Sender: |
|
X-To: |
|
Date: |
Mon, 25 Jun 2007 07:35:07 -0500 |
MIME-version: |
1.0 |
Reply-To: |
|
Content-type: |
text/plain; charset=us-ascii; format=flowed |
Subject: |
|
From: |
|
In-Reply-To: |
<001401c7b71f$42dd7f70$2f01a8c0@DSGN03> |
Content-transfer-encoding: |
7BIT |
Parts/Attachments: |
|
|
Scott, freebie calculators are not a good idea if you want to use
them to build a stack-up that is accurate. They should be uses only
for a ballpark idea. They can be off sometimes more then 30%.
If you need a stack-up for a differential pair at 50 ohms then you
should be using a 2d field solver like one from Polar, or one that is
built into your EDA software. Keep in mind that a 2d field solver is
more accurate then a set of equations setup for certain conditions,
conditions that may not exist in your design. Also, EDA software
packages don't necessarily have a 2d field solver built in.
In the end, you should use what your PWB manufacturer suggests. They
will use a 2d field solver to calculate the impedance based on the
parameters you give them and then suggest a layer stack-up to achieve
that impedance.
Hope this helps
Donald Kyle
At 06:52 AM 6/25/2007, you wrote:
>I've got a design that needs to have 50ohm differential pair on a two layer
>flex design (microstrip). I need to come up with the material stack up for
>this application. I came up with line width/space and material thickness
>using a calculator that I purchased.
>
>
>
>Dielectric constant = 3.5
>
>Substrate thickness = 0.002"
>
>Cu weight 1oz (0.0014")
>
>LW = 0.007"
>
>Space = 0.004"
>
>Calculated differential impedance = 56.7ohm
>
>* This calculator also takes into account the cover film/solder mask
>
>
>
>However, I ran the same numbers through a different calculator (a freebie)
>and there is a warning that the results are not valid unless the trace width
>is less than twice the dielectric thickness. The resulting impedance with
>this tool is 39.1 ohm.
>
>
>
>I get no errors or warning from the "store bought" calculator, but before I
>proceed I want to be confident that the numbers I'm getting are indeed
>valid. I'm assuming that all the calculators use the same general formulas,
>but get significantly different results using these two. I also asked
>another designer to try the numbers in his tool (Polar I believe), and he
>gets completely different results.
>
>
>
>Question: Is the "trace width must be less than 2x the material thickness" a
>valid rule for all calculators, or is this something that might be unique to
>this specific tool ?
>
>
>
>
>
>
>
>
>
>Scott D. Riley
>Technical Design Service
>607-658-9322 - Office
>607-785-1696 - FAX
> <http://www.techdesignservice.com> www.techdesignservice.com
>
>
>
>
>
>
>
>
>---------------------------------------------------------------------------------
>DesignerCouncil Mail List provided as a free service by IPC using
>LISTSERV 1.8d
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
>To temporarily stop/(restart) delivery of DesignerCouncil send: SET
>DesignerCouncil NOMAIL/(MAIL)
>Search previous postings at: www.ipc.org > On-Line Resources &
>Databases > E-mail Archives
>Please visit IPC web site
>http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
>information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>---------------------------------------------------------------------------------
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------
|
|
|