I've been looking and not finding anything, so I'll try posting the
  question here...
  We're doing some investigation of PWB constraint, and as part are doing
  some thermal expansion coefficient measurements.  On the reference sets,
  (bare polyimide PWB, 6 & 12 layer unmounted and 6 & 12 layer mounted on
  aluminum) we're seeing what looks like a hiccup in the TCE curve at about
  15 degrees C.  (The TCE runs are from -40 C to 100 C).  The setup has
  strain gages mounted on the PWB and a titanium silicate reference slide,
  then is being heated in at 2 degrees per minute.  From -30 to 15 degrees
  there's a longitudinal TCE of about 18 ppm/C and a transverse TCE of
  about 15 ppm/C.  From 20 to 90 C the longintudinal TCE is 19 and the
  transverse is about 16 ppm/C.  But there's this pronounced dip in the
  range of 15 to 20 C.

  It doesn't show up on the other boards, which are rigidly bonded to low
  TCE substrates, just the soft bond to aluminum and the boards with no
  bonding at all!  There is soldermask on the panels, any chance that that
  might have a glass transition at about 15 C?

  Thanks,
  Steve Axdal             [log in to unmask]

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