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The main types of lead-free solders which are commercially available are:
Alloy composition |
M.pt. °C |
Comments |
Sn0.7Cu |
227 |
Recommended for wave soldering applications (known as 99C) |
Sn3.5Ag |
221 |
Wetting inferior to SnAgCu but used where higher melting point is required |
Sn3.5Ag0.7Cu (and variations on this) |
217 |
Most widely used lead-free alloy. Various percentages of silver and copper are used. Recommended by NEMI for surface mount |
SnAgBi alloys (some with Cu) |
~ 210 -215 |
Better wetting properties than SnAgCu but must not be used with lead. Mainly used as solder paste but has been used for wave soldering, mainly in Japan. Wire not available so rework difficult |
Sn9Zn |
198 |
Zinc-containing alloys are difficult to use, need special fluxes and are susceptible to corrosion but new solder pastes with reasonable soldering performance have recently been developed. |
Sn8Zn3Bi |
~ 191 |
Used by several Japanese manufacturers where heat sensitive components are used. Includes NEC and Matsushita. Paste made by Senju. Difficult to use, needs nitrogen for SMT |
58Bi42Sn |
138 |
Low melting point, hard, brittle alloy but performed well in reliability trials |
The main difference between tin/lead and the most commonly used lead-free solders is melting point. The melting point of SnAgCu is 34°C higher than that of eutectic SnPb. Increased temperature can damage components and laminate and so it is important to:
- use as low a temperature as possible
- avoid heat sensitive components – damage includes plastics melting, delamination of ICs, cracking of brittle ceramic devices, loss of fluid from electrolytic capacitors, and
- consider using high T g laminate for thicker PCBs with many layers and large high thermal mass components – laminate damage can include warping, delamination, and cracks in plated through holes.
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