The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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47 T he success of the manufacturing process is based upon the successful execution and ability to measure the health of each process step. The machines on assembly lines install thousands of parts every minute; therefore, throughout the assembly process, manufacturing checks must be completed. These checks verify many important items pertaining to assembly manufacturing. However, final testing—in-circuit testing capability—must be designed into the PCB at the layout stage. DFT allows capability for testing which includes component connectivity, electrical value, and proper component orientation. Many of these tests can be run without required modifications to the PCB layout; however In-Circuit Testing (ICT) does require some additional features added to the PCB, so this will be the focus of this section. Remember how the virtual production of Ian's first PCB design went? How it looked good on screen, but as the assembler could see, there were just too many test inconsistencies with the BOM, the part footprints, and the routing. With only the non-intelligent Gerber file data provided (and no test points included) it was impossible to test Ian's design. Critical defects were not detected along the way. Test engineers understand production test fixtures and the development of testing software can get expensive. Incorporation of simple DFT for ICT basics (such as bed-of-nails testing for single- sided testing or clamshell fixtures to test two sides of a PCB assembly simultaneously) can greatly help assembly teams to check and verify their production work. This allows parts which pass the electrical tests to move on to their next production phases with zero defects. Like Ian, you might be left asking yourself: "Should I have taken the time and spent more money incorporating DFT for ICT? " Chapter 6 DFT for In-Circuit Test and JTAG

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