Question for layout experts

[Speakerguy]

I am working on a board right now and I have a question.

In the attached image there are 3 bypass capacitors for an IC. The leftmost is circled in yellow, and the other two are immediately to its right. The cap in the middle of the trio is the bypass cap for power to the chip; the other two caps are external bypass caps for voltages developed on the chip itself.

The cap circled in yellow is a small bypass cap (220nF) for one of the on-chip generated voltages (compensation cap for an internal regulator). I 'split' its pads with a trace for the power supply to the chip.

Is this generally an OK practice, or do I need to worry about running traces under components like that? Is that a no-no? These will be professionally made boards so I won't have any problems etching or anything like that. I'm more interested in potential problems with signal coupling, EMI, stuff like that.

Thanks for any help.

 

[Speakerguy]

Well, I tweaked my component layout so the track won't run underneath any components. But if anyone has an answer it's something I'd be interested in knowing.

 

[Mikebits]

Since it is a non switching voltage, I think it would not present a problem as you had it. My opinion.

 

[crutschow]

Running a trace under a component is usually not a problem unless there are some very high frequency signals involved and you are worried about crosstalk.

A concern: The bottom three pads are tied together but seem to go nowhere else, which would leave the capacitors floating and non-functional. I believe those pads should be connected to a ground/common point.

 

[Speakerguy]

Thanks guys. This is an audio PWM processor and that particular section is the PLL loop filter (above) and some voltages that are supposed to be tied to the same ground, so I wanted to be careful about it.

Crutschow,

My bad on that one. I forgot to turn the airwires back on for the ground signal for that section of the board before taking the screenshot. I'm going to have a ground plane pour underneath all of this, and I find it easier to route if I just turn off the ground signal airwires until all of the signal and power routing is done. When I finish each sub-section of the board I put in the ground plane underneath and tie them in to it with vias where necessary.

 

[Diver300]

I wouldn't say that there is anything wrong with running tracks under components.

Pad shape is vital for correct reflowing. For 0603 components and smaller the track underneath may restrict the pad shape, so that can lead to tombstoning.

My suggestion would be to get rid of all the neat lines of components if that can shorten the traces at all. You will almost certainly be sending the data to the pick and place machine electronically, so neat lines and only having components at 0, 90, 180 and 270 degrees doesn't actually make life easier for anyone.

It can also help the soldering to keep the number of connections per pad down to 1 if possible, by joining tracks away from the pad.

 

[Speakerguy]

Thanks Diver. This is for a home project, so no pick & place (unless I win the lottery, at which point I would consider it ). But it is very nice to know that kind of information. I would like to become something of a layout expert, so I appreciate knowing those kinds of things.

I'll post another pic of the updated layout later. I've made it much tighter. The 0603 components are spaced such that only one 10 mil trace will run between them and keep 10 mil trace/pad spacing.

 

[3v0]

I would like to see the board and how well it builds. I am still new to SMD and have been too chicken to run traces between the pads on 805's.

 

[Speakerguy]

I'll be sure to post about it when it's done. It's a huge project and will take me a while though.

 

[3v0]

A few days ago I order some 1/2 copper PCB FR4. Pulsar (Frank) is doing 5 mil traces with the stuff.
3v0