Designer's Den - Design Process 2

The PCB Design Process - Section 2

Draw the board
You start the design by setting-up the physical board in the design database. The first step is to to draw the outline of the board. Some ECAD systems require you to create the outline as a separate symbol, others require it to be drawn in the same program as the layout. You also need to set-up the route keep-in areas inside the outline. If you are inserting the board into a card rack you need to base these areas on the amount of the board that sits in the slides. Even for a card that sits inside a housing you need to allow space around the edge of the board for manufacturing tolerances. For this kind of design a keep-out of .050" around the board edge should be more than enough.

More advanced ECAD systems will also need to have placement keep-outs, and your system may support special keep-out areas that impose height limitations or limit what parts and signals may be routed within them. All these areas help you place and route without having to worry if you are staying within the limits imposed by the circuit designer. There may also be other properties related to manufacturing, testing, and assembly issues. RTFM

You can now insert the symbols for the mounting holes and any fixed location parts at their required coordinates. You should place three tooling holes so that they are in three corners of the board, two on the same vertical axis and the third on the same horizontal axis as one of the first. The hole at the intersection of the horizontal and vertical axis should be designated as the datum (0,0) coordinate of the PCB. This allows extraction of placement, hole, and interconnection locations. All board dimensions should be in reference to this hole. For a dense design you may be able to use only two tooling holes, but you need to check with the assembly people to see if they will have problems with this before you do it.

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Import the netlist
Next, read in the netlist and associate it to any pre-existing fixed parts on the board. Any errors you have made in processes before this usually become very apparent here, so be prepared to go back and fix any errors the ECAD system discovers in your work. This points up one of the useful reasons for using ECAD in the first place - detection of errors.

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Place the Parts
Now you can begin placing the parts on the board. There are a number of factors to be considered when placing parts -- electrical function, physical size, temperature factors, and routability are a few. You will have to decide on a placement strategy and how the system will place the parts.

Many ECAD programs now have autoplacement options that let the computer place the parts without human intervention. This sometimes even works. The biggest problem is that, even on systems costing tens of thousands of dollars, the program can not look at many of the constraints that will be obvious to a human mind. If a board has few parts on it, autoplacement may be a viable option. If you are trying to place a very dense design, you probably will have more luck placing the parts yourself. As computers become more powerful, autoplacement will improve, but it has a long way to go right now.

Most ECAD systems have aids for manually placing the board. One of the most common is the ratsnest display. This will show all connections imported in the netlist, allowing you to see a general "flow" duting placement. Some ECAD software allows you to show only the nets you are interested in for a particular part. This makes it much easier to place a part in a functional group. Some ECAD packages allow you to specify "rooms" where only certain classes of parts (i.e. "high speed") parts go. Some of the more advanced aids include density histograms, thermal maps, and signal simulation.
Some general placement guidelines:
  • Parts in this kind of design go on one side of the board, refered to in the "old days" as the component side. With the advent of surface mount this is now refered to as the "primary" side, since SMD components may be placed on either side of the board.
  • Use a standard grid while placing the board. For this kind of design a .025" grid is good for placement.
  • Try to place integrated circuits in an even matrix. This allows you to place support parts around them and connect power and ground more easily. Place the ICs so that they are functionally grouped, and that they are near any fixed-location parts associated with them.
  • Place support parts such as resistors and capacitors where they are needed (i.e. input ESD filters should go near their associated connectors, not the IC). Arrange the parts so they are evenly spaced, so they do not overlap, and so their pads are a sufficient distance apart to allow the interconnecting traces to pass between. Note that pads should have at least a .020" gap between each other for a general purpose board.
  • Place parts in only one or two orientations, vertical and horizontal. It is best to place polarized parts such as caps and diodes in the same orientation (i.e. all diode cathodes to the right or up) to keep assembly and repair errors to a minimum. Do not place parts at non-orthogonal angles unless absolutely necessary to the function of the board as it is much harder to assemble these components.
  • Axial parts such as resistors and diodes should be placed so there is an equal amount of lead wire on either side of the body, and the two pads are at a similar distance as other axial parts. A good general distance is .500" for the smaller parts. In most cases you should not allow a lead to bend less than two times the lead diameter from the body of the part.
  • Allow sufficient diameter around mounting holes for the hardware to be inserted and tightened without affecting parts placed around it.
  • Allow sufficient distance between parts that their leads will not touch when bent over to secure them before soldering.
It is a good idea to show the placed design to another knowledgable person for a critique after the board is completely placed. There are many factors affecting parts placement that are not covered here, sombody less familiar with your design may notice things you have overlooked. When satisfied with the placement you can begin routing.
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