by Arthur Ed LeBouthillier

This article appeared in the October 1999 issue of The Robot Builder

Soldering is the use of a low-melting point, conducting alloy in order to create a mechanical and electrical joint. Notice that there are two important connection aspects: mechanical AND electrical; a good solder joint provides both.

Solder is an alloy composed usually of tin and lead but can also sometimes contain silver. Because of the unique properties of these elements, the melting point of solder is lower than the individual melting points of each constituent material. For  soldering electronic components, you should use a 40/60 or 60/40 solder. You should also use a flux compound, which reduces oxidation and helps apply heat. Rosin flux or rosin flux core solder is best. Never use acid-core flux.

Use a 25 to 40 Watt soldering iron with a good clean tip. Cleanliness of the tip and area to be soldered is vital to a successful solder joint. To ensure a clean tip, you should use a damp solder sponge regularly during your soldering session. Bend the leads of the components so that they fit neatly into the whole. Trim away the excess lead. For the best mechanical and electrical connection you should have a little bend on the pad side as figure 1 shows.

The basic idea behind soldering is to heat both elements to the melting point of the solder. Apply your soldering iron as shown in figure 1. You should apply the soldering iron so as to quickly heat both the component lead and the copper pad to the solder melting point, while applying small amounts of solder. When the solder melts, let it flow around the component lead as shown in figure 2.

You need to be careful about applying either too much solder or too much heat. Too much solder creates opportunities for shorts. Too much heat can destroy the component, the board, or break the bond between the copper pad and the circuit board. A little practice will help you identify how to apply heat properly. Do not allow the component to move while the solder is fluid since it will create a weak solder joint.

A cold solder joint is a bad solder joint where the solder did not bond completely to both the component lead and the solder pad. As its name implies, it is caused by the components being too cold when the solder was applied. Because of this,
the solder could not flow properly before it solidified. Cold joints are bumpy, grainy, and dull gray rather than smooth, shiny and silvery.

A perfect solder joint will be smooth and shiny. The solder will  have flown evenly between the component and the pad and will not bulge out. In a good solder joint, the solder will have an inward slope and will spread evenly out onto the copper pad of the circuit board.