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Flux Both alcohol-based and water-based (volatile organic compound [VOC]-free) fluxes are compatible with most Pb-free alloys. The longer preheat times and slightly higher wave temperature are more of a challenge for alcohol-based fluxes, as dry-out is a concern. Higher flux solids content will help in this regard. Water-based fluxes are the direction in which the industry is moving, and a higher boiling point is widely used for Pb-free soldering. Pb-free wave-solder joint appearance such as duller solder joints Just as in reflow soldering, wave-soldered joints tend to be duller than those obtained from eutectic Sn-Pb. This is not a defect but a reality. Fillet lifting This has been previously covered, but it is worth noting that the effect is more prevalent in Pb-free alloys than in Sn-Pb alloys. Fillet lifting occurs when the solder cools at a different rate than the board. The Sn-Bi system is notorious for resulting in lifted leads. Bi expands upon freezing and can rip itself off the bonding pad or rip the bonding pad from the laminate. Fillet lifting is generally controllable by optimization of the cooling ramp rate after soldering.
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When associated with the primary side of the board, fillet lifting is not considered a defect. If found on the secondary side (bottom) of the PWB, it is considered a workmanship defect with reliability consequences. Hot tears and shrink holes Most solders shrink upon solidification, and this is true of SAC solder.This alloy is known to form microcracks (see Fig. 47.30) on its surface during the phase change from liquid to solid. These cracks are called hot tears or shrink holes. They are generally benign as long as the crack is shallow (that is, the bottom of crack can be seen) and the crack neither extends to contact component lead nor PTH features (annular pad or barrel).
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FIGURE 47.30 Hot tear (shrink hole) on an SAC wavesoldered PTH joint. (Courtesy of Hewlett-Packard).
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Equipment There is little difference between wave-solder equipment set up for Sn-Pb and that configured for Pb-free soldering. There are two points to consider before converting equipment. First, the level of Pb contamination will be high in the Sn-Pb equipment. This can be remedied by draining the system of Sn-Pb solder and purging the system with a charge of pure tin. The tin scavenges any Pb from the solder pot, delivery lines, and nozzles. Good cleaning of the wave machine will take care of any trace Pb contaminants, but proper protective gear (respirator and clothing) should be donned for this cleanup.
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Sn is known to dissolve many metals. With the high Sn content of the SAC solders (SAC305 is 96.5 wt percent Sn), check to ensure the machine is compatible with the higher tin content. The solder pot, impeller, delivery lines, and nozzles are all subject to iron leaching. This can interfere with the process by changing the composition of the solder. It can also be dangerous if a leak develops in the system allowing molten solder to leak from the solder pot. Some coated steels (such as ceramic and nitride) and cast iron are generally compatible. Changing over older systems to Pb-free compatibility can be an expensive proposition.
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Because of safety and environmental concerns and compliance with the Montreal Protocol for the reduction of ozone-depleting chemicals, this soldering technique had fallen out of favor. With the advent of Pb-free soldering, there is increased interest in vapor-phase soldering for SMT applications, but it is likely that it will remain a niche application. Due to its continued diminished status, vapor-phase reflow is covered only briefly.
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Basic Process As in any other reflow technique, the board must be supplied with sufficient solder for joint formation. Most commonly, solder paste is stenciled onto circuit board pads. Components are placed onto the solder paste in preparation for reflow. The board is conveyed into the reflow chamber, where it is exposed to the vapor-phase of a boiling liquid. This liquid is inert with respect to the solder and board. It is a dense synthetic of high boiling point (slightly higher than that of solder liquidus but not high enough to damage the circuit board or components). Historically, chlorofluorocarbons were used, but now only hydrofluorocarbons (HFCs) are acceptable due to environmental concerns. Material costs are high, at several hundred dollars per pound. When conditions are optimized, the hot vapors begin to condense on the cooler PWB, heating it. As the process progresses, sufficient energy for sustained solder reflow results. Solder wets to lead and pad and, as the board is removed from the hot vapor, the molten solder solidifies, bonding component leads to circuit board pads.
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Machine Subsystems The vapor reflow soldering machine is composed of three main subsystems: conveyor, reservoir vessel, and heaters.The vessel has cooling coils surrounding it, placed well above the level of the liquid in the reservoir. These condense the vapors, returning the majority to the reservoir, as shown in Fig. 47.31.
Advantages and Disadvantages of Vapor-Phase Reflow Soldering The process is exceptionally uniform in heating and is also very precise in temperature. This method of reflow negates problems associated with varying topologies and areas of high thermal mass that confound other soldering processes. Since the soldering occurs in an inert atmosphere, fluxes of lower activation level can be used. In the absence of air, solder-joint quality is generally excellent. Although vapor phase reflow has the appearance of being a fast process, solder paste reflow recommendations must still be followed. If the paste is heated too rapidly, paste volatiles may boil, resulting in explosive solder ball formation. As in other soldering methods,
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