Friends -
Last month I posted a pcb-based layout of the attachment of
a Spartan3 FT256 (BGA) to a 6-layer board, with routing of
all 184 non-power pins to the perimeter. I got some useful
feedback from this crowd, thanks!
I just updated that page. Changes:
Uses a more realistic footprint for the 0603 caps
Uses fatter power traces
Orients all capacitors with pad 2 at ground
More tuning of my patches to pcb-20040903
The URL is still the same:
http://recycle.lbl.gov/~ldoolitt/ft256/
I used this BGA fanout as a base for a board design. While
that design as a whole is not done, I did most of the FPGA
routing, and this fanout seems easy to work with.
Other followup points, based on the discussion here:
The 0603 footprint I cobbled together (based on last month's
"pcb SMT footprints" thread) is at
http://recycle.lbl.gov/~ldoolitt/ft256/0603_lrd
It's supposed to be a neutral average of the parameters
out there, and hand-solder friendly.
Norwood suggested I look into low inductance capacitors,
AVX LIC in particular. After an hour or two of study, it
looks like they don't benefit a design like this, because
their inductance gets swamped by the via inductance.
Hal2000 suggested I ask for 0.002" separation between power
and ground plane, to enhance the natural decoupling ability
of that distributed capacitance. The obvious gEDA/pcb friendly
choice for fab, Advanced Circuits, only has laminate down to
0.005". Bummer.
Thanks again for your comments and suggestions!
- Larry
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