Written by Miles Budimir
Senior Editor, WTWH Media
Soldering is a technique used to join together two separate metals using another metal (or metal alloy) with a low melting point. A heat source such as a soldering iron raises the temperature of the two metals to be joined together. When the solder makes contact with the heated metals, it melts. The molten solder flows over the metals and quickly solidifies, bonding them together.
This melting metal alloy, the solder itself, comes in different varieties, depending on the intended use. So solder for electrical applications differs from solder used with copper plumbing, for example. Historically, lead has been a common component of solder due to its relatively low melting point. Other adjunct metals common in solder are tin, copper, and silver, among others.
Solder can be split generally into two categories; lead-based solders (usually a combination of lead and tin) and newer lead-free solders, which use tin along with other metals such as silver, copper, bismuth, or antimony.
Electrical solders have a lower melting point (around 360˚F), while solders used in plumbing have higher melting points, usually well over 400˚F. This is why electrical soldering can be done using a soldering iron while most plumbing soldering uses a gas torch to heat up the metals. Electrical solders also tend to be smaller gage, which means they melt faster, as opposed to plumbing solder which is usually thicker gage and requires more heat to melt.
Within plumbing applications, another distinction is whether the pipes are used in drainage or for drinking water. For the former, lead-based solder is acceptable. However, most local codes call for lead-free solder to be used on all potable water plumbing connections.
Perhaps the key difference between electrical and plumbing solder is the type of flux used in each application. Flux is used to clean the metals to be joined, removing any oxidation and preventing any from forming. The flux also aids wetting, helping the solder to flow and bond with the metals more readily.
Typically, electrical solder contains rosin core flux; plumbing solder uses an acid-based flux. So it’s not a good idea to use plumbing solder for electrical connections because the acid in the flux can damage the wiring and lead to connection failure.