[SI-LIST] Re: On the effect of solder mask on Zo
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: ariazi@xxxxxxxxxxxxxxx
- Date: Fri, 13 Feb 2004 11:07:38 -0800
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
Abe Riazi wrote:
>Dear Charles and others
>
>In a column by Dr. Eric Bogatin ( "When Accuracy Counts" Printed Circuit
>Design & Manufacture", May 2003, P. 36 ), it is stated that Zo decreases by
>about one Ohm per one mil of soldermask thickness.
>
>Regards,
>
>Abe Riazi
>ServerWorks
>
>-----Original Message-----
>From: Grasso, Charles [SMTP:Charles.Grasso@xxxxxxxxxxxx]
>Sent: Friday, February 13, 2004 7:49 AM
>To: 'si-list@xxxxxxxxxxxxx'
>Subject: [SI-LIST] On the effect of solder mask on Zo
>
>Its about 3ohms per mil. The upper limit is 7mils at this point the
>soldermask is essentially infinite. Quite significant I would think.
>
>
>Source : Verifying the accuracy of 2S solvers on Zo Calculation - Dr Eric
>Bogatin.
>
>Slides available from the GigaTest website : http://www.gigatest.com
><http://www.gigatest.com/>
>
>
>
>Best Regards
>Charles Grasso
>Senior Compliance Engineer
>Echostar Communications Corp.
>Tel: 303-706-5467
>Fax: 303-799-6222
>Cell: 303-204-2974
>Email: charles.grasso@xxxxxxxxxxxx; <mailto:charles.grasso@xxxxxxxxxxxx;
>%20>
>Email Alternate: chasgrasso@xxxxxxxx <mailto:chasgrasso@xxxxxxxx>
>
>
>
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