Hi, I’m only just on the learning curve, but struggling with this question. I added SMD decoupling capacitors to my schematic with ground nets and want them placed as close to the component pins as possible and connecting across to a polygon fill ground plane.
Before autorouting, I expected to do manual moves for the decoupling caps to get them physically close to component pins as they seem to be all over the place. I even expect to tidy up the autorouted result, but I’ve got polylines connecting the grounds of my decoupling caps to other components with gdn nets and these become tracks after autorouting.
I assume the polygon ground plane is overlaid after the track lines have been autorouted, so how can I stop the autorouter making ground connections to the decoupling caps before I do the ground plane pour? I want to make the connection from the ground end of the smd cap to the nearest ground from the pour? I’m only working with single sided layouts. I could leave decoupling caps off the schematic and add them manually to the routed pcb after adding the ground plane pour, but that doesn’t seem right.
I guess this is something common to r.f boards and ICs requiring decoupling close to their pins but I haven’t worked it out yet.
voxmagna:
I assume the polygon ground plane is overlaid after the track lines have been autorouted…
I’m taking a guess that you are using Eagle. If that’s the case, you need to make your GND plane before you autoroute. The autorouter will create a full plane and remove material from it as needed to allow for traces to be routed. The end result of that is your SMD GND pad will either blend into the plane if it is on the same layer (or multiple perpendicular connections if thermals are turned on), or will have a trace just long enough to allow for a via to the other layer.
As an example of what you will end up with, here is a board I recently made. Don’t let the size of the picture fool you, it is only 1.25" wide and 1.5" tall. If you look just below the upper right mounting hole you will see capacitor C1 in a 402 SMD package. The top pad is GND and the bottom pad is 3.3V. My GND plane is on the back and 3.3V is on the front. You can see that the GND pad has a small trace with a via to the other side while the 3.3V pad just blends into the top layer (thermals off). I routed this board by hand but the autorouter will create similar results which can be tuned with all the adjustable parameters it has.
My mistake was assuming the GND plane was made AFTER autorouting the tracks which is what I had done once before, since I had assumed that the copper is literally ‘poured’ around tracks after they had been made.
I still have a lot to learn, thanks for the example.
One final question, when a decoupling cap is drawn on the schematic, can it be forced to place it on the actual pin where it is drawn? I have this feeling that since Eagle doesn’t autoplace, it starts by just putting the components in lines and it’s down to you to juggle them about manually.
voxmagna:
One final question, when a decoupling cap is drawn on the schematic, can it be forced to place it on the actual pin where it is drawn? I have this feeling that since Eagle doesn’t autoplace, it starts by just putting the components in lines and it’s down to you to juggle them about manually.
Eagle doesn’t look at your schematic as a bunch of parts connected to pins. All it sees is a database of netlists that describes how signals are connected to each other. There are a number of ways you can approach component placement and ultimately it is up to you to decide what works best for you. I usually approach it in one of 2 ways. For boards where I know I’ll have plenty of space, I usually enter the schematic all at once, print it out, and use it to keep track of component names while looking at the board layout. If I had dual monitors it would be a lot easier but I have to roll with what I have. For boards where space is a concern (like the board I posted above) I usually add a component to the schematic and immediately switch to the board view and move the part to the relative position I would like to see it. Once the schematic is done, I tweak all the placements while routing the board manually.
Ultimately if you keep practicing you will develop a routine that works for you. Like anything else, it takes time and experience.
I made a couple of simple boards using Eagle, printed both of them from laser transfers and they came out ok. I thought I’d trial about seven or eight other CAD layout programs and what surprised me was whilst I thought Eagle was a bit clunky with it’s DOS command design and bolt on GUI, many of the others were worse. One package was good at everything but lousy at rule checks, as a novice I couldn’t work with it. Others were vastly expensive looking every bit professional. Its basic design ‘engine’ was no better than Eagle but what made it worse was it was smothered in stuff I would never use and it seemed to me like some team of management and marketing gurus had bloated it to charge a high price. One thing they all had in common was a tortuous approach to finding library components and making custom components from the built in library. However whilst I succeeded in making some custom components with Eagle, others were even more difficult.
I’m still finding Eagle library searches a bit hit and miss. Often I know what I want is there, but I can’t find it. Eventually I exported the full default library to an Excel sheet and now I find searching easier since it tells me which package has the component I want.
So I’m back to investing more time and learning with Eagle.
