[PCB_FORUM] Re: Is silkcreen affects the traces with impedance

Hi

You've touched on something that, if you understand or accept, will likely put 
you ahead of many other PCB designers or engineers.  My apologies that this is 
not specifically Cadence-related or if you know this already, but it may be of 
benefit to some others.  (Besides, I love the science!)

You wrote "Silkscreen doesn't have any electrical properties".  Well, at high 
frequencies, *everything* had electrical properties.

Direct current (DC) voltages are really all about the copper.  (Unless you want 
to talk about heat build-up.  Then of course you should be looking at the 
surrounding materials.)

However, at high frequencies the energy of the signal (digital or RF) tends to 
travel *around* the trace, and not so much in the copper itself.  Air has a low 
dielectric and does not "assist" the signal that much.  But if there is 
material around the copper then it may actually assist the signal and lower the 
impedance.

So, anything physical next to a copper trace will have an effect regarding the 
impedance.  It depends on the frequency and the dielectric value and thickness 
of the material.  (Don't forget that "impedance" basically means "AC 
resistance".)  However, the signal will also have a "loss" - that is, it will 
be degraded because of what is around it.  The more physical material, the 
greater the loss.  (In very basic terms.)

A few silkscreen lines or bits of text are like a pebble on the road.  They 
won't have much effect to what's running over them, at least not to the 
impedance.  Signal loss is different - you have to be careful here, especially 
with RF.  It depends how much material is covering the trace, and what the 
material is.

Disclaimer:  
there opinions are my own but I think they're not too bad.

Final disclaimer:  
RF is in a world of its own.  If an engineer wants nothing to cover the traces, 
then he may well be justified in what he's asking.  (As long as it's not gold 
over nickel, because nickel has magnetic properties at high frequencies and 
will likely screw up the signal.  Another story again :-)


I hope I haven't bored anyone here.  I've covered this a lot to various people 
in the last few years so I thought it may be useful.  If it isn't , well...

Cheers,
Richard




  
__________________________
Richard Moffat
PCB CAD Manager
Allied Telesis Labs
ph. +64 (3) 3393000
richard.moffat@xxxxxxxxxxxxxxxxxxx
>>> "malou teoxon" <malou.teoxon@xxxxxxxxx> 09/30/08 8:33 PM >>>
Hello to all

Thanks for the explanation.

I have a  RF design, most of the traces need to match the impedance, My EE
ask me to remove the silkscreen on top of those traces that need to match
the impedance, basically the silkscreen is only those component notations,
text , lines . which my EE wants me to remove as he said it  affects the
impedance of that traces. I was just confused as this particular silkscreen
is white color paint and dont have any electrical propertirs, or might not
affects even the thickness of the materials. Yap Richard soldermask
sometimes might affects the impedance but the silkscreen (text, lines,
component notataion) the white color paint , I am a little bit confused if
this one affects impedance.

On Mon, Sep 29, 2008 at 9:22 AM, richard moffat <
richard.moffat@xxxxxxxxxxxxxxxxxxx> wrote:

> Hello Marie
>
> I just ran a model on our software.  The impedance may drop a bit more, say
> 2-3 ohms, but it really, really depends how thick the soldermask is.
>
> It would pay to have a word to your friendly fabricator.  They will easily
> help you and they will have better software than I've currently got.
>
> Hope this helps.
>
> Cheers,
> Richard
>
> >>> "richard moffat" <richard.moffat@xxxxxxxxxxxxxxxxxxx> 29/09/2008 2:02
> p.m. >>>
>  Hi Jerry
>
> That's true - but I was speaking in terms of impedance, not loss.  I think
> that was what Marie was asking.
>
> I do not believe the impedance will not change much, at least not because
> of a coating such as soldermask or silkscreen.  In real terms the impedance
> sometimes goes all over the show but for different reasons, usually the
> material.  Correct me if I'm wrong.
>
> The loss from covering the strips with soldermask or silkscreen certainly
> will be greater to sensitive RF designs or digital multi-multi-GHz.
>
> Cheers,
> Richard
>
> >>> "Schwartz, Jerome" <jschwa01@xxxxxxxxxx> 29/09/2008 1:35 p.m. >>>
> Richard,
>
> Solder mask and silkscreen, even in close proximity at high frequencies can
> have an effect.
> Lot's of  my designs are in the 6-80 GHz range. We will use solder dams for
> assembly instead of blanket solder mask. Silkscreen on active components
> only and
> far, far away from RF traces and elements. These are PCB's not PWB's.
>
> General statements do not apply. The frequency's must be supplied for a
> proper answer.
>
> Jerry
>
> ________________________________
>
> From: icu-pcb-forum-bounce@xxxxxxxxxxxxx on behalf of richard moffat
> Sent: Sun 9/28/2008 5:43 PM
> To: icu-pcb-forum@xxxxxxxxxxxxx
> Subject: [PCB_FORUM] Re: Is silkcreen affects the traces with impedance
>
>
>
>
> Hi
>
> The silkscreen will have little effect unless you have the silkscreen
> absolutely plastered over everything.  Even then it will not lower it very
> much at all.  Bits of text or lines will have no measurable effect.
>  Soldermask will most likely lower it more, by 1-2 ohms say (depending on
> the type of mask.)
>
> Cheers,
> Richard
>
> __________________________
> Richard Moffat
> PCB Specialist Engineer
> Allied Telesis Labs
> ph. +64 (3) 3393000
> richard.moffat@xxxxxxxxxxxxxxxxxxx
>
>
> >>> "malou teoxon" <malou.teoxon@xxxxxxxxx> 26/09/2008 8:27 p.m. >>>
> Hi friends
>
> I have a question here. Do you have any idea if the silkscreen on  top of
> traces with impedance affects the impedance matching of that traces.
>
> Regards
> Marie.
>
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NOTICE: This message contains privileged and confidential
information intended only for the use of the addressee
named above. If you are not the intended recipient of
this message you are hereby notified that you must not
disseminate, copy or take any action in reliance on it.
If you have received this message in error please
notify Allied Telesis Labs Ltd immediately.
Any views expressed in this message are those of the
individual sender, except where the sender has the
authority to issue and specifically states them to
be the views of Allied Telesis Labs.
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