Lead Free Technology

The primary choice for SMT assembly is lead free solder. Lead free solder is typically comprised from an alloy of tin, silver and copper.

Working with Lead-Free Products

Lead free solder doesn’t look or behave like their lead solder counterparts. As the electronics industry changes over to lead free solders, they will need to address several issues related to hand soldering and reworking. These include the physical properties such as: the melting point, surface tension, oxidation potential, metallurgy and metal leaching potential.

The main differences between lead based SMT soldering and lead-free SMT soldering are:

• Physical properties; the melting point, surface tension and oxidation potential, as well as metallurgy and metal leaching potential
• Higher peak temperatures
• Higher preheat temperatures
• Lead-free finishes for boards and components
• Solder cosmetics and surface effects
• The differences between solder ability, such as the speed of wetting and spread
• Less self-centering or alignment of components

SAC alloys have a liquidus temperature between 423 -428° F (217-220° C) which is approximately 93° F (34° C) above the melting point of eutectic solder (63/37). This requires higher peak temperatures of 455-473° F (235-245° C) to achieve wetting and wicking. For boards or assemblies with lower overall thermal masses, which do not have large thermal mass differentials, lower peak temperatures of 444° F (229° C) can be used with SAC solders. This lower peak temperature will also require longer heating periods above liquidus (TAL)

Common defects with lead-free reflow or reworking are:

• Bridging
• Solder balls
• Mid-chip balling
• Poor wetting
• Voids
• Tomb-stoning
• De-wetting