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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:

The main differences between
a leaded and lead-free SMT process are summarized below :

Successful rework and PCB assembly methods have been developed with lead-free solders for all types of components. During the transition period to lead-free manufacturing rework practices have to be developed that deal with the issues related to handling, reworking, tracking and inspecting lead free PCB assemblies.

Lead-free solder rework is different than leaded (Sn63/Pb37)solders as the lead-free solder alloys typically do not wet or wick as easily.

Successful rework methods have been developed with lead-free solders (including Sn/Ag/Cu or Sn/Ag alloys) including components removal/replacement, jumper wires, trace and pad repairs and BGAs. Most of your existing rework equipment for Sn63-based alloys can still be used for the lead-free solder.

The soldering parameters must be adjusted to accommodate the higher melting temperature and lesser wettability of the lead-free solder. Studies have shown that reliable lead-free solder joints, with proper grain structures and intermetallic formation, can be produced using appropriate rework processes.

The liquidus temperature of SAC alloys is 217-220 °C; this is about 34 °C above the melting point of leaded (Sn63/Pb37). 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 liquidous (TAL).