Advanced Circuit Materials Division
100 N. Dobson Road
Chandler, AZ 85224
Tel: 480-961-1382, Fax: 480-917-5256
www.rogerscorporation.com
Test Procedures
Advanced Circuit Materials
Selecting Engineered Materials
The Circuit Challenge: What's the right way to choose the best material for flexible
printed circuit applications?
Background
Flexible printed circuits (flex
circuits) are widely used to solve a
broad range of interconnection
and packaging challenges. They
face a demanding combination
of chemical, electrical, thermal,
and mechanical performance
requirements. These various and
often competing aspects of
performance call for careful
selection of materials optimized
for the application.
There are two fundamentally
different approaches to material
selection. The traditional approach is to find a material that
works for the majority of applications, and then use it for everything. This has the advantage of
simplifying inventories and streamlining the manufacturing process.
Unfortunately, it can lead to
unexpected product failures when
the weakness of a general solution
fails to meet the demands of a
tough new design.
The alternative method is to select
an appropriate material for each
new application. This optimizes
the chance of success, but requires more organizational effort
to be implemented effectively. The
material selection process requires
additional resources, and the
processing techniques must be
adaptable.
Selection Process
Once the performance requirements of an application are
defined the flex circuit material
can be selected. First the component materials are chosen, based
on their relevant properties, then
the specific construction of those
components is determined. It is
important to communicate the
application's performance require-
Making Informed
Choices
Define Performance
Requirements Of The
Application
Determine Controlling
Material Properties
Evaluate Relative
Influence Of Key
Properties
Select Optimum
Component Materials
Design Component Layer
Sequences Based On Circuit
Performance And
Manufacturability
Considerations
Specify Specific
Material Constructions
ment to the materials manufacturer accurately. Over-specifying
the material's performance may
add unnecessary cost to the
manufacturing process without
adding value.
Component Materials
A comprehensive matrix of materials is available to the specifier to
meet the needs of the next
generation of flex circuits (Figure
1). These choices provide numerous combinations of conductor,
adhesive, and dielectric to create
a custom solution optimized for
each application.
Metal foil, typically copper, is used
to provide conductive electrical
properties. It also plays a role in
the mechanical structure of the
circuit. The choice of foil is driven
by factors such as cost, strength,
fatigue ductility, conductivity,
solderability and surface finish.
Various types of adhesive are used
to give the flex circuit adequate
mechanical strength, resistance to
solder temperatures, and good
chemical resistance. Additionally,
they are an integral part of the
dielectric packaging of the signal,
power, and ground circuit traces.
Several properties are crucial to
the performance of an adhesive in
a particular application. These
include flammability, coefficient of
thermal expansion (CTE), bond
strength, temperature resistance,
modulus, moisture and insulation
resistance, and flex life. It may also
be important that the adhesive be
compatible with special processes
like laser, plasma, or wire bonding.
Polyimides are used for the base
dielectric film because they are
dimensionally stable, non-flammable, can withstand soldering
temperatures, and have high
initiation tear strength. The principal characteristicsthat differentiate the various types of polyimide
are; stiffness, thermal conductivity,
oxygen and water vapor permeability, and CTE. The differences in
these qualities between film types
can be dramatic.
Tables of component material
properties that are essential to the
selection process are located in
the Rogers Circuit Materials
Manual in the chapters;
“Polyimide Film”, Adhesives”, and
“Copper Foil”.
Construction Options
The thickness of each component
layer is a function of the physical
and electrical requirements of the
package. Similarly, the layer
sequences are derived from circuit
performance and manfacturability concerns. If these determinations are made independently
from the choice of component
materials, the selection may not
be fully optimized.
For example, eliminating the
adhesive from the laminate
altogether may help solve a
thermal expansion problem. Or
perhaps several manufacturing
steps could be saved if an adhesive layer was preapplied onto
the laminate.
A summary of these properties
for R/flex® and FLEX-I-MID®
materials is available in table
form in the Rogers Selector
Guide for Circuit Materials.
Assistance with the material
selection process is also available directly from the Rogers
customer support team.
Informed Choices
In order to make informed
choices the appropriate information must be available. It should
be organized to allow a direct
comparison of the key properties
that are critical to the success of
the application.
Rogers has developed a comprehensive family of materials to
facilitate an engineered approach to materials selection. An
intelligent part numbering system
is used to create custom solutions
for each requirement. This system
is detailed in the Rogers Circuit
Material Manual in the chapter
“R/flex® Material Systems and
FLEX-I-MID® Adhesiveless Laminate
System”.
Theassembly of the component
materials into laminate,
coversheet, bonding film, and
unsupported bonding film can be
tailored to meet the needs of the
finished circuit. Novel approaches to constructing the
circuit can be considered by
taking the engineered materials
approach.
Figure 1. Selection Matrix for Flex Circuit Materials
The information contained in this bulletin is intended to assist you in designing with Rogers’ Circuit materials. It is not intended to
and does not create any warranties, express or implied, including any warranty of merchantability or fitness for a particular
purpose or that the results shown on this bulletin will be achieved by a user for a particular purpose. The user should determine
the suitability of Rogers’ circuit materials for each application.
These commodities, technology and software are exported from the United States in accordance with the Export Administration
regulations. Diversion contrary to U.S. law prohibited.
R/flex and FLEX-I-MID are licensed trademarks of Rogers Corporation
Kapton is a registered trademark of Du Pont
Upilex is a registered trademark of Ube Industries, Ltd.
Apical is a registered trademark of Kanegafuchi Chemical
© 1996-2003, Rogers Corporation,Printed in U.S.A.
1289-0.5-088-SS, Pub# 14-109