Because it melts easily and is good in a solder mixture to hold wires together.
It's the other way round, soldering is used in electronics.
In batteries and soldering.
solder tin, lead,
Borax is used as a flux in soldering to clean the metal surfaces being joined and to prevent oxidation during the soldering process. It helps the solder flow more easily and create a strong bond between the metals.
Soldering irons are usually used with rosin core solder. Sometimes it has lead in it but often not since its toxic. Most of the time its for heavy soldering like heavy gauge wires or older circuit boards since the tips are fairly big. They also have a decent temperature control so anything that is delicate like large SMD soldering.
Soldering wire is usually made of a combination of tin and lead. The most common ratio is 60% tin and 40% lead. However, due to environmental and health concerns, lead-free solder wire made of tin, silver, and copper is also widely used now.
Lead is unlikely to become airborne during hand soldering because the operation occurs at a temperature much lower than the boiling point of the lead.
lead burning or lap joint soldering
Soldering is the process of using a bubble of hot metal to bond two electronic components or circuits together.
Soldering irons are designed to heat up the lead in solder to its melting point, allowing it to flow and create electrical connections. The high temperature of the soldering iron transfers heat to the lead in the solder, causing it to melt and bond with the materials being soldered.
The purpose of a cold heat soldering iron is to fuse electronic components, pipes and other metal materials together by melting solder as a connector which hardens as it cools. You can learn how a cold heat soldering iron works and it's different purpose at the HowStuffWorks website.
salammoniac is used to clean and tin the soldering iron that solders the lead that holds the stained glass together.