Saturday, January 22, 2011

Practical guidelines for PCB design

This is a very basic list of points important in the design of printed circuit boards (PCB). It is based in a lecture given by Jhonny Chee from the Polytechnic Ngee Ann from Singapore in the Chiba University in October 2010

There are two main problems to deal with when we are working on the design of PCB´s. 

  1. Reduce the noise that might affect the circuit behavior itself
  2. Avoid to produce electric that contaminate the power supply network or the surrounding devices.

in the successive we will name some common sources of the above mentioned problems and the way to minimize their undesired effects.

Stray capacitances: It exists between any two conductors. If the stray capacitance is large enough, aimage small current can “jump” trough the capacitance and contaminate the signal traveling in the near conductors. This is specially annoying when we have high frequency components in our circuit.

Techniques to minimize this capacitance:

  • Widening the separation between conductors
  • Shortening the conductors lines.
  • Guarding (Putting ground lines “covering” the signal lines)

Radiated emission: Electromagnetic noise originated by the conductors in our circuit that could act as a antenna.

Techniques to minimize this problem:

  • Try to reduce the area covered by the conductors carrying high frequency signals, for example, always try to put the supply and return conductors running parallel.
  • Shield the electromagnetic sensible circuits (covered with a metallic cap connected to ground, or surround it by a ground plane)
  • Use ferrite choke. Ferrite increase the magnetic permeability increasing  the inductance imagewhich  is “see” as impedance by the high frequency currents. Using ferrite choke at the input or output of the power supply will “clean” the signal of high frequency components. This is very effective for common mode interference but do not affect differential signals.

General tips.

- Always try to use a ground plane (a side of the PCB as ground)

- Create your own library including all the components you will use. This tip reduce time when designing the PCB and minimize errors selecting wrong devices not in stock.

- Use always decoupling capacitors ( minimize the noise in the power supply for each device,usually 0.1uF, 0.01uF) and try to attach it as close as possible to the device using them. Decoupling capacitors are not effective far from the “protected device”.

-  Try to allocate pin 1 of each connector as ground. This practice makes easier to locate the ground point in each connector.

- Use a spreadsheet to aid in the pin assignment listing functions of each pin.

- Include tests points  and one or more ground pads. It makes easier to debug your circuit. Only use tests points for stages of the circuit or for the most important signals, do not over use the test points. Place the ground pads close to the edge of the PCB, is easier to identify and to connect using crocodile clips.

- Always try to use connectors to connect your circuit with the real world. Is easier to test and debug than is you connect your circuit using soldered wires.

- Group components together according to their intended function giving priority to the shorter path rather than aesthetics.

- Prioritize component placement in the next order:

  1. Decoupling (closest to the IC keeping copper paths short)
  2. High frequency (place components to minimize track length. This reduces radiated interference)
  3. High sensitivity (Put inputs of signals close to their amplifiers to avoid pick up noise and amplifier it)

- Routing order priority:

  1. Within functional group (first route between components of a functional stage and the interconnections with other stages )
  2. inter-group

- Minimize length of tracks in the ground plane. This preserves the properties of the ground plane.

- Use via holes to connect ground components to the ground plane. (shorter routes to ground allowing space on the routing plane for interconnect components signals)

-  Use thicker tracks for power lines. (power lines carry more current and need more cooper to do that efficiently)

- Avoid use breaks in the power lines. Using many breaks  will increase the track impedance.

- place text strings to identify components.

- Check and optimize your layout.

  • Verify footprints match with the real component in stock.
  • Verify connections looking weird.
  • Verify that the decoupling capacitors are near to the IC´s-
  • Incoming power has a large capacitor?
  • Adjust clearance between tracks and vias.
  • Check the ground plane (usually the bottom side), it is not braked by signal tracks?
  • Remove any island of floating copper (This increase the crosstalk). If it can not be removed connect it to ground.
  • Attach connecting vias around the edge to ground in order to block the EMI (electromagnetic interference)

Happy PCB design, soldering and use! Sonrisa

1 comment:

Laraib said...

hello!! Very interesting discussion glad that I came such informative post. Keep up the good work friend. Glad to be part of your net community. UETPCB

Related Posts Plugin for WordPress, Blogger...