General guidelines to producing a good component placement

By far the most important part of PCB design is component placement however inexperienced designers tend to rush this process.  Typically, 60% to 70% of the time spent on the design should be allocated to component placement.  It’s a bit like playing chess; you have to think several moves ahead otherwise you will have difficulty routing the board with the end result being poor electrical performance and possibly a board that cannot be manufactured. A well thought out component placement will allow for much better signal integration and in some cases allow you to even reduce the board layer count. 


General guidelines to producing a good component placement 

Firstly, layout the PCB outline on a mechanical layer and set the component grid to 25mil or larger. Load the components from the schematic into the work area.  At this stage you should be able to gauge if all the components will fit within your PCB area comfortably. If it appears that the density will be a problem do a density calculation.

Density Calculation

A density calculation needs to be done so that you know how tightly you have to place your components together to utilize the working area evenly across the board.  If the density is very high a tight component placement will be required.  If the density is over 80% the size of the PCB working area may need to be increased.  If the area cannot be increased consider the following options:

1.     Use smaller component F/P’s

2.     Place the components on both sides

3.     Remove circuitry which is not absolutely necessary

At this stage it is a good idea to divide the PCB working area into functional groups such as analogue, digital and power. If any isolation is required determine where you can allow for this to be placed now. 

Component Grouping

The next step is to group components as per functional schematic groups outside the work area of the board. During this procedure you should use the schematic as a guide and also switch on the connected nets in you PCB editor. Each functional schematic group will then be treated as a small PCB layout, where you will be arranging, rotating and placing components so that the track layout will be short and direct. In some cases you will need to refer to the PCB design guide lines of the particular devices that are used, eg. (Switch Mode Power Supplies) etc. You can also find actual recommended layouts that you should be able to reproduce.       
Don’t concern yourself too much about where the actual groups will go on the board yet.  Placement can be done of each group independently as each group can be treated as a small layout.  At this stage you can begin to route tracks within the groups as well. Once this is completed move, arrange and rotate the groups into your PCB area according to pre-determined functional groups. If you have placed your components and groups logically 70% to 80% of your design will be completed.  Then it’s just a matter of routing the nets between the groups.

Final Overview

When all the routing is completed do a design rule check (DRC) which will check for correct connectivity of tracks, widths and clearance.  Get someone to check your board because a fresh pair of eyes will pick up errors that have been overlooked. 

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