Lead-Free Solders
Lead-free solders are tin alloyed with a few percent of silver, copper, antimony or zinc to bring the melting point down from 232 °C and to strengthen the joint. The tin-silver-copper (SAC) family melts at 217–220 °C and is the RoHS electronics standard; tin-copper eutectic at 227 °C is the low-cost wave and plumbing alloy; tin-silver at 221 °C is the strongest of the common copper-joining grades. All of them run 30–40 °C hotter than tin-lead, wet less eagerly and need more active flux or nitrogen to give the same joint.
Popular lead-free solder grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| SAC305 | 32 | 48 | 7.38 | Poor | Poor | Default RoHS reflow, wave and hand solder |
| SAC405 | 40 | 52 | 7.44 | Poor | Poor | Earlier SAC standard; slightly better wetting |
| SAC105 | 24 | 33 | 7.38 | Poor | Poor | Drop-shock resistance in handheld BGAs |
| SAC0307 | 25 | 32 | 7.38 | Poor | Poor | Low-silver wave and selective soldering |
| Sn99.3Cu0.7 | 15 | 28 | 7.31 | Poor | — | Copper plumbing and low-cost wave pots |
| Sn96.5Ag3.5 | 35 | 55 | 7.37 | Poor | — | Strongest common copper-tube joint |
| Sn95Sb5 | 30 | 40 | 7.25 | Poor | Poor | Joints that stay solid at 200 °C |
Choose lead-free solder for
Every electronics assembly without a lead exemption, copper plumbing and refrigeration joints, food and potable-water fittings, stainless and copper heat exchangers, and any product that must pass RoHS, REACH or drinking-water rules — SAC305 on the board, Sn-Cu or Sn-Ag on the pipe.
Look elsewhere when
The assembly is exempt and already qualified on tin-lead (Sn63Pb37 wets better, melts at 183 °C and has a longer field record), a component cannot survive a 245 °C reflow peak (Sn42Bi58 at 138 °C), the joint must hold above about 150 °C (Pb95Sn5 under the high-lead exemption, or a silver braze), or the joint carries real load (braze or weld — no solder exceeds about 60 MPa and all creep at room temperature).
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Lead-free solder by application
- Reflowed circuit boards
- SAC305 paste, Type 3 or Type 4 powder, no-clean or water-soluble flux; SAC405 only where the process is already qualified on it.
- Wave and selective soldering
- SAC0307 or Sn99.3Cu0.7 in the pot for through-hole and mixed assemblies — lower silver cost and slower copper dissolution than SAC305, at a 260 °C pot temperature.
- Portable and automotive electronics
- SAC105 under BGAs that see drops and vibration; the softer low-silver joint absorbs shock that cracks SAC305 at the intermetallic layer.
- Copper pipe and heat exchangers
- Sn96.5Ag3.5 or Sn95Ag5 for potable water and refrigeration fittings, Sn99.3Cu0.7 for general plumbing; Sn95Sb5 where the line runs hot.
Working with lead-free solder
- Process
- Reflow SAC305 with a peak of 235–250 °C and 45–90 s above 217 °C; wave and selective pots run 255–265 °C. Hand soldering needs a 350–400 °C tip and a more active flux than tin-lead (ROL1 or a water-soluble class per J-STD-004). Nitrogen improves wetting and reduces dross on wave lines; Sn91Zn9 is unusable without it.
- Joint reliability
- The joint forms a Cu₆Sn₅ and Cu₃Sn intermetallic layer that thickens with time and temperature and is where brittle fracture starts — keep it to a few microns by limiting time above liquidus and the number of reflows. SAC305 outlasts tin-lead in thermal cycling to 125 °C but is stiffer, so drop performance is worse; SAC105 reverses that trade. Pure-tin finishes grow whiskers — specify matte tin over a nickel barrier, or a SAC finish.
- RoHS and regulatory
- RoHS limits lead to 0.1 % of any homogeneous material; SAC and Sn-Cu grades meet it with margin. Potable-water solders must be lead-free under drinking-water rules and are usually Sn-Ag or Sn-Cu with a certified flux. Specify the alloy to J-STD-006 or ISO 9453 and the flux class to J-STD-004 on the drawing.
- Storage and handling
- Solder paste is stored at 0–10 °C, has a 6-month shelf life and must warm sealed for 4 h before opening; once on the stencil it is good for 8–12 h. Wire and bar keep indefinitely if dry. Never top up a lead-free pot with tin-lead bar — a few percent lead forms a 179 °C phase that fails at the joint; keep the two processes on separate lines and tips.
Other solder & low-melting alloy series
Lead-Free Solders — frequently asked
What is lead-free solder?
Lead-free solders are tin alloyed with a few percent of silver, copper, antimony or zinc to bring the melting point down from 232 °C and to strengthen the joint. The tin-silver-copper (SAC) family melts at 217–220 °C and is the RoHS electronics standard; tin-copper eutectic at 227 °C is the low-cost wave and plumbing alloy; tin-silver at 221 °C is the strongest of the common copper-joining grades. All of them run 30–40 °C hotter than tin-lead, wet less eagerly and need more active flux or nitrogen to give the same joint.
Which grade of lead-free solder should I use?
SAC305 for reflow and hand soldering of anything sold into RoHS markets — the default paste, wire and bar, with the widest process data behind it. SAC405 wets a little better and was the earlier standard; keep it where a qualified process already uses it. SAC105 trades thermal-cycling life for drop-shock resistance, so it is the choice for handheld and automotive body electronics with BGAs. SAC0307 and Sn99.3Cu0.7 are the low-silver wave and selective-soldering alloys where pot cost and copper dissolution matter more than joint strength. Sn96.5Ag3.5 and Sn95Ag5 join copper tube, heat exchangers and potable-water fittings with the highest strength of the group; Sn95Sb5 is the high-temperature grade for joints that must stay solid at 200 °C; Sn91Zn9 melts at 199 °C and solders aluminum but oxidizes fast and needs a covered pot.
Which grade of lead-free solder is the strongest?
Sn91Zn9 has the highest yield strength in the series at 42 MPa; SAC405 follows at 40 MPa. Values are for each grade's default condition.
Which grade of lead-free solder is easiest to machine?
Sn99.9 (machinability fair, 45/100), then SAC0307 (30/100).
SAC305 or SAC405?
Same alloy family with 3 % or 4 % silver; both melt at 217–220 °C and process identically. SAC405 wets slightly better and was the first industry standard; SAC305 replaced it because the extra silver bought little joint life. Use SAC305 for new work and SAC405 only where an existing qualification names it.
Why do lead-free joints look dull and grainy?
Because SAC solidifies over a range rather than at a sharp eutectic point, and the tin dendrites shrink as they freeze, so the surface is matte and slightly rough even on a good joint. Judge the joint by wetting angle and fillet shape, not shine — IPC-A-610 accepts the dull finish. A truly disturbed joint shows cracks or a lifted fillet, not just a grainy surface.