is relatively low, the processes using hot gas are in most cases slow. The temperature of gas is set to about 4000 C. The high temperature of the gas is necessary to attain an acceptable heating rate, but this introduces the risk of damaging components in the vicinity of the place to be soldered. If these components are not heat resistant, they can be screened from the hot gas. Hot gas provides a suitable method of heating, especially in small scale production. If this method can be used, it is always a cheap solution. Alumina substrates are usually preheated to a temperature of 1000 C to 1500 C before hot-gas soldering is carried out. The bodies of the components positioned in the stream of gas are often heated more rapidly than the terminations to be soldered, so that for this reason also overheating may be encountered. The advantage of hot gas soldering is that the components cannot be mechanically damaged. One of the main disadvantages is that the energy transfer is limited and so consequently is the speed of heating. The size of the objects to be heated is also limited. Also, there is a risk of blowing components out of position. A particular field of application is repair or rework of soldered boards, especially surface mounted boards. Here the use of hot gas minimizes the risk of damaging the sensitive pad areas.
2.2 Resistance soldering:
In resistance soldering an electric current is passed either through the parts to be soldered or through a heater element, which is gently pressed against the spots to be heated. In the first case the heat is developed owing to the resistance of the parts themselves or to their contact resistances, whilst in the second case the heat of the element is transferred by conduction. In resistance soldering, the heater elements in contact with the parts to be soldered are made of material (e.g. molybdenum, tantalum, cobalt copper, beryllium copper) that is not wetted by molten solder.
Advantages of resistance soldering:
i) The amount of heat can be accurately controlled.
ii) Heating times are short
iii) Heating is very local
i) The heat may not reach the right spots owing to variations in contact resistance.
ii) Components are soldered sequentially, whereas the process time per component is rather long.
The resistance soldering is particularly suited for QFP packages or other multi-lead packages with gull wing leads, where it is possible to place the resistive element(s) on top of the lead(s). In this way automatic correction of non-coplanarity of the leads is obtained by the vertical force of the element while the solder is in the molten state. The heater assemblies are also called thermodes.
The technique of multiple-lead reflow soldering has been specially developed for the soldering of integrated circuit flatpacks to printed boards. It is an extension of single point resistance soldering. Here the soldering tip has been built in such a way that a number of leads can be soldered simultaneously. The heater element is usually a strip of metal, which is placed vertically with its edge on the places to be soldered. The strip is heated either by a controlled current to maintain a constant temperature or by brief electrical impulse each time the strip is placed in the soldering. position. This kind of resistance soldering where a strip of metal heated is called hot-bar soldering.