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Erik Simon
Analysis of Contact Resistance Change of Embroidered Interconnections
Untersuchung der Kontaktwiderstandsänderung bei gestickten Kontakten
ISBN: 978-3-8366-3871-5
Herstellung: Diplomica® Verlag GmbH, Hamburg, 2010
Zugl. Technische Universität Berlin, Berlin, Deutschland, Diplomarbeit, 2009
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V
48B
Abstract
Integrating electronics into textiles, for diverse applications, is a topic that various
research institutes and companies are increasingly working on. State of the art is still the
use of conventional wiring techniques. However, for a number of applications it is
important to preserve the textile character while integrating these new functions.
Embroidering electrically conductive yarn is one approach. Since it has the same
mechanical properties as ordinary yarns, it will not alter the textile character
significantly. It can be embroidered directly to an electronic module. Hence, the yarn
acts as an embroidered wire, which can transmit electrical signals, and furthermore acts
as a means to provide an interconnection to electronic modules.
Embroidered contacts have been investigated in earlier research projects. Their
feasibility could be successfully shown but it was observed that the resistances of
embroidered contacts are changing widely during reliability tests and may even fail.
The focus of this study is to reveal causes for the changing contact resistances and
failures of embroidered interconnections, and to propose possibilities to improve them.
In the first part of this study, the mechanical and electrical properties of the conductive
yarn are analyzed. Their behavior due to different environmental parameters as well as
their interrelation is discussed. This is done by means of thermomechanical analyses,
and resistance measurements at constant temperatures and in a climatic test chamber.
The focus lies then on the embroidered contacts. At first, the measurement technique
and a theoretical model of the measured contact resistance are presented. Furthermore, a
simplified physical embroidery contact model is proposed, which excludes disturbing
parameters. The behavior of both structures is then analyzed in the climatic test
chamber using an in-situ measurement system. The results of the experiments reveal
that due to the particular behavior of the yarn an embroidered contact will initially be
enhanced but may degrade during repeated temperature cycles and may even fail. A
negative coefficient of thermal expansion, reversible and irreversible shrinkage as well
as creep and stress relaxation effects can be made accountable for these contact
resistance changes.
It is concluded that the tested embroidered contacts do not show sufficient reliability in
temperature cycling tests. From the identified causes, improvements to the embroidered
contacts are proposed, and tested. It is shown that these techniques result in very
confined resistance changes and thus strongly increase the reliability of embroidered
contacts.
VII
49B
Kurzfassung
Integration von Elektronik in Textilien ist ein Thema, das zunehmend in den Blickpunkt
verschiedener Forschungsinstitute und Unternehmen rückt. Für diese sehr aktuelle
Technik werden allerdings bisher nur herkömmliche Methoden der Verdrahtung
verwendet. Um die Beschaffenheit eines Textils nicht zu verändern, und damit den
Tragekomfort und das Erscheinungsbild nicht negativ zu beeinflussen, müssen neue
Technologien verwendet werden.
Ein Ansatz ist die Verwendung von gesticktem, elektrisch leitfähigem Garn. Dieses hat
die gleichen mechanischen Eigenschaften wie herkömmliche Garne und stellt damit
keinen Fremdkörper in einem Textil dar. Zusätzlich leitet es den elektrischen Strom und
kann direkt durch Sticken mit einem elektronischen Modul verbunden werden. Dadurch
dient das Garn als elektrischer Leiter für Signale und kann gleichzeitig eine Verbindung
zu elektronischen Modulen herstellen.
In vorangegangenen Forschungsprojekten wurde die Machbarkeit und Zuverlässigkeit
von gestickten Kontakten untersucht. Es hat sich gezeigt, dass das Ankontaktieren und
Verbinden von Substraten mit elektrisch leitfähigen Fäden eine mögliche Alternative zu
den herkömmlichen Verdrahtungstechniken darstellt. Allerdings zeigten sich starke
Schwankungen und auch Ausfälle der gemessenen Kontakwiderstände während der
Zuverlässigkeitstests.
