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Potted QFP Rework Challenge
Potted QFP Rework Challenge
Image 1. LED circuit board with silicone potting gel encapsulation.
A manufacturer of outdoor electronic billboards had hundreds of circuit board assemblies that were returned from the field. They all required the removal and replacement of a faulty 44 pin fine pitch QFP device.

These 15" x 10" circuit board assemblies were populated with over 1500 surface mount LEDs, along with hundreds of other surface mount devices.

Ordinarily, the removal and replacement of a QFP would be a relatively straightforward rework project. In this case however, the boards were completely encapsulated with a 0.20" thick layer of silicone potting gel, making for a significant challenge.

The engineers and technicians at Circuit Technology Center debated several approaches that could be used to selectively remove the potting gel; ultimately, mechanical removal of the gel was chosen as the safest and most reliable way to proceed.

Potted QFP Rework Challenge
Image 2. Silicone potting gel removed, QFP removed and surface mount pads cleaned and prepared for resoldering.
Highly skilled technicians, with the aid of a zoom microscope and using sharp Exacto type knife tools, extracted a 0.45" x 0.59" section of the gel to expose the QFP.

Great care was taken not to damage the base board material or adjacent LED devices during this process. Once the potting gel was removed and QFP exposed, the boards were sent to the hot gas rework department to remove the defective QFPs.

Circuit Technology Center has five top-line Air-Vac DRS 24 and 25 programmable hot gas rework systems. These systems are used for a variety of surface mount and lead-less device removal and replacement including BGAs, QFNs, LGAs, and more. 

Potted QFP Rework Challenge
Image 3. Completed rework with new QFP soldered in place.
These recipe driven machines, in the hands of trained operators, provide a very stable and highly repeatable rework process. Once the defective QFPs were removed, the sites were inspected, cleaned, and prepped for new device placement.

Because the component rework site was located in a relatively deep and narrow well within the silicone gel, the re-soldering process was more challenging than usual.

Circuit Technology Center's most highly skilled soldering technicians were assigned to this final step of the rework process.

Several members of the Circuit Technology Center team contributed to this feature story.