[SI-LIST] Re: Skin Effect, Au, and Cu-->More details...

Good point.  I was forgetting how thin the gold plating was w.r.t.
typical skin depth of copper at a few GHz.  Things are starting to make
sense now.  

Time to start reading some more articles from all the replies.  Thanks
to all.

Richard


  
__________________________
Richard Moffat
PCB CAD Manager
Allied Telesyn Research Ltd, NZ.
ph. +64 (3) 3393000
richard.moffat@xxxxxxxxxxxxxxxxxxx

>>> "Salkow, Steven" <steven.salkow@xxxxxxxx> 22/02/2005 10:44:46 a.m.
>>>
The whole picture includes vias, board dielectric, skin effect,
pre-emphasis compensation. When gold is used it usually is very thin
(sometimes 15 microinches) hence most conduction is in the cu layer.
Dielectric losses can be mitigated by choosing a better (but more
expensive) pcb material. The dielectric layer of the solder mask need
to
be thin as possible but not thin enough to present soldering problems
either first pass of BGA rework. Since roll off is highest at the
highest frequency pre-emphasis can crank in frequency compensation.
Articles I have pointed to add to the understanding


<www.fairchildsemi.com/products/ backplane/syschip/fr4loss.pdf>

<www.interactive-products.com/presentations/ 
SPICE-Loss-Models-pcb-west-2003-5.pdf>

http://bwrc.eecs.berkeley.edu/Classes/EE290C_S04/lectures/Lecture2_Chann

els_modeling.9.pdf

http://www.bychoice.com/calculators.htm (see skin effect calculator)


Steve Salkow 
Lockheed Martin 
3200 Zanker Road 
San Jose, CA 95134 
(408) 473-4058 (san Jose) 
(408) 742-4162 (Sunnyvale) 
steven.salkow@xxxxxxxx 
salkow@xxxxxxxxxxxx 
(925) 462-1075 Home 
(925-) 487-5946 Cell 
(408) 468-7271 Numeric Pager 



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] 
On Behalf Of richard moffat
Sent: Monday, February 21, 2005 12:23 PM
To: si-list@xxxxxxxxxxxxx 
Subject: [SI-LIST] Skin Effect, Au, and Cu


Hi all

This discussion developed in another forum, so I thought I'd throw it
into this one to get some more comments.

For GHz signals, some people leave off the resist covering and have
traces gold-plated instead:  "At high enough frequencies (we do 18 GHZ
on rogers) the soldermask changes the impedance of the line."  I have
a
bit of trouble with this reasoning, as (non-corroded) copper has less
resistivity than gold.  

I would have thought that copper at skin effect freqs would be a
better
conductor than gold.  Is the loss due to the soldermask coating
greater
than the skin effect loss at very high frequencies?

Cheers,
Richard

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