Solder is a material that is used to join soft metals, such as copper, gold, and silver. In electronics we're mostly concerned with copper. Most commonly we're using eutectic solder, which is a combination of tin and lead. This is eutectic solder, this is 63 percent tin, 37 percent lead, and this is made with a formulation that has a very minimum melting point for this combination of metals. The melting point is somewhere 360 degrees Fahrenheit. It's a very low temperature, and something we can easily achieve with a soldering iron.
There are some slightly different formulations of solder that are commonly encountered in electronics. A non-eutectic solder, such as 60, 40, which is 60 percent tin, and 40 percent lead, has a slightly higher melting point, something around 370 degrees Fahrenheit. And it also has a range of temperatures where the solid and the liquid tend to co-exist together, and so you can form a kind of pasty substance as it cools. It's often useful for shaping the solder.
You can see that solder is very flexible. This is a property known as ductility. And like the metals that it's used to join with, such as copper, gold, and silver, the ductility allows the metals to interpenetrate one another when the solder is melted. Solder is very dense, it's a very heavy material, mostly because of it's large lead content. Solder comes in a number of different gauges or wire diameters. This is a very heavy gauge here. Smaller. Smaller still. And finally some very fine solder, which could be useful for very delicate work, such as service mount devices.
In recent years there's been a concern that the amount of lead in solder is hazarous to the environment, and there's been a drive to reduce that, t hrough the reduction of hazardous substances, ROHS program. And with that we have lead free solder to discuss. This is a formulation that has three different metals, instead of two, this has tin, silver, and copper. It's mostly tin, but it does have a low melting point, and you can use this type of solder with common electronics using low temperature soldering irons. Lead free solder has a higher melting point, which makes it a little more difficult to work with. It's also a relatively new material and most people are unfamiliar with it's properties.
So lead solder happens to be the most common solder in use today, although that trend is changing.