[SI-LIST] Re: Trace width selection for high speed serial links
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: wolfgang.maichen@xxxxxxxxxxxx
- Date: Mon, 31 Aug 2009 09:40:12 -0700
just to avoid possible misunderstandings, in the previous post "thick
traces" was supposed to say "wide traces".
Wolfgang
wolfgang.maichen@xxxxxxxxxxxx
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08/31/2009 09:33 AM
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[SI-LIST] Re: Trace width selection for high speed serial links
JF,
purely for minimization of skin effect loss, the wider the traces, the
better. So 10 mils will have lower losses than 4 mils (although the
improvement is less than you may expect, which is due to the fact that the
current tends to crowd at the - sharp - edges of the trace). The trace
width that gets used eventually is always a compromise between many
different factors, and skin loss is just one of them. Some possible
problems with thick traces are
- require thick dielectric layers to achieve 50 Ohm impedance - that makes
for thicker boards, which in turn will worsen via parasitics
- can be difficult to route inside a dense BGA via grid; if you have to
neck down the wide trices to make them pass between adjacent vias you
incure impedance discontinuities. Due to same reason (dense BGA grid), the
traces will come very close to each other, which will increase crosstalk
compared to thinner traces.
- require wider trace spacings, which reduces routing density and possibly
forces you to use more layers (added cost and even thicker boards)
Advantages are lower skin losses and also smaller relative etch
tolerances, thus potentially better impedance control.
Of course there are many more parameters to look out for on a real design.
Wolfgang
jhasson@xxxxxxxxxxxxxxxxxxx
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08/31/2009 09:21 AM
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[SI-LIST] Trace width selection for high speed serial links
Hi,
It is often mentionned in guidelines for routing high speed differential
serial links to have a trace width between 4 and 8 mils to reduce skin
effect losses. What would be the drawback of having even larger traces
like 10 or 12 mils if any ? The setup would be to use loose coupling and
the board density is not the main concern.
Best regards,
JF Hasson
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