[SI-LIST] Re: On the effect of solder mask on Zo
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: scott@xxxxxxxxxxxxx
- Date: Fri, 13 Feb 2004 16:09:20 -0800
That should have read
soldermask is not the
limiting factor for impedance control of FR4 PCBs, but it does tend to
decrease trace impedance slightly ...
not increase as I said
Scott McMorrow wrote:
>All,
>
>It turns out that the precentage variation in impedance due to
>soldermask thickness is generally less than the variation in impedance
>due to local variation of FR4 laminate. For typical microstrip
>construction, there is up to a +/- 10% variation in Er across even very
>short distances on a PCB. This can be seen with accurate TDR trace
>profiles using high quality launches. This amounts to a fundamental
>limit of around +/- 5% for impedance control. Soldermask is not the
>limiting factor for impedance control of FR4 PCBs, but it does tend to
>increase trace impedance slightly, as Dr Bogatin points out. For
>soldermask that is applied with a spray process, the thickness is
>reasonably uniform. Unfortunately, woven fiber laminates are not.
>
>If anyone is interested, we have posted slides from our DesignCon
>presentation on our website. In our presentation titled:
>
>"Hybrid Solver and Measurement Based Design"
>
>you will find on slides 34 and 35 some high quality measurements made
>with a 20 GHz TDR and TDA Systems IConnect software which shows the
>phenomena of local trace impedance variations due to Er variation of the
>underlying FR4 woven fiber laminate. At some future venue, we'll be
>publishing our findings showing correlation to material microsections.
>
>best regards,
>
>scott
>
>
>
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
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