Good morning.  We are in the process of evaluating flexible printed
circuits at cryogenic (LN2 and lower) temperatures.  We are particularly
interested in the response of various coverlay materials and their response
to these extremes.  We have subjected three different coverlay materials to
rather brutal cold temperature tests with some interesting results.  Our
sample is rather small and we wonder if any one else has subjected flex
circuits in a similar manner and would be willing to share their results.

In the spirit of sharing information (in the hopes of obtaining more) I
would like to summarize out test procedure and results.

All test circuits were two-layer with 5/50/5 adhesiveless-polyimide based
material from various manufacturers.  Upper and lower coverlays are
installed on all circuits. We are most concerned with cracking/breaking of
the coverlay, exposing copper layer below.

Procedure:

Test 1.  Resistance of circuit traces were measured in LN2.  Circuits were
cooled to 8K in a cryostat and resistance was measured. Circuits were twice
swirled in LN2 to gauge their low temperature flexibility.

Test 2.  Circuits were wrapped around a 1/2 inch diameter tube and
temperature cycled twice by immersion into LN2.

Test 3. Circuits were wrapped around a 1/2 inch diameter tube and left over
night in LN2. The estimated submerged time (until LN2 boil off) was 2
hours.

Test 4. Circuits were reverse wrapped (copper layer previously in
compression now in tension) around a 1/2 inch diameter tube an temperature
cycled 10 times by immersion into LN2.

Test 5. Each circuit was submerged in LN2 then bent through 180 degrees
with an estimated bend radius of 3/16 inch.

Note that each of the circuits were subjected to all tests indicated above.

Results:

Circuit 1:  We believe that the coverlay is PC1020.  Extreme evidence of
crazing observed in the coverlay.  Evidence of open traces was observed.

Circuit 2:  We believe that the coverlay is PC1520.  Extreme evidence of
crazing observed in the coverlay.  Evidence of open traces was observed.

Circuit 3:  Coverlay is FR7001.  No evidence of crazing observed in
coverlay.

Circuit 4:  Coverlay is R/flex8080LP1.  Were able to observer what are
described as scuff marks in coverlay only after cold temperature bend test
was repeated with about 5 pound of tension at the bend.

Caveats:

Circuits 1 and 2 have been around here for a long time.  Siblings of these
two circuits were subjected to similar, but less extensive, cold
temperature testing when they were first delivered (1 to 2 years ago).  No
such damage was observed at that time.  During the interim, each of these
circuits were installed in a cryostat were one end of the circuit was
cooled to about 10K and the other end was left at room temperature. Several
temperature cycles are likely to have occurred.  It is possible that aging
has affected the coverlay material in some manner.  That we observed open
traces in these two samples is a new source of concern.  We had anticipated
that the adhesiveless copper attach would minimize this problem.  We will
examine the circuits more closely to attempt to determine the source of
these failures.

Circuits 3 and 4 have recently been fabricated.

In actual use, the flexible printed circuit would be flexed once to
install, through a 90 degree bend with a bend radius of about 0.1 inch.
This bent area would be subjected to liquid helium temperatures over an
approximate 5 year period with maybe 20 temperature cycles.

We are interested in obtaining more information about coverlay/circuit
material which can withstand cryogenic temperatures.  If you have
additional information, or can direct us to a source of additional
information on very low temperature flex circuit testing,  it would be
greatly appreciated.

                                                Mike

Michael S. Matulik
Engineer   [\]
Fermi National Accelerator Laboratory
630 840-4091 [log in to unmask]

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