scan barcode mobile SOLDERING REPAIR AND REWORK in Software


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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. 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. Reflow and Removal. The same considerations apply for this as for any reflow step. The ramp, soak, peak temperature and time above liquidus are dictated by the solder alloy used and should be roughly the same profile parameters that were used for the initial assembly of the board. Once the component has reached complete reflow, the vacuum pickup tool is engaged and the component is lifted vertically. Care should be taken to minimize the time above liquidus to minimize intermetallic compound thickness. Site Dressing. Following component removal and while the board is still hot, the solder lands on the PWB can be prepared for component replacement by means of a hand soldering iron and a braided copper solder wick or a solder vacuum machine that simultaneously heats the pad by hot gas and vacuums up any excess solder. During the component removal step, solder surface tension capriciously allows some of the solder from the joint to be removed with the package leads and some to be left on the bonding pads. In the case of area-array packages (BGA, ceramic ball grid array [CBGA], etc.), the ball may remain with the package or with the land. This can make solder land dressing a labor-intensive operation. Care at this step is critical for success in resoldering a fresh component and for reliability. The dressed bonding pads should be smooth so as not to interfere with fresh component placement. Part Bake-Out. As with first pass soldering, it is important to observe rules concerning component bake-out to remove any absorbed moisture and preclude part cracking during the reflow heating cycle. Guidance for part baking can be found in IPC/JEDEC J-STD-033.1 Solder Paste or Flux Deposition. Although this is not part of the hot-gas station, it is part of the hot-gas work cell and needs to be discussed. For most SMT components, solder paste is added to either the board or the component to effect a reliable solder joint for component replacement. This is generally done by ministencil similar to what is used for SMT solder paste deposition, but on a smaller scale; the stencil is only slightly larger than the component footprint. The paste is pushed through the stencil apertures with a razor blade or a miniature stainless-steel squeegee. Programmable syringe-dispenser machines are sometimes used, but solder paste is known to separate when under pressure, i.e., solder alloy separates from the solder flux. Choosing the largest needle bore, slowest dispense rates, and lowest pressures will help to mitigate paste separation. Also, use of direct drive auger pump rather than an air-pressure-driven dispenser will help to reduce paste separation. Sometimes in BGA repair, solder paste is not used. Instead, solder from the BGA ball is used to make the solder joint, and solder flux must be added for this bonding step. The flaw in this methodology is that lowered solder volume results as compared to the original solder joint and the BGA reliability may not be as robust as when first soldered. In fact, any repaired joint is less reliable than a first-pass solder joint due to thickening intermetallic layer formation that comes with each reflow cycle. Addition of solder paste results in a better process because the paste compensates for small differences in BGA ball sizes. It domes upon reflow, adding some compliance to make up for non-coplanarity solder balls. Opens are less likely to occur, although more process precision is required to dispense the paste and place the BGA
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