Wednesday, March 04, 2009

Solder-joins metallic surfaces

A solder is a fusible metal alloy with a melting point or melting range of 90 to 450 °C (200 to 840 °F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful in electronics and plumbing. Alloys that melt between 180 and 190 °C are the most commonly used.

The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning '‘to make solid’'. Solder can contain lead and or flux but in many applications solder is now lead free.

Tin/lead solders are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. At the retail level, the two most common alloys are 60/40 Sn/Pb and 63/37 Sn/Pb used principally in electrical work. The 63/37 ratio is notable in that it is a eutectic mixture, which means:

1. It has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and
2. The melting point is truly a point — not a range

At a eutectic composition, the liquid solder solidifies as a eutectic, which consists of fine grains of nearly pure lead and nearly pure tin phases, but in no way is it an intermetallic, since there are no tin/lead intermetallics, as can be seen from a tin/lead equilibrium diagram.

No comments: