Soldering, compared with other

Resistance of LCP measured on a 56 mm long plug connection with 40 contacts during gas phase soldering compared with other materials (L = length in flow direction, W = width (length perpendicular to flow direction), H = height) [695]... 

Although there has been significantly more research carried out on Pb-based solders than Pb-free solders, the literature is scanty compared with other conductors such as Cu, Al, and their alloys. Lead-based solders are two-phase alloys (Fig. 6), usually combined with Sn, but sometimes alloyed with other metals depending on the specific application. In many applications, such as in microelectronics, where low melting points are desired, eutectic (63% Sn/Pb) or near-eutectic solders are used, but high-Pb (>90%Pb) solders are typically used for flip-chip solder bumps owing to their resistance to electromigration. 

The coefficient of hydride bed effective heat transfer aef is proportional in general to layer effective thermal conductivity and inversely proportional to layer thickness. The analysis of this equation shows that the heat transfer coefficient is less than a smaller coefficient of heat emission and, consequently, it is meaningless to increase strongly one of them without changing the other. The experimental results show that for the soldered and diffusion welded connections of sorber case and heat-conducting insertion R = (0.5-1.5)-10 5 (m2 K)/W. If contact between insertion and a case is tight fit, then R increases in 10-100 times and influence of contact resistance becomes comparable with influence of reduced heat emission of a hydride bed. 

Tape automated bonding (TAB) and controlled collapse chip connection (C4) are two other common first-level packaging technologies that allow increased I/O density compared with wirebonding in molded plastic packages. In TAB, chips are thermocompression bonded to the inside ends of radially patterned metal leads on pol5uner tape. The outside ends of the leads are then soldered to a second-level package and the chip is covered with an encapsulant. 

The low surface tension (and low 7lf interfacial tension) of Pb-Sn alloys also contributes to excellent capillary flow. Molten Pb n solders can readily enter small clearances associated with through-hole solder interconnections and other configurations. This behavior was documented in a series of capillary flow experiments carried out between parallel Cu plates, using the specimen shown in Fig. 18a the superior meniscus (capillary) rise of Pb-Sn solders as a function of gap thickness, when compared to several Pb-free solders, is shown in Fig. 18b [5,39]. 

The steady-state creep rate of eutectic In-Sn solder joints is reported in Table 6, in comparison with Pb-Sn and other solders. Among these solders, it was determined that In-Sn joints display the weakest resistance against creep deformation [26,29,61,94-96]. In contrast, eutectic In-3Sn solder exhibited the largest total deformation to fracture among them. Some superplastic behavior of this solder is reported under shear loading at temperatures above 65°C [29]. This finding may explain why In-Sn solder joints possess a superior fatigue life compared to other solders, as discussed earlier. 

For eutectic Sn-Zn, the low melting point of its ternary and quaternary alloys is very favorable compared to other Sn-base alloys in replacing eutectic Sn-Pb. However, since Zn readily oxidizes during soldering which causes poor wetting, their wettability must be improved to be used in microelectronic applications. A proper flux/metallization system compatible with the SMT soldering technology is required. 

The size of the particles in a solder paste determines the print characteristics, amongst other things. The particles are produced by a variety of methods, but they are most commonly made by dispersion of a stream of molten solder onto a rotating disk. The particles fall into a tank filled with an inert atmosphere where they solidify and then are collected at the bottom of the chamber. The collected particles are separated by size utilizing in a series of wire-mesh sieves. The mesh size is typically given in wires or holes per square inch. The Joint Industry Standard, J-STD-005, provides for solder paste particle size classification as listed in Table 1. The choice of solder-powder particle size for SMT applications is based on component pitch, part mix and pad arrangement. For example, the paste particle size required for an area array device is smaller compared to a peripheral-leaded device with the same pitch. A 0.5-mm pitch area array device may have 0.25-mm diameter pads which require a 0.25-mm to 0.3-mm stencil aperture to print a Type rv solder paste with an approximately at 60-80% transfer efficiency. Comparatively, a 0.5— mm pitch quad flat pack device would typically have an 0.2 mm to 0.2.5mm wide pad, but require a 0.15 to 0.2mm wide stencil aperture to print a Type III paste with approximately a 80 90% efficiency. 

Figure 8 shows the maximum load values for SOIC device leads pulled from assemblies made with each Pb-free solder as a function of circuit board surface finish. Considering all board surface finish conditions and accounting for the experimental variability of the data, it was concluded that the performance of all solders is nearly equivalent and, with the exception of Sn58Bi, the differences observed are not statistically significant. Compared with all other down-selected solders, the maximum load values with Sn-58Bi are the lowest for imidazole, Ni/Au, Ni/ Pd, and Pd, and second lowest for Sn. 

Sn-4.7Ag-l.7Cu Compared with five other promising lead-free solders in microstructure, creep, fracture, and thermomechanical fatigue behavior 42... 

Sn-Ag-Cu (composition unknown) Compared with eight other promising lead-free solders in wettability, fatigue resistance, and T Sn-Ag-Cu was claimed as the best 45... 

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