For years, solder was made from a combination of lead and tin, but concerns about lead exposure have led to a shift toward lead-free alternatives, which are safer and more eco-friendly. Lead-free solders have a higher melting point than lead-based ones, making them more heat-resistant and requiring higher temperatures for application. Lead solder appears shinier and heavier, while lead-free solder looks duller and rougher. In terms of performance, lead-based solders provide smoother application and stronger bonding, making them easier to work with, but lead-free solders are less prone to spreading during the reflow process. A downside of lead-free solder is the potential for tin whiskers, tiny conductive structures that can cause short circuits, an issue mitigated in lead-based solders.
Lead-based solders are also more affordable, as lead is cheaper than tin or silver, but lead-free solders eliminate the health risks associated with lead exposure. Longevity-wise, lead-based solder forms a thinner, more flexible intermetallic layer, which is less prone to cracking, while lead-free solder forms a thicker, more brittle layer, which skilled professionals can manage to minimize potential issues.
source: https://www.myemssolutions.com/lead-and-lead-free-solder-use-in-pcb-manufacturing/
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