Solder (/ˈsoʊldə/, /ˈsɒldə/ or sometimes in USA /ˈsɒdər/) is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece(s).

Soft solder is typically thought of when solder or soldering is mentioned, with a typical melting range of 90 to 450 °C (190 to 840 °F). It is commonly used in electronics and plumbing, and when manually applied is often done so using a soldering iron or soldering gun. Alloys that melt between 180 and 190 °C (360 and 370 °F) are the most commonly used. Soldering performed using alloys with melting point above 450 °C (840 °F) is called 'hard soldering', 'silver soldering', or brazing.

For certain proportions an alloy becomes eutectic and melts 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. In electrical work, if the joint is disturbed in the pasty state before it has solidified totally, a poor electrical connection may result; use of eutectic solder reduces this problem. The pasty state of a non-eutectic solder can be exploited in plumbing as it allows molding of the solder during cooling, e.g. for ensuring watertight joint of pipes, resulting in a so-called 'wiped joint'.

For electrical and electronics work solder wire is available in a range of thicknesses for hand-soldering, and with cores containing flux. It is also available as a paste or as a preformed foil shaped to match the workpiece, more suitable for mechanized mass-production. Alloys of lead and tin were universally used in the past, and are still available; they are particularly convenient for hand-soldering. Lead-free solder, somewhat less convenient for hand-soldering, is often used to avoid the environmental effect of lead.

Plumbers often use bars of solder, much thicker than the wire used for electrical applications. Jewelers often use solder in thin sheets which they cut into snippets.

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

With the reduction of the size of circuit board 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.

Contact with molten solder can cause 'solder embrittlement' of materials, a type of liquid metal embrittlement.

Read more about Solder:  Lead Solder, Lead-free Solder, Flux-core Solder, Hard Solder, Solder Alloys, Intermetallics in Solders, Glass Solder, Preform

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