Solder candidate alloys

Lead-Free Solder Alloys. Because of the toxicity of lead and the concern that the lead in electronic products may end up in landfills, and ultimately in the water supply, the electronic industry is exploring alternative solder alloys that do not contain lead. These alternative solder alloys are typically composed of tin (Sn), with one, two, or three additives such as copper (Cu), silver (Ag), bismuth (Bi), antimony (Sb), zinc (Zn), or indium (In). Typical tin lead-free candidate solder alloys include Sn-Cu, Sn-Ag, Sn-Ag-Cu, Sn-Ag-Cu-Sb, Sn-In, and Sn-Cu-Bi-Sb. Some of these are more suitable for wave solder... 

It is generally agreed upon that none of the lead-free binary systems investigated is likely to be a consensus replacement for eutectic Sn Pb in assembly reflow applications, although this may be the case for wave soldering. Viable candidates typically consist of a small quantity of a third or fourth element added to a lead-free binary alloy compared to the relatively uncomplicated eutectic Sn-Pb solder. 

D. Major Lead-Free Candidate Solder Alloys... 

Taking the foregoing concerns into account this chapter discusses in detail the key metallurgical aspects including as-solidified microstructure, thermal stability characteristics, effects of elemental additions, and thermal aging, etc. as they relate to the Sn-Ag, Sn-Cu and their ternary alloys (Sn Ag Cu) into account. These alloy systems are the subject of much attention as solder replacement candidates for solders of the Pb-Sn system, particularly eutectic Sn-Pb. In addition, the chapter discusses the major mechanical properties of interest for electronic packaging applications as a consequence of their metallurgical characteristics. 

For flip-chip applications, the eutectic Bi-Sn solder alloy was electroplated to form solder bumps on various under-bump metallizations (UBMs) [37]. Based on the nature of the interfacial reactions and ball shear strength exhibited when exposed to multiple reflow cycles, Bi-Sn solder bumps were identified as a potential candidate for replacing the Pb-Sn eutectic solder bumps. 

Several Pb-free solders were reviewed in light of replacing the conventional Pb-Sn solders particularly the eutectic alloy used for various electronic packaging applications. These candidates include the binary solder alloys of the Bi-Sn, Sn-Sb, Sn-Cu, Sn-Zn, and Sn-In solder systems and their ternary and quaternary compositions. The Sn-Ag system including its ternary and quaternary compositions are discussed in detail in Chap. 8. 

A number of Pb-free binary, ternary, and quaternary alloys with combinations of potential candidate elements solder alloys have been surveyed and the differences in their mechanical... 

There are no drop-in replacement solders for Sn-Pb. Although there are many candidates, the most widely considered alloys require higher process temperatures and necessitate changes to components, circuit board laminates, and even process equipment. 

The electroplated nickel (Ni) serves as a barrier between the copper and the gold. Ni on its own is a poor candidate for a solder pad coating as it oxidizes rapidly and its native oxide is very stable—chemically tough to dissolve or react away. Unoxidized Ni can alloy with tin, which means that it can be soldered. Were the gold deposited over the copper directly, the gold layer would begin to interdiffuse with the copper even at room temperature forming an... 

Investigations into a solder for lead-free production have not highlighted an ideal alloy for PCB manufacturers. Because of its processing temperature of260°C, the leading candidate for use in many facilities is tin/ copper. This temperature is nearly identical to that of processes running on many leaded machines. Many board manufacturers rely on optional finishes, such as ENIG, instead of lead-free solder. 

TABLE 12 Relative Cost of Some Candidate Lead-Free Alloys to Replace Eutectic Sn-Pb Solders in Electronic Assemblies Based on Their Densities... 

Sn-Ag-Cu Leading candidate system for reflow soldering Lower melt point than Sn-Ag, Sn-Cu binary alloys Adequate wetting, mechanical properties Much reduced Cu scavenger characteristics compared to Sn-Ag... 

Most lead-free alloys of interest have a melt temperature that is about 40°C higher than eutectic Sn-Pb (mp = 183°C), which can have a drastic effect on the integrity, reliability, and functionality of printed wiring boards, components, and other attachments (e.g., connectors) [18]. The bulk of lead-free research has focused on identifying suitable solder candidate alloys to replace eutectic Sn-Pb. Solder pastes, wires, and bar stocks are only a part of the issue. The effects of the increase in process temperature necessary for utilizing most Pb-free solders are listed in Table 26 and discussed in the following sections. 

Bosch is involved with lead-free activities including solder paste alternatives. The likely candidates are alloys within the Sn Ag Cu or Sn-Ag Bi solder systems. Some automotive electronics products at Bosch are aheady lead-free. According to material suppliers, Bosch introduced conductive adhesives in its hybrid devices to a significant extent. 

Chap. 8 discussed the Sn-Ag and Sn-Ag-Cu based systems, whose alloys are of key interest as substitutes for Pb-containing solders. However, there are several other alloy systems of interest as well. Some lead-free solders have been utilized commercially, while others developed as Pb-free substitutes [2-11]. Several examples are Sn-3.5Ag, Sn-3.5Ag-0.7Cu, Sn-3.5Ag-4.8Bi, Sn-0.7Cu, Sn-5Sb, Sn-20In-3Ag, Sn-8Zn-3Bi, Sn-10In-3Ag-lCu, and Bi 3Sn among others. The list has been reduced to three or four promising candidates as a result of accelerated development efforts in the microelectronics industry [12-15]. 

For more examples of lead-free solder candidate alloys, see Refs. 99 and 100. 

Extensive activities are underway in both industry and academia to identify and implement alternatives to Pb for the various aspects of Sn-Pb solder electronic assemblies. In the area of Pb-free solder pastes, a ternary Sn-Ag-Cu alloy has been generally identified as the most promising candidate and so for the solder paste portion of electronics assemblies, the selection of a Pb-free material appears to have a clear direction. 

---