In general, design rules for lead free soldering are the same as those currently in existence for tin lead soldering. However, the designer should be aware of some exceptions and should consult with the printed wiring board fabricator and assembler to minimize manufacturing concerns.

1. Check components MSL (derate Tin-Lead version if unknown) and package temperature limits.

2. Mixing of dissimilar alloys & platings results in low reliability solder joints.

3. Thermal Balance: Distribute larger components evenly across the board. Heat-sinks and power planes should evenly balanced across the board.

4. Use 1oz copper or greater during PCB fabrication.

5. Avoid placing smaller components in isolated positions as these are at risk of overheating during the reflow soldering process.

6. Place large components away from the board edge as these areas tend to be cooler than the center of the board at peak reflow temperature.

7. At minimum, board laminate and prepreg materials must have a Tg (Glass Transition Temperature) greater than 170°C for standard board thickness (1.6mm). Thicker boards (> 1.6mm) must be designed with laminate systems specifically developed for lead-free use.

8 If a lead-free component that is resistant to reflow temperatures in excess of 250°C (260°C preferred) cannot be found, then; alternatively the component may be hand soldered after SMT and wave soldering stages. If this option is used then the board design / layout must accommodate sufficient clearance to access the leads with a soldering iron.

9. Connectors that are not resistant to reflow temperatures in excess of 250°C cannot be used for intrusive (paste in hole) reflow methods. These connectors must therefore be wave or hand soldered.

10. Double-sided boards having bottom-side components that are glued and wave soldered must withstand solder temperatures of 250°C. Suitable clearances are required in the board design / layout to permit selective soldering if components cannot withstand these temperatures.

11. Thru-hole via diameters should be as large as the board design will tolerate to allow vias to expand in the Z-direction during soldering at elevated reflow temperatures. For board designs greater than .062 (1.6 mm), consult with bare board manufacturers to ensure selecting the correct combination of via diameter and suitable laminate material.

12. Decrease the solder paste mask stencil aperture openings as much as 50% for large copper areas used for soldering a component leads.