What is Circuit Design

Circuit Designers try to make connections.

6. Putting it all Together

6.2. Eutectic Compositions

Solder is a type of metal chosen for its electrical, mechanical, and thermodynamic properties.  It is generally composed of two or more metals.  Historically, those metals have been tin (Sn) and lead (Pb).  When tin and lead are mixed together, they form an alloy whose melting point is lower than the melting point of either individual metal. 

Most alloy concentrations of lead and tin have an intermediate stage of matter that appears to be a type of sludge or paste -- neither completely solid nor completely liquid.  But when the two elements are mixed together in an exact proportion of 63% Tin and 37% Lead (Sn63Pb37), the metals transition directly from solid to liquid and from liquid to solid at a lower temperature and without the intermediate states-of-matter ever appearing.  This precise mixture transitions at a single, low temperature and the alloy is referred to as an Eutectic composition.

Eutectic Compositions

This is a common Eutectic Diagram that shows the mass percentage of Lead and Tin on the horizontal axis and the phase transition temperatures on the vertical axis.

Not all solder compositions are eutectic -- but they all do try to minimize the transition temperatures into and out of the sludge/paste phase.  

Solder Alloys

Assorted Solder Compositions are shown in the table above.

The theory for the rapid transition is that it can keep contaminants from congealing into large clumps during the cool-down phase.  Large clumps of contaminants become mechanically weak areas where a solder-joint might break.