Re: What does the rosin core of solder do that makes solder stick to metal?

Soldering is a joining of two metallic pieces with a low melting point alloy (solder). During the process, the metal parts being join partially dissolve into the solder to form a new alloy which is chemically and mechanically bonded to the metal parts.

All metals, regardless of how shiny they look, have a thin layer of oxide on their surface. That oxide will prevent the solder from reaching the metal and dissolving it. This gives either no bonding or a very weak bond.

Flux is a highly corrosive chemical that will chemically attack and destroy the oxide layer. Once the layer is removed, the solder can interact with the metal and form a strong metallurg ical bond.

There are several types of fluxes, but two general types are most used: acid and rosin. Acid is commonly used in plumbing and other applications where a small amount of corrosion from any residual flux is not critical.

Rosin is used when corrosion is a concern. An example would be electrical circuit boards. The metal in the boards is very thin. Any corrosion can effectively destroy the circuit board. Rosin is noncorrosive at room temperature, so any flux left over from the soldering process will not damage the computer, other electronics devices, etc. At high temperatures, just below 183°C (361°F)for lead-tin solders, the rosin becomes highly corrosive and will attack the surfaces to remove and lift (flux) the oxide away from the part.

An excellent guide on soldering including illustrations and videos can be found at Circuit Technology Center