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At present, the internationally recognized definition of lead-free solder is: Sn as the matrix, adding Ag, Cu, Sb, In and other alloy elements, while the mass fraction of Pb below 0,2 is mainly used for electronic assembly of solder alloys. There are strict performance requirements for solders used in the field of microelectronics, including not only electrical and mechanical properties, but also ideal melting temperature. Therefore, lead-free solder can be used in SMT, MCM and microelectronics industry only when the following conditions are fully met.
(1) The melting point of lead-free solder should be as low as possible, and should be as close as possible to the eutectic temperature l83 of 638n/37Pb alloy. C.
(2) Non-toxic, alloys and their components are non-toxic, so cadmium, thallium and mercury can not be used. Bismuth is also excluded from use because it is mainly a by-product of lead extraction.
(3) The conductivity and thermal conductivity after welding are good, which should be close to 63/37 Sn-Pb alloy solder. (4) Lead-free solders should have good wettability to ensure quality.
(5) Easy to be processed into all the required forms, including wire for manual welding and repair, solder powder for solder paste, solder rod for wave soldering, not all alloys can be processed into all forms, such as the increase of bismuth content will lead to brittleness of alloys instead of drawing into wires.
(6) Good compatibility with various flux. At the same time, it is compatible with the existing component substrate/lead and PCB material in metallurgical properties.
(7) Lead-free solders should have good physical properties (strength, tensile strength, fatigue, etc.). Alloys must be able to provide mechanical strength and reliability that 63Sn/37Pb can achieve, and there will be no protruding fillet welds on through-hole devices (especially for alloys with large solid-liquid coexistence temperature range).
(8) Appearance of solder joints: The appearance of solder joints should be close to that of tin-lead solder. It is easy to repair the solder joints after welding. (9) There is sufficient storage capacity to fully meet market demand.
(10) The price is suitable and the cost is as low as possible. It can be controlled at 1,5-2 times of tin/lead alloy, which is an ideal price.
