Lead-free Solder Composition

Soldering

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Editorial Team - PCB Directory

Sep 27, 2020

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

Solder Alloy Composition (% Weight)	Composition	Melting Temperature	Eutectic Solder
96.5% Sn/3% Ag/0.5% Cu (SAC305)	Tin-Silver-Copper	217-220°C	No
95.5% Sn/3.8% Ag/0.7% Cu	Tin-Silver-Copper	217°C	Yes
95.5Sn4.0Ag0.5Cu	Tin-Silver-Copper	217-220°C	No
93.5% Sn/3% Sb/2% Bi/1.5% Cu	Tin-Antimony-Bismuth-copper	218°C	Yes
99.3Sn0.7Cu0.06Ni0.005Ge(SN100C)	Tin-Copper-Nickel-Germanium	227°C	Yes
95.5% Sn/3.5% Ag/1% Zn	Tin-Silver-zinc	218°C – 221°C	NO
96.5% Sn/3.5% Ag	Tin-Silver	221°C	Yes
95% Sn/5% Ag	Tin-Silver	221°C – 240°C	NO
96% Sn/4% Ag	Tin-Silver	221°C- -225°C	No
97.5% Sn/2.5% Ag	Tin-Silver	221°C – 226°C	No
97% Sn/2% Cu/0.8% Sb/0.2% Ag	Tin-Copper-silver	226°C – 228°C	No
99.3% Sn/0.7% Cu	Tin-Copper	227°C	Yes
97% Sn/3% Cu	Tin-Copper	227°C – 300°C	No
95% Sn/5% Sb	Tin-Antimony	232°C – 240°C	No
91.5% Sn/8.5% Sb	Tin-Antimony	232°C – 240°C	No
65% Sn/25% Ag/10% Sb	Tin-Silver-Antimony	233°C	Yes
42% Sn/58% Bi	Tin-Bismuth	138°C	Yes
91% Sn/9% Zn	Tin-Zinc	199°C	Yes
11%Ag/ 89 % Bi	Silver-Bismuth	262°C – 360°C	No

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.