For pcb assembly,talk about smt processing lead-free solder’s development status

At present, the widely used alloy for replacing tin-lead solder is mainly Sn, and metal elements such as silver (Ag), zinc (Zn), copper (Cu), bismuth (Sb), bismuth (Bi), and indium (In) are added. , constitute ternary alloy or multi-alloy.

The choice of these metallic materials reduces the melting point of the solder when it is alloyed with tin, giving it the desired physical properties. Several alloy systems that have been successfully developed at present are described below.

(1)Tin-zinc system (Sn-Zn). The tin-zinc solder has a melting point of only 199 ° C, which is the only one of the lead-free solders that is close to the eutectic melting point of the tin-lead solder, and can be used for soldering components with poor heat resistance, and the cost is low. However, when using the surface in the atmosphere results in a very thick zinc oxide film, which must be used under nitrogen or a strong active flux that dissolves the zinc oxide film to ensure solder quality. Poor wettability can not be ignored, and a large amount of scum will produce when used it for wave soldering. When the solder paste is made, the reaction activity of zinc is strong, and it’s troublesome to ensure the storage stability of the solder paste and increase its wettability. It can be said that such solder will not be promoted in the short term.

(2)Tin-copper system (Sn-Cu). Tin-copper (Sn99.3Cu0.7) solder has no difference in solder joint brightness, solder joint molding and pad wetting after welding with conventional tin-lead solder, and because tin-copper solder is simple in composition and good in supply. The cost is low, so it is widely used for wave soldering and dip soldering of PCBs, and is suitable as a soft solder for rosin. It has better strength and fatigue resistance than tin-lead solder, and it is better than tin-lead solder, that is, there is no bridging connection in IC flow soldering of fine pitch QFP, and there is no proprietary lead-free solder. Needle crystals and pores provide shiny solder joints.

(3) Tin-silver system (Snr-Ag). Tin-silver solder has been used in the PCB manufacturing electronics industry for many years as a tin-lead replacement. Sn-Ag alloy is a high-temperature solder that people have been afraid of in the early days. Due to the high welding temperature, it has not been widely used. With the promotion of lead-free solders, people have re-recognized and studied Sn-Ag alloys and improved them. It provides good adhesion over a long period of time. It does not require nitrogen protection during reflow soldering. Its wettability and diffusibility are similar to those of tin-lead solder, and the tin-silver flux residue looks better than tin-lead residues. , basically colorless and transparent. The alloy's electrical conductivity, thermal conductivity and surface tension are comparable to those of tin-lead alloys.

(4) Tin-Sb system. Tin-based solder is a high-temperature solder with a melting point of 235 to 243 °C. At present, there are not many types of formulas, and almost only the Sn95Sb5 formula is used. Its tensile strength is not as good as Sn63Pb37, but the plastic strain is very good, so the overall life is better than Sn63Pb37 about 1.4 times. The wettability of Sn95Sb5 is not as good as that of Sn63Pb37, but industry test results are considered acceptable.

(5) Tin-bis system(Sn-Bi). The binary alloy Sn42Bi58 is a material commonly used for low temperature soldering. The usage of niobium can lower the melting point temperature (a research focus on lead-free technology), reduce surface tension (so there is better wettability), and enhance the life of solder joints. However, the influence of the composition of niobium on the mechanical properties of the alloy varies greatly, and it is easy to have the problem of “lead” pollution. At the same time, its natural supply is small and the cost is high. The addition of Ag can solve some of the characteristics of instability, so the usage of bismuth ternary or quaternary alloys is more popular.

(6) Tin-silver-copper system (Sn-Ag-Cu). Tin-silver-copper solder is currently the best alternative to tin-lead solder and has good physical properties. Compared with the tin-silver system (Sn96.5Ag3.5), the tin-silver-copper system (Sn96.5Ag3Cu0.5) has a minimum melting temperature of 216 to 217 ° C, which is about 4 ° C lower than the eutectic Sn 96.5 Ag 3.5. When compared to the basic mechanical properties of Sn96.5Ag3.5, the specific alloy composition under research performed better in strength and fatigue life. Compared with the tin-copper series (Sn99.3Cu0.7), the tin-silver-copper series has good strength and anti-workability, but the plasticity is not as high as Sn99.3Cu0.7.

In recent years, the more practical lead-free standard alloys are based on tin-silver-copper series. Sn96.Ag3Cu0.5 has become the mainstream lead-free solder that is widely used and recognized.