Lead-free solder alloys typically have melting points that are 30-40°C higher than the melting point of lead alloys. This temperature difference complicates the assembly process if the PCB is populated with both lead and lead-free components (often the norm). In particular, subjecting the board to a range of temperatures can affect fluxes, pastes, and plastic component packages, and higher temperatures can damage sensitive components. So, mixing lead and lead-free components must be accompanied by tighter monitoring and control of the entire PCB assembly and solder process.

Be sure to contact your wave-solder equipment manufacturer to review the processes needed to maintain effective control of PCB quality and reliability. For example, testing should address any differences required in wetting, dwell time, pre-heat times, feeder speeds, cool-down times, and other processes. Also be aware that the use of pure tin solders may damage the stainless steel components in the wave solder equipment.

References: www.smta.org and www.smtinfocus.com 

Cross contamination might occur during wave soldering if the manufacturing process involves both lead and lead-free components. The solder pot may become contaminated with a fractional amount of lead, which then gets re-deposited on the PCB during the wave solder step.

Determine whether or not separate solder steps and pots are required to maintain solder free products.

The mandate for the continued use of lead in solder for military components is partially based on several potential reliability issues. The most significant concern regards to risk of inter-metallic migration in the termination of the electronic component and the growth of tin whiskers which could short out the electronic PCB. [www.calce.umd.edu/lead-free/tin-whiskers/]

Contact your solder supplier to see how this may affect your PCB reliability.

References: www.aimsolder.com www.alphametals.com

Given that lead-free components require changes to the PCB assembly process, manufacturers need a method to identify components that are lead-free. Here are different approaches currently being used by component manufacturers:

Create a new part number

• Manufacturers will change the part number when replacing a leaded product with its lead-free equivalent.
• The leaded product may or may not continue to be available under the old part number.

Continue to use the existing part number

• Manufacturers utilizing the same part numbers, will indicate lead-free components by the use of specific date code, lot code, or a lead-free symbol on the label.
• The leaded part may or may not be available under a new part number.

If your ERP and MRP systems only identify parts based on their part number (i.e., no lot codes or date codes) you may be ?blind? to the part?s lead/lead-free status. In these situations, be sure to develop extra procedures to keep track of leaded and lead-free versions of a part.