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CAD PCB Design v1.9 Walter Shawlee 2, Sphere Research Corporation


CAD PCB Design v1.9 Walter Shawlee 2, Sphere Research Corporation What is the real point of CAD for PCB Design? Main Answer: Only one single data entry drives all ... – PowerPoint PPT presentation

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Title: CAD PCB Design v1.9 Walter Shawlee 2, Sphere Research Corporation

CAD PCB Designv1.9Walter Shawlee 2, Sphere
Research Corporation
  • What is the real point of CAD for PCB Design?
  • Main Answer Only one single data entry drives
    all downstream functions.
  • And Easy design re-use and modification, high
    complexity plus easy to check, store and share

CAD PCB Design
  • On the CAD negative side It is NOT cheap to
    commercially acquire (1K-15K now typical), or
    fast to use, and often has a steep learning curve
    and serious long term deployment problems.
  • The significant short term benefits are generally
    felt to offset these issues in a corporate
    setting. The continual software upgrade,
    compatibility, recurrent training and OS
    headaches never really go away, however. Keeping
    files usable and users qualified long term is
    very difficult to achieve.

Manual PCB Design
  • How was it done before CAD?
  • Rubylith/Amberlith film, tape, and pads to make
    over-sized photo-tools.
  • Resulting boards have a distinct low density
    appearance. Even double sided art is difficult to
    align well.

Manual PCB Design
  • Double sided hand-taped board artwork.
  • This then created (reduced) photo tools for
    exposing sensitized PCB material.

CAD PCB Design
  • How does it work? First, start with a circuit
  • It doesnt have to be fully developed, this is
    very important. You can keep modifying and adding
    throughout the process.
  • The initial tool to start the idea in the CAD
    process is called Schematic Capture.

CAD PCB Design
  • The Schematic Capture phase associates PARTS
    (symbols) with a specific pin-by-pin
    interconnection, that association forms the NET

CAD PCB Design
  • Each PART is also associated with a physical
    FOOTPRINT. This data, coupled with the NET LIST
    make it possible to create a printed circuit
    board or PCB.

CAD PCB Design
  • There is an almost infinite variety in terms of
    PART FOOTPRINTS. A resistor can be through-hole
    or surface mount, and can come in almost any size
    and shape.

CAD PCB Design
  • Any given schematic could produce many boards,
    simply by altering the FOOTPRINT of each part or
    its value to suit a given application

CAD PCB Design
  • All too often, the parts needed DO NOT EXIST in
    the CAPTURE software, then the LIBRARY EDITOR
    must be used to create them.
  • This is a major roadblock in all CAD software,
    and a significant delay to using new parts.

CAD PCB Design
  • Poor LIBRARY EDITOR functions, or library errors
    can dramatically slow down CAD designs.
  • Weak, incomplete, inconsistent or incorrect
    LIBRARIES are the perpetual problem of all PCB
    CAD software. 1-2, 2-1, A-C, A-K, anode, cathode
    issues remain everywhere.

CAD PCB Design
  • Common library problems include mismatched
    connection points, wrong hole sizes, missing
    pins, incorrect sizing, lack of polarization
    marks and much more.
  • It is easy to make new parts, but it is not a
    quick process to do it well, and Spice data
    cannot be done by the user, thus halting

CAD PCB Design
  • If the CAPTURE program does not internally link
    to and store the FOOTPRINT data, then moving the
    files to another computer with a different
    library may be problematic.
  • The ideal situation is to have project files
    which are internally self-sufficient, and
    compatible shared libraries between users.

CAD PCB Design
  • The next step in the CAD PCB process is to take
    the data generated from the SCHEMATIC CAPTURE,
    and feed it into the next program process, which
    is the PCB LAYOUT TOOL. In addition, a BOM or
    bill of material can be generated for purchasing
    and production, listing all the parts and their
    identifiers or IDENTS.
  • LAYOUT will automatically take the physical data
    (net list footprints), and create a rats
    nest basic layout.

CAD PCB Design
  • The Rats Nest simply tosses all the footprints
    onto the workspace and shows where connections
    should go. The rest is up to you.

CAD PCB Design
  • The Rats Nest can ALSO contain some design tool
    features, stored in a Technology File or other
    predefined Template.
  • These can include colors, layers, required
    symbols, tables, predefined board outlines, etc.

CAD PCB Design
  • Going from the Rats Nest to a usable board
  • A defined Board Size
  • A layout Strategy
  • Design Rules
  • Physical Routing

CAD PCB Design
  • Board Size (or at least a defined area) is needed
    to allow Autorouting or Pouring. You have to
    determine the size, or just let it flow naturally
    from the layout as it progresses.
  • Size can be changed at any time, and is not
    really important until the end, and as needed to
    fit any specific project requirements.

CAD PCB Design
  • Board Strategy is how you intend to order the
    parts on the board, how you want it to look, and
    any issues you think are important for a workable
  • Many boards turn out poorly because too little
    forethought goes into them. Think through
    assembly, test and end use carefully.

CAD PCB Design
  • Board Strategy may dictate sensitive part
    separation, the use of large areas for heat
    sinks, shields, or require careful control or
    partition of power and ground traces or pours for
    minimum signal noise and radiated emissions in
    the final design.

