scan barcode asp.net mobile SOLDERING REPAIR AND REWORK in Software

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FIGURE 48.2 Soldering steps required for first-pass soldering of a bottomside component on a PWB and success reflow steps encountered during first-pass soldering and repair. Note that the intermetallic compound formation thickness is not truly linear with each step. Thickness depends on materials of the soldering system, time above solder alloy liquidus, peak temperature, etc. The illustration is meant to show that with each reflow cycle, there is increased intermetallic layer thickness.
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intermetallic layer thickness, Therefore, careful process control is needed to minimize time above solder liquidus. Figure 48.2 depicts soldering and repair steps and a hypothetical impact to intermetallic layer thickness common to any soldering and repair cycles. The thermal profile used for component replacement should be predicated on the four things: (1) the solder paste manufacturer s recommendations for reflow, (2) component manufacturer s specification for peak temperature and heating ramp rate, (3) optimization of the reflow profile, and (4) results of thermal profile established from trials with the instrumented product profile board. For no-clean boards, the same solder paste used for the initial assembly should be used for the repair. For aqueous-cleaned boards, often a no-clean solder chemistry is used to preclude the need for another wash cycle. If solder paste is not added, then a paste flux (identical to that comprising the solder paste) should be used. This will obviate solder chemistry interactions and reduce likelihood of long-term corrosion.
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MANUAL SOLDER FOUNTAIN
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PTH components are generally repaired by solder fountain, which is akin to a wave-soldering system with a vertically pumped jet of solder. It is significantly smaller than a wave-soldering machine and generally lacks a conveyor, a fluxer, and preheaters. Since the equipment is much smaller, often benchtop-mounted, it permits an operator to manually remove parts and replace them. The classic solder fountain is a manually controlled system. Nozzles are chosen to shape the molten solder jet to match closely the size of the area to be soldered. Nonetheless, a shield or pallet is often used to localize the soldering and to prevent contact of the molten solder with previously soldered components on the board. The board requiring repair is shielded, except in the area of component replacement, and positioned over the solder fountain. The shaped solder fountain is directed at the repair site. Once the component solder joints go molten, the component can be pulled out with gloved hand or pliers, some flux can be added, and a new component can be inserted to complete the rework within one thermal excursion. Care must be taken to ensure that all solder joints have gone molten on the component being reworked. If not, PTH barrels can be ripped out of the board or barrel annuli can be damaged. To aid the solder fountain process, dense assemblies can be batch-heated in a box oven to make the repair process more efficient and limit exposure of the board to the turbulence of the molten solder wave. Limiting the soldered assembly s time near or above solder liquidus is key for solder joint and product reliability. As mentioned previously, time above solder liquidus along with peak temperature will define the intermetallic thickness for a given metals system, and intermetallic thickness will in part define the solder joint reliability. The turbulence of the
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solder fountain is known to enhance dissolution of certain metals into the molten solder of the fountain. Copper PTH barrels can be thinned and PTH annuli (bonding pad atop the PTH) can be thinned or completely dissolved, impairing the degrading solder joint either to make a connection or to ensure long-term reliability. The high turbulence, high temperature, and long dwell times all hasten dissolution in this fashion. For this reason, precise process control, including strict exposure time limits and minimum solder pot temperature, must be imposed for this largely manual operation. This process is best suited for repair and should be avoided, if at all possible, for first-pass soldering of printed circuit assemblies.
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