The various soldering methods which are used with electronic assemblies differ in the sequence in which solder, flux, and heat are brought to the joint, and in the way in which the soldering heat is brought to the joint or joints.

With hand soldering, the heat source is the tip of a soldering iron, which is heated to 300–350 °C/570 660 °F. A small amount of flux may have been applied to the joint members before they are placed together.

The assembled joint is heated by placing the tip of the soldering iron on it or close to it. Solder and flux are then applied together, in the form of a hollow solderwire, which carries a core of flux, commonly based on rosin.

The end of the cored wire is placed against the entry into the joint gap. As soon as its temperature has reached about 100 °C/200 °F, the rosin melts and flows out of the solderwire into the joint. Soon afterwards, the joint temperature will have risen above 183 °C/361 °F; the solder begins to melt too, and follows the flux into the joint gap.

As soon as the joint is satisfactorily filled, the soldering iron is lifted clear, and the joint is allowed to solidify. Thus, with hands oldering, the sequence of requirements is as follows:

1. Sometimes, a small amount of flux.
2. Heat, transmitted by conduction.
3. Solder, together with the bulk of the flux.

Clearly, this operation requires skill, a sure hand, and an experienced eye. On the other hand, it carries an in-built quality assurance: until the operator has seen the solder flow into a joint and neatly fill it, he – or more frequently she – will not lift the soldering iron and proceed to the next joint.

Before the advent of the circuit board in the late forties and of mechanized wavesoldering in the mid fifties, this was the only method for putting electronic assemblies together. Uncounted millions of good and reliable joints were made in this way.

Hand soldering is of course still practised daily in the reworking of faulty joints. Mechanized versions of hands oldering in the form of soldering robots have become established to cope with situations, where single joints have to be made in locations other than on a flat circuit board, and which therefore do not fit into a wave soldering or paste-printing routine.

These robots apply a soldering iron together with a metered amount of flux-cored solder wire to joints on three-dimensional assemblies, which because of their geometry do not lend themselves to wave soldering nor to the printing down of solder paste.

Naturally, soldering with a robot demands either a precise spatial reproducibility of the location of the joints, or else complex vision and guidance systems, to target the soldering iron on to the joints.

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