April 22, 2009, Minneapolis, Minnesota
IPC International Conference on Flexible Circuits
Embedded passives Components built on Flexible Substrates
Dominique Numakura,
([email protected])
DKN Research, Haverhill Massachusetts
Abstract
A series of advanced screen-printing (APS) processes with ink materials have been
developed to build functional high-density flexible circuits. They are not only fine
conductor traces, but also embedded passive and more functionaries. Utilizing the whole
process, an electronic system can be built on thin plastic flexible substrate. The printed
resistors built on flexible substrates have range of resistance up to 100 mega ohms with
good linearity. Multiple screen-printing of the conductive ink and dielectric ink has
showing large capacitance up to 2 nano Farads that was impossible by the
lamination/etching process with copper laminates. The high precision process shows
good linearity of the capacitance with the area size of the pattern.
Introduction
Several types of embedded passive
technologies have been developed for rigid
circuit boards and commercialized since
1980s. Both of lamination/etching process
and screen-printing processes have been
employed as the major technologies.
However, they could not be the common
technologies in the industry because of
limited applicability or high processing cost.
On the other hand, recent screenprinting process has been becoming more
capable to generate patterns with fewer steps
compared photolithography and etching
process. A series of advanced screenprinting process have been developed to
build functional circuit constructions for not
only passive components, but also
semiconductor and optical components. In
this study, the advanced screen-printing
technology was applied to generate wide
ranges of resistors, capacitors and inductors
on the flexible substrates to show the
possibilities of the embedded passive
flexible circuits.
Advanced Screen-Printing Process
Previously, screen-printing process
was used for low density polymer thick film
circuits such as membrane switches or for
the low-resolution coating such as solder
mask of the printed circuit boards. However,
there have been a lot of technical progresses
not only with resolutions, but also with
material
capabilities. Therefore,
the
advanced screen-printing has been getting
more values other than fine pattern
generation. Fig. 1 shows the comparison
between
advanced
screen-printing
technologies and traditional screen-printing
technologies and photolithography/etching
technologies in the printed circuit
manufacturing by a radar chart. Nowadays,
the advanced screen-printing process can
generate fine lines down to 10 microns with
high mesh number screen masks. Double
layer and multi-layer constructions are
available with low cost micro via holes. The
new processes provide broader choices for
the substrates for both of rigid and flexible
circuits. The process does not need extra
chemicals such as etching resist or striping
chemicals, therefore the process does not
produce large amount of chemical waste.
The process is very environmentally
friendly. It eliminates the cost of the waste
treatment significantly.
QuickTime™ and a
decompressor
are needed to see this picture.
components and optical devices. The screenprinting process is much simpler compared
to the photolithography/etching process,
therefore the total manufacturing cost can be
much smaller.
Basic Constructions of the passive
components
Fig. 2 illustrates the basic idea of the
layer construction of the printed resistors
built on a flexible substrate. All of the layers
can be built by screen-printing. Screenprinting process is more capable to manage
varieties of the ink materials compared to
the ink-jet printing, especially, for broad
range of ink viscosities. The first layer
screen-printed is the conductor traces that
are basically the same functions as the
copper foil circuits. The second layer is the
resistor built instead of discrete components.
Fig. 1 Comparison of the processes
The material processing capabilities
of the APS process is the major advantage
compared to the traditional photolithography
and etching technologies. The process is
able to manage not only passive materials
such as conductive and insulation materials,
but also functional materials including
photo-active materials and dielectric
materials if they can be prepared as the
condition of liquid or paste. As the results of
the new capabilities, the APS process is able
to generate many kinds of electronic devices
on the flexible substrates including passive
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 2 Construction of printed resistor
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 3 Construction of printed capacitor
Fig. 3 illustrates the basic layer
construction of the printed capacitors built
on the flexible substrates. There are three
layers to form a printed capacitor on a
flexible substrate. The first layer is the lower
electrode. But it can be the same layer as the
conductor trace of the printed resistor. The
second layer is the insulator layer with high
dielectric constant material. The third layer
is the upper electrode, generally the similar
material as conductor layer, but it should
have good affinity with the dielectric layer.
Fig. 4 illustrates an example of
printed inductors. The inductors can be
formed by only conductor traces, but they
need two or more layers with appropriate
insulation layers.
