The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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94 F rom start to finish, Ian saw proper design consideration is critical for supplying viable data and documentation to drive the hundreds of steps involved in fabricating a PCB at the board shop. With the help of the assembly and test stakeholders, Ian was amazed to discover that the viable data driving the PCB assembly processes after the PCB is fabricated is just as important. After the PCB layout is complete and the data and documentation for the PCB has been generated, the fabrication package has most likely been released for fabrication. PCB fabrication cycle, or turnaround time, can vary depending on whether the design is being considered for quick-turn or standard fabrication. Quick-turn times for PCB's can be accomplished at specialized PCB fabrication shops and are measured in days, sometimes hours. Standard times are typically measured in weeks. Timing of the PCB fabrication cycle is mentioned here because it is a good indicator of how much time a designer needs to finish his or her final project design responsibility and generate assembly data and documentation. Data drives the process. It is very important a complete package of data is supplied to the assembly stakeholder. As in any other process, without complete and accurate data, the process will stall or become confused, resulting in time-consuming queries between the supplier and the customer. The following describes the main items of a complete assembly data package. The Bill of Materials File (BOM) After investing so much thought, effort, and time into the layout it is sometimes hard for a designer to consider that once the PCB is fabricated, it will simply be treated as another component commodity —one which must be ordered along with many other components. This will then feed into the PCBA using the BOM. The assembly process starts with the evaluation of all materials published in a BOM report which is exported as data output from the schematic end of the design and layout process. A BOM which lists all PCBA components, their manufacturing part numbers, descriptions, and corresponding reference designators is a powerful tool for use by a supplier management purchasing agent. The sooner the component information can be captured and sent to them, the better. Challenges will arise and can include item costliness, long lead time, discontinuation, and obsolescence. The ability to flag potential procurement problems requires visibility. Chapter 10 Assembly Data & Documentation, Process Overview

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