70 DESIGN007 MAGAZINE I JANUARY 2021
Planner, iCD Coplanar Waveguide Planner and
the more recent iCD Materials Planner software.
Over the years, I have also written many
stackup planning and material selection appli-
cation notes and columns. Rather than repeat
all the valuable points, made in these articles,
I will list them below:
1. Arguably the most downloaded (and most
plagiarized) is Multilayer PCB Stackups.
You can find this article in numerous sites
on the internet but sadly not attributed to
me. It covers stackup planning basics and
looks at the most common multilayer con-
figurations (4-16 layers) and their associ-
ated pros and cons.
2. The Perfect Stackup: This application
note discusses how to plan a multilayer
PCB stackup to obtain the ideal stackup
for high-speed design. Stackup Planning
Parts 5 and 6 elaborate on this.
3. Stackup Planning and the Fabrication
Process: Before starting a PCB design,
we need to plan the PCB stackup for
optimized performance, ensure that the
selected substrate materials are available
and clearly document the stackup so
that it can be fabricated to engineering
specifications.
4. Stackup Planning Part 1: The PCB
substrate must be selected based on
specifications to achieve the best
possible performance of the product.
5. Stackup Planning 2: Comprises defini-
tions of basic stackups starting with four
and six layers. Of course, this methodol-
ogy can be used for higher layer count
boards—36, 72 layers and beyond.
6. Stackup Planning Part 3: Looks at higher
layer-count stackups. As the layer count
increases, these rules become easier to
implement but decisions regarding return
current paths become more challenging.
7. Stackup Planning Part 4: Ten plus layers
require very thin dielectrics to reduce the
total board thickness. This naturally pro-
vides tight coupling between the adjacent
signal and plane layers reducing crosstalk
and electromagnetic emissions.
8. Stackup Planning Part 5: To achieve the
next level in stackup design, one needs
to not only consider the placement of
signal and plane layers in the stackup,
but to visualize the electromagnetic fields
that propagate the signals through the
substrate.
9. Stackup Planning Part 6: Impedance
variables. Interconnect Impedance is a
trade-off between the variables—trace
width, trace (copper) thickness, dielectric
thickness and dielectric constant. Then,
if you also need to include differential
impedance, the trace clearance comes into
play. For minimum crosstalk, coupling
also must be considered.
10. Material Selection for Digital Design:
What types of materials are commonly
used for digital design and how to select
an adequate material to minimize costs.
11. Material Selection for SERDES Design:
Many challenges face the designer work-
ing with new technologies. For SERDES—
high-speed serial links—loss, in the trans-
mission lines, is a major cause of signal
integrity issues. Loss can be mitigated by
the correct selection of materials.
12. It's a Material World: Precise material
selection is crucial to the performance of
the today's multi-gigabit designs.
Design techniques constantly change and the
current best practice for high-speed stackup
design is:
I. Closely couple GND/PWR planes on
layers 2 and 3 and the second and third
layers from the bottom (Figure 2). This
lowers the AC impedance of the PDN
and provides low inductance power to
the devices.
Figure 2: Plane pairs close to BGAs.
