Lead-free solder candidate alloys

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

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... 

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. 

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. 

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]. 

Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases p-Sn, AgsSn, and CueSns. Starting from the fully liquid state in solidifying near-eutectic Sn-Ag-Cu alloys, the equilibrium eutectic transformation is kinetically inhibited. The AgsSn phase nucleates with minimal undercooling, but the (J-Sn phase requires a typical undercooling of 15-30°C for nucleation. Because of this disparity in the required... 

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

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... 

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. 

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