Potted QFP Rework Challenge
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.
|Image 1. LED circuit board with silicone potting gel encapsulation.|
These 15" x 10" circuit board assemblies were populated with
over 1500 surface mount LEDs, along with hundreds of other surface mount
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.
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
|Image 2. Silicone potting gel removed, QFP removed and surface mount pads cleaned and prepared for resoldering.|
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.
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.
|Image 3. Completed rework with new QFP soldered in place.|
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
Several members of the Circuit Technology Center team contributed to this feature story.