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Schematic and PCB Design Process March 7, 2008

Question:
I would like to know how a circuit designer in OrCAD plans the schematic and layout for the digital, analog, power, low speed high speed portion of the PCBs?

Answer:
Wow, this is a big but good question! For anyone considering undertaking a schematic capture and PCB layout for the first time, it can seem like a daunting task. A modern PCB serves not just as a means to interconnect components, but also as a mechanical structure, heat conductor, noise shield, and even as a circuit element, and must go through a complex automated manufacturing process to boot.

Perhaps it is best to start with a simple project to get your feet wet! Aside from that, it also helps to have some background knowledge about how electronic stuff works. Typically, at least a two year degree in electronics is recommended for anyone serious about PCB layout as a career. I can’t describe the whole process – that would take a book. What I can do is briefly touch on some of the basics to help you get started or even decide if PCB layout is something you really want to do.

Assuming you have a circuit design already and you just need to capture the schematic in OrCAD (or a similar tool) and do a PCB layout, I can propose some basic guidelines for how to approach it. As you mentioned, the design is partitioned into logical blocks like digital, analog, power, high speed, etc. It is good to keep the schematic and PCB layout also partitioned in this way. This will allow you to focus on the specific critical aspects of each type of circuitry throughout the process and avoid getting everything all tangled up.

If you are working on a medium to large scale project, there is generally a project team, so it couldn’t hurt to ask the Electrical and Mechanical Engineers who designed the thing and the Manufacturing Engineer who will build the thing and the Test Engineer who will test the thing what their design requirements are. The best way to do this is to have a PCB design kick-off meeting. Other team members may include the Customer, Quality, Reliability, Purchasing, Planning, and Safety representatives. As you go along, have the team sign-off at critical points.

Before drawing anything, start by reading the data sheets and application notes of any critical components to see what the recommendations are for PCB layout. A little bit of research up front can save a lot of time down the road!

Next, it is great to start by drawing a block diagram to use as a guide along the way. Ideally, the block diagram should be part of a hierarchical schematic that drives the design, but it can also be a separate sketch. It should have a logical flow e.g. from left-to-right and top-to-bottom. It should be the type of thing that can be used to show people how the thing works and updated as the project moves along.

Now you are ready to fire up OrCAD or whatever design tools you are going to use.

Draw any new schematic symbols needed
• To save time, first check for built in libraries or ones from part vendors or user groups
• Rather than start from scratch, modify an existing library part when possible
• Use a left-to-right and top-to-bottom flow of signals on the symbols
• Typically you place power pins on top and GND on bottom
• Double check all the symbols for correct pins! A mistake here can make a lot of mistakes in the PCB times the number of PCBs – You don’t want to go there.

Capture each section of the circuit into the schematic
• Again, use a left-to-right and top-to-bottom flow of signals on the schematic
• Each block can be on a page
• Draw critical circuitry like you want it to be laid out on the PCB (i.e. short lines where you want short traces etc.)
• Note any special design rules like length limits, required widths, or controlled impedances
• Name signals in a way that makes it easy to understand the schematic
• Add test points to critical signals (and lots of GND test points)
• Double check schematic against design source with a highlighter etc.
• Get approval from team

Draw any required new PCB footprints and pad stacks
• To save time, first check for built in libraries or ones from part vendors or user groups
• Follow vendor guidelines if available, else use industry standards
• Consult with the manufacturing engineer about process specific requirements

Proceeding to PCB Layout
• Draw board outline with critical cut-outs, mounting holes, and keep outs
• Sketch “rooms” corresponding to each block onto the board outline
• Decide on number of layers, copper thickness, and board stack-up arrangement. This is a function of board density, the sheer number of signals and power planes needed, the controlled impedance scheme, heat conduction requirements, etc.
• Consider routing adjacent signal layers with horizontal / vertical routes to obtain perpendicular overlap. This reduces cross talk and eases routing.
• Place critical components and get approval on the initial placement from the team
• Route critical traces and planes, and then route the rest
• Do a clean up pass to fix any odd things, improve silk screen, etc.

Here are some bullets to keep in mind when doing PCB layout for each type of circuit:

Low Speed Digital
• Not very susceptible to noise
• Generates a medium amount of noise, so keep it away from the most noise sensitive circuits
• The most active signals are the most noisy
• Other than that, layout is not too critical and this can be placed around the board as kind of a “filler” in places where nothing else will want to go and can be easily auto-routed.

High Speed Digital
• Same as low speed digital, plus the following…
• There can be a very high number of interconnecting traces
• Generally uses controlled impedance traces
• Requires controlled, minimized, or matched signal delay times and proper location of termination devices
• Can generate some serious heat – added copper planes to conduct heat away and heat sinks will need to be designed in

Analog
• Usually very sensitive to noise pickup
• Keep away from noise generators
• High impedance nodes and nodes followed by a lot of gain are generally most sensitive to noise
• Look for recommended grounding and shielding in application notes
• May also be sensitive to surface leakage currents, temperature, etc
• Power amplifiers may generate significant heat – added copper planes to conduct heat away and heat sinks will need to be designed in

Power
• Can generate a lot of noise (esp. switching power supplies, class D amplifiers, etc)
• At the same time, some nets are very sensitive to noise (i.e. sense and feedback nets)
• Can generate a lot of heat – added copper planes to conduct heat away and heat sinks will need to be designed in
• Switching nodes are sensitive to layout parasitic inductance and capacitance
• Keep high AC currents confined to small loop areas to reduce noise
• May involve high voltage and safety issues, special spacing and insulation requirements

RF
• Uses controlled impedance traces
• May require traces to be designed as antennas (requires RF design background)
• Receivers are very sensitive to noise
• Transmitters generate noise and dissipate heat proportional to the power involved
• Again, added copper planes to conduct heat away and heat sinks will need to be designed in
• Requires special attention to shielding and grounding

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