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Solder is a metal alloy used to connect two metal object at a relatively low temperature, below 450°C (840°F). Most commonly is solder used for connecting electronic components to a circuit board. Another solder is the indium solder used for mounting the heat spreader on modern CPU:s, for example pinless P4 CPU:s. For temperatures higher than 450°C(840°F) it is called braze.
The increased environmental awareness brought with it lead free solders that was mostly tin with a few trace metals to adjust the properties of the solder. Common additions in lead free solders are a few percent of copper and silver, but not all solders contains silver.
When gold or silver plated boards or component legs are soldered, the gold or silver is dissolved in the solder. For wave soldered boards the gold could go back into the solder bath and build up over time. At 0.5-1% Au in the alloy the solder joints becomes brittle and the bath has to be replaced. If several different boards are soldered in the same production line even boards without gold plating can end up with gold in the solder.
Refining of tin - lead solders
The refining of tin-led solder is discussed in Ammen, second edition page 277. The method recommended is to do electrolysis in a molten chloride electrolyte, made up of potassium chloride, lead chloride and tin chloride. Copper, gold, silver and other contaminants are concentrated at the anode. The anode and cathode leads are made up of tungsten.