Lead-free Solder Composition

Lead-free solder materials as the name suggest do not have any lead in their composition. Originally when solder materials and solder paste were developed they almost always used lead. However, lead is a harmful/poisonous material and thus in 2006 the use of lead based soldering materials were prohibited by the RoHS directive (2002/95/EC).

Lead-free solder materials usually comprise of lead-free solder and flux. Lead-free solder can be made from different compositions (% weight) of metals, such as Tin (usually the main element), Silver, Copper, Antimony, Bismuth, Cobalt, Nickel, Indium, Zinc, Germanium, and rare earth elements. The different compositions decide the properties of the solder paste. For example, the composition 96.5% Sn/3% Ag/0.5% Cu (SAC305) has high strength and thermal fatigue; similarly, the composition 91% Sn/9% Zn has high drossing and corrosion potential.

As per IPC standard J-STD 004, three types of flux available; they are Rosin and Rosin substitute type flux, Water-soluble type flux, and No-clean type flux. Anyone type of flux (either rosin or water-soluble or no-clean) is used in a solder paste.

Popular Lead-Free Solder Material Composition

Eutectic solder is a solder alloy that melts and freezes at one single temperature. For example, the solder alloy composition 95.5% Sn/3.8% Ag/0.7% Cu is a eutectic that melts and freezes at a temperature of 217°C. Non-eutectic solder has a melting range. For example, 96.5% Sn/3% Ag/0.5% Cu (SAC305) is a non-eutectic solder that starts melt at 217°C and becomes fully liquid at 220°C.