Casr Studies: PCB Repair Station Reduces Scrap
The rigor of pass-fail is not found in just the pressure-cooker of a classroom. In the business world, the difference between Pass Inspection and Failing to Meet Standards could amount to thousands or millions of dollars. One commercial pioneer, Electronic Pack-aging Co. (Dallas, EPC), has developed a new system that considerably reduces the size of the proverbial ?scrap heap.? EPC?s particular application involves the repairing of ?failed? printed circuit boards (PCBs), which is often not a failure, but a result of the technician being unable to locate the short. Using their conventional repair stations, an average of 65% of the boards were classified as ?irreparable.?
Now, using a CNC Gantry from Techno-isel (New Hyde Park, NY) as the base platform, EPC?s new computer-aided PCB repair system has significantly reduced scrapped boards. EPC?s new station, the Model 5500, projects any ?problem traces? onto the board with an LCD projector, which indicates the most likely areas or faults and it also detects recurring errors. The CNC Gantry allows the images from the LCD to be positioned with accuracy and precision that is essential in today?s densely packed PCBs.
EPC?s products aid circuit board manufacturers in the analysis and re-pair of errors. The company now offers a long list of tools for the electronic products industry including computer-aided re-pair systems, inner-layer inspection tools, loaded board systems and an audible resistance meter. EPC?s customers range from Fortune 100 companies to smaller, family-owned shops.
An automatic approach
When a failed board is brought to this repair station, the operator enters the two X, Y coordinates from the test report. The system¿s fault prediction software examines the CAD data used to manufacture the board and checks all of the pads and traces associated with the two coordinates to determine where the wires come in close proximity. When the fault prediction analysis is complete, the repair station uses a color LCD, high-resolution video graphics projector to display repair data such as traces, pads, components and other information, directly onto the circuit board. The colors of the projected image can be changed so that there is maximum contrast between the circuit board and the graphic data.
The system does not just display images, however. "It incorporates intelligence so that it actually walks the operator through the process," explains Evan Evans, engineering manager at EPC. "The system highlights the first place to put the test probe, then the second place. It presents the problem areas one after the other so the operator can find the fault." The failure analysis software takes into account whether there might be internal shorts so the repair person does not waste time repairing these external faults only to find that the PCB must be scraped because of a nonrepairable fault. A stereo zoom microscope is included so that location, verification and repair of small faults not seen by the naked eye can be performed. The repair station also features fault-ranking software that collects a history of the faults and uses this data to determine the repairability of subsequent PCBs.
A key feature of the system is the accuracy with which it displays the images. Positioning accuracy is critical because of the close proximity of the traces and components on today¿s PCBs.
This servomotor-controlled gantry system delivers 0.0005-inch resolution and repeatability, and 0.003 inches per foot absolute accuracy. It is constructed on steel stress-relieved bases with hardened steel linear ways. Its shaft-and-bearing system produces very smooth, play-free motion and is an extremely rigid system that produces high-accuracy positioning. The gantry¿s design includes heavy cast-aluminum side plates supporting the Y axis, giving extra stiffness for accuracy in positioning. Antibacklash ball screws and nuts are standard. These screws have excellent power transmission due to the rolling ball contact between the nut and screws, and this type of contact ensures low friction, low wear, and long life, says Evans.
- The servo-controlled gantry system delivers 0.0005-inch resolution and repeatability, and 0.003 inches per foot of absolute accuracy.
- Fault prediction software examines the CAD data used to manufacture the board and checks all of the pads and traces associated with the X, Y coordinates to determine where wires come in close proximity.
- When the fault prediction analysis is complete, the repair station uses a color LCD high-resolution video graphics projector to display repair data directly onto the circuit board. The colors of the projected image can be changed so that there is maximum contrast between the circuit board and the graphic