Question about curved PCB traces...

[Cameron]

Hey. I read a long long time ago about how 486 motherboards couldn't go past 33MHz front size buss because a lot of the traces had right angles. And there were these new special motherboards which could run at 40MHz FSB because they had 45 degree angles or less only.

My understanding is that any hard angles in a PCB trace would cause RFI and probably reflections. Between leakage and smearing, it apparently can effectively destroy the signal. Is this basically correct? Does anyone know exactly what a hard angle does to the signal from a transmission line perspective? Would it maybe look like ESR and/or ESL?

So wouldn't it be best to have nice soft curves instead of hard angles in the traces? I seems logical to me. Do any digital PCB guys use curved traces in practice? Do any of the PCB fabrication houses someone like a DIYer might use support them?

Lastly, I am curious what if any effect curved traces might have on audio bandwidth analog signals? I am guessing virtually none.

[jwb]

Hi Cameron,

90° angles in PCB traces won't work with fast edges, as you correctly state. The right-angle represents an impedance change in the line, because the conductor is wider at a right angle, and therefore the parasitic capacitance is higher. A right angle in a transmission line behaves like a gate attached to the transmission line at the same point.

If for whatever reason you need the line to bend 90° at a point, don't worry, there's an easy workaround. You can either radius the outside of the bend, to keep the conductor width constant, or chamfer the outside at a 45° angle to 57% of the line width. Either way will maintain good impedance control. Using two 45° angles close together is probably easier.

If you look at a fast PC motherboard you can see curved traces, mostly in delay lines. Some PCB layout programs can generate these curves and other can't. Similarly, only some prototype PCB manufacturers are capable of producing them.

For digital audio, where you will normally be working at speeds of < 50MHz and rise times > 5ns, the rule of thumb is to always use 45&deg; angles in traces. Beyond that, there are more important improvements you can make, none of which involve curved traces.

[dmfraser]

I use PADS 3.5.1 to design PCBs and I use curved corners all the time on my traces. At an EMI class I was told they are less prone to radiate and they are no big deal to use. If you can't do this, then, always miter the corners with 2 45°angles instead of a single 90° bend. And any story about some motherboards having 90° angles would maybe have applied to one manufacturer with a stupid board layout person singe mitering the corners with 2 45° bends has been common practice singe the very first PC boards.

[tiroth]

Originally posted by dmfraser
At an EMI class I was told they are less prone to radiate and they are no big deal to use.

This is a myth:
http://www.signalintegrity.com/Pubs%...bigbadbend.htm

[dmfraser]

So they have no effect according to the link posted. Nice to hear another legend destroyed. However, mitered corners, whether radiused or done with dual 45° bends look prettier. At least I think so.

But then I think some village in Texas is missing its Harvard educated idiot and others disagree with me about that too as is their right.

[jwb]

Originally posted by tiroth

This is a myth:
http://www.signalintegrity.com/Pubs%...bigbadbend.htm

That seems like a weird rebuttal, because on the very same guy (Howard Johnson) advises against 90&deg; corners in his book High-Speed Digital Design. I'd also propose that most DIY designs don't use 8-mil microstrip no FR-4. You're much more likely to be working at 20-mil or larger.

Mainly i think 90&deg; bends look bad!

[janneman]

Originally posted by jwb
Hi Cameron,

[snip]
If you look at a fast PC motherboard you can see curved traces, mostly in delay lines. Some PCB layout programs can generate these curves and other can't. Similarly, only some prototype PCB manufacturers are capable of producing them.
[snip].

I think if you can put it on the layout, ANY pcb house should be able to make it, after all it's just a photographic process. You should see the things some people put on layouts, weird component shapes, company logo's etc. They always come out beatifully IME.

Jan Didden

[tiroth]

I don't think there are any arguments FOR 90 degree bends; I think there are good reasons to use 45 degree corners. It just seems that there has been this mythology attached to it that doesn't seem to be true for sub-microwave designs.

[jwb]

Originally posted by janneman

I think if you can put it on the layout, ANY pcb house should be able to make it, after all it's just a photographic process. You should see the things some people put on layouts, weird component shapes, company logo's etc. They always come out beatifully IME.

The problem seems to be that some of the more poorly designed layout software generates huge Gerber files for any kind of curve.

[janneman]

Originally posted by jwb


The problem seems to be that some of the more poorly designed layout software generates huge Gerber files for any kind of curve.

Agreed, I have seen that also, like generating thick tracks by repeated flash-commands to the laser printer. Very time consuming, but it should work. Not popular with the pcb bureau, for sure.

Jan Didden