CAD PCB Design
  • Good Strategies include
  • The use of large planes (pours) for power and
    ground to reduce supply noise and reduce
    emissions. Insure adequate trace width for power
    and ground paths.
  • Partitions of ground and power to keep digital
    noise from sensitive analog sub-systems.
  • Clean signal flow from inputs to outputs.

CAD PCB Design
  • Good PCB Design is ART, it transcends simple
    technical skill in exactly the same way a good
    book is not simply the use of language.
  • This skill improves with use and the study of
    other good work. You will know your work is of
    good quality when other people start to copy the
    way you work.

CAD PCB Design
  • Use Thermal Reliefs to attach parts to heavy
    tracks and fills or soldering may become
  • Document and mark EVERYTHING both on the board
    and the schematics.

CAD PCB Design
  • Design Rules set key factors like minimum track
    size, track to track and pour spacing, via
    definitions, and other variables that can be
    controlled and defined globally in the design.
  • Mistakes here can make the design very difficult,
    so think about what will work best for this task.
    These are set by you, but are controlled in

CAD PCB Design
  • Routing can be done by you manually, a track at a
    time, or to some degree by Auto-routing (if
  • Auto-routing is best for logic array or memory
    wiring where there are repetitive patterns, but
    is not very effective in analog design.

CAD PCB Design
  • Routed boards have LAYERS, representing copper,
    silk screens, solder masks, outline information,
    drill information and assembly data.
  • You can only really see and work on one layer at
    a time, because they are visually stacked on top
    of each other like pages in a book.

CAD PCB Design
  • Layers have unique names, somewhat standardized
    over time, they are (in sequence)
  • Colors are not standardized, but a black
    background workspace is much easier on your eyes
    than white. The associated layer Gerber Files DO
    have standardized names.

CAD PCB Design
  • Once the board is routed, checked (use any DRC
    DESIGN RULE CHECKING features available), all
    cosmetic features touched up and reviewed by
    another person, then you can go to the production
    phase. Peer review is critical to catch errors.
  • For PCB production, you can run the Gerber
    Post-Processor from the LAYOUT tool to generate
    Gerber files, drill charts and design files.
    Purchasing and assembly get the BOM or Bill of
    Materials file.

CAD PCB Design
  • Gerber Files contain the layer-by-layer data plus
    the drill info and outline to make a finished
  • Make sure you have EVERY FILE required, and none
    that are not required, then add any needed notes,
    and archive to send out for board manufacturing.

CAD PCB Design
  • Gerber Files drive a Gerber (or other)
    photoplotter which makes the PHOTOTOOL to expose
    the sensitized PCB material or screen and mark
    pads and tracks.
  • The Drill File runs an Excellon (or other)
    automated drill which matches hole sizes to
    locations, and drills the board.

CAD PCB Design
  • The CAD PCB work flow is shown at left, red paths
    are manual, blue are automated.
  • Some programs also support Back-Annotation, where
    a change made downstream will automatically
    reflect back to the source files.

Common CAD Tools
  • Protel and the later Altium Designer platform
    from Altium.
  • Cadence Orcad.
  • CadSoft Eagle.
  • Mentor Graphics PADS.
  • Public Open Source (no charge) tools
    DesignSpark, KiCAD, PCB123, ExpressPCB

CAD PCB Design Tips
  • Cost is directly proportional to size, layers,
    holes and speed of delivery.
  • Single sided boards are cheap, but solder joints
    are very weak mechanically. Double sided board
    production has become the default industry
    standard, with plated through holes.
  • Adding each pair of additional layers more than
    doubles costs, and may require electrical
    testing, another cost adder.

CAD PCB Design Tips
  • Copper thickness is 1 oz (per sq.ft.) standard,
    for more current, it is usually plated up during
    the double sided process.
  • Thickness
  • ½ oz. 0.7mils
  • 1 oz. 1.4mils
  • 2 oz. 2.8mils

CAD PCB Design Tips
  • Surface finishing has changed considerably in the
    post-RoHS world.
  • HASL tin-lead solder was standard for many years,
    but is now being replaced with other
    enviro-friendlier finishes including tin, silver
    and ENIG (electroless nickel, immersion gold).
  • A process called SMOBC is also possible (solder
    mask over bare copper), with plating only at pads
    for soldering.

PCB Extras
  • Most boards now have a SOLDER MASK, usually
    applied to both sides. This is to improve
    solderability (reduce bridging), and to prevent
    corrosion after manufacture, defined in Gerber
    Files. SMT, SMB.
  • Boards also typically have IDENTS, to mark parts
    and locations, generally in silk-screened epoxy
    inks, on one or both sides, also defined in
    Gerber Files. SST, SSB.
  • SMD boards may also have SOLDER PASTE tools, made
    in a separate metal stencil, for the screening of
    solder paste for assembly, also defined in Gerber
    Files. SPT, SPB.

  • Association of Connecting Electronics Industries
    IPC-2221/A/B. This is a total design guide for
    aspects of layout and manufacturing.
  • MIL-STD-275E, MIL-PRF-31032B, IEC-60297-3
    (Eurocard) and IEEE 1101.10
  • Most are now already uploaded to the study files
    area to review.

General Design Tips
  • We keep a large reference page at Sphere Research
    where you can often find very useful design
    information links, it is located here
  • http//

You Should Read
  • Donald A. Normans The Design of Everyday

You Should Read
  • W. Edwards Demings
  • Out Of The Crisis

And possibly
  • 1 boat
  • http//

Thank you!
  • Class materials are at
  • http//
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