Solder

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Solder ( /ˈsɒdər/ or /ˈsɒldər/) is a fusible metal alloy with a melting point or melting range of 90 to 450 degree Celsius (190 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 (360 and 370 °F) are the most commonly used. By definition, using alloys with melting point above 450 °C (840 °F) is called brazing. Solder can contain lead and/or flux but in many applications solder is now lead free.

The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning "to make solid".

Eutectic alloys melt at a single temperature. Non-eutectic alloys have markedly different solidus and liquidus temperature, and within that range they exist as a paste of solid particles in a melt of the lower-melting phase. The pasty state causes some problems during handling; it can however be exploited as it allows molding of the solder during cooling, e.g. for ensuring watertight joint of pipes, resulting in a so called wiped joint.

With the reduction of the size of circuitboard features, the size of interconnects shrinks as well. Current densities above 104 A/cm2 are often achieved and electromigration becomes a concern. At such current densities the Sn63Pb37 solder balls form hillocks on the anode side and voids on the cathode side; the increased content of lead on the anode side suggests lead is the primary migrating species.[1]

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