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Why lead-free process is inevitable?
Lead is a poisonous soft and malleable metal, lead may impact on human intelligence, nervous system and reproductive system.Annually about 60,000 tons solder consumed by global electronics assembly industry , but also increased year by year .the thus formed lead salt industrial waste causes serious environmental pollution, thus reducing the use of lead has become the focus of the world, many large companies in Europe, Japan are vigorously accelerate the development of alloys lead-free alternatives and plan to reduce the use of lead gradually in the assembly of electronic products,in 2004 completely eliminated. (Traditional solder composition 63Sn/37Pb, in the electronic assembly industry, Pb is widely used).
 Poisonous substance to animals, including humans. It damages the nervous system and causes brain disorders. Excessive lead also causes blood disorders in mammals. Like the element mercury, another heavy metal, lead is a neurotoxin that accumulates both in soft tissues and the bones.  
Requirements of lead-free substitute:
1,Price: Many vendors requires the substitute prices not exceed price of 63Sn/37Pn, but finished lead-free alternatives prices are 35% higher than 63Sn/37Pb .
2, Melting point: Most manufacturers require a minimum of 150 ° C solidus temperature to meet the work requirements of the electronic equipment. The liquidus temperature depending on the application.The Wave soldering electrode: successful wave soldering, liquid temperature should be lower than 265 ° C.The manual welding solder wire: liquid temperature should be lower than the soldering iron operating temperature 345 ° C.Solder paste: The liquidus temperature should be lower than 250 ° C.
3,Conductive.
4, Good thermal conductivity.
5, The smaller the solid-liquid: mostly expert recommend this temperature range within 10 ° C in  order to form a good solder joint, if the solidification range is too wide, there may occur the solder joint cracking, easily cause Electronics damaged.
6, Low toxicity: alloy composition must be non-toxic.
7, Hhas a good wettability.
8, Good physical properties (strength, tensile): alloy must be able to to provide Sn63/Pb37 attainable strength and reliability, and not by the device on the projection angle welds.
9, Production repeatability and consistency of the solder: As the electronic assembly process is a high-volume manufacturing process, requiring the repeatability and consistency to maintain a high level, if certain alloy composition is not in bulk conditions under repetitive manufacturing, or its melting point in the mass production of larger changes due to the change of the ingredients, it can not be taken into account.
10, Joint Appearance: solder joint appearance should be close to the appearance of the tin / lead solder.
11, Supply capacity.
12, Compatibility with lead: it will not immediately be transferred to a lead-free system in a short time, so the lead may still be used in PCB pads and component terminals, such as doped solder ,may cause the melting point of solder alloy is very low, the intensity is greatly reduced.
鏂囩珷From:http://www.etasmt.com/te_news_industry/2013-07-03/5361.chtml