Tin-Silver-Copper alloys are the primary choice for lead-free SMT assembly.

Lead-free solders do not behave or look like their lead containing counterparts. As our industry changes over to lead-free solders, individual PCB assemblers will need to address several issues relating to hand soldering and rework. These issues include:

- Solder physical properties, melting point, surface tension, oxidation

potential, metallurgy and metal leaching potential.

- Higher peak temperatures.

- Higher preheat temperatures.

- Lead-free finishes for boards and components (preferred).

- Solder cosmetics and surface effects.

- Solder ability differences such as speed of wetting and spread.

- Less self-centering or alignment of components.

The liquidus temperature of SAC alloys is 217-220 °C; this is about 34 °C above the melting point of eutectic 63/37. This higher melting range requires peak temperatures to achieve wetting and wicking to be in the range of 235-245 °C. Lower peak temperatures can be used with SAC solders such as 229 ° C. This lower peak temperature often can only be used for boards with lower overall thermal masses or assemblies, which do not have a large thermal mass differential across the board. This lower peak temperature may also require extended times above liquidus (TAL).

• Bridging

• Solder balls

• Mid-chip balling

• Poor wetting

• Voids

• Tomb-stoning

• De-wetting