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
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