Solder eutectic composition

Because of the ease with which they can be soldered, electroplated tin—lead coatings of near eutectic composition (62 wt % tin) are extensively used in the electronics industry for coating printed circuit boards and electrical coimectors, lead wires, capacitor and condenser cases, and chassis. 

The preferred composition contains a minimum of 55 percent and a maximum of 70 percent tin. This alloy is near the tin-lead eutectic, which fuses at a temperature lower than the melting point of either tin or lead and, thus, makes it easy to reflow (fuse) and solder. (The composition of the eutectic is 60 percent tin, 37 percent lead, with a melting point of 361 °F.)... 

The lead-free solder whose composition is nominally 95.5Sn-3.9Ag-0.6Cu will be the focus of the analysis. The material property information presented should be appropriate for slight deviations from this composition. For simplicity, the alloy will be referred to as Sn-Ag-Cu. Select comparisons will be made with standard eutectic lead-tin solder with 2%Ag (62Sn-36Pb-2Ag). For convenience, the lead-tin eutectic material with 2%Ag will be designated as Sn-Pb. 

On occasion, low-melting-temperature alloys are prepared having near-eutectic compositions. A famihar example is 60-40 solder, which contains 60 wt% Sn and 40 wt% Pb. Figure 9.8 indicates that an aUoy of this composition is completely molten at about 185°C (365°F), which makes this material especially attractive as a low-temperature solder because it is easily melted. 

The Sn-Zn binary system has a eutectic composition of Sn-8.8Zn at 198.5°C [81], which is close to eutectic Sn-Pb solder, 183°C. Because of this reason, the Sn-Zn system has been investigated as a promising candidate in replacing eutectic Sn-Pb solder. The microstructure of eutectic Sn-Zn consists of a needle-like Zn-rich phase uniformly dispersed in the primary Sn-rich matrix as shown in Fig. 5 [82]. Since there is little solid solubility of Zn in Sn, most Zn atoms precipitate out as large needles during solidification. 

The formation of intermetallic compounds in solder joints during both reflow and subsequent annealing below the solidus temperature is also discussed. The classic Pb-Sn/metallization systems serve as a reference point [4—13]. In this context, the case of thermal cycling of solder joints of Cu/Ni/Au/Pb-Sn and the formation of Au-Ni-Sn intermetallic compounds at metallization interfaces is considered in detail [5,41-62]. The fast diffusion of Au controls the formation of Au-Ni-Sn alloys in this system [54—56]. In fact, some fundamental observations of the diffusion characteristics in Sn provide useful generalizations for all Sn-based solder systems, including Pb-free solders. Many Pb-free solders have near-eutectic compositions of Sn and various elements, often in ternary compositions. Those elements are often in relatively low concentrations, so the reactions between pure Sn and various metallizations are discussed as a basis for comparison. The formation of intermetalhc compounds at Pb-free solder/metaUization interfaces is discussed, and an attempt is made to understand the basic mechanisms of the growth processes and their relation to the reliability of interconnections. 

Lithium chloride is used in the production of lithium metal by electrolysis. It also is used in metallurgy as a eutectic melting composition with potassium chloride (LiCl 41 mol% KCl 59 mol%). Other applications are in low temperature dry-cell batteries as a dehumidifier in air conditioning in welding and soldering flux as a desiccant in fireworks and in mineral waters and soft drinks. 

There can be, however, some interim mixture that has just the right composition to show an abrupt, decisive melting point. In chemistry, this single-melting-point mixture is called the eutectic mixture. In cheese, it s called mozzarella. Solder is a eutectic mixture. 

Conventional tin/lead solders have the approximate composition Sn63/Pb37 by weight, corresponding to the eutectic mixture, which is close to SnsPb, with a melting point of 183 °C. Lead-free solders are often composed of tin with 3 % silver and 0.5-1% copper, and have a melting point of 215-220 °C. 

Solder is an important material used in electronic systems. The tin-lead solder system is the most used solder compositions. The system s equilibrium diagram shows a typical eutectic at 61.9% Sn. Alloys around the eutectic are useful for general soldering. High Pb content solders have up to 10% Sn and are useful as high-temperature solders. High Sn solders are used in special cases such as in high corrosive environments. Some useful alloys are listed in Table 2.14. 

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