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At present, the research on lead-free solders mainly focuses on the following three aspects:
(1) Development of new lead-free solder alloys;
(2) Research and design of fatigue characteristics and reliability of welded joints and solder joints;
(3) Study on the technology of lead-free solder.
Through nearly 20 years of research and development, countries have achieved certain research results. Lead-free solders can partly replace tin-lead solders under the premise of limited changes in technological conditions. At present, more than 100 kinds of lead-free solders have been developed. Almost all lead-free solders use Sn as matrix material because of its low cost, abundant supply and ideal physical properties, such as conductivity/heat and wettability. At the same time, Sn is also the base metal of 63Sn/37Pb alloy. By adding Bi, Cd, In, Zn, An, TI, Ga, Hg, Cu, Sb and Ag, suitable melting point and mechanical properties can be obtained.
For existing lead-free solders, the selection of appropriate lead-free solder alloy substitutes is the basis of lead-free assembly technology. From a purely technical point of view, the lead-free solder products that replace the traditional Sn63-Pb37 or Sn60-Pb40 lead solder products have matured. At present, Sn-Cu, Sn-Ag, Sn-Ag-Cu or Sn-Zn series are mostly composed of Sn as matrix.
It is generally accepted that Sn-Cu, Sn-Ag or Sn-Ag-Cu system should be used in manual soldering, Sn-Cu system should be used in dipping and wave soldering, Sn-Ag system and Sn-Ag-Cu system should be used in reflow soldering, and Sn-Zn-Bi system should be considered in low temperature soldering. Although there are many kinds of lead-free solders available now, there is no solution that can completely and directly replace SnPb solders. For some special process, some lead-free solders can be directly replaced.
At present, the most attractive lead-free solder is the Sn-Ag-Cu series. The advantages of tin/silver/copper alloy lie in its simple composition, abundant raw materials, low cost and relatively friendly to the environment. The melting point temperature (217 C) is higher than that of Sn-Pb alloy, but the overall performance is better. Other potential combinations including Sn-0.7Cu, Sn-3.5Ag and Sn-Ag-Bi are under development. At the same time, there is no suitable lead-free substitute for high lead and high melting point solders.
