Re: [SI-LIST] : FPC impedance control

john thomas (jthomas@staktek.com)
Tue, 04 May 1999 07:39:48 -0500

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Michael,

I performed some experiments several years ago on silver epoxy conductivity comparing it to that of gold epoxy. The conductivity of silver epoxy at that time was an issue due to a migration phenomenon. Over time in the presence
of a very low stimulus, the conductivity would decrease drastically. We didn't see the same occurring in the gold epoxy. We did note that the conductivity of new samples depended very much on the mixture density, so the answer
would not be a constant even without the migration effects. The same was noted of the gold epoxy, and we actually saw mixtures of silver which were comparable to gold. The trade-off was the adhesion strength. The greater the
density, the lower the adhesion strength.

Note that this was back around 1986 that these experiments were performed. The state-of-the-art stuff may have formulations which work much better now.

john thomas

"Fox, Michael J" wrote:

> I have a question: What is the conductivity of a thin silver epoxy layer?
> Higher than copper?
>
> Michael
>
> -----Original Message-----
> From: Andy Burkhardt [mailto:mail@polar.co.uk]
> Sent: Wednesday, April 21, 1999 10:22 AM
> To: si-list@silab.eng.sun.com
> Cc: johnlin@ccmail.arima.com.tw; Casey; Richard S Smith
> Subject: Re: [SI-LIST] : FPC impedance control
>
> At 04:34 PM 16/03/99 +0000, you wrote:
> >From: johnlin@ccmail.arima.com.tw
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> >Date: Tue, 16 Mar 1999 09:23:14 +0800
> >To: <si-list@silab.eng.sun.com>
> >Subject: [SI-LIST] : FPC impedance control
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> >Dear all SI gurus,
> >
> >One question.
> >
> >Recently, I design a stackup structure for a FPC, flexible printed circuit
> >board, to get right controlled impedance.
> >The FPC is an embedded microstrip structure with a thin silver epoxy layer
> as
> >the ground layer and 20cm trace length.
> >
> >Then I measure the trace impedance of the prototype of the cable from one
> >end of the trace with TDR.
> >
> >I find that its impedance smoothly rises up from 50 to 70 ohms.
> >However, measuring from the other end of the same trace, I find that the
> >impedance curve looks flat ,around 60 ohms.
> >(The FPC cable has a U turn at its tail).
> >
> >Why I got two different results by measuring the two ends of the same
> trace?
> >What causes the impedance ramp up?
> >
> >Any comments on this phenomenon?
> >
> >Thank you for your helps in advance.
> >
> >John Lin
> >CAE Engineer @ Arima
> >
> >**** To unsubscribe from si-list: send e-mail to
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>
> Dear John,
>
> Sorry for the late response. (I had a great vacation!)
>
> The rise in impedance can come from two sources:
> (1) Skin effect losses due to very thin traces (or in your case
> perhaps a lossey GND return path).
> (2) A true change in impedance of the structure along it's length.
> eg a tapered trace (thick to thin) in your case.
> (other progressive changes in structure geometry will cause similar
> effects.)
>
> Resistive losses are linear, so you should see the same rise when
> testing from either end of the test trace.
>
> A tapered trace will might cause an impedance change of 10 ohms
> over its length, but add to that another 10 ohms of resistive skin effect
> loss and this gives you your 50 to 70 ohm rise.
>
> When testing from the other end you might expect to see the 10 ohm
> drop due to taper, but you must add 10 ohms of resistive skin effect
> loss in a linear manner over length, so this gives a flat 60 ohm.
>
> I have seen similar effects on PCBs, so a close inspection by
> microsection at various points may be in order. Non-tapered
> traces will still exhibit skin effect loss, so other sources of
> geometry variation can also cause such results.
> The GND plane provides the return path for current flow, so any
> form of cross-hatching will increase the inductance of the GND
> plane and reduce capacitance leading to an increase in Zo.
>
> Hope this helps.
>
> Best regards
>
> Andy Burkhardt
> Product Manager
> Email: mail@polar.co.uk
> Tel: + 44 1481 253081
> Fax: + 44 1481 252476
> http://www.polar.co.uk
> =====================================================
> World leaders in PCB faultfinding and controlled impedance measurement
> =====================================================
>
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