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the lead-pad combinations and heated to reflow. Precise control of the heating process is required to prevent flux spattering or solder bridging. A mechanical stop is employed to keep the bonding head from exerting too much force on the lead and pad combination. Excessive force can cause the component leads to slide off pre-reflowed (domed) solder-coated pads. It can also enhance solder bridge formation and solder-starved joints by physically displacing solder from between leads and pads. One approach to preventing bridged or solder-starved joints is to retract the hot-bar by several hundredths of a millimeter once the solder starts to reflow. This results in a surface tension coupling of solder between lead and land, reduced propensity for solder bridge formation, and robust joint formation upon cooling. As in any other soldering process, it is important to maintain proper reflow time-temperature characteristics. In hot-bar soldering, it is of paramount importance that the blade seating plane is maintained normal to the surface place of the PWB. On the other hand, it should be noted that the coplanarity of solder on the PWB pads does not have to be exact.The hot-bar will reflow this solder and it will self-planarize, as does any liquid, and remain planarized into the solid phase. 47.10.4 Hot-Bar Construction The hot-bar bonding head can be composed of one or several blades. They are designed to solder one side, two sides, or all four sides of a gull-wing leaded component simultaneously. Generally the hot-bar assembly is configured to accommodate the maximum span of a lead set, permitting simultaneous bonding of all leads on a package side. In the case of some very long connectors or other large packages, the thermal uniformity of a single blade may not be adequate for the process. This is sometimes remedied by using a smaller hot-bar and stepping it along the length of the lead set until all leads are bonded. Another approach uses multiple hot-bars side by side, each individually controlled, to achieve the span and required thermal uniformity. Bar configuration is designed for the lead form to be bonded. It should sit flatly on the foot of the lead, neither contacting the radiused area of the lead form nor significantly overhanging the lead toe, as shown in Fig. 47.38.
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FIGURE 47.38 A hot-bar in contact with leads: (a) transverse view; (b) lateral view. The hot-bar must be positioned so as not to interfere with the leg of the formed component lead. It should sit squarely on the foot of the lead.
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The bar itself can be manufactured to nearly any dimension, but there are limitations on its size; the longer the blade, the worse its longitudinal thermal uniformity. Variations in blade temperature may cause it to distort due to differential thermal expansion and contraction. Thermal uniformity is critical for consistent lead-to-lead soldering and joint quality per component side. The allowable variation in bar temperature is limited by the component specification, the damage threshold of the board to be soldered, the solder, and product reliability requirements. As in any other soldering process, the variation in process temperature must be understood and controlled to ensure highest board assembly yields and best-quality solder joints. Most hot-bar systems are configured with a fine-gauge thermocouples welded to the bar, an integral part of a closed-loop, temperature-controlled hot-bar heater system. There may be more than one thermocouple per bar. Numerous bar designs and materials of construction are available; tungsten, titanium, and molybdenum are commonly chosen for their electrical resistance, durability, thermal conductivity, immunity to flux damage, and solder nonwetting characteristics. Some ceramics are also used for blade construction. The bar must be designed for uniform heating across its length and it must be able to shed heat rapidly to allow the solder to resolidify within a reasonable time. The blade has to expand and contract uniformly throughout the soldering cycle. Some blades have been seen to develop a frowning or smiling profile in their z-axis during heating due to differential thermal expansion or built-in stress in the metal, as shown in Fig. 47.39. For the same reasons, transverse warping of the blade is also commonplace.
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FIGURE 47.39 If not constructed properly, the hot-bar can develop a smiling profile (a), a frowning profile (b), or may warp along its length (c). (a) and (b) are front views whereas (c) is a top view. The dashed line in (a) and (b) shows the desired seating plane.
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These conditions detract from proper blade contact with lead and pad combinations and are indicative of poor hot-bar design or poor choice of materials. Blade warpage, if the curvature is sufficiently large, may result in solder opens. There have been many solutions proposed in the realm of structure, materials of construction, and electrical input to permit highest uniformity of blade heating and contact to the bonding pads. D. Waller et al.25 have patented a rigid molybdenum truss blade that has been found to be stable dimensionally and thermally uniform over long spans (in excess of 76 mm). Usually the hot-bar blade is attached to a self-leveling, spring-loaded bonding head that allows the blade to self-planarize in relation to the underlying component leads and circuit board surface. Nonetheless, variations in board contour, such as a localized high or low spot, can render the concept of self-planarization useless and solder opens can result. Rigid-member hot-bars are excellent for accommodating differences in lead coplanarity because the blade
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