Printed Circuit Boards
Written by a world-renowned electronics expert, this reader-friendly guide helps engineers and technicians solve issues in PCB layout, fabrication, assembly, and testing.
TABLE OF CONTENTS Printed Circuit Boards
13.2: Theory of Soldering
13.2 Theory of Soldering
It is essential to understand the theory of soldering so as to visualize as to what happens when a solder joint is formed. Soldering is not a simple physical attachment of one metal to another, but chemically form an intermolecular bond. There are number of important variables in soldering process, which need to be controlled for proper soldering results.
13.2.1 The Wetting Action
When the hot liquid solder dissolves and penetrates the metal surface to be soldered, it is referred to as "wets the metal" or "the metal is wetted". The molecules of solder and say copper blend to form a new alloy, one that is part copper and part solder. It is this solvent action which is called wetting, that forms an intermolecular bond between the parts, an inter-metallic compound. The forming of a proper intermolecular bond is the heart of soldering process that determines the quality and the strength of the solder joint. Wetting can only occur if the surface of the copper is free from contaminations and from any oxide film that forms when the metal is exposed to air. Also, the solder and work surface need to have reached the proper temperature.
13.2.2 Surface Tension
We are familiar with the surface tension of water as a force that retains the cold water in globules on a greasy plate. The adhesive force that tends to spread the liquid on the solid is, in this case, less than the cohesive force. Washing with warm...
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