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forces lead to pad. Driving a heavily sprung lead onto its corresponding bonding pad can, however, lock stress into the resultant solder joint because the solder is working to resist the spring force of the lead. This can detract from the joint reliability and cause premature joint failure.
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Maintenance and Diagnostic Methods As with any process equipment, it is important to maintain the hot-bar properly. Check it frequently for distortion. Ensure that it remains flat and perpendicular to the circuit board to be bonded. Scrub the hot-bar on a ceramic flat to remove any baked-on flux residue. This may need to be done every few bonding cycles, depending on the flux and the criticality of the assembly. Several diagnostic tools can be applied to assess hot-bar conditions. The two most important characteristics that must be understood and monitored are thermal performance and blade planarity.
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Thermal Monitoring One of the most common methods of assessing hot-bar performance is to use a thermocoupleinstrumented board, as is the case with any other soldering method. Preparing such a board for thermode use has its own requirements. Fine-gauge thermocouples are attached to leads of the component or the bonding pad of the circuit board to be soldered. Positioning thermocouples at both ends of the lead set and also near its center helps quantify the longitudinal thermal uniformity of hot-bar performance during soldering. It is also useful to deploy thermocouples on surrounding components to ensure that the hot-bar soldering process does not jeopardize the integrity of adjacent, previously soldered joints. When preparing the thermal profile board for hot-bar soldering, avoid placing the thermocouple bead(s) between component lead and bonding pad.The added height of the bead would prevent the bar from contacting adjacent component leads and results in point-contact heating that is not indicative of the bar s normal operation. Instead, place the thermocouple bead at the pad extension area in front of the lead tip or at the lead heel as indicated in Fig. 47.40.
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FIGURE 47.40 Just as in any other soldering method, a thermal profile board is needed for hot-bar soldering. Care must be taken that the thermocouple bead does not interfere with seating of the lead to pad or with the hot-bar blade to the lead and pad combination (a, b). Instead, the thermocouple can be attached with a hightemperature solder alloy in front of or behind the component lead (c) so that a common lead, pad, and bar seating plane is maintained.
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Blade Planarity As stressed throughout this section, the coplanarity of the hot-bar blade or multiblade assembly with respect to the bonding surface is of extreme importance to preclude solder opens during hot-bar soldering. There are numerous measurement methods that can help diagnose this; unfortunately, though, they are meant for evaluation of the blade(s) at room temperature, not at bonding temperature. As previously mentioned, a blade may distort temporarily or permanently during heating, but planarity measurements on hot blades are impractical. Therefore, the majority of techniques encountered are performed at room temperature. J. A. Wilkins26 suggests the use of a colorant, such as from a marking pen, applied to the cold bonding surface of a freshly cleaned thermode. Once the ink is dry, the blades are then scrubbed over a clean, flat ceramic plate. Low spots on the bar are indicated by the presence of colorant remaining on the bar after several circular swipes on the ceramic flat. Single-bar and two- or four-sided blade assembly planarity can also be evaluated using an array of ground, leveled, rigidly mounted pressure transducers. Bar pressure differential, an indicator of blade planarity, can be adjusted so as to be uniform from end to end and from blade to blade on the two-up or four-up hot-bar assembly. Of course, some hot-bar assemblies are selfleveling, but even these should be checked for planarity and force per blade to ensure best uniformity during soldering. The use of pressure transducers has poor spatial resolution.
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Process-Induced Defects Solder bridging is the most prevalent problem associated with this method of soldering. Recall that the solder can be squeezed out of the solder joint and that conditions may favor solder bridge formation. Solder opens result from lack of coplanarity between the hot-bar and the plane of the circuit board surface. Lead misalignment during the bonding cycle is another defect that detracts from this method.As pressure is applied to component leads by the hot-bar prior to the onset of solder liquidus, leads are sometimes forced to slide down any domed pre-reflow the solder deposits. This displacement causes misregistration of component leads to bonding pads, and may build stress into soldered joints. If the forces are great enough, it may also cause the whole package to move and misalign the entire lead set. Because the heating is rapid, the thermode temperature is necessarily well above the solder liquidus temperature. If the time-temperature cycle is not carefully controlled, overly thick intermetallic compound formation can be a problem. This is especially true in this process, where the solder may be largely displaced from between lead and pad, and bond lines are excessively thin. Within the joint, the volume of intermetallic compound (hard and brittle) may be large compared to the remaining solder (soft and compliant). If this is the case, solder joints will be less reliable and more susceptible to brittle fracture. All of these obstacles have prevented the widespread acceptance of hot-bar bonding in manufacturing except in some niche applications where soldering by other methods may be difficult. Hot-bar bonding is most useful for low-volume, fine-pitch surface-mount soldering and rework.
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