Die vorliegende Arbeit befasst sich mit den Ursachen der Widerstandschwankungen
und Ausfälle von gestickten Kontakten. Ziel ist es, diese zu analysieren und gezielt
Verbesserungsvorschläge zu machen. Die erhöhte Zuverlässigkeit der verbesserten
Kontakte soll abschließend im Experiment validiert werden.
Anfangs werden die thermomechanischen und elektrischen Eigenschaften des
verwendeten Garns analysiert und der Zusammenhang beider Größen erläutert. Dies
geschieht auf Basis von thermomechanischen Analysen und Widerstandsmessungen,
sowohl bei konstanter Temperatur als auch im Temperaturzyklentest. Weiterhin werden
die gestickten Kontakte betrachtet. Das verwendete Messverfahren und ein
theoretisches Modell des Kontaktwiderstands werden vorgestellt. Um die
Einflussgrößen des gestickten Kontakts zu reduzieren, wird ein vereinfachtes
physikalisches Modell eingeführt. Beide Strukturen werden anschließend in
Temperaturzyklentests mit In-situ-Messsystemen untersucht. Die Ergebnisse des
Experiments zeigen, dass sich der Widerstand der Kontakte anfangs verbessert, dann
aber mit der Zeit verschlechtert. Die Degradation führt bishin zum Ausfall. Dies kann
auf die besonderen Eigenschaften des Garns zurückgeführt werden. Die beteiligten
VIII
Effekte eines negativen Temperaturausdehnungskoeffizienten, reversibler und
irreversibler Schrumpf sowie Kriech- und Relaxationseffekte sind dafür verantwortlich.
Es wird geschlussfolgert, dass die getesteten Kontakte keine ausreichende
Zuverlässigkeit in den Temperaturzyklentests zeigen. Auf Grundlage der gewonnenen
Kenntnisse werden verschiedene Verbesserungsmaßnahmen vorgeschlagen und deren
Zuverlässigkeit getestet. Abschließend wird gezeigt, dass Widerstandsschwankungen
durch die Verbesserungen sehr stark begrenzt werden können. Somit ist es möglich,
gestickte Kontakte mit hoher Zuverlässigkeit herzustellen.
IX
Table of Contents
Abstract
...
V
Kurzfassung
...
VII
List of Tables ... XI
List of Figures ... XIII
U
1
U
U
Introduction
U
... 1
U
2
U
U
Motivation
U
... 3
U
3
U
U
Components and Assembly of Test Structures
U
... 5
U
3.1
U
U
Components
U
... 5
U
3.2
U
U
Assembly
U
... 6
U
3.3
U
U
Embroidery Technology
U
... 9
U
4
U
U
Introduction to the Electrical Conductive Yarn
U
... 11
U
4.1
U
U
A Short Introduction to Fibers and Yarns
U
... 11
U
4.2
U
U
Fiber and Yarn Manufacturing
U
... 12
U
4.3
U
U
Fiber Formation and Molecular Fiber Models
U
... 13
U
4.3.1
U
U
Basic requirements for fiber formation
U
... 13
U
4.3.2
U
U
Molecular Models of Drawn Fibers
U
... 14
U
4.4
U
U
Shieldex Yarn
U
... 15
U
5
U
U
Physical and Electrical Properties of Shieldex
U
... 17
U
5.1
U
U
Shrinkage of Fibers
U
... 17
U
5.2
U
U
TMA of Shieldex
U
... 19
U
5.3
U
U
Stress-Strain Behavior
U
... 22
U
5.4
U
U
Creep and Stress Relaxation of Nylon Yarns
U
... 23
U
5.5
U
U
The Effects of Water on Fibers
U
... 25
U
5.6
U
U
Applications of Electrical Conductive Yarns
U
... 26
U
5.7
U
U
Making Fibers Conductive
U
... 26
U
5.8
U
U
Configuration of Shieldex
U
... 27
U
5.8.1
U
U
Electrical Resistance of Untreated Shieldex
U
... 30
U
5.8.2
U
U
Electrical Resistance of Treated Shieldex
U
... 31
U
6
U
U
Relation of Electrical and Physical Behaviors
U
... 33
U
6.1
U
U
