On Mon, 2007-11-12 at 07:39 -0800, Vasile Surducan wrote:
> So, how you're doing for 1700+ balls where there is no room for
> capacitors on the side where the BGA part is soldered ?
> Usually could be the same distance to the supply planes to the top or
> to the bottom...(imagine a 12 layer board with supply planes on layers
> 4,5 and 8,9.
> On actual technology there is no other way than styffing on both sides.

Agreed, a couple years ago we did a board, not sure how many layers (20
+) and the only option was to put decoupling for a very large FPGA on
the "other" side. Xilinx really went a little nuts with decoupling
requirements on the Vertex4 FPGAs, there are whole pages of our
schematics dedicated just to decoupling. The datasheet has formulas to
determine how many caps you need. And they aren't all the same value
either. Those 0201 caps are REALLY small!

Fortunately, the decoupling requirements were reduced for the Vertex5
FPGAs, although since they have more capacity the actual number of caps
didn't go down that much! :)

While I try to reduce the number of components on the "other side" of
boards I design to a relatively small number, it isn't very high on my
priority list, especially for "small" components. The extra cost isn't
an issue for me, the issue is just the annoyance of having to flip a
board to probe something or remove/install a part on the underside.

TTYL
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