There are two differences between Pb-Free components and Sn/Pb components:
Care has been taken to ensure that all components are solderable using the current Sn/Pb PCB assembly processes. This means that Pb-Free components can be integrated with Sn/Pb components whenever the assembly processes have not been converted to Pb-free. As a consequence, production assembly lines will not be affected by the use of Sn products and all stocks will be compatible with the various Sn/Pb solder assemblies and components.
Before September 2004, a universal standard for Pb-Free solderability testing had not been established. There were several Pb-Free assembly processes and components used with these processes prior to this time that were not tested to any Pb-Free solderability process. This includes Sn/Pb components. While most Pb-free solder pastes will be acceptable for Pb-Free products, the customer is cautioned to evaluate the Pb-Free pastes selected for production, particularly if the intent is to include Pb components.
Extensive testing performed to date ensures that Pb-free products will survive the Pb-free soldering temperatures. However, when customers convert their production line to Pb-free solder paste, many devices containing Pb will not survive the higher reflow temperature (235°C to 260°C) required for the Pb-free solder paste. This is because mold compounds and die attach materials designed to maintain the package's moisture sensitivity level must be changed. If a customer needs to use leaded parts with lead-free process, it is the customer's responsibility to make certain that Pb components are qualified to their Pb-free soldering process. This includes reflow temperature and solder paste.