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Last update:

10/12/2006

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What constitutes a complete design package to be delivered to the PCB designer…?

Here are a collection of thoughts for you to consider…

Aspects of the design to be communicated are electrical, physical, environmental, operational, and functional.  A design package must consist of a complete description of the circuit to be designed and the environment into which it must survive and it must consist of all the aspects of the design… not just a hand drawn schematic on a paper napkin, (although I have worked from those too) .

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This level of communication can be accomplished through the use of several face to face meetings and

The use of the following documentation vehicles…

1) Physical or Mechanical constraints drawing… complete mechanical drawing indicating location of critical components, mounting holes, tooling holes, test connectors, thermal paths, high current high voltage circuit locations, shielding, and the physical board outline, interconnection requirements, location of RF sources, heat sources, physical interfaces, component height clearances, keep out areas, board material and thickness requirements,  etc.

2) Complete schematic… all components identified, Company part numbers for the components, definitions for power, ground, critical signals, high current high voltage circuits, shielding, sensitive circuits defined and rules communicated, high speed or RF circuits, gate and pin swapping rules, clock lines, data bus requirements, bypassing, filtering, etc…  

Things to consider – second sources for components? Hot parts? Sensitive parts? Grounding? Power distribution? Matched Impedance needs?

3) Bill of materials, accompanied by all the data sheets needed indicating the physical size, and electrical properties, thermal and environmental needs and mounting requirements etc…   

4) Test requirements document.  – what does the designer need to know to support how you intend to test this board?

The PCB designer needs to be made aware of the environment into which the product will be expected to operate, to all extremes, temp, vibrations, humidity, EMI/EMC, high voltage, high current, etc… he/she must know how the board is mounted or housed, how it will be assembled, how it will be tested and what sort of environment it will need to function in. This data shapes the approach and solutions proposed to packaging and creation of the board design. The board is no longer a replacement for a rats nest of wires behind the chassis… it is an integral component in the product, and how it is created can make or break its development.

Electrical requirements are very important… voltage spacing, adequate conductor widths for higher current needs, EMI protection, RF coupling, proper grounding, decoupling, filtering, local supply regulation, impedance, over voltage protection, the list goes on…

Mechanical requirements are also important… the board must be able to survive in heat/cold/vibration/humidity/organic or fungal growth, salt or corrosion, vacuum of space, where ever it must function… requirements to survive might include, lock washers, vibration dampening, localized heat or cooling, conformal coating or anti corona dope, epoxy mounting, thermal transfer materials, application of fans, heat sinks, extra board stiffeners, brackets or supports, flex circuits, shielding, ruggedized connectors, sealants, the list goes on…

The physical parts must fit on the board leaving clearances for automated component placement and assembly and test. Tooling holes must be provided, fiducial marks for automated equipment must be supplied as needed, often-times boards must be designed to fit into assembly panels that are designed to fit specific assembly equipment tooling for ease and efficiency of the manufacturing process… pads or connectors may be needed for test probes with proper spacing for access and reliability during test…

Human engineering needs to be considered, can the tool required to tighten or loosen a screw or bolt be inserted into the assembly with the appropriate clearances? Can the adjustable parts be accessed to adjust the levels in the circuit while it is installed in the product?  Parts should not be stacked upon each other… you should not have to remove one part to get at another… can a finger or hand get into the space available to do what needs to be done?  Are humans protected from shock hazard? Fire hazard? Can you test this design? Will the equipment, probes, etc. be able to access the test points?

Remember fit form and function… all must be in harmony in order to have a producible product that can be assembled, tested and relied upon to operate in the field.

DFX = Design for Manufacture – Design for Assembly - Design for Testability