Last week, at the Embedded Systems Conference, a gentleman came up to me and asked me about component spacing. I started rattling off things about IPC standards when we interrupted me and said: "Not IPC. I want to know the real rules - how close together can I really put my parts."
That's a difficult discussion to have. By our policy here at Screaming Circuits, we guarantee our service if the spacing and all of the other things meet IPC standards. Then we'll build it to either IPC-A-610 Class II or IPC-A-610-Class III standards. But he pointed out that whenever he asks us to do something non-standard, we pretty much always say we'll do it.
So, really... How close is close? Well, we want it to have IPC spacing. You will get a better product if you stick with IPC spacing, but yes. We often do weird things. There are limits though. In image "A", there's a little passive part under a connector that needs to be flush to the PCB. That's not gonna work. Sure, maybe for a one-off when you just need to see if the design works and if the leads on the connector are long enough, then maybe it will work and you can avoid an extra board spin. Maybe. But I wouldn't count on it for something like this.
The board in image "B" was a little luckier. The SOIC sits up high enough for the little passive to fit snugly underneath. The bigger passive is too close as well. In the face-paced world of prototyping, sometimes the "we'll make it fit" approach can get the job done, but, really. Don't go there intentionally. You want a design that will hold up to your testing and handling. You want a board that's mechanically ready to go so you can focus your testing and revision efforts on the actual circuit.
These problems are reasonably easy to avoid. Just make sure your CAD footprints have the IPC keep-out distances correct. If in doubt, get the actual part and a micrometer and measure it out. You could save a board spin and lots of dollars by doing that.
Measure it with a micrometer
Mark it with chalk
And cut it with a Type III Phaser rifle