The detective on your favorite police television show will often be looking for the plain facts of the case. In PCB design tools, we have some “plane” facts to look for. One of those would be getting a better understanding of what it takes to create the power and ground planes.
As designers, we know that planes provide power and ground to the components on our circuit boards. We also know how to manipulate our tools in order to create those planes. But do we know why certain terminology is used, or why our tools create the planes the way they do? You may have heard the term “D-code” or even used it yourself, but do you know where it came from and what it is for?
Hang on, we’re about to take a trip down memory lane that will give you some background history on PCB planes and how software performance has affected their creation. Our goal is to expose some of the plain facts about planes so that you can plainly see how all of these planes work together today. Are you with me? Good, then I’ve got some ‘splaining to do.
A brief history of the plane
There are two ways that we create power and ground planes in PCB design tools, as a positive image and as a negative image. There are lots of names associated with planes such as area fills, copper pours, and floods. For clarity though, let’s stick with positive and negative planes so we are all on the same page.
Positive Planes: A positive plane is usually created by designating a polygon shape on the board, and the layout tool will fill it in giving you a solid plane image. There is a lot going on under-the-hood of the layout tools in order to do this. Object clearances must be calculated, pads must be connected to the plane, and proper net connectivity must be maintained to prevent shorts to other net objects. As a result, creating a positive plane is one of the more complex functions performed by your layout tools.
Negative Planes: Because of these complexities, the first generation of CAD layout tools had difficulties creating positive planes. It was too much data for them to process. To overcome this, the negative plane was invented. A negative plane is a reverse image of a positive plane. Polygons are not filled to create metal, instead pads and other shapes are used to create clearances in the metal.
A positive plane on the exterior of a board
Positive planes require higher performance
The computers using those early CAD tools were nothing compared to what we have available today. At one point I was using Recal-Redac software on a PDP-11 with a 16-bit processor. Each day I would have to install my personal disk platter in the disk drive, which was about the size of a dishwasher, to access my 10 megabytes of design storage.
Those systems simply didn’t have the performance, the memory, or the disk space to handle positive planes like we can today. Even when they had the capability for positive planes, you ran the chance of crashing the system if your plane was too large. Because of that, designers depended on negative planes. These negative planes often wouldn’t have actual contours or shapes. As well, connections to the planes would be represented by a simple “X”. The designer would then be responsible for assigning these shapes and characters to the correct photo plotter aperture position.
Early photo plotters
Another deterrent to creating positive planes was the photoplotter itself. Early photo plotters used a bright lamp that was focused through a physical aperture in order to create an image on the film. This film was then used by the manufacturer to fabricate the board. Since these were vector plotters instead of raster plotters, it could take a lot of time to paint the images on the film, especially if you were trying plot a positive plane.
Another challenge with positive planes was giving the photoplotter the correct commands to work with. Early photo plotters used the original Gerber format for their input, and the apertures were assigned to “D-codes”. This was before the aperture information was embedded in the Gerber file, and as a designer you were responsible for creating an accurate aperture list. You had to know the proper D-code assignments, and then assign those codes to specific functions such as filling in positive planes. I have seen a two-hour photo plot turn into an overnight ordeal because a 10 mil D-code was specified as the aperture instead of a 100 mil D-code.
Today’s high performance PCB software does the hard work of creating planes for us
Planes in high performance PCB software
Fortunately, in today’s PCB design tools we don’t have to worry about D-code assignments, or how our design will affect the photoplotter. When we work on a positive or negative plane, all of the work to create the fills and assign the D-codes is handled automatically for us. Today’s raster laser plotters are also vastly superior and will plot large positive planes in minutes instead of overnight.
Now you should have a better idea of what a D-code is and why it is important, as well as some historical context for negative planes. There is a lot more that can be said about this subject, but we are plain out of time.
Altium Designer is leveling up to 64 bit. Its high performance and powerful functionality let you choose whether you want to work with both positive and negative planes. This will help you to create the power connectivity that you need for your design.
Would you like to find out more about how Altium can help you design your power and ground planes? Talk to an expert at Altium.
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PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.More Content by Altium Designer