Lead-Free Solder: Alloys, Melting Points & Process Selection

What is lead-free solder?

Under the EU RoHS and WEEE directives, lead, mercury, cadmium and hexavalent chromium are restricted in electrical and electronic equipment. Solder with a lead content below 0.1 wt% (1000 ppm) is generally described as lead-free.

STAR.I controls lead below 1000 ppm and cadmium below 20 ppm in its lead-free series. Lead-free solder bars, wires and pastes are SGS-tested and suitable for RoHS-compliant electronic assembly.

RoHS compliance is assessed at the homogeneous-material and finished-product level. Refer to the latest regulations, customer requirements and batch test reports.

Common lead-free alloys and melting points

Alloys differ in melting point, wettability, mechanical strength and cost. The table below provides a starting point for selection; validation with your PCB finish, component temperature limits and reliability tests is still recommended.

Process requirements for lead-free solder

• Use metals with a lower environmental risk; the usual safety order is bismuth > zinc/indium > copper > antimony > silver > lead.
• Wet common lead-free components and PCB materials reliably.
• Form sound joints with minimal oxide inclusions and porosity.
• Resist corrosion and electrochemical migration in service.
• Be compatible with copper substrates, immersion alloy finishes and a range of lead-free and leaded boards.
• Suit different product forms: solder wire, bar, preforms, BGA spheres and SMT paste.
• Provide good electrical and thermal conductivity, a melting point typically below 240°C and adequate strength.
• Be recyclable and cost-effective.

Product forms and applications

• Solder bars: wave soldering, hot-dip tinning, transformer and wire tinning; Sn-Cu-Ni-Ce recommended.
• Solder wire: hand soldering, iron soldering and rework; rosin-core, no-clean or water-soluble flux.
• Solder paste: SMT stencil printing and reflow; SAC305, Sn-Cu and other alloys.
• Electroplating anodes, tin-plated copper wire and alloy wire: plating, component leads and decorative applications.

How to choose a lead-free alloy

• General SMT reflow: Sn-Ag-Cu such as SAC305 (217–220°C) offers a mature process window and balanced properties.
• Cost-sensitive wave soldering: evaluate Sn-Cu0.7 (227°C) with a compatible flux and pot temperature.
• Stable, low-oxidation wave soldering: STAR.I recommends Sn-Cu-Ni-Ce (227°C).
• Low-temperature soldering: Sn-58Bi (138°C) or Sn-50In (117–125°C), with brittleness and long-term reliability assessed.
• High-temperature or high thermal mass: Sn-Sb systems (230–240°C), subject to component and PCB temperature limits.
• Zinc-containing alloys: about 191–199°C, oxidation-sensitive and demanding on flux and atmosphere control.

Confirm the final choice with a pilot run, wettability test, joint inspection and reliability validation. Do not select an alloy from melting-point data alone.

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