The flow of solder on a metal surface is known as "wetting." Reliable solder connections show thorough wetting. Poor wetting demonstrates unreliable soldering. (Poor wetting is often - and incorrectly - referred to as "cold solder" even though the condition is not caused by lack of heat.) This sample Science of Soldering© video lesson explains the forces that determine solder wetting and reliability. Every operator, technician and engineer requires knowledge of wetting and wetting forces but only Science of Soldering© provides this information properly.