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Definition and classification of lead-free solder

Definition and classification of lead-free solder

Lead-free solder is relative to lead-containing solder. In lead-containing solders, lead exists as a matrix element, but it does not mean that lead-free solder is absolutely free of lead, and Pb can exist as an impurity element. It can be seen that there is an upper limit to the content of Pb in “lead-free solder”, so “the problem of defining lead-free solder” can be converted into “the problem of the upper limit of lead content in lead-free solder”, but there is currently no lead-free solder international standard. Table 1 shows the regulations for toxic substances in the RoHS Directive.


Around 1990, ISO 9453, JIS Z 3282 and other international standards have specified the impurity content of binary alloy lead-free solders, that is, the lead mass fraction is less than 0.1%, and the EU recently introduced the RoHS directive the draft proposes to control the lead mass fraction below 0.1%. Therefore, in PCB manufacturing, the solder paste of the circuit board requires that the lead content of the lead-free solder should generally be less than 0.1%.


Lead-free solders are generally classified by component, composition, and melting temperature. According to the component classification, it is usually divided into binary alloy, ternary alloy and multi-alloy alloy, wherein the binary alloy includes SnZn, SnCu, SnBi, SnAg, SnIn, SnSb, SnAu, etc.; the ternary alloy mainly includes SnAgCu, SnAgBi, SnZnBi Etc.; the multi-alloy mainly includes SnAgCuBi, SnAgCuRE, SnZnBiRE, SnAgCuZn etc.



Toxic substance  

Maximum limits(wt.%)

Heavy metal Lead 0.1%(1000ppm)
Mercury 0.1%(1000ppm)
Cadmium 0.1%(1000ppm)
Chromium(VI) 0.1%(1000ppm)
Organics Polybrominated biphenyls(PBB) 0.1%(1000ppm)
Polybrominated biphenylethers(PBDE’s) 0.1%(1000ppm)


Sn3.5Ag0.75Cu、 Sn (3.4~4.1 ) Ag(0.45 ~0.9) Cu, Sn3Ag2Bi, Sn57BilCu, Sn3.5Ag0.5Cuzn etc.

According to the composition, lead-free solder can be classified into Sn0.7Cu, Sn3.5Ag, Sn58Bi, Sn3.5Ag0.75Cu, Sn (3.4~4.1) Ag (0.45-0.9) Cu, Sn3Ag2Bi, Sn57BilCu, Sn3.5Ag0.5Cuzn etc.

According to the melting temperature, lead-free solder can be divided into low-temperature lead-free solder, medium-temperature lead-free solder and high-temperature lead-free solder, but the boundary of the melting temperature is not clear. Lead-free solders with a melting point lower than 138 ° C are generally considered to be low-temperature solders, such as Sn52In; high-temperature lead-free solders with a melting point higher than 250 ° C, such as Au20Sn; etc.; the rest are medium-temperature lead-free soft materials, such as Sn3.5Ag, Sn0 .7Cu and so on.