28 The PCB Design Magazine • June 2015
•The outer prepreg has a copper foil
coating.
•Prepreg layers may be combined with
(multiple) sheets of prepreg separating
them. For instance, two sheets of 1080
and one sheet of 106 prepreg material
may be required to achieve the desired
thickness.
So where do we start in an attempt to build
the perfect stackup for our project? Initially,
virtual materials are used to get the rough
numbers. Obviously, every digital board will
require 50 ohms impedance and generally a
100 ohm differential pair. This is our target im
-
pedance. However, multiple technologies are
often used on complex designs.
Keep
these tips in mind when planning the
board stackup:
•All signal layers should be adjacent to, and
closely coupled to, an uninterrupted
reference plane, which creates a clear return
path and eliminates broadside crosstalk.
•There is good planar capacitance to
reduce AC impedance at high frequencies.
Closely coupled planes reduce AC
impedance at the top end and dramatically
reduce electromagnetic radiation.
•High-speed signals should be routed
between the planes to reduce radiation.
•Reducing the dielectric height will result
in a large reduction in your crosstalk
without having a negative impact on
available space on your board.
•The substrate should accommodate a
number of different technologies. For
example: 50/100 ohm digital, 40/80 ohm
DDR4, 90 ohm USB.
Unfortunately, not all of these rules can
be accommodated on a four-layer or six-layer
board simply because we have to use a buffer
core in the center to realize the total board
thickness of 62 mil. However, as the layer
count increases, these rules become more criti-
cal and should be adhered to.
Part 2 of the Stackup Planning series will con-
tinue detailing the construction of typical, high
layer-count stackups and build-up technology.
STACkuP PLANNING, PART 1 continues
Points to Remember
• The PCB substrate is the most critical
component of the electronics assembly.
•Ensuring that your board stackup and im-
pedances are correctly configured is a good ba-
sis for stable performance.
• Dielectric material may be in the form
of core or prepreg (pre-impregnated) material.
The core material is thin dielectric (cured fiber-
glass epoxy resin) with copper foil bonded to
one or both sides. The prepreg material is thin
sheets of fiberglass impregnated with uncured
epoxy resin which hardens when heated and
pressed.
• The total substrate thickness is generally
62 mil (1.6 mm) but may vary according to the
application.
•When the board is cured, the resin in the
prepreg flows around the signal traces below,
thus enveloping the trace completely and also
thinning the prepreg material. This alters the
impedance of the signal traces.
• To construct a stackup: Initially, virtual
materials are used to get the rough numbers
then exact materials from the library are intro
-
duced to improve accuracy.
PCBDESIGN
References
1. Barry Olney Beyond Design columns:
Material Selection for SERDES Design, Material
Selection for Digital Design, The Perfect Stack
-
up for High-Speed Design.
2. Henry Ott: Electromagnetic Compatibil-
ity Engineering.
3. Bob Tarzwell: Controlled Impedance.
4. The ICD Stackup and PDN Planner: www.
icd.com.au.
Barry Olney is managing
director of In-Circuit Design Pty
Ltd (ICD), Australia. The compa-
ny developed the ICD stackup
Planner and ICD PDn Planner
software, is a PCB Design
service Bureau and specializes in board level
simulation.
To read past columns, or to
contact olney, click here.
beyond design
