Lead-free soldering Potential solder alloys

Standards will require revision to accommodate the potential for a variety of leaded and lead-free solder alloys. Acceptable test measurements including inspection criteria must be established including shear and tensile testing, wetting angle measurements, visual criteria defining the acceptable surface appearance of the solidified alloy and retraining will be required. 

Figs. 69, 70, and 71 show that the use of lead-free solder (Sn-Ag-Bi-In) has not resulted in reliability problems. However, some joints have indicated a concern with the potential precipitation of bismuth from the alloy over time. Therefore the bismuth content of the solder used to assemble the MD player will be reduced from 2.5% bismuth to 0.5% bismuth with 6% indium. 

Lead-free solders available on the market today can be classified into three general categories binary, ternary, and complex multicomponent systems. These terms are descriptive of the number of elements initially used to create the alloys. Among the binary alloys of Sn-Bi, Sn-Sb, Sn-Zn, Sn-Cu, and Sn-Ag, the Sn-Ag alloys have shown the greatest potential for application in electronic assemblies. Ternary alloys of interest are Sn-Zn-Bi, Sn-Ag-Cu, Sn-Ag-Bi, Sn-Ag-In, Sn-Zn-In, etc., with Sn-Ag-Cu becoming the popular choice for many applications. The lead-free alloys of interest are discussed in detail in Chaps. 8 and 9. In pure form, the alloys are studied to determine the bulk properties and their potential for applications in the electronics industry. 

In an ideal world, the transition from SnPb to Pb-free could happen overnight. However, in reality, the transition is not likely to happen instantly instead, this is expected to occur over a period of at least 6 to 12 months. During this time, SnPb and lead-free alloys are likely to coexist on the same printed circuit board assembly (PCBA), which poses potential issues with component soldering and reliabiUty. 

Many component terminations are either plated or dipped into molten tin-lead solder to preserve their solderability, providing yet another source of lead. This is the normal procedure for component terminations that are difficult to solder, such as nickel-iron or nickel plating. Alternative lead-free finishes for components include palladium (Pd), and gold (Au) applied by electroless plating over nickel (Ni), and hot-dipped coatings of Sn-Ag, Sn-Cu, or Sn-Ag-Cu alloy. Some component metallizations are coated with Pd-Ag, Pd-Au, or Pd-Ni. These coating alternatives for tin-lead add new materials to the electronics waste stream, and potentially make recycling or reclamation of metals in electronic products more difficult and costly due to the increased number of separations required. 

---