Hi Earl, the MoonMan,
First, I have problems accessing Steve's website--please send me the pics
directly.
Second,
>Thermally asymetric? Does this mean in the Z axis as a primary function of
>the Al heat sink and the Silicon chip placed on a BT substrate, or?
A: What it means is that the various layers [overmold, die, die attach,
interposer, package substrate] are made of different materials all having
different CTEs and modulii of elasticity. By proper variation of the
thicknesses of these materials one can achieve a thermally balanced [or at
least nearly balanced] design where the various layers will thermally
expand/contract without significantly warping [of course there are shear
forces between these layers] the package--with a metal heat spreader this
becomes essentially impossible with the heat spreader on the outside.
Third, the die attach on thin BGAs is typically some polyimide-based adhesive.
Fourth,
>Sure wish Werner well with grandkiddies, but I need some more help. Maybe
>Dave Hillman has some more educated ideas since I sent him my first report
>from fellow Rockwell types. Also, I still await Joe Felstad's comments about
>the chip attachment, its method, and the lack of an apparent
>"compliant/flex" layer as he helped develop at Tessera.
A: The "compliant/flex" interposer layer of the Tessera patents is needed
most for CSPs and thin BGAs with a thin package substrate not including woven
glass-fiber reinforcement and prevents the large die/PCB expansion mismatch
from loading the SJs [the failures would occur at the die corners]--the large
die/PCB expansion mismatch is accommodated in the "compliant/flex" interposer
layer(s) [in some designs to the extent that the copper traces broke]. The
BLR of the larger BGA packages depends on the much thicker woven glass-fiber
reinforced BT to be stiff enough to shield the solder joints from the much
larger die/PCB CTE-mismatch. This works particularly well for packages much
larger than the die [it is the package corner SJs that fail first] and those
with SJ arrays depopulated under the die.

I hope this helps--you do need to get out of your cave more often--there is a
whole world out here

Werner Engelmaier

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