[SI-LIST] Re: Use of Ceramic substrates at High Frequencies
- From: "Perry Qu" <perry.qu@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 18 Sep 2001 09:32:20 -0400
Hi!
Ceramic is widely used in packaging industry for better thermal performance and
reliability. However, it's not used very often in high speed digital PCBs. One
of the point to note is, the dielectric constant of ceramic is much higher than
FR4 or other commonly-used material. Thus we need a much thinner trace or
thicker board to get the same impedance control compared with FR4.
Regards
Perry
"Raymond W. Waugh" wrote:
> Anil...
>
> Of the commonly used microwave and RF boards, FR4 and other cheap materials
> have the highest dielectric loss. Ceramics such as 99.4% alumina are in the
> middle, and certain organics such as Teflon based boards have the lowest.
> Dielectric losses dominate the total loss picture at microwave frequencies.
>
> The reason ceramics are used (so called MIC or microwave integrated
> circuits) is twofold. First, they are sometimes chosen for high temperature
> processing (die attach, wire bond). This permits semiconductors to be used
> without packages, greatly reducing parasitic reactances and permitting wider
> bandwidths. There is no way one can make a 6 - 18 GHz GaAs FET amplifier
> using packaged FETs. Second, the high dielectric constant of alumina
> (around ten) shrinks the size of distributed transmission line structures
> such as backward wave couplers.
>
> Ray
> ------------------------------------------------------
> Raymond W. Waugh - WSD Applications Engineering
> E-mail: ray_waugh@xxxxxxxxxxx
>
> USPS : Wireless Semiconductor Division
> 39201 Cherry Street, MS NK20
> Newark, California 94560
> ------------------------------------------------------
>
> -----Original Message-----
> From: Anil Pannikkat <APANNIKK@xxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx <si-list@xxxxxxxxxxxxx>
> Date: Monday, September 17, 2001 4:43 PM
> Subject: [SI-LIST] Re: Use of Ceramic substrates at High Frequencies
>
> Hello
> This might be a very naive question, but can anyone please tell me as to
> why ceramic substrates are used so much in the microwave world as compared
> to organic substrates. I know the dielectric loss in ceramic substrates is
> much lower than the organic substrates, but the skin resistance in ceramic
> substrates (due to W metallization) is much higher. So why is the dielectric
> loss more important than skin R?. Or is the choice more due to other factors
> such as cost?
> Thanks
> Anil
>
> Anil Pannikkat
> MTS Package Development Ph : 408-544-7542
> Altera Corporation Fax: 408-544-6419
> 101, Innovation Drive, M/S 4101 Email: apannikk@xxxxxxxxxx
> San Jose, CA 95134 WWW: http://www.altera.com
>
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