Point #3: Tin whiskers
The purpose of adding lead to soldering alloys was to prevent tin whiskers from popping up. While the reason for whisker formation remains unknown, we do know these crystalline structures of tin sometimes grow from surfaces where tin, especially electroplated tin, is used as a final finish. It is also suggested that voltage and soldering temperature have something to do with tin whisker growth.
Point #4: Reliability
Soldering professionals all agree that that for critical applications where reliability is paramount, such as in military and aerospace electronics, the use of lead-bearing solders is the wiser choice. Lead-free solders have a tendency of failing earlier as compared to lead-bearing solders. For reliable electronic components that have sound circuits, lead-bearing solder alloys are better; they ensure the electrical properties are preserved over a longer time period.
Lead-free soldering is continually evolving. In the future, you might find that lead-free solder alloys can compete better with lead-bearing alloys in terms of reliability.
About the Author:
Alpha Assembly Solutions, Inc. is the global leader in the development, manufacturing and sales of innovative specialty materials used in a wide range of industry segments, including electronics assembly, power electronics, die attach, LED lighting, photovoltaics, semiconductor packaging, automotive and others. Since its founding in 1872, Alpha has been committed to developing and manufacturing the highest quality specialty materials.