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Bismuth containing solder
Bismuth-containing solders have poor solderability, such as tin shrinkage, because their soldering temperature is very low, sometimes the flux can not fully play its activity. Furthermore, Bismuth-containing solder joints are easily oxidized, resulting in insufficient strength of solder joints. This point is particularly important for fuses that use electricity safely. Once oxidized, it often causes the situation that the fuses break without interruption, and the safety will be greatly discounted.
Indium containing solder
Soldering tin containing indium also has the trouble of insufficient strength of solder joints, and the price is not very expensive, but it also has some advantages, such as:
(1) Wettability is very good.
(2) Ductility is good, and it exhibits excellent Fatigue Resistance, even better than Sn-Pb eutectic alloy.
(3) Compared with Sn-Pb eutectic solder, indium-containing solder joints are slightly absent due to the melting of gold.
Silver containing solder
When the surface of the part foot or plate pad is treated as silver plated surface, the defect of external silver melting can be greatly alleviated if a little silver is added to the solder. However, the solderability of silver-containing solders with higher melting points is usually poor, the appearance of solder joints is dim, and the mechanical strength is insufficient.
Solder and process
The solder of tin-lead binary alloy is actually tin melted into lead, and the so-called Solder is just the "solution" of the two. In high temperature welding, the copper in the plate surface cushion will also be incorporated into lead and tin, that is, copper atoms will diffuse into the melted solder, and form a middle interface layer IMC (Cu6 Sn5) between the solder and the base copper, which is also the only way to truly solder. Once copper oxide or other surface contaminants occur on the surface of the pad, the diffusion of copper will be prevented without IMC, so that the pad can not be soldered firmly. The so-called "Dewetting" or "Non-Wetting" solderability was also found.
Tin coating process
Wetting, also known as Tining, is a fast and slow action. First of all, the activity of flux in high temperature can remove oxides or dirt on the surface of metal pad or organic copper protector (e.g. Entek) quickly, so that a thin layer of "interfacial alloy co-oxides" (Inter Matalic Com) between molten solder and base copper (or other solderable metals such as base nickel) can be produced rapidly. Pound Intermetallic Compound Cu6 Sn5), and tin and solder fast.
In the appearance of solder joints, we can see the action of solder expanding outward and upward. There is a "solid/liquid/gas" three-phase junction on the outer edge of solder joints. It seems that there is a small angle of "potential ready" running out. It is called Contact Angle (theta), also known as the double backslant angle (Dihedral Angle) like a jet. The smaller the contact angle, the better the tinning or solderability.
