[SI-LIST] Re: Regarding etch factor calculation

  • From: sai krishnan <sai_krishnan_s@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx, steve weir <weirsi@xxxxxxxxxx>
  • Date: Wed, 25 May 2011 05:36:09 -0700 (PDT)




 
Hi Steve,
 
Thanks for the reply. For microstrip, I included soldermask thickness as 
0.6mils.  I have not done any measurement. As mentioned in my email, my 
observations are based on the caculcations from the Polar tool.  the 
difference is around 5ohms.   
 
I am seeing in my previous posting, alignment got distorted. I am posting my 
query again: 
 
 
I am trying to understand the EtchFactor value in the stackup. I am  checking 
the PCB stackup received from fab house.    The impedance value resulting 
from my calculation using Polar for microstrip lines,  differs from the 
values  provided by the fab house.  For striplines, it comes closely to the 
fabhouse calculations.
 
I am using the following approach:   
Etch factor = W1 (bottom side width of a trace )  â?? W ( top side width of a 
trace)    = copper thickness  (T) 
 
For microstrip traces, I included the plating thickness and copper thickness 
for the total thickness (T).   For stripline traces, just the copper 
thickness of trace.

Does my approach on calculating Etchfactor requires any modifications.  Why I 
am getting different impedance value for microstrip  .  Any inputs will be 
helpful.
 
Thanks & Regards,
Sai 

--- On Wed, 5/25/11, steve weir <weirsi@xxxxxxxxxx> wrote:


From: steve weir <weirsi@xxxxxxxxxx>
Subject: [SI-LIST] Re: Regarding etch factor calculation
To: si-list@xxxxxxxxxxxxx
Date: Wednesday, May 25, 2011, 5:46 PM


How far off are you from what your fab house says? Have you measured 
test traces?

Microstrip effective dielectric constant is affected by the solder mask. 
If the surface treatment includes Ni, that will also have an impact.

Steve

On 5/25/2011 5:07 AM, sai krishnan wrote:
> Hi All,
> Ã?
> I am trying to understand the EtchFactor value in the stackup. Ã? Ã?
> Ã?
> I am � checking the PCB stackup received from fab house.� � �  The 
> impedance value resulting from my calculation using Polar for microstrip 
> lines, � differs from the values � provided by the fab house.�  For 
> striplines, it comes closely to the fabhouse calculations.
> Ã?
> I am using the following approach:Ã? Ã?
> Etch factor = W1 (bottom side width of a trace ) Ã? ââ?¬â?? W ( top side 
> width of a trace) � � � = copper thickness�  (T) .
> Ã?
> For microstrip traces, I included the plating thickness and copper thickness 
> for the total thickness (T). Ã? Ã? For stripline traces, just the copper 
> thickness of trace.
> Ã?
> Does my approach on calcualating Etchfactor requires any modifications.�  
> Why I am getting different impedance value for microstrip � .�  Any inputs 
> will be helpful.
> Ã?
> Regards,
> Sai
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Steve Weir
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