[SI-LIST] Re: Use of Ceramic substrates at High Frequencies
- From: Scott McMorrow <scott@xxxxxxxxxxxxxxxx>
- To: APANNIKK@xxxxxxxxxx,"si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 17 Sep 2001 17:19:05 -0700
Anil,
Loss due to skin effect is proportional to the square root of frequency.
Dielectric loss is proportional to frequency.
At frequencies beyond 200 to 1000 MHz, dielectric loss dominates.
To reduce the effect of dielectric loss, ceramic substrates are used
at Microwave frequencies.
For example, for a 4 mil, 1/2 oz copper, 50 ohm stripline trace on FR4 or BT
material
with a loss tangent of 0.016 the dielectric and copper losses are as follows:
dielectric loss copper loss
100 MHz 0.0074 dB/in 0.0381 dB/in
500 MHz 0.0370 dB/in 0.0854 dB/in
1000 MHz 0.0740 dB/in 0.1208 dB/in
2000 MHz 0.1480 dB/in 0.1708 dB/in
2500 MHz 0.1850 dB/in 0.1910 dB/in
5000 MHz 0.3699 dB/in 0.2701 dB/in
10000 MHz 0.7398 dB/in 0.3820 dB/in
For this particular trace configuration, above about 2.5GHz, the dielectric
losses begin to dominate for fairly high quality FR4 with a low resin content.
If the resin content is higher, the loss tangent is increased, with the
crossover
frequency being reduced.
If the trace width is widened, the the crossover frequency will
also be reduced.
(the crossover frequency being defined as the point where dielectric losses
and copper losses are equal. Above this point, dielectric loss dominates
and should be minimized. See a paper on this by Ed Sayre and associates
at NESA.)
As Ray Waugh points out in his posting, ceramic substrates have
much higher dielectric constants, allowing smaller resonant structures
to be built in less space. This, however, is at the expense of increased
copper losses. PTFE type materials and some of the hybrid derivatives
have lower dielectric constants (3.2 to 3.5) and lower loss tangents,
and allow for optimization of long, high bandwidth interconnect
structures.
Below the crossover point, you get the most bang for your bucks by using
wider traces. Above the crossover point, you get the most bang for your
bucks by reducing dielectric loss. At extremely high frequencies, you
get the most bang for your bucks by using low loss, low dielectric constant
materials (like Rogers RO4350) and wide traces.
regards,
scott
--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR 97062-3090
(503) 885-1231
http://www.siqual.com
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