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Micromachined structures are emerging as useful instruments in the fields of medicine and surgery as precision sensing is crucial and can be a matter of life and death Various types of biosensors shown in Figure 826 are useful for different functions One example is the electrochemical sensor array developed by Teknekron Sensor Development Corp The biosensor is small enough to fit inside a blood vessel and can measure the levels of oxygen, carbon dioxide and pH of the blood [9] The development of micromachined neural probes

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.

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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

limits described by the solder paste manufacturer, the component specifications for thermal exposure, best solder wetting, and minimized intermetallic formation In the case of repair, a dummy board is used with thermocouples attached to the component to be removed and critical adjacent components to ensure that reflow conditions are correct 48222 Component Removal Preheat As discussed previously, preheating may be necessary to overcome the thermal mass of the assembly and to ready the board for the reflow step Preheat temperature is set well below the reflow temperature of the solder alloy in use to confine reflow to the device being removed or replaced and to discourage widespread reflow in the vicinity of the removal or repair 48223 Reflow and Removal The same considerations apply for this as for any reflow step.

Fig 826: A micromachined transducer for application in medical treatment and diagnostics [2]

(LSB)

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