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without causing solder bridges. Eliminating solder paste is not an option for area-array devices with noncollapsible interconnects, such as ceramic column grid arrays (CCGAs) or CBGAs. Solder has to be added for rebonding of this type of component since the interconnects (balls or columns) are of a high-temperature solder alloy and will not reflow. Most commonly, the necessary solder is added by the aforementioned ministencil method. 48.2.2.7 Soldering Preheat. After flux deposition, the board is preheated to a temperature about 50 to 70 C below solder liquidus. Generally, the hot-gas rework station has its own preheater and component and board preheating is part of the reflow profile. 48.2.2.8 Component Pickup. As the board is preheating, the component is picked up and readied for placement.After the component is placed on a pickup station, the nozzle with vacuum pickup is placed and lowered over the component and the vacuum is activated. The component is raised up and readied for the alignment step. 48.2.2.9 Component-to-PWB Alignment, Placement, and Reflow. Generally, a split vision optics system is used and the semitransparent image of the component to be placed is superimposed over the video image of the PWB bonding pads for that component. When the two are manually aligned, the component is lowered on to the solder paste- or flux-coated bonding pads and the thermal cycle begins. Once reflowed and the solder reaches solidus, the rework process is complete. The board is then ready for inspection. 48.2.2.10 Inspection. Visual inspection should be conducted as practical to ensure integrity of any visible solder joints and also the area surrounding the rework. Ensure that solder mask is not charred or delaminated. Also check whether bonding pads have delaminated (not possible on inner rows of area-array packages). For BGAs and other area-array devices, following a visual check, use transmission x-ray to inspect for solder bridges. Sometimes this method also reveals lack of solder. Lastly, if in-circuit testing or functional testing is done on the first-build product, then it should be applied to the reworked product also.
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Hot-Gas Rework Guidelines Generally the number of SMT solder rework cycles per component per board is kept to two or three. Beyond that, the board should be scrapped to avoid reliability problems. The key in all reflow steps is to limit the amount of time at or near solder liquidus in order to minimize intermetallic compound (IMC) formation. IMC thickness increases with each reflow cycle and is temperature-dependent also. IMC is brittle the weakest link in terms of solder joint structure and resultant joint strength. During component rework and replacement, minimizing thermal excursion is all important. Too often, overly high temperatures are used during rework and dwell at liquidus are prolonged as compared to a normal mass reflow soldering cycle. When a component is removed and replaced, IMC from previous soldering operations or other thermal excursions remain on the pad since the IMC becomes an integral part of the pad. For illustrative purposes, consider the case of a surface-mount component with a copper lead-frame and copper bonding pad and either a leaded (Pb d) or lead-free (Pb-free) solder alloy on the bottom side of the PWB. During the first-pass assembly, the bottom side of the board is built first. During that soldering process, an intermetallic layer forms at the copper lead-to-solder interface and also at the solder-to-bond pad interface. During topside soldering, the components on the bottom side of the board undergo a second reflow. Assume that a part soldered during the bottomside assembly fails and requires removal. The removal and replacement processes necessitate additional reflow cycles to the affected component site, including component removal; site repair, and component replacement. With each soldering cycle, the intermetallic layer builds and reliability generally degrades with increasing
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