SOLDERING REPAIR AND REWORK in Software

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48.1 INTRODUCTION
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As printed circuit assemblies (PCAs) become more densely populated, the probability of rework increases. So does the complexity of rework steps. Rework deserves consideration both in terms of board design as well as in conducting the repair. Deep preheating cycles, careful thermal profiling, and thermal masking of adjacent components become critical. PCAs are often mix-mount (surface-mount technology [SMT] and plated through-hole [PTH]). Additionally, they may contain press-fit components or other subassemblies that may be sensitive to overheating. Aside from hand repair with a soldering iron, there are two techniques that are the mainstays of repair: solder fountain and hot-gas. The former is a variant of wave soldering and is tailored to removal or post-soldering addition of PTH (solder-tail) components. Hot-gas (also known as hot-air) soldering is used primarily for removal and replacement of SMT components. Because repair of dense assemblies is difficult, this chapter covers some additional descriptions and tips for repair. The onset of Pb-free soldering has increased the difficulty of repair processes due to generally higher melting temperatures and slower wetting times that are characteristic of the most prevalent Pb-free solders. Otherwise, repair involving Pb-containing solders and Pb-free solders is the same.
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48.2 HOT-GAS REPAIR
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This technique consists of directing a heated stream of air or other process gas such as nitrogen at the component to be removed. Modern hot-gas repair stations are sophisticated, with computer control, predictive reflow profiling, vision systems to assist in component alignment to bonding pads on the printed wiring board (PWB), preheating stations, and accommodations for component removal. Hot-gas repair is most often used for SMT components but can be adapted for PTH component removal.
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Hot-Gas Repair Equipment
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48.2.1.1 Hot-Gas Nozzle. The heated gas is generally delivered through a component-specific nozzle. The gas stream is directed at the top of the component, under the component, or a combination of the two, depending on the nozzle design (see Fig. 48.1). 48.2.1.2 Preheater. Preheaters are used to help overcome the thermal inertia of the board. Since the nozzle exposes only a small amount of the board to a high-temperature gas stream, heating through this mechanism is limited. The preheater uses heated gas or infrared heating but heats a much larger area of the board. The preheating is set to attain a temperature much lower than the reflow temperature of the solder, but high enough that when nozzle heating is applied, reflow can take place in a reasonable period of time without overheating the component to be replaced or other components adjacent to the repair site. Most hot-gas repair machines have a built-in bottomside preheater. An alternative to preheating would be to bake the board in a box oven. The problem with this approach is rapid cooling of the board from the time of removal from the oven and during setup of the board on the hot-gas repair station.
FIGURE 48.1 Schematic of hot-gas repair of a ball grid array (BGA) (B) soldered to a circuit board (C). Hot-gas (air or nitrogen) is injected through a baffled nozzle. In this diagram, the heated gas is directed under and on top of the BGA.A heat-resistant suction cup (A) is used to remove the BGA once all joints have reached reflow temperature. Gas injected through a heated platen (D) is used to preheat the circuit board, lessening the thermal load and shortening the rework cycle.
48.2.1.3 Vacuum Pickup Device. This is generally a silicone rubber suction cup or a solid metal tube nested within the nozzle and connected to a vacuum pump. Silicone is chosen for its heat resistance and compliance. The vacuum can be toggled on and off to pick up and release a component. The vacuum pickup device serves two purposes. First, it is used to remove a component from the circuit board once the component has been heated by the hot-gas to the point of reflow. Following dressing of the repair site, the suction cup is used to hold the replacement component rigidly during lead-to-pad alignment and placement steps. 48.2.1.4 Adjustable Stage. This permits alignment of the nozzle to the PWB in the x-, y-. and q-axes. It is important for component removal but even more critical for component replacement and alignment of component leads to PWB bonding pads. 48.2.1.5 Vision System. This is used to align component leads to PWB bonding pads manually. Usually a split-image video system is used to view both component leads and board bonding pads simultaneously. The images are superimposed and the operator manually adjusts them for the best lead-to-pad alignment. 48.2.1.6 Computer Control. This is the heart of most hot-gas machines. It monitors and controls the various operations of the system, such as preheating and thermal profile. The rework engineer programs the profile manually.
Hot-Gas Repair Process 48.2.2.1 Thermal Profiling. As with any reflow process, whether for first-pass build or for repair, it is necessary to construct a thermal profile board as previously described to assess the impact of heating and to ensure that the heating is adequate, in accordance with the thermal
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