I'm final-tweaking an 8-layer board that The Brat laid out. There's one Xilinx Spartan6 FPGA that we use almost all the pins of, and I noticed that its power pours are very cut up.
ftp://jjlarkin.lmi.net/Board51.jpg
Here's the "before" 1.2v core voltage pour
ftp://jjlarkin.lmi.net/L5_before.jpg
Notice the cut-off peninsula feeding from the right, which really isolates the bypass caps to the left.
I tweaked this to...
ftp://jjlarkin.lmi.net/L5_after.jpg
by futzing with the via pad stack, the pour parameters, and by pushing traces and vias around - very tedious - to open up some flood channels.
We're using 1mm pitch BGAs, 6 mil design rules, and the bgavia is 10 mil drill, 18 od.
We'll have to ping some board houses to see what sort of design rules and vias they can do nowadays without the cost exploding. We're already getting quotes like $1600 for 5 boards in one week.
Any thoughts on this? What are you guys doing to get good floods under dense BGAs?
We're using dumbell pad+via combos on all the connected balls except the outer rows. It would be fabulous to eliminate the vias, namely to drill the BGA pads. Is this ever done?
The Brat is still learning PCB layout, and I'm a simple engineer. I wonder if some real layout expert close to San Francisco could come up and give us some pointers, for money or beer.
John
(back to checking)
BOARD STATISTICS REPORT -- 26D150A_51.pcb -- Sat Nov 07 10:40:04 2009
Job Design Time: 291:31
Part Types: 66 Parts TopSide: 516 BottomSide: 0 Total: 516 Drilled pads: 478 Undrilled pads: 2659 Total: 3137 Via Name : STANDARDVIA Via Count : 727 Via Name : POWERVIA Via Count : 123 Via Name : BGAVIA Via Count : 946 Signal Nets: 714 Connections Routed: 1116 Partially 373 Unrouted: 35 Total: 1524
Plane Nets: 1 Connections Routed: 40 Partially 599 Unrouted: 20 Total: 659
Routed Connection Length (inches) X: 907.86 Y: 813.33 Total:
1721.19 Unrouted Connection Length (inches) X: 1741.48 Y: 1275.80 Total: 3017.28Number of copper clearance errors: 0
Number of Routing Layers: 8 Size of Board (square inches): 61.63 Equivalent IC count (1-IC/14 pins): 224.07 Board Density(boardsize/14pin-components): 0.28