PCB Design

PCB ( Printed Circuit Board ) Design and Manufacturing Overview

  • Double Sided & Multilayer Designs
  • Radial Layouts and Non-standard Geometries
  • SMT, Thru Hole, Hybrid boards
  • BGAs
  • Digital, Analog, RF, Power, Mixed Technologies
  • High Speed/High Density/Fine Pitch
  • Impedance Control
  • Matched Line Lengths
  • Differential Pairs
  • High Current
  • Flex Circuits
  • ANSI/IPC compliance
  • OrCAD Capture
  • OrCAD Layout
  • Cadence Allegro
  • PCAD
  • Altium Designer
  • AutoCAD
  • TurboCAD

Usually an electronics or electrical engineer designs the circuit, and a layout specialist designs the PCB. PCB design is a specialized skill. There are numerous techniques and standards used to design a PCB that is easy to manufacture and yet small and inexpensive.

PCB layout Basic guidelines

It is often a good idea to have made a prototype circuit using point-to-point construction or wire wrap, as you will have solved certain basic issues to do with component selection: (eg: should I use a 1/4 watt resistor here, or do I need 1/2 watt? etc.)

  • Consider physical constraints on the assembled board's size and heat dissipation requirements; choose your heat sinks if needed.
  • Consider carefully the physical size of the components you are laying out; the circuit schematic doesn't tell you this. Equivalent components often have different packages.
  • How do the components attach to the board? Are they surface mount components? or do they require holes, screws, washers, etc?
  • Are there mechanical parts directly mounted to the board? eg: switches or variable resistors?
  • How will the board mount in its container? What stresses (shock, strain, shear) will there be upon it and upon components?
  • How will the board connect to its power source? What other connectors will be required (e.g: signal inputs, outputs)?
  • Use construction paper and a pencil and sketch the board in its actual size; or use component layout software that includes information about the component outlines.
  • Decide appropriate widths for each of the signal traces; this depends on the current each trace is expected to carry.
  • Decide whether you will have a single-layer board, 2-layer, or multi-layer based on the circuit complexity and fabrication costs.
  • Begin by placing component outlines, then by placing signal traces; leave a little room around each for tolerances.
  • For a single layer board, spend more effort to avoid having traces cross each other; play with component placement or run traces underneath components; sometimes a jumper wire is needed.
  • In 2-layer and multilayer boards simply run the traces on different layers, and use plated-through holes to jump from one layer to another.
  • Try to predict and avoid assembly errors: where there are multiple components of the same kind, or where pins have a polarity (eg: electrolytic capacitors), try to place them in parallel and orient the positive pin in the same direction.
  • If your PWB design software has a DRC (design rule check), use it.

PCB layout guidelines for RF circuits on a 2-layer or multilayer board:

  • Identify the critical parts of the circuit and lay them out first
  • Have one of the layers act as a continuous ground plane (usually the 'bottom' side).
  • If signal traces are constant width and height above the ground plane, and are properly terminated, then their characteristic impedance is more well-behaved and may be calculated.
  • Avoid sharp corners.
  • Keep signal traces and component leads as short as possible.
  • Inputs and outputs should be far apart, so that RF energy will not leak back from output to input. stages should line up, rather than snake around.
  • Decouple the RF parts of the circuit from the DC parts of the circuit.
  • Shield AF and IF components from RF components.

Cost Cutting Design Tips for PCB Manufacturer

  • Limit the number of layers as much as possible to reduce overall costs
  • Utilize hole sizes larger than .012 (smaller drill sizes increase drill time)
  • Try to provide Annular ring pads that are .010 larger than hole size
  • Standard panel size is 18 x 24 with the manufacturing usable area of 16.00 x 22.00
  • Specify hole size tolerances of at least +/- .003. Tighter tolerances just increase problems and lower yields