What exactly is soldering?
Simply put, we when we solder (in electronics at least) our objective is to make a conductive connection between two other pieces of metal. That might be two wires or components on a Circuit Board or maybe you need to solder wire to a component on a Circuit board. In order to make this connection, we've got to melt down a little bit of metal between the two pieces and let it cool. Easy, right? Well, sorta.
Alloy Compositions
The metal that we melt down is called Solder and oftentimes is composed of a ratio of Tin and Lead. For the purposes of this blog, we will stick with leaded solder, and perhaps we will expand on other alloys in a later post. Because of Tin's low melting point, it is particularly useful because we want our solder to melt without melting down the metal components we are connecting. The two most popular ratios are 60% tin 40% Lead and 63% tin 37% lead. These alloys have a low melting point, between 183 and 191 degrees Celsius for 60/40. 63/37 is a special ratio because it is what is called a eutectic solder, which means it's melting and freezing point are both 183 degrees Celsius.
Record scratch okay what does THAT mean? How can something melt and freeze at the same time? Well, it may be easier to think of this as a pot of water boiling on your stove. While the water is heating up, the temperature will increase, but once it reaches it's boiling point of 100 Degrees Celsius, it will no longer increase in temperature. Instead, the increase in thermal energy will be used to begin it's transition to vapor. You can think of the temperature more as the degree at which something enters its transitions between states of matter. Essentially, if you heat this solder up to 183 C, it will melt if you continue applying thermal energy, and once you stop, it'll solidify.
So which solder should I pick 60/40 or 63/37?
Both 63/37 and 60/40 solders have low melting points which makes either alloy ideal for electrical components which have a lot of small parts that could melt or break apart when exposed to too much heat. The main differentiator between the two is this eutectic property of 63/37. This in effect means that the solder will go from solid to liquid and then back to solid again quickly, where as 60/40 will have a bit of a grace period where it becomes slushy in between 183 and 191 degrees C. For this reason, 63/37 might be preferable for precision soldering as it the fast cooling can help prevent cold joints, or joints where the solder doesn't yield a suitable connection between component and board. However, some may feel that because of this slushy period in between temperatures, 60/40 is a more forgiving alloy for beginners. If you are using 60/40, it is important to make sure you keep the board you are soldering still until it has cooled, to prevent getting a cold joint.
What about Diameter?
I will quickly touch on the diameter, as it is another variable that you will quickly notice when you are shopping around for materials. The Diameter is how thick the solder wire is, and which size is right for you is largely dependent on the type of job you're working on. When looking at electrical solders you will see a range from .010" to .092". Simply, the smaller the components you're working with, the smaller the diameter you will need. If you are working with mid sized to larger components, a small diameter will prove difficult as you will end up spending more time wetting enough solder to complete your joints. You want to avoid exposing the parts you are soldering to too much heat, so spending too long making your joints can ultimately be counter productive. So, consider the job while making this choice, and generally a good middle diameter of .031" is a solid pace to start.
Regardless of which you choose, it will ultimately come down to personal preference. As with any skill, working with your tools and materials is as much a learning experience as acquiring the skill is, and some degree of finding what works for you is inevitable.
Hopefully, this post will give you enough fundamental information to help you make an educated decision when shopping around.
