For military and aerospace applications, Lead-free is a general term defined as the use of Tin (Sn) with the absence of Lead (Pb) from a system previously requiring use of Lead (Pb). The referenced system may be specifically plating or other finish process or the attaching media process used to install components to include but not limited to electronics parts, terminals, lugs, and mounting hardware associated with avionics systems.
The term Lead (Pb) – free electronics is actually a misnomer. The term "Lead-free" was coined in response to the European Union's banning of certain specific hazardous materials [including Lead (Pb)] through their Reduction of Hazardous Materials (RoHs) and Waste Electrical and Electronic Equipment (WEEE) directives. Although neither directive specifically use the term "Lead-free,” the phrase has been widely accepted with various meanings. Current RoHS and WEEE related regulatory information are provided as separate contributions.
DoD has been using components without Lead (Pb) for decades. However, these materials and alloys have had minimal Tin (Sn) content and were primarily made with Copper (Cu), Iron (Fe) & Nickel (Ni), with some exotics such as Palladium and Gold (Au) being used (Reference MIL-STD-1276). In this case when Lead (Pb) – free electronics are referenced, in reality the concern is the use of Tin (Sn) with the absence of Lead (Pb); in other words, pure or nearly pure Tin (Sn). Lead (Pb) acts as a poison and inhibits the growth of tin (Sn) whiskers, while providing many good metallurgical properties when used in solders. Lead (Pb) enhances inter-metallic growth, which in turn improves joint integrity. Solders with high Tin (Sn) content are harder and less ductile compared to high Lead (Pb) content solders (SN63-37Pb). Also, Lead (Pb) use in solder alloys, lowers the melting point to around 185 degrees C – depending upon alloy mixture. Lead (Pb) also reduces the service tension of the solder, thereby providing excellent wetting properties. It is important to note the Printed Wiring Boards (PWBs) used in our legacy systems were designed and manufactured around the mechanical properties associated with (Tin (Sn) – Lead (Pb) solders.
This definition provides a common sense interpretation of the term "Lead-free" as it applies to military and aerospace applications. See also ACQuipedia Article entitled “Lead Free Electronics” and DAU continuous learning module CLL 007 “Lead-Free Electronics Impact on DoD Systems” for additional information
