WP484 Datasheet, PDF (6/12 Pages) Xilinx, Inc – DDR2/DDR3 Low-Cost PCB Design

DDR2/DDR3 Low-Cost PCB Design Guidelines for Artix-7 and Spartan-7 FPGAs
X-Ref Target - Figure 4
Top Layer
Layer 2
Layer 3
Blind
Via
Layer 4
Layer 5
Buried
Via
Through Hole
Via
Through Hole Via
with Back Drilling
Layer 6
Layer 7
Blind
Via
Bottom Layer
Figure 4: Different Via Types
WP484_04_091416
Via Aspect Ratio – The ratio of PCB thickness to the smallest unplated via drill hole diameter. This
is used as a guide to ensure that the PCB fabricator does not exceed the mechanical capabilities of
the drilling equipment. A via aspect ratio of 10:1 is fairly common with the standard PCB
fabrication. The via aspect ratio can be increased to 20:1 using advanced PCB fabrication while
maintaining the design for manufacturing (DFM) rules.
Back-drilled Vias – A back-drilled via is a through-hole via that has a portion of its length “drilled
out” such that it is no longer conductive. This improves signal integrity because it removes an
unneeded stub from the route. The typical cost adder for back-drilling vias ranges from 5–10% of
the total PCB fabrication cost.
Via-in-Pad – A Via-in-Pad is a via drilled directly beneath a pad. This removes the need for a
separate metal trace (stringer) to drop down a via. This can help with breakout routing and
improved signal integrity at the expense of higher board fabrication cost. The cost adder varies
from +10–15% of the PCB fabrication cost and is dependent on the via aspect ratio.
Buried and Blind Vias – A buried via is located entirely inside the printed circuit board and does
not touch the top or bottom layers; a blind via travels from either the top or bottom layer to an
inner signal layer. Both types of vias free up room above or below for other routing. This differs
from a through-hole via, which travels all the way from the top to the bottom layer. The cost adder
for a buried or blind via depends on the number of different types of buried or blind vias that exist
on the PCB. Each type of buried / blind via requires a separate lamination cycle, resulting in extra
cost. For example, a PCB with three different types of buried / blind vias (L1 – L4, L16 – L12, L4 – L8)
on a 16 layer PCB results in a 30% cost adder per each type of buried / blind via.
WP484 (v1.0) September 27, 2016
www.xilinx.com
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