There are a lot of compounds used in soldering. Consult the manufacturer's MSDS information for each product, and handle each correctly to minimize your exposure to toxins and carcinogens.
There are a lot of soldering irons available. Most professional irons will allow you to adjust the heat, and may include a stand to keep the hot iron in safely. This particular model also turns off automatically if it's not used.
There are probably as many types of solder as there are soldering irons. Solder can be divided into two categories: lead, and lead-free.
Lead-free solder is becoming more and more common. It's often made up of an alloy of tin (96.3%), silver (3%) and copper (0.7%). It has a relatively high melting point, and it can be difficult to get it to flow as well as lead alloys, but it may be required to be in compliance with hazardous substances rules.
Lead solder can also be categorized by its tin/lead content. 60/40 is a very common solder containing 60% tin and 40% lead. 50/50, pictured here, is 50% tin and 50% lead.
63/37, which is shown here, is used for fine electronic work. It has a slightly lower melting point, and both metals melt at the same time. When it solidifies, 63/37 solidifies immediately and uniformly without any parts of the joint remaining liquid. For our purposes, this means that our solder joint will go directly from liquid to solid.
Oxidation and other impurities are the enemy of a good solder joint. Even when you clean your solder pads and leads well, the temperature that you are working at can re-introduce oxidization, which will reject the solder. This is where flux comes in.
Flux chemically dissolves oxides, and can also act as a wetting agent. You'll notice that the solder "flows" well into the workpieces when flux is present.
Many solders today contain a flux core, so additional flux may be unnecessary.
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