Hello Wee Mei,

Pleased allow me to take a swing at answering . your questions

1. Beside doing a "doll-house" method to model a 3-D flex design, is there
any CAD software in the market that can do it in a less painful way?

There have been some efforts to facilitate layout using CAD and it can be
done but I would not pan use of a mock up.  "Paper doll" modeling is very
instructional relative to the actual assembly and can highlight a lot of issues that
might not be so obvious on a computer screen.

2. What is the highest frequency range a flex material can be used for? or
which flex material currently in the market can achieve 4GHz range?

You should not have a problem with 4 GHz depending on the distance We have
already demonstrated the ability to run a signal at 10Gbps (5GHz) over distances
of 30 inches through 2 connectors at very low power. 20 inches of that path
was in flexible circuit. Our construction was with polyimide but LCP and PTFE
offer still better performance.  Detail can be found at the link below

http://cs.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subsection
=Display&ARTICLE_ID=230649

3. Is there any difference between designing RF & video signal on flex as
compare to rigid?

No real differences, High speed digital and RF are fundamentally the same,
however, one needs to be mindful of material properties and design based
discontunities.

4. If I have EMI requirement, what would be a better choice?

(a) Shield the critical signals between two ground plane (hatched or full) -
3-layer flex

If impedance control is critical, full metal is better.

(b) Critical signals on single flex with two single ground planes - bookbind
method

I don't think that bookbinder constructions are advised as the second gound
reference will not be consistent. Go for a stripline construction if you must
use two metals

(c) Critical signals on one side and other side with silver plating -
two-layer flex

I am not certain of the purpose of the silver plating. Were you thinking of
silver ink? If so it is not as conductive as copper an not likely a good choice
for controlled impedance construction.

Good luck with your project,

Kind regards,
Joe

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