There are two choices of dielectric tape for use with BeO ceramic. These
ESL102 (blue) compatible with silver and gold inks.
ESL101 (white) compatible with gold ink only.
The prototype hybrds, which are bowed more than is tolerable for SVXII,
have 4 layers of blue tape and 2 layers white tape.
To test the best way to reduce bowing of production hybrids, 2 blank layers
of each tape were laminated and baked on bare 3.5" square BeO substrates.
The Blue tape caused a bow given by the formula D=.001L**2, where L is
a length in the plane of the hybrid and D is the sagita caused by bow in that
length, whereas the White tape caused a bow D=.00025L**2
The production hybrid design now has 1 layer of blue tape and 3 layers of
white tape. Of the 4 blue tape layers in the prototype hybrids, one has
been replaced by white tape because it was not in contact with silver.
Another two layers have been replaced by a printed dielectric, since these
layers do not have any fine features that require the use of tape.
The printed dielectric is ESL4913G, which has been developed to match the
CTE of BeO, and should therefore produce no bow.
Because there is one layer of blue tape in the hybrid design, a dummy layer
of blue tape will be added to the back of the production hybrids, to
"cancel" the bow. Thus the only bow in the production hybrids will be from the
white tape layers. These layers have only partial coverage of the surface area
and additionally some of their bowing effect will be cancelled by the metal
layres, which by themselves induce bow in the opposite direction. Therefore,
the production hybrid bow is predicted to be less than D=.001L**2.
The production hybrid thickness will be equal to the prototype hybrid
thickness, because the added thickness from the dummy tape layer on the back
is compensated by a reduction due to the printed dielectric, which has less
fired thickness than tape.