[SI-LIST] Re: Substrates

Munish,
Table D-2 in my book Robust Electronic Design Reference Book, Volume 2,
lists the following Thermal Conductivities for insulators at room
temperature:
*  Alumina (aluminum oxide) 2.8 to 45 watts/meter-degreeCelsius.
*  FR-5 (epoxy/fiberglass) 0.16 to 1.7 watts/meter-degreeCelsius.
*  FR-4 (epoxy/fiberglass) 0.16 to 1.7 watts/meter-degreeCelsius.
*  Dry air 0.016 to 0.030 watts/meter-degreeCelsius.

These are the lowest and highest values that I found listed in
manufacturers' datasheets, application notes, and the scientific and
engineering literature.  FR-5 has a higher degree of cure than FR-4,
with more cross-linking between the polymer chains, so that it can be
used up to 170 degrees Celsius versus the 140 degrees Celsius service
limit for FR-4.  Otherwise their properties are identical.

For comparison, Table C-2 lists the following Thermal Conductivities for
conductors at room temperature:
*  Copper 190 to 420 (nominally 398) watts/meter-degreeCelsius.
*  Aluminum 97 to 247 watts/meter-degreeCelsius.
*  Tin-lead solder 30 to 63 watts/meter-degreeCelsius.

For printed circuit boards (PCB's) we can essentially ignore heat flow
(and thus device cooling) through the insulator.  High-power devices may
be soldered straight to a large pad on the board, with a dense array of
vias under the heat spreader to conduct heat to another large pad on the
backside of the board, giving us heatsinks on both sides.

For hybrid modules, resistors are deposited directly on the substrate
using a thick-film or thin-film process.  This provides intimate contact
between the resistive element and the substrate, so we can get
significant heat flow through the insulator.

If you are interested, I have a web page about the Robust Electronic
Design Reference Book, Volumes 1 and 2, at 
   http://www.r-e-d-inc.com/book-out.htm
which includes a link to a sample chapter on Kluwer's web site.  So far
readers have found 8 errors in the two volumes.  The locations of these
errors, and the corrections to them, are listed in my web page
   http://www.r-e-d-inc.com/errata.htm

                John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE
                dBi Corporation
                http://www.dbicorporation.com/
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