Designing Modular

[pyromaniac4382]

I have seen many designs with micro-controllers and micro-processors on the internet that are modular with many interconnected PCBs. One example is a base PCB that routes traces between multiple PC-card connectors, and other PCBs with edge copper traces that so neatly fit into the connectors of the base pc-card. (kind of like a mother board and a PCI card.)

Are there any standard options for these? I have found so many connectors on digi-key and mouser, but I just don't understand how I could design my PCB to fit them. Are there options in design software to accommodate this, or should I just throw some rectangle pads down at the pitch of the connector?

I think this way of designing could be very beneficial in expanding on a design without scrapping your progress and making a whole new PCB to carry your previous idea and your new idea.

Maybe I just don't have the right terminology, but it seems like this is something that is used often in electronics design. I could always just use male-female header pins, but that just seems clunky.

Are there any modular designs you know of that offer a better solution to making PCB modular? 

[Laertes]

One very common standard for low-frequency backplane connections is DIN 41612 connectors. You'll find them on digikey and pretty much anywhere else. They're useful and widely available, but not exactly high-density and certainly not high-frequency. In comparison to using PCI-Style board edge connections, they have the advantage of not requiring special gold plating and routing techniques during PCB manufacturing and a bit more mechanical support for plugging them in.

Board Edge connections like PCI have the advantage of being able to support very high frequencies if done right and potentially a very high density. If you do go for them, you could just use a PCI connector, that would make things easy and PCI connectors are widely available as well (and a standard). Also, depending on your PCB package, you will most likely find an example project somewhere on the interwebs where someone made a PCI card with the edge pads so you can copy that.
Be aware, though, that for these connections, you might have to use special PCB manufacturing techniques that might cost you a fair bit extra. For example, if you want to make sure the PCB can be plugged in/out a fair number of times, you'll have to make sure the gold plating on these pads is extra thick, which many manufacturers offer under a name like "Connector Pads" or something like that. Also, normal PCI PCBs have a beveled edge(20° angle, I believe) that will usually cost you a little extra milling fee. Many backplane connectors for the purpose don't mind unmitered edges though, it just isn't as easy to get the board into the connector then.
If you think you will just try it out and maybe not plug the daughter board in and out a hundret times, you can just mill the PCB off a tad short, ignore the mitering and put normal gold plating on the pads(no HAL finishing, though) and you'll probably be fine without the extra cost.

Connectors for the backplane for PCI or any kind of board edge connections you can find in the digikey cathegory Card Edge Connectors - Edgeboard Connectors.

Also, in the datasheets for those connectors(especially the ones that aren't a standard shape like PCI), you will usually find a drawing of what the PCB should look like, including the miter and all... then, yes, you do just place the pads at the places the manufacturer recommends and be happy about it. Don't forget to remove soldermask from the entire connector area, though, not just the pad itself...