[SI-LIST] Re: Board design adjustments by manufacturers

  • From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 9 Sep 2014 14:24:28 -0700

Thank you all for the responses.

I agree with Jeff that the only practical option so far is the cross-sectioning 
(it does not matter who does it). Also, it looks like Gary Brist's works is the 
only source of data that is usable to develop proper models that account for 
the distribution of geometry parameters. Though, such data are not readily 
available from the manufacturers. 
3D CT scan is probably another option, but I have no idea how accessible such 
equipment and how much it would cost. There was a good paper on that subject by 
J. Hillebrand et al from Stuttgart University at DesignCon 2011.

Just a few words to clarify my position. As the software vendors we are usually 
involved into the analysis to measurement validation only at the very last 
stage of the project - when everything is already designed, manufactured and 
measured (an example of such project is the "Lessons learned..." paper #2014_01 
at http://www.simberian.com/AppNotes.php). Thus, all advices on working with 
the manufacturer at early stages are good, but not applicable. Usually, our 
customers put a few test structures on the board to do formal broadband 
dielectric and conductor roughness model identification with GMS-parameters. 
But even accurate material model identification up to 50 GHz already requires 
accurate cross-section geometry or high accuracy in manufacturing. It means 
that the board has to be cut into pieces or reliable geometry data available 
after the manufacturing to achieve reasonable correlation at 10-50 GHz 
frequency range (may be not so important at lower frequencies). Just changing 
geometry to fit the measured data (S-parameters or impedance) is very ambiguous 
technique - there are always multiple choices of what can be changed. For 
instance, if measured and modeled GMS-parameters are matched, but TDR 
impedances are off, changes in trace width and shape or spacing or dielectric 
thickness may be used to adjust geometry to match the impedance or reflection 
parameters. But if we change wrong parameter, it will work for only one 
structure and not for the others. Just tuning models for each structure is not 
an option. I think there is clear need for systematic approach to the analysis 
to measurement validation with the numbers from manufacturers. 

Best regards,
Yuriy

Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA
Office +1-702-876-2882; Fax +1-702-482-7903
Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev

www.simberian.com 
Simbeor – Accurate, Fast, Easy and Affordable Electromagnetic Signal Integrity 
Software
2010 and 2011 DesignVision Award Winner


-----Original Message-----
From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx] 
Sent: Tuesday, September 9, 2014 7:39 AM
To: shlepnev@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Board design adjustments by manufacturers

Hello Yuriy,
The only data I would trust is from cross-sections of the actual trace I was 
trying to characterize, although I would argue that the identical trace from a 
different board (as long as it is from the same location on the panel) is going 
to have approximately the same geometries.  And, the geometry of the trace 
might vary dramatically along the length of the trace.  For instance, prepreg 
thickness will vary depending on local copper density.

Variation is going to be very vendor-specific, so I don't think you'll ever be 
able to apply an assumed distribution to a particular trace.  For instance, RTF 
etches better than standard copper (due to the nature of the grain structures), 
thus you would have to comprehend that variable in your assumption.

For these reasons (and more), I've been very grateful for access to 
cross-sectioning ability; I don't know of an adequate substitute.
Hint to entrepreneurs...  If someone could come up with a compact, clean, cheap 
cross-sectioning tool, there might be significant interest.  If you already 
have that available, let me know (and no, I don't mean a hacksaw and emery 
cloth :-)).

If anyone has published data on the distribution of trace dimensions, it would 
probably be Gary Brist.  A quick search revealed "Design Optimization of 
Single-Ended and Differential Impedance PCB Transmission Lines" which has some 
nice distribution data on slides 23, 24 , etc.  He might have published more 
nuggets.

Best wishes,
Jeff Loyer


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Yuriy Shlepnev
Sent: Monday, September 08, 2014 1:49 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Board design adjustments by manufacturers

Hello Everyone,

After going through a multiple analysis to measurement validation projects, I 
realized that almost all boards are not manufactured as designed (what a 
surprise :-)). As a consequence the analysis to measurement validation may 
require investigation of the manufactured board geometry, that includes cutting 
the board and taking measurements on the cross-sections (usually important only 
above 10 GHz). Now I am trying to figure out what board design adjustments are 
needed and where to get those numbers without cross-sectioning for the accurate 
post-layout electromagnetic analysis.
In particular, is there any way to get the following numbers without cutting 
the board?
- change thickness of layers (account for plating and variations in thickness 
of laminates);
- change trace width and shape (account for etching and plating);
- change spacing between traces in diff pairs (if adjusted for the "impedance 
control"); How board manufacturers actually adjust trace widths or spacing for 
a given design in the "impedance controlled" process if they have just Gerber 
or ODB++ data? Is there any software that does it?

A relevant question - are there board manufacturers out there who provide 
probability distribution data for stackup layer thicknesses, strip widths, 
back-drilling depth etc.?

Can anybody point me to a source of information on this subject.

Best regards,
Yuriy

Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA Office +1-702-876-2882; Fax 
+1-702-482-7903 Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev

www.simberian.com
Simbeor – Accurate, Fast, Easy and Affordable Electromagnetic Signal Integrity 
Software
2010 and 2011 DesignVision Award Winner


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