Solders, fluxes, and solderability


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4. Solders, fluxes, and solderability

            1. Solder Alloys and applications
           2. Availability of solders
                        2.1 Cored wire
                        2.2 Solder Paste (Solder Cream)
                        2.3 Solder preforms
                        2.4 Solder rods
             3. Flux and its requirement
                        3.1 Inorganic acid
                        3.2 Organic acid
                        3.3 Rosin
                        3.4 No clean
            4. Storage of solder paste
            5. Solderability
                        5.1 Solderability testing, edge dip method

1. Solder Alloys and applications

The alloy of tin and lead is the one mainly used and other alloys are considered to fulfill any specific requirements as given earlier. The main constituents of the solder alloy are 60% tin and 40% lead, but they also often contain amounts of other relatively low melting metals such as bismuth, indium, and silver.

The extensive use of the Tin / Lead alloy is due the following reasons.

1. The melting point (183⁰ C- 189⁰ C) is low enough to permit the design of components that can endure the high temperatures associated with the soldering process.

2. Though the solder oxidizes quite rapidly, the characteristics of the tin oxide films pose relatively few problems compared with the oxide films of some other low-melting metals.

3. Because of the affinity between tin and many other metals, good wetting can be achieved with the aid of only mildly active fluxes.

4. It provides reasonably good mechanical strength to the solder joint, which can be expected with soft-soldering.

5. Because of the affinity between tin and many other metals, good wetting can be achieved with the aid of only mildly active fluxes. We will discuss fluxes and wetting at a later stage. Given below are some of the solder alloy configurations for various applications.

Composition of solder alloy	Eutectic/ Liquidous temperature	Properties and uses
63 Sn/ 37 Pb 60 Sn/ 40 Pb	183E 188L	Widely used tin-lead solders for surface mounting and general circuit assembly; low cost and good bonding. Not recommended for silver and gold soldering because of high leaching rate.
62 Sn/ 36 Pb/ 2 Ag	179E	Widely used. Small amount of silver is added to prevent leaching of silver conductors and leads. Also gives shiny appearance to the soldered joints. Not recommended for gold conductors and leads.
50 Pb/ 50 In 25 Pb/ 75 In 48 Sn/ 52 In 43 Sn/ 43 Pb/ 14	209L 165L 118L 163L	Less gold leaching, more ductile than Sn/Pb alloys. Low melting points of less than 160⁰ C can be achieved. Used for die attachment, and general circuit assembly. These are being used increasingly for second side solder on double side assemblies.
42 Sn/ 58 Bi	138E	Low temperature solder with high strength.

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