by Brian Dodson

Created on: January 08, 2010

Soldering is an ancient technique for affixing multiple pieces of metal, which may have existed as early as 5000 years ago in Mesopotamia.  Two or more pieces of metal are joined by heating them and, often with the aid of a flux (surface cleaner), joining them with a fillet of a third metal.  When simply describing soldering, soft soldering is usually the topic, using filler metals having a melting point below about 400 °C.  Such a filler metal is called solder.

In use, solder is melted and wicked between the pieces of metal which are being soldered together.  The resulting joint is typically not as strong as the metals being soldered, but can have adequate strength, electrical conductivity, and water properties for many applications. 

Traditional solders are made of a tin/lead alloy, at times with additional additives.  For electronics applications the ideal is the eutectic alloy.  This alloy, having 37% lead and 63% tin by weight, melts at 183 °C, and melts and freezes all at once as the temperature changes.  This is unlike other tin/lead alloys, which have an intermediate temperature range between being totally solid and totally liquid. 

Tin being about 7 times as expensive as lead, alloys having higher lead content are used when possible.  A 60/40 solder with a plastic range of 183-190 °C is used for most general applications, despite the small difference in composition from the eutectic alloy.  Part of the reason is that for many applications, it is useful to have the small plastic temperature range between solid and liquid.  It allows one to adjust the soldered joint and perhaps clean it up before it sets completely solid.  This advantage is large enough that most soft solders used in plumbing are 50/50 alloys.  The melting point is 185-215 °C, which provides a long period of time during which the solder can be spread evenly over the joint, adding to the reliability of the water seal. 

To make a sound soldered joint, the adjoining surfaces of the metals to be soldered must be clean and free of oxide layers.  This can in some cases be accomplished simply by sanding or wire brushing the surfaces, but often a flux should be used.  Flux is a reducing agent which cleans the impurities from those surfaces.  There are two main types of flux, an acid flux which is used for most structural soldering, and rosin flux, whose use is primarily limited to electronics

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