Component Placement

An old saying is that PCB design is 90% placement and 10% routing. Whilst the actual figures are of no importance, the concept that component placement is by far the most important aspect of laying out a board certainly holds true. Good component placement will make your layout job easier and give the best electrical performance. Bad component placement can turn your routing job into a nightmare and give poor electrical performance. It may even make your board unmanufacturable. So there is a lot to think about when placing components!

Every designer will have their own method of placing components, and if you gave the same circuit (no matter how simple) to 100 different experienced designers you’d get a 100 different PCB layouts every time. So there is no absolute right way to place your components. But there are quite a few basic rules which will help ease your routing, give you the best electrical performance, and simplify large and complex designs.

At this point it is a good idea to give you an idea of the basic steps required to go about laying out a complete board:

Ø Set your snap grid, visible grid, and default track/pad sizes.

Ø Throw down all the components onto the board.

Ø Divide and place your components into functional "building blocks" where possible.

Ø Identify layout critical tracks on your circuit and route them first.

Ø Place and route each building block separately, off the board.

Ø Move completed building blocks into position on your main board.

Ø Route the remaining signal and power connections between blocks.

Ø Do a general "tidy up" of the board.

Ø Do a Design Rule Check.

Ø Get someone to check it

This is by no means a be-all and end-all check list, it’s highly variable depending on many factors. But it is a good general guide to producing a professional first-class layout. We have already looked at the grids and track/pad sizes, these should be the first things that you set up before you start doing anything. 

Many people like to jump straight into placing all the components into what they think is the most optimum position on the board, all in one hit. Whilst this can work for small circuits, you don’t have much of a hope when you have more complex circuits with hundreds of components spread across many functional circuit blocks.

Why?, because it’s very easy to run out of "routing space", which is the room to lay down all your tracks. If you fix all your component positions and then try to route everything, you can easily paint yourself into a corner so to speak. Alternatively, if you space the components out too much, you can end up with a large board that does not make efficient use of space.

The hallmark of an inexperienced designer is a board that has every component evenly spaced out, and then has thousands of tracks and vias crisscrossing the board. It might work, but it can be ugly and inefficient, not to mention bigger and more expensive to manufacture.

The best way to start your layout is to get ALL of your components onto the screen first. If you have a companion schematic package, then the simplest way to do this is to get your PCB program to import your schematic design and select all the components automatically. This will also be discussed later. If all you have is a PCB program, then you’ll have to select each component from the library and place them down manually.