Kester - The Effect Of Metallic Impurities On The Wetting Properties Of Solder

Our thanks to Kester Company for allowing us to reprint the following article.
by Dennis Bernier, Vice President, Research & Development Kester Solder Company


Since the development of wave soldering machines about forty years ago, the electronics industry has become more aware that certain metallic impurities have an effect on the wetting properties of solder. Traditionally the maximum amounts of contaminating metals -- such as copper, gold, antimony, iron, zinc, aluminum and cadmium -- have been established by trial and error over many years. There is a need in the industry for substantiated data so specifications can be established for replacing contaminated solder at the proper time.

This paper is a report of a study made to determine the maximum allowable impurities in solder used for wave soldering applications. This report concludes with a list of impurities compiled from actual analyses of solder which caused production problems. A list of recommended maximum allowable impurities will assist in establishing reliable quality controls on the purity level of the solder in a wave soldering machine.

With the introduction of wave soldering machines in the late fifties, the printed circuit industry moved into the age of automation and high productivity. Like any new process, there was a period of trial and error which extended over several years to determine the operating parameters of the equipment. The technician or assembler using a hand soldering iron had much better control over the soldering situation since he could control join-to-joint heating to complete properly soldered connections. Heating on the wave soldering machine, however, could not be variable if automation and productivity were to be accomplished. All joints were exposed to the same amount of heat, and only the mass of component parts being soldered determined the temperatures to which the surfaces were heated. The wave soldering machines and the problems associated with them helped advance soldering technology since the speed of soldering required more careful control of the other parameters for soldering. Studies began and still co tinue on the solderability of surfaces, the proper application of flux, faster acting fluxes, types of preheating and conveyor mechanics. The temperature of the solder pot rather quickly was established at 60°-80°C higher than the melting point of the solder. This, like other parameters, was by experimentation. By far the most commonly used solder alloys were, and still are, 63/37 and 60/40 tin/lead. These alloys are low melting, bond to copper surfaces most readily, and melt over a narrow temperature range. There are many soldering fluxes to select, but after the choice is made, control is fairly simple by checking density or acid number and adding the correct amount of thinner...

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