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 5 Manufacturing process of thick
film conductor
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 6 Manufacturing process of copper
foil conductor
Fig. 4 Construction of printed inductor
Manufacturing Process
The ASP process has been employed
to build embedded passive components on
the flexible substrates. Nowadays, screenprinting process is not the major technology
for the pattern generation in the printed
circuit manufacturing because of lower
resolution and higher conductor resistance
compared to the photolithography/ etching
process.
However, the ASP process
developed recently has an equivalent pattern
resolution with more advantages as shown in
Fig. 1. The major advantage of the screenprinting process is that both of material
formation and patterning can be processed in
two steps as shown in Fig. 5. It needs
several steps by photolithography/etching
process with additional processing materials
such as photo resist and etching solution as
shown in Fig. 6.
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 7 Process sequence of advance thick
film circuits with embedded passive
Fig. 7 shows the basic manufacturing
process of the printed passive components
on the flexible substrates. The total process
is much simpler and less expensive
compared to the photolithography/etching
process with special resistor and capacitor
laminates. The process needs only screenprinters and baking ovens. The standard
equipment of the solder mask process for the
traditional printed circuit boards is available.
The process does not need any processing
materials such as photo resist or strong
chemical solution, therefore it does not
produce chemical waste. It does not need
extra facilities for the waste treatment.
Trials and results
The trial was conducted on 50micron
thick polyimide and PEN film substrates
supplied by Du Pont and Teijin. Several ink
materials produced by Fujikura Kasei and
Asahi Chemical Laboratory were used as the
resistor and dielectric layer. Five kinds of
carbon pastes were employed to cover
broader range of the resistances. The
dielectric constant of the dielectric material
is higher than 50. A traditional silver
conductive paste was used for the electrode
layer. A standard solder mask material of
the flexible circuits was used as the
protection layer.
Fig. 8 shows the examples of the
printed resistors build on flexible polyimide
film. The curve of Fig. 9 shows the
geometrical performance of the printed
resistors using one carbon ink for 5 mm long
elements. The hyperbolic line indicates
exact inverse proportion of the resistance
against the width of the printed resistors. It
means that one order range of the resistances
can be covered by one screen-printing
process. The standard deviation can be in +/10% in a same work sheet. Changing the
carbon inks, the printed resistors cover a
broad range of the resistance from 100 ohms
to 10 mega ohms in 2 mm squares. It is
much broader and more precious compared
to the embedded resistors made by
traditional screen-printing process or
laminate/etching process.
Fig. 10 shows the examples of the
printed capacitors build on flexible
polyimide film. Fig. 11 shows the
geometrical performance of the printed
capacitors
including
the
frequency
dependence. The data indicates that the
capacitance of the printed devices have clear
linearity under the same dielectric material.
The capacitances have a trend of frequency
dependency. The results indicate that printed
capacitors can have over 2 nano Farads in a
40 square millimeter space. It is one order
larger compared to the traditional printed
capacitors.
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 8 Example of printed resistors
Fig. 10 Example of printed capacitors
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 9 Geometrical performances of the
printed resistors
Fig. 12 shows an example of the
printed inductor built on a flexible
polyimide film. It is possible to add more
layers to increase the inductance of the coil.
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 11 Geometrical performances of the
printed capacitors
QuickTime™ and a
decompressor
are needed to see this picture.
Fig. 12 Example of printed inductors
Conclusion
The trials show the evidence that the
APS process with new ink materials is
capable to produce boarder ranges of
embedded passive components, especially
for printed resistors and printed capacitors
with small deviations on thin flexible
substrates compared to the traditional
embedded passive circuits made by
laminate/etching process.
All of process can be applied to RTR
(Roll to Roll) system without big
investment. The total manufacturing cost
could be remarkably lower compared to the
photolithography/etching process for the
volume productions.
More examples with detailed data
will be introduced during the presentation.
Reference
1. “Advanced Screen Printing Process” Practical Approaches for Printable &
Flexible
Electronics”,
Dominique
Numakura, 3rd IMPACT and the 10th
EMAP, Taipei/Taiwan, October 2008
2. “Introduction of Printable Electronics”,
Dominique Numakura, Nikkan Kogyo
Shinbun, January 2009
3. “Fine Line Thick Film Circuits with High
Conductivity Built on Flexible Substrates
are Capable of Soldering”, Robert Turunen
and Dominique Numakura, IPC APEX
EXPO, April, 2009
4. “Flexible LED Array made by All ScreenPrinting”, Masafumi Nakayama and
Dominique Numakura, IPC APEX EXPO,
April, 2009