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SOLDERING TECHNIQUES
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defect unique to laser soldering is charring or burning of the circuit board, which occurs if too high an energy density is used and the laser damage threshold of the PWB is exceeded. Charring or burning of the PWB can also occur if the circuit board is grossly contaminated with grease or other organic contaminants. This is not to say that boards for laser soldering have extraordinary requirements for cleanliness. Requirements should be considered the same for this technique as for any other reflow process. 47.8.5 Laser Safety Issues Lasers are categorized by their safety hazard potential. A full review of these will not be provided, but suffice it to say that Class 1 is an intrinsically safe laser, posing no intra-ocular danger, while Class 4 lasers pose the greatest hazard. All lasers considered for soldering use are of the Class 4 variety. Because of this, they are generally embedded in appropriate interlocked cabinets with laser-safe viewing ports or a video camera incorporated to preclude direct ocular exposure to the laser s intense beam. When the Class 4 system is embedded in an interlocked cabinet, it is considered a Class 1 system and poses no hazard to the immediate area; laser-safe eyewear need not be worn except when interlocks are overridden for system maintenance. As mentioned previously, lasers, especially Nd:YAG, are known for their high uptime and lack of required expendables.
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ADDITIONAL INFORMATION SOURCES
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There are numerous excellent supplemental texts available for additional detail and instruction in laser technology. Those by S. Charschan,16 Hecht,17 and Ready18 are particularly useful. Additional information on CO2 lasers in the realm of soldering can be found in references.19, 20, 21, 22, 23, 24
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HOT-BAR SOLDERING
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Specifically suited to surface-mount assembly of packages with lead frames, hot-bar soldering has been in use for several years. The technique relies on a resistance-heated element to push component leads into contact with solder and bonding pads, simultaneously reflowing the solder. Compression of the leads onto the circuit board lands is continued as the heat is ramped down. Upon cooling, the solder solidifies and the heating element is withdrawn from the newly formed solder joints. The heated element is commonly referred to as the hot-bar, although the term thermode is also in widespread use. Hot-bar soldering is best suited for low-thermal-mass, single-sided surface-mount assemblies. Each component type requires its own hot-bar bonding head assembly. These can be expensive, with the price dependent on the complexity, materials of construction, precision required, and overall size of the bonding head.
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Solder Application The use of solder paste is discouraged because blade heatup is fast and may result in explosive solder ball formation from rapidly volatilized paste constituents. Also, the paste is likely to squeeze out from between the component lead and circuit board pad, which can cause the solder to bridge to adjacent conductors. In fact, even with solid solder coatings on the circuit board, bridging can be problematic in hot-bar bonding. This is usually a function of the volume of solder on the pad, the quantity of flux, its degree of activity, and lead-pad pitch. As the
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PRINTED CIRCUITS HANDBOOK
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solder is melted and displaced by the hot-bar, it bulges laterally to the point that the solder masses of two or more adjacent pads may touch one another, forming a solder bridge. Once the bridge has formed, the forces associated with lead-pad wetting and capillarity may not be strong enough to overcome the surface tension conditions established during bridge formation. If that is the case, the bridging defect(s) will persist (Fig. 47.37).
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FIGURE 47.37 As the hot-bar is forced into contact with component leads and circuit board pads (a), the solder may bulge laterally as shown in (b), resulting in solder bridges (c). Careful control of solder volume, applied pressure, and temperature profile during hot-bar soldering is required to preclude this defect.
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Solid solder coatings such as hot air-leveled pads, solder-plated boards, or stenciled solder paste reflowed prior to hot-bar bonding are preferred for this bonding method. 47.10.2 Fluxes and Fluxing for Hot-Bar Soldering Soldering flux is applied prior to component placement and soldering. Flux choice should be tailored to the hot-bar process. The flux can be either liquid or paste. The flux chosen should be tested to resist charring and the development of polymerized decomposition products, or lacquers, which adhere to both circuit board and hot-bar. Residue build-up on the bar can adversely inhibit hot-bar performance by diminishing thermal transfer. The residue can also become thick and uneven enough to prevent the hot-bar from squarely contacting component leads. Baked-on aqueous-clean flux residues can make flux cleaning difficult.With no-clean flux, overheated residues may detract from the visual appearance of the printed circuit assembly. 47.10.3 The Hot-Bar Soldering Operation Generally, the board is preheated on a hot stage beneath the hot-bar, off to the side or even in a box oven prior to soldering. Sometimes hot air is used for preheating. This step is generally done after flux application, so the thermal profile has to be adjusted to prevent flux dry-out. Once component leads are aligned with solder-coated PWB pads, the hot-bar is pressed against
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