Introduction to Basic Manufacturing Processes and ... - always yours

Welding 343
exceeding 450°C and below the solidus of the base material. Though soldering obtains a good
joint between the two plates, the strength of the joint is limited by the strength of the filler
metal used.
Solders are essentially alloys of lead and tin. To improve the mechanical properties and
temperature resistance, solders are added to other alloying elements such as zinc, cadmium
and silver in various proportions. Soldering is normally used for obtaining a neat leak proof
joint or a low resistance electrical joint. The soldered joints are not suitable for high temperature
service because of the low melting temperatures of the filler metals used. The soldering joints
also need to be cleaned meticulously to provide chemically clean surfaces to obtain a proper
bond. Solvent cleaning, acid pickling and even mechanical cleaning are applied before soldering.
To remove the oxides from the joint surfaces and to prevent the filler metal from oxidizing,
fluxes are generally used in soldering. Rosin and rosin plus alcohol based fluxes are least
active type and are generally used for electrical soldering work. Because of the content of
acids, these are corrosive at soldering temperature. They can be easily cleaned after the
soldering. The organic fluxes such as zinc chloride and ammonium chloride are quick acting
and produce efficient joints. But because of their corrosive nature the joint should be thoroughly
cleaned of the entire flux residue from the joint. These are to be used for only non-electrical
soldering work. Fluxes are normally available in the form of powder, paste, liquid or in the
form of core in the solder metal. It is necessary that the flux should remain in the liquid form
at the soldering temperature and be reactive to be of proper use.
The most commonly used soldering methods include soldering iron (flame or electrically
heated), dip soldering, and wave soldering. A soldering iron is a copper rod with a thin tip
which can be used for flattening the soldering material. The soldering iron can be heated by
keeping in a furnace or by means of an internal electrical resistance whose power rating may
range from 15 W for the electronic applications to 200 W for sheet metal joining. This is the
most convenient method of soldering but somewhat slower compared to the other methods.
In dip soldering, a large amount of solder is melted in a tank which is closed. The parts that
are to be soldered are first cleaned properly and dipped in a flux bath as per the requirement.
These are then dipped into the molten solder pool and lifted with the soldering complete. The
wave soldering is a variant of this method wherein the part to be soldered (e.g.” an electronic
printed circuit board, PCB) is not dipped into the solder tank, but a wave is generated in the
tank so that the solder comes up and makes a necessary joint.
17.14.1 Basic Operations in Soldering
For making soldered joints, following operations are required to be performed sequentially.
1. Shaping and fitting of metal parts together
Filler metal on heating flows between the closely placed adjacent surfaces due to capillary
action, thus, closer the parts the more is solder penetration. This means that the two parts
should be shaped to fit closely so that the space between them is extremely small to be filled
completely with solder by the capillary action. If a large gap is present, capillary action will
not take place and the joint will not be strong.
2. Cleaning of surfaces
This is done to remove dirt, grease or any other foreign material from the surface pieces
to be soldered, in order to get a sound joint. If surfaces are not clean, strong atomic bonds
will not form.