RE: FW: [SI-LIST] : FPC impedance control

Alderete, Michael (MICHAEL.ALDERETE@Aerojet.com)
Fri, 7 May 1999 07:15:11 -0700

Hello Radu-

I may have misunderstood your question...

FR-4 is a dielectric, glass fiber mat embedded in an epoxy, and is thus an
insulator. When you ask for FR-4's conductivity, are you really asking for
its dielectric breakdown voltage, or perhaps (in ohms) the insulation
resistance value for FR-4?

Michael Alderete

-----Original Message-----
From: Radu Talkad [mailto:radu.talkad@siara.com]
Sent: Thursday, May 06, 1999 2:29 PM
To: Alderete, Michael
Subject: Re: FW: [SI-LIST] : FPC impedance control

Michael,

As you seem to have some answers to questions regarding Conductivity
numbers for dielectric, I thought I would bother you for the
conductivity of FR4. I have looked on the web, asked some PCB fab houses
but could not get it. I need this number for calculating dielectric
losses. I would appreciate any info / pointers you could give me. If
there are any graphs, I would appreciate a copy of it (them?) also (FAX
number 650-237-2179).

Thanks very much for your help and regards,

-radu

"Alderete, Michael" wrote:
>
> sending again...
>
> -----Original Message-----
> From: Alderete, Michael [mailto:msamcm@pe.net]
> Sent: Wednesday, May 05, 1999 7:33 PM
> To: SI-LIST
> Subject: RE: [SI-LIST] : FPC impedance control
>
> Michael-
>
> Don't know if anyone tried to answer your question on conductive epoxy.
>
> An experiment which tested three silver filled one part epoxies was
> discussed in a new text, "Conductive Adhesives for Electronics
> Packaging", Ed. by Dr. Johan Liu, Electrochemical Publications, 1999.
> Chapter 5 of the text describes Adhesives A, B, and C, with
> (advertised?) volume resistivities of 1e-3, 1 to 3e-4, and 3e-4 ohm-cm.
> This chapter studied effect of degree of cure on isotropic conductive
> adhesive (ICA) properties.
>
> One graph showed resistivity of each adhesive, versus Conversion Degree
> (%). At 100% (cure) ICA's A, B, and C had the following measured
> Resistivities:
> A: 3.0e-4 ohm-cm
> B: 0.8e-4 ohm-cm
> C: 0.9e-4 ohm-cm
>
> The chapter authors also studied the effect of 85degC/85%RH exposure
> times on ICA volume resistivity, for different cure schedules.
>
> I think that ICAs, Ag filled, are the most common conductive epoxies, as
> compared to anisotropically conductive adhesives (ACA), though I believe
> there are numerous high volume applications using ACA for fine pitch
> flex circuit termination to LCDs and in many low cost consumer gadgets.
>
> Hope the published data on Ag filled ICAs gives you a rough idea of
> connection qualities achievable with commercial compounds.
>
> [Ref, Chapter 5, "Curing of Isotropic Electrically Conductive
> Adhesives," by Li Li (Motorola SPS, Tempe AZ) and James E. Morris (SUNY
> at Binghamton, NY), in text, "Conductive Adhesives for Electronics
> Packaging",]
>
> Regards,
> Michael Alderete
> Sr. Engineer, Elec Pkg.
> Aerojet, Azusa CA.
> michael.alderete@aerojet.com
>
> -----Original Message-----
> From: Fox, Michael J [mailto:michael.j.fox@intel.com]
> Sent: Monday, May 03, 1999 6:11 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: RE: [SI-LIST] : FPC impedance control
>
> 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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-- 
========================================================================
Radhakrshna (Radu) Talkad                          Phone: (650) 237-3177
Siara Systems, Inc.                                FAX:   (650) 237-2179
300 Ferguson Dr.                                   mailto:radu@Siara.Com
Mountain View, CA 94043                             http://www.siara.com
========================================================================

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