Different PCB laminates used to comply with higher temperature Pb-free fabrication requirements can affect impedance control calculations. Normally, PCB materials like FR-4, FR406 or FR408 can withstand upwards of 270 degrees C or higher. But on rare occasions, a different laminate may be required. In those cases, impedance control calculation changes would impact design layout.
Hot air solder leveling or HASL is a surface finish used for eutectic boards, but is not conducive to Pb-free assembly. Pb-free assembly requires surface finishes such as electroless nickel immersion gold (ENIG), immersion silver, organic solderability protectants (OSP), and a special Pb-free brand of HASL used mainly in Asia to do Pb-free assembly.
Surface finishes are important to Pb-free PCB fabrication because conductivity of immersion silver and immersion gold is considerably higher compared to the tin-lead variety used for eutectic soldering. These finishes withstand higher temperatures, and there is much less likelihood of the pads peeling away from the board surface when it is exposed multiple times to higher temperature cycles.