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FIGURE 47.18 Solder-joint defects can result from uneven solder distribution: (a) A solder-starved pin-in-paste solder-joint showing large void; (b) a good pin-in-paste solder joint showing nearly 100 percent barrel fill and no voids. (Courtesy of Hewlett-Packard).
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Stencil designs that preclude solder paste from entering the PTH also result in a significant reduction of solder paste on the top surface and in the PTH. These reductions may make the pin-in-paste technique impractical for some boards. If the solder volume is too low, the available solder distributes unevenly around the PTH pin and the PTH barrel, resulting in voids and inferior solder joints (see Fig. 47.18a, b). A normal incident x-ray will highlight circumferential voids in PTHs, as evidenced in Fig. 47.19. 47.3.11.3 Pin-in-Paste on Thick Boards. Since the solder volume increases proportionately with increasing board thickness, pin-in-paste soldering is best suited for thin PWBs ( 1.6 mm). The technique is limited by available solder and required solder volume for proper barrel fill. There are two ways to deal with the added volume.
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FIGURE 47.19 A normal incident x-ray photo of a connector. The light spots around the pins are areas of no solder or voids (reduced solder thickness). (Courtesy of Hewlett-Packard).
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FIGURE 47.20 Application of solder pre-form foil washer to augment pin-in-paste solder volume. Note that the flux from the solder paste is sufficient for pre-form soldering.
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47.3.11.4 Addition of Solder through Pre-Forms. Solder foil pre-forms (stamped solder foil) can be added to through-hole parts to augment solder volume (see Fig. 47.20). This approach is expensive and application of the pre-forms is tedious and time-consuming. 47.3.11.5 Buried Intrusive Method. There is a new method for formation of a somewhat nonstandard solder joint in thick PWBs.2 A foreshortened pin, on the order of that used for a board 1.6 mm in thickness, is used in the thick board. So, too, solder volume is treated as if for a board 1.6 mm thick. When the solder melts, it coalesces and wets to the pin and barrel as in any through-hole soldering process. Conventional thinking would have the molten solder running through the barrel and out the bottom side of the board. Instead, nearly all the solder is retained by surface tension forces around pin and barrel. If the solder volume is calculated properly and aptly delivered, there is 100 percent pin wetting both longitudinally and circumferentially (see Fig. 47.21). Accelerated thermal cycling followed by tensile testing has shown that resultant solder-joint reliability is sufficient and equivalent to conventionally formed pinin-paste or wave-soldered joints of twice the pin-wetted length. 47.3.11.6 Temperature Compatibility. If pin-in-paste soldering is to be used, check that components are temperature-compatible with the oven reflow process. The high temperatures and long exposures associated with oven reflow soldering may cause unsuited molded component bodies to melt or warp. Connector contact normal force may be affected if the molded connector body softens or distorts. Solder joints or wire bonds internal to some devices may become disbanded, and some, such as electrolytic capacitors, may leak or even explode as a result of an oven reflow cycle. Check the component manufacturer s specification for thermal limits and compatibility with oven reflow soldering.
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FIGURE 47.21 Cross-sectional micrograph showing pin-in-paste buried intrusive soldering. Note that very little solder has drained.
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Once the predominant method for mass assembly of circuit boards, wave soldering has taken a back seat to oven-based reflow soldering. The ease of surface mounting led to a rapid rise in popularity of oven-based technology, and proliferation of surface-mountable packages has led to a decreased reliance on wave soldering. Nonetheless, through-hole componentry persists and mixed-mount (surface-mount plus through-hole) technology still may be the only alternative for some assemblies, and it is unlikely that wave soldering will disappear from PWB manufacturing in the short term. Since wave soldering, compared to reflow soldering, is fraught with defects, it behooves the designer and assembler to minimize the number of PTH components on a board.
Wave-Solder Process Basics Wave soldering utilizes a reservoir of molten solder pumped and circulated to form a standing wave. The circuit board is prepared with devices for wave soldering in one of three ways:
Coarse-pitch surface-mount components, especially passive devices, are affixed to the bottom side of the PWB using surface-mount adhesive. The adhesive is cured prior to wave soldering. The adhesive is in contact with the body of the component, which is aligned with its respective pads on the circuit board in preparation for wave soldering. Solder-tailed components such as connectors, PGAs, or other through-hole devices are inserted into PTHs from the top side of the board. Solder-tailed components such as axially leaded devices are inserted from the top side of the circuit board, whereas the leads are clinched on the bottom side of the board.
The circuit board is placed on a motorized, edge-hold conveyor where it is fluxed, then preheated both to activate the flux and give the PWB a thermal boost.The board is next skimmed over the crest of the molten solder wave. Only the bottom of the circuit board is exposed to the molten solder (see Fig. 47.22).
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