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temperatures required by Pb-free solders, and (2) the poorer solderability of these alloys (owing largely to the absence of Pb). With respect to the five general process steps noted previously, the use of Pb-free solders primarily affects steps 3, 4, and 5. The need for higher processing temperatures limits the assembly process window of Pb-free solders. A higher nominal temperature is needed to accommodate the temperature variation at components across a circuit board to ensure melting of the solder and adequate wetting and spreading at each interconnection. On the other hand, the maximum temperature must be limited to prevent thermal damage to heat-sensitive devices and the circuit board. The poorer solderability of the Pb-free solders presents several challenges as well.Although the longer time needed to heat the component lead or termination and circuit board is often cited as the underlying source of poor solderability for Pb-free solders, it is primarily the higher surface tension of the Sn-based alloys (in the absence of Pb) that restricts wetting and spreading behavior. The need for longer heatup times is a particular issue with faster assembly processes such as wave soldering and hand soldering. However, the intrinsically poorer solderability impacts all assembly processes because it can degrade the quality of hole fill and fillet development for both short and relatively long (e.g., reflow) assembly processes. The intrinsic solderability performance of Pb-free solders is being improved by two means. First, new flux formulations are available that more effectively reduce the surface tension of the solder. Second, alternative surface finishes can be specified for the component I/Os and/or circuit boards that improve wetting and spreading activity exhibited by the Pb-free alloys. Strictly from an assembly process point of view, the mixing of Pb-free and traditional Sn-Pb solder can be beneficial. The Sn-Pb solder can improve the wetting and spreading performance of the Pb-free solder by two phenomena. First, Pb contamination lowers the molten solder surface tension of the solder. Second, the Pb contamination reduces the melting temperature of the Pb-free alloy. However, concerns are raised by the mixing of Sn-Pb and Pb-free solders and its effect on the long-term reliability of interconnections under thermalmechanical fatigue environments. Lastly, the use of Pb-free solders impacts the post-assembly cleaning step (step 4) and the inspection step (step 5). The higher process temperatures can produce more tenacious flux residues that require more aggressive cleaning steps to ensure their removal. Also, the more tenacious residues affect the ability of test probes to contact test site pads on the circuit board. Poor contact can be responsible for detecting false opens on the assembly.
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THROUGH-HOLE TECHNOLOGY
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Through-hole technology refers to assemblies having components with leads that are inserted into holes in the circuit board and soldered into place.This technology has been used since the early days of electronics (the 1920s). The leap in automation came in the early 1960s with the advent of wave soldering. The particular drawback of through-hole printed circuit boards, which led to the introduction of surface-mount technology (see the next subsection), was low assembly densities. The relatively large devices, and the circuit board real estate required for holes, limited the density of components, which in turn restricted product functionality and further miniaturization. A photograph of a through-hole circuit board is shown in Fig. 40.1 along with an optical micrograph that shows the through-hole solder joint of the component lead extending through the circuit board. An advantage of through-hole technology is reduced cost in some applications. That cost benefit may be realized by lower labor costs in some parts of the world that support hand assembly, which is relatively easy with the larger components and products, and lower board densities. Even when fully automated either by wave soldering, selective soldering, or pastein-hole/reflow the capital equipment and manufacturing costs can still be lower than those required for surface-mount assembly.
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