FW: [SI-LIST] : FPC impedance control

Alderete, Michael (MICHAEL.ALDERETE@Aerojet.com)
Thu, 6 May 1999 10:21:52 -0700

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

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>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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