Sidactor Protection Thyristor Products Page 26 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 Solder Joint Inspection Inspection of QFNs on a PCB is commonly accomplished with the use of an X-ray inspection system. In most cases, 100% inspection is not performed. Typically, X -ray inspection is used to establish process parameters and then to monitor the production equipment and process. The X-ray inspection system can detect bridging, shorts, opens, and solder voids. In addition to searching for defects, the mounted device should be rotated on its side to inspect the sides of the solder joints. These joints should have enough solder volume with the proper stand-of height so than an "hour glass" shaped connection is not formed (see Fig. 1.14 below). Rework Methodology Due to the fact that the QFN is a leadless device, the entire package must be removed from the PCB if there is an issue with the solder joints. It is important to minimize the chance of overheating neighboring devices during the removal of the package since the devices are typically in close proximity with each other. Standard SMT rework systems are recommended for this procedure since the airflow and temperature gradients can be carefully controlled. It is also recommended that the PCB be placed in an oven at 125 C for 4 to 8 hours prior to heating the parts to remove excess moisture from the packages. Component Removal The gas nozzle used during this process surrounds the device and seals against the board. The QFN is heated from the topside with hot gas while residual heat is exhausted up and away from adjacent components. The anti-crushing feature in the nozzle prevents excessive topside force from being applied to the QFN. The entire assembly is also heated from the bottom side with an under-board heater to help prevent warpage. Preheating the board to a fixed temperature before the component is heated also helps to ensure process repeatability. Once the reflow process is complete, the nozzle vacuum cup is automatically activated and the component is slowly lifted off the pads. The vacuum cup in the nozzle is designed to disengage if the component has not fully reflowed for any reason. This prevents the potential for lifting pads. Site Redress Once the QFN has been removed, the residual solder that remains on the pads must be removed. The QFN PCB site is very fragile because of its small pad sizes. To avoid damaging the pads or solder mask, the site redress process must be performed very carefully. "No Clean" flux is applied to the site after component removal. Using a temperature- controlled soldering iron fitted with a small flat blade, gently apply solder braid that has been presoaked in flux over the PCB pads. Residual flux is removed from the site with alcohol and a lint-free swab. This site is then inspected prior to the replacement process. Component Replacement and Reflow Due to the small pad configurations of the QFN, and since the pads are on the underside of the package, a manual pick and place procedure without the aid of magnification is not recommended. A dual image optical system where the underside of the package can be aligned to the PC board should be used instead. Reflowing the component onto the board can be accomplished by either passing the board through the original reflow profile, or by selectively heating the QFN with the same process that was used to remove it. The benefit of subjecting the entire board to a second reflow is that the QFNs will be mounted consistently and by a profile that was already defined. The disadvantage is that all of the other devices mounted with the same solder type will be reflowed for a second time. If subjecting all of the parts to a second reflow is either a concern or unacceptable for a specific application, then the localized reflow option would be the recommended procedure. Figure 1.14: Desirable vs. Hour Glass Solder Joint PCB Placement Guidelines for QFN - Quad Flatpak No-Lead Package (continued)

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