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Solder is a brazing material, its melting point is lower than that of the base metal. In the brazing process, the solder is heated to a temperature higher than the solder melting point, but lower than the base metal melting point. After the solder melts, the gap between the joints is filled and the metallurgical effect occurs with the base metal, thus realizing the connection of the material.
1. Solder is mainly composed of tin and lead, and a few other components are silver, bismuth and indium, which have different melting points (M.P.). However, the lowest melting point of Sn63/Pb37 binary alloy is 183 (?) C. Because of its liquefaction melting point (Liquidus Point) and solidification melting point (Solidus Point), there is no pasty during the transition period, that is to say, it will be liquefied higher. The melting point and the lower solidification melting point are combined, so they are called eutectic alloys. It is also called "eutectic alloy" because of the simultaneous appearance of tin and lead in its coarse crystals. This kind of impurity-free alloy has a bright "Eutectic Composition" or "Eutectic Solder" appearance. After solidification, the structure of the alloy is very uniform and there are few particles. Different alloying ratios will affect the melting point change. The "Phase Diagram" of this change can be found in the 12th issue of TPCA.
Another Sn60/Pb40 alloy, which is close to the eutectic point, is more widely used in the electronic industry. The main reason is that Sn is more expensive. With almost no difference between solderability and Joint Strength, the cost of Sn60/Pb40 alloy is reduced by 3%, which is naturally conducive to cost reduction. Compared with the former eutectic alloy, the 60/40 alloy has to undergo a little slurry, so its solidification time is a little longer and its appearance is not bright, but its solder joint strength is not different. However, if the latter is shocked by external force during its curing process, there will be a phenomenon of "Disturbed" solder joints with rough surface particles, and even a bad situation of "Dewetting".
2. Characteristics of solders
In addition to the "solderability" will cause production line problems, "Joint Strength" is the focus of product follow-up life. However, it is not practical to discuss the Tensile Strength and Shear Strength of finished solder only from the viewpoint of material mechanics. On the contrary, in the long-term thermal cycle (Thermal Cycling, also known as thermal shock Thermal Shock), plastic deformation, Creep and even fatigue are the key points of the equal solder joints due to their different thermal expansion coefficients (TCE) from the solder. Sooner or later, such worries will lead to irremediable cracking of solder joints (Crack), which will do great harm to the reliability of solder joints.
The thermal expansion coefficients (TCE) between the metal pins of the components and the body of the components and the welding pads on the plate surface are not the same. Thermal stress will be generated in the thermal cycle, and then, like Strain in response, will be "fatigued" due to repeated strains after multiple thermal stresses, which will eventually lead to the rupture of solder joints or packages. This crisis will affect the feetless SMD components. Louder. The general mechanical properties of common eutectic solders are summarized as follows:
2.1 Tensile Strength of 63/37 solder at room temperature is 7250 PSI, while cold rolled steel is 64,000 PSI. However, the effect of tensile strength on the strength of solder joint is not as great as Shear Strength. If a small amount of antimony is added, the result will be better. As for Ductility and Elastic Modulus, 63/37 are better than other binary alloys with high melting point, while the conductivity and heat conductivity of the two alloys are worse than pure tin, and will decrease slightly with the increase of lead content. The strength of 63/37 is better than that of other proportions. Tin is also better than lead.
2.2 The strength of tin-lead alloy solders with various proportions is better than that of tin-lead solders alone. The specific gravity increases with the increase of lead content, and there is little relationship between surface tension and alloy proportion when it is liquid.
2.3 The Creep capability of solder joints will be more important to reliability than tensile strength. Unfortunately, the closer the eutectic ratio is, the bigger the grain size is, the bigger the latency is. The latent transformation resistance of columnar crystals is not as good as that of Equiaxial crystals. Atom Lattice reorganization will occur in solder joint alloys under long-term load; that is, after long-term deterioration of solder joints, eventually failure will occur, of course, due to excessive "strain" caused by long-term stress.
2.4 The strength of solder joints is related to flux, solderability and IMC. Many experimental results show that the strength has nothing to do with the amount of filling tin, but too much tin is not beneficial. Welding time should not exceed 5 seconds, the longer the worse, the welding temperature should not be too high.
