Surface Finishes and Pb-Free PCB Fabrication
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.
There are several trade-offs involved with Pb-Free surface finishes used for PCB fabrication: shelf life, cost, reflow cycles and solder joint flatness. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost five to 10 percent extra, depending on the amount of exposed surfaces. Therefore, it is best from a cost/performance point of view to select a finish that can be cost-justified in a particular end-product. HASL has a shelf life of about 18 months versus only six months for OSP. Immersion silver has a shelf life ranging from 12 to 16 months, while immersion gold is the most durable, clocking in at 24 months.
An OEM should cast a wary eye on the EMS provider’s system for separating eutectic components from Pb-free components. The last thing an OEM wants is for his boards, supposedly populated with Pb-free components, to go through the Pb-free reflow oven and come out damaged. This relates to how QC is conducted for incoming components, how well auditors are trained, and the level of professionalism applied to the inventory system.