Lead-Free Soldering

Lead has been identified as a harmful element and has been eliminated from many applications where it was previously used such as lead compounds in gasoline (or petrol) and paint.  However, new regulations have forced manufacturers to remove lead from solders as well.  Regulations such as RoHS (Restriction on use of Hazardous Substances) initiative in Europe and similar such regulations all over the world are establishing new restrictions to use of lead.  Research into alternatives have narrowed down to a family of alloys called SAC for Sn (Tin) - Ag (Silver) - Cu (Copper).  Different continents seems to have zeroed in on slightly different compositions but the overall composition is 95-96.5% Sn, 3-4%Ag, and 0.5-0.7% Cu.  For wave soldering, the 0.5% Cu - SAC alloys are competing with 99.3%Sn-0.7%Cu alloys with small amount of Ni for stabilization.

With lead-free, one has to worry about not only the solder alloy but the plating as well.  For ease of solderability, most electrical components are plated; plating helps prevent oxidation of base copper alloys and provide good compatibility with the solder alloy.  Gold flash over Ni is a choice but can cause embrittlement of the joint by gold migration;  Silver platings have their own problems including tarnishing, migration, and formation of silver-sulfide dendrites.  Organic inhibitor coatings can mitigate problems with limited shelf life of silver and tin.  Surface finish of choice seems to be Sn since it is compatible with SAC alloys but poses challenges of tin whisker formation.  Tin finishes can be applied by electrolytic (1 to 13 microns), electroless ( less than 5 microns), and immersion (0.3 to 1.5 microns) techniques.

Lead-free solders have higher melting temperatures (217C or 423F) than traditional tin-lead eutectics alloys, collateral damage to components becomes an issue.  Higher melting point reduces the operating process window for the soldering operation since temperatures have to be high enough to melt the solder but low enough to avoid component damage.  Wave soldering operations have adapted well to a lead-free environment since the operating temperatures (250-270C) have remained the same as lead-based alloys. 

Reliability of solder joints with lead-free compositions is also of concern.  For consumer and telecommunications applications (0-100C hardware), SAC solders reportedly perform better than tin-lead compositions.  However, for military and underhood applications, (-55 to 125C) SAC solders do not perform as well.  Cross-compatibility of mixing leaded and lead-free solders and surface platings is also a concern.  Growth of tin-whiskers form 100% tin platings also remains a concern.  Issues with using lead-free solders in high-temperature (die-attach) and low temperature (optoelectronic and MEMS) applications are still being worked out.