CCD-world: Socket for the THOMSON TH7899 CCD

[Tom Droege]

The following was posted to CCD-world:

I use "Single In-Line Strip" sockets for this purpose.  I can then make a
printed circuit board with a large cut out (for cold finger cooling) and
mount four socket strips around the cut out hole.  (I am looking at them on
p-120 of the current Digi-Key Catalog.)

One would like to use a zero insertion force socket for this application.
But I do not know of any that allow you to get at the back of the CCD which
I assume you want to access for some sort of cold finger.  

Using SIP sockes allows making connections to the CCD chip with printed
circuit wiring with the possibility of very good impedance matching, and
more rugged design than that achieved by using fine wire (for low thermal
loss) to connect to the CCD.  Some of the clock drive circuitry can be put
right on this board

If you use a thin multi-layer printed circuit board, conduction losses will
be quite low.  This is the material most often used for structural supports
in superconducting magnets. 
A great source for thermal information to compute losses is:
 
http://www.kkassoc.com/~takinfo/

This is the largest collection of thermal information that I have ever seen. 

Thermal conductivity of printed circuit board material is of order 0.001
that of copper.  The copper leads, even though quite fine, will dominate
the thermal impedance for a properly designed PC board.  One can put a nice
ground plane at the socket, then approach the socked with two sides strip
line with very narrow conductor.  (Which will have low thermal conduction.)
 One can also cut appropriate slots in the PC board to further reduce
thermal conduction.  This is quite cheap to do in manufacturer, as it is a
CAD operation.  i.e the printed circuit board will not cost much more if
made with a lot of slots cut in it to reduce the thermal conductivity.

This method of mounting CCDs has been the subject of controversial debate
between myself and those advocating constantan wire and the like to make
connections to the CCD.  If you do the computations, you will find that for
the same resistance, copper wire connections are superior to materials like
constantan.  i.e. the ratio of electrical conductivity to thermal
conductivity is better for copper.  What constantan allows is the use of a
larger wire size (which is then more rugged and has lower impedance) to
make the socket connection.  Putting the wire on a printed circuit
substrate solves the "rugged" problem and allows using strip line
techniques which are difficult to implement with wire.  With strip lines
and thin printed circuit boards one can make low impedance connections and
put the clock line edge shaping where you want it without ringing problems
from the interconnecting leads.

I hope this does not start a fist fight.  In other groups I have found an
"attachment" to constantan for the connections.  I have been unable to
persuade anyone to make computations.  It is not a clear cut decision.
There are trade offs.  This group should be better at evaluating such
considerations.  I am also taking the liberty of answering more that what
is asked.  One selecting a socket should also be thinking about the thermal
problems.  ;^)

Tom Droege  

At 11:38 AM 12/3/99 +0800, you wrote:
>The following was posted to CCD-world:
>
>We are designing a CCD camera, intended to use the THOMSON TH7899 CCD.
>according the outline drawing, 15 pins are located in each side of the CCD
>
>  Anybody have used the THOMSON TH7899 CCD and what kid socket is
>suitable for this CCD?
>
>Best Regards
>
>Zhaowang Zhao
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