The role of each metal element in the solder paste
Generally speaking, there are different formulations, different processes and different types of solder pastes. There are common solder pastes such as low temperature solder paste, medium temperature solder paste and high temperature solder paste. So many people are confused: what is the difference between low-temperature, medium-temperature and high-temperature solder pastes? How to distinguish them?
Low temperature solder paste
Low temperature solder paste melting point of 138 ℃ solder paste is called low temperature solder paste, when the SMD components can not withstand the temperature of 200 ℃ and above and the need for SMD reflow process, the use of low temperature solder paste for soldering process. Played a protection can not withstand high temperature reflow soldering original and PCB, very popular LED, its alloy composition is tin-bismuth alloy. Low-temperature solder paste reflow soldering peak temperature at 170-200 ℃.
Medium temperature solder paste
Medium-temperature solder paste is a solder paste for SMT lead-free process. Its alloy composition is Sn/Ag/Bi, with a particle size between 25-45um and a melting point of 172°C. Medium temperature solder paste is mainly characterized by the use of imported special rosin, good adhesion, can effectively prevent collapse, less residue after soldering, and transparent, does not hinder ICT test.
High temperature solder paste
High-temperature solder paste is generally composed of tin, silver, copper and other metal elements, high-temperature solder paste melting point 210-227 degrees C. LED or recommend the use of high-temperature lead-free solder paste, high reliability, not easy to desoldering cracking.
1、Antimony
Add antimony to increase the strength without affecting the wettability. Prevent tin worms. Should avoid the use of zinc, cadmium or galvanized metal, because these will lead to brittle solder joints.
2, bismuth
Bismuth can significantly reduce the melting point and improve wettability. In the case of sufficient lead and tin, bismuth will form a melting point of only 95 ℃ Sn16Pb32Bi52 crystals, these crystals along the grain boundary diffusion, and may cause failure of the solder joint at lower temperatures. Therefore, when soldering with bismuth-containing solder, high-power components pre-plated with lead alloy are desulfurized under load. Such solder joints are also prone to cracking. Alloys with more than 47% Bi expand on cooling, which can be used to counteract thermal expansion mismatch stresses. It prevents the growth of tin whiskers. However, it is relatively expensive and availability is limited.
3、Cu
Copper lowers the melting point, improves thermal cyclic fatigue performance and improves the wetting properties of the molten solder. It also reduces the rate of dissolution of copper from the board and slows down some of the leads in the liquid solder to form metal compounds. It promotes the growth of tin whiskers. A solution of (about 1%) copper in tin can be used to inhibit the dissolution of metallization under the film bumps of BGA chips, for example, as Sn94Ag3Cu3.
4、Nickel
Nickel can be added to the solder alloy to form a supersaturated solution to inhibit the dissolution of the metalization under the film bump.
5、Indium
Indium can reduce the melting point and extend the ductility. In the presence of lead, it forms a ternary compound that undergoes a phase change at 114°C. Very high cost (several times the silver color) and low availability. Prone to oxidation, which leads to repair and remanufacturing problems, especially when flux cannot be removed using oxides. During GaAs chip attachment. Indium alloys are mainly used in low temperature applications and are used to dissolve gold much less than in tin. Indium can also be used to solder many non-metals (e.g. glass, mica, alumina, magnesium oxide, titanium dioxide, zirconia, porcelain, brick, concrete and marble). Indium-based solder is easy to corrode, especially in the presence of chloride ions.
6、Lead
Lead is cheap and has suitable performance. It is more wettable than tin. However, it is toxic and has been eliminated in some countries. It can stop the growth of tin whiskers and inhibit tin pests. Reduce the solubility of copper and other metals in tin.
7、Silver
Silver provides mechanical strength, but is less ductile than lead. In the absence of lead, it improves resistance to fatigue from thermal cycling. Using SnAg solder with HASL-SnPb coated leads has a melting point of 179°C. Adding silver to tin can significantly reduce the solubility of the silver coating in the tin phase. In eutectic tin - silver (3.5% Ag) alloys, it tends to form platelets of Ag3Sn, which can act as crack initiation sites if formed near high stress points; the silver content needs to be kept below 3% to suppress these problems.
8、Tin
Tin is usually the basic ingredient in solder paste. It has good strength and wettability. But itself is prone to tin damage, tin crying, and the growth of tin whiskers. Easy to dissolve silver, gold, and other metals, for example, copper for a higher melting point and reflow temperature of the tin alloy, which is a particular problem.
9、Zinc
Zinc can lower the melting point and is cost effective. However, it is very easy to corrode and oxidize in air, so zinc-containing alloys are not suitable for some soldering, for example, the shelf life of zinc-containing solder paste is shorter than that of zinc-free. A brittle Cu-Zn intermetallic compound layer can be formed in contact with copper.
10、Germanium
Germanium in lead-free solder of tin type can inhibit the formation of oxides; below 0.002% will increase the formation of oxides. The best concentration to inhibit oxidation is 0.005 percent.
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