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Using Reverse Engineering to Recreate Lost CAD Data
C-Link DTM software can show the real image of the board on the background of CAD data.
By Ebeid Francis Safein, Applications Manager, Digitaltest, Concord, CA
What do you do when your customer, a major transportation equipment manufacturer has a big problem with its electronics? The company has a large number of older printed circuit boards that are still used for the company's legacy equipment. There is no PC board data in any form. CAD, BOM or schematics are not available. What the company does have is a large pile of boards that need to be repaired so they can be put back to use.
A military branch using legacy equipment has a stockpile of unusable boards. No CAD is available but schematic data is somewhat usable although not 100 percent correct. They require a lot of functional and digital testing including flash programming, Boundary Scan, frequency and other measurements. For these boards in-circuit test fixtures or flying probe testing is a necessity to repair or redesign and rebuild.
Many companies and repair depots have situations similar to those just described. They face the daunting task of testing and repairing these boards with little or no data. In many cases they simply have the board in hand. These boards can represent a significant financial and engineering investment — far too much to just throw them into the trash.
A new solution for these situations is now available using flying probe test and reverse engineering software. The solution is a unique way to reverse engineer the unit under test without causing any damage to the board, even if the board is multi-layered.
Shortening the Timeframe
Reverse engineering of a printed circuit board used to mean months of manual labor to check connectivity (point-to-point), component locations, wiring, and functions before manually rebuilding a new board using CAD tools. This process was made even longer by human error, trials, and multiple prototypes and in some cases this resulted in complete failure to accomplish the job.
With the introduction of new technology added to flying prober testers including high accuracy for landing on small pads of very small components including SMD components and FPGAs — along with higher camera resolution, speed of probes, precise measuring capabilities, softlanding technology to prevent board damage, more powerful software tools — manufacturers are able to integrate the reverse engineering software. Using this new technology, they are now able to combine the powerful functions of the flying prober into tools that are capable of 100 percent reverse engineering in most cases providing capability of testing this board and also adding more capabilities to redesign and build the boards again.
Using a powerful high resolution camera mounted on the head of the flying prober allows for hundreds of magnified pictures to be taken of the top and bottom of the board. These pictures will be stitched together to create a complete and accurate linear picture of the complete board.
Using this stitched image, the reverse engineering software will take these images and convert them to CAD dimensions and build the X-Y locations of test points and all the dimensions of the component outlines, including all the locations that can be accessed by the flying probe, and then converts them to a usable CAD format like GenCAD, ORCAD, Protel, or Mentor. This cataloging includes surface mount components, through-hole connections, vias — every visible characteristic.
Using a high resolution camera, hundreds of pictures are taken of the board and "stitched" together.
Using the flying prober's movable heads, the tester will go to every accessible location and highlight the center of the pad using the zoom feature of the camera. The user will be able to easily fine-tune any non-linear images in the conversion of the picture or that may be due to the presence of irregular components or solder — fine-tuning by moving the probe head using the software features or a joystick to adjust the location to the center of the pad and saving it. This action serves to enhance the accuracy of the outcome.
The tester will now probe all of these accessible locations automatically since the X-Y locations are defined by simply learning three points on the board that the user defines as fiducials. These cab be reference points like vias, thru-hole pads or test points. Using these three reference points, the software will match the scale of the picture to the real printed circuit board. The flying prober will land on every single location and checks all possible connectivity of the board — building all the net connections and layers (net list). Every two points that are connected are considered one location in order to optimize the process and save time during extraction of the net list.
Once probing is completed, an engineer can build the component packages, referencing the thousands of saved libraries in the reverse engineering software. If the Bill of Materials is available, it can then be imported directly, or default values will be assigned to the component values. Default values are learned using the flying probe auto-learn measurements. Using in-circuit test guarding techniques, the flying prober will be able to measure the real value of the analog components like capacitors, resistors, inductors and prevent the effect of parallel electrical paths. At this point, you have X-Y locations, net connectivity, component locations, and component values. Once these steps are completed, CAD data will be generated automatically. This CAD data can immediately be used to test the board using any test equipment: AOI, flying probe, ICT, analog, digital, functional testing.
Designing the Boards
In order to rebuild, remanufacture or even design these boards, more steps will be needed. The output will have provided enough information for complete testing on a flying prober or even for a bed-of-nails fixture, but it will be missing the net routing needed for the manufacturing process, especially on internal layers. So using the CAD auto-routing or CAD tools manual routing, the output of the reverse engineered net list is converted into complete CAD data that can be converted to Gerber files that allow the board to be manufactured from scratch.
Using a golden board, the firmware/programs or data saved on the microcontroller flash or EEPROM can be extracted and these data can be used to reprogram a repaired or a newly manufactured board. In one example, a large, expensive military/aerospace board was processed and the only available data was the printed circuit board itself and schematics. The schematics appeared to contain many errors, and the software discovered that these schematics were actually for a different version of the board. After processing the board using reverse engineering software, a complete fixture was manufactured to test the board on a bed-of-nails tester and also to power on the board and test it functionally — including programming of the flash memory and boundary scan testing. This allowed for the remanufacture of this board with slight modifications because of obsolete parts, and thereby continue production. A bed-of-nails tester was used due to heavy digital and functional test requirements, and necessary programming for the microcontroller. It is also possible to draw schematics using the output of the reverse engineering software, which will make troubleshooting much easier to handle.
When combined with new and advanced flying prober technology, reverse engineering software tools offer an opportunity to rescue valuable boards from oblivion. Instead of throwing these boards away, is now possible to repair, replace, and rebuild, and most importantly this is done without damaging or destroying the original golden board.
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