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

Hi Richard
Thanks ..well explained
Regards
Marie

On Thu, Oct 2, 2008 at 4:55 PM, richard moffat <
richard.moffat@xxxxxxxxxxxxxxxxxxx> wrote:

> 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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