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

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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
  files.

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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.

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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.

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Manual PCB Design

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

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CAD PCB Design

• How does it work? First, start with a circuit
  IDEA.
• 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.

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CAD PCB Design

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

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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.

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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.

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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

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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.

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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.

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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
  simulation.

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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.

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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.

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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.

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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.

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CAD PCB Design

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

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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.

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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
  design.
• Many boards turn out poorly because too little
  forethought goes into them. Think through
  assembly, test and end use carefully.

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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.

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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.

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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.

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CAD PCB Design

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

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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
  software.

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CAD PCB Design

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

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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.

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CAD PCB Design

• Layers have unique names, somewhat standardized
  over time, they are (in sequence)
• TOP SILK SCREEN, TOP SOLDER MASK, TOP, POWER,
  INNER1-16, GROUND, BOTTOM, BOTTOM SOLDER MASK,
  BOTTOM SILKSCREEN.
• 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.

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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.

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CAD PCB Design

• Gerber Files contain the layer-by-layer data plus
  the drill info and outline to make a finished
  PCB.
• 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.

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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.

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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.

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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

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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.

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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

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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.

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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.

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Standards

• 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.

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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//www.sphere.bc.ca/test/data.html

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You Should Read

• Donald A. Normans The Design of Everyday
  Things

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You Should Read

• W. Edwards Demings
• Out Of The Crisis

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And possibly

• 1 boat
• http//www.sphere.bc.ca/document.html

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Thank you!

• Class materials are at
• http//www.sphere.bc.ca/class