Strain-Resistance Measurements
U
... 33
U
6.1.1
U
U
Compression of Fibers
U
... 35
U
6.2
U
U
The Effects of Temperature
U
... 36
X
U
7
U
U
Metalized Pad
U
... 41
U
8
U
U
Embroidered Interconnection
U
... 43
U
8.1
U
U
Geometrical Formation of the Interconnection
U
... 43
U
8.2
U
U
Measurement of Contact Resistance
U
... 45
U
8.3
U
U
The Electrical Contact Resistance in General
U
... 46
U
8.3.1
U
U
Contact Area
U
... 47
U
8.3.2
U
U
Contact Resistance
U
... 48
U
8.3.3
U
U
Contact Resistance of an Embroidered Interconnection
U
... 50
U
8.4
U
U
Embroidered Contact Model (ECM)
U
... 53
U
8.4.1
U
U
Assembly of the ECM
U
... 54
U
8.5
U
U
Experiment
U
... 56
U
8.6
U
U
Results of Embroidered Contacts
U
... 58
U
8.7
U
U
Results of the Contact Models (ECMs)
U
... 64
U
8.8
U
U
Explanation of the Yarn Loosening Effect
U
... 66
U
9
U
U
Encapsulation
U
... 69
U
10
U
U
Contact Improvements
U
... 71
U
10.1
U
U
Techniques and Experiment
U
... 71
U
10.2
U
U
Results
U
... 73
U
10.3
U
U
Other Techniques
U
... 78
U
11
U
U
Conclusion
U
... 79
References
...
81
Appendix A: Shieldex Technical Data Sheet ... 85
Appendix B: Recorded Contact Resistances ... 87
XI
51B
List of Tables
U
Table 1: Resistance measurements of Shieldex yarns and fibers
U
... 31
U
Table 2: CTE values of the used materials; values taken from [17] and [33]
U
... 42
U
Table 3: Minimal resistances of embroidered contacts at 85 °C; all values in [m]
U
... 63
U
Table 4: Mean value of all contacts and standard deviations of ICA improved contacts
in [m] at RT
U
... 74
U
Table 5: Mean value of all contacts and standard deviations of NCA improved contacts
in [m] at 85 °C
U
... 75
XIII
52B
List of Figures
U
Fig. 1: (a) Philips Emotions Vest; (b) Fully Integrated EKG-Shirt [1]
U
... 2
U
Fig. 2: Example of Embroidered Interconnections
U
... 4
U
Fig. 3: (a) Electronic module with assembled components; (b) SEFA thermopress
U
... 7
U
Fig. 4: (a) embossing tool; (b) laminated module on fabric; (c) embroidery machine; (d)
embroidered module; (e) hot-melt molding machine; (f) encapsulated module
U
... 8
U
Fig. 5: Stitch formation in embroidery
U
... 10
U
Fig. 6: (a) Melt spinning of Nylon from a polymer melt; (b) orientation of microfibrils
by drawing
U
... 12
U
Fig. 7: (a) Disordered arrangement of long-chain molecules in rubber. (b) Oriented
arrangement of molecules in stretched rubber.
U
... 13
U
Fig. 8: Fringed micelle structures: (a) in undrawn fiber; (b) in drawn fiber [11]
U
... 14
U
Fig. 9: Models of drawn Nylon: (a) by Hearle and Greer [12]; (b) by Murthy et al. [13]
U
... 15
U
Fig. 10: Overviews of Shieldex yarns. (a) Unprocessed yarn sample; (b) 3-times
embroidered yarn; (c) Cross-section view of fibers
U
... 16
U
Fig. 11: Changes in length of drawn Nylon 6.6 yarn with change in temperature [14]
U
. 17
U
Fig. 12: Model of drawn yarn [15]
U
... 19
U
Fig. 13: Model of annealed yarn [15]
U
... 19
U
Fig. 14: Change of length with temperature [15]
U
... 19
U
Fig. 15: TMA 1: Three heating cycles of the same Shieldex yarn sample
U
... 20
U
Fig. 16: TMA 2: Heating and subsequent cooling cycle of the same Shieldex yarn
sample
U
... 20
U
Fig. 17: Typical load versus strain curve of Shieldex yarn
U
... 22
U
Fig. 18: Stress-strain curves for high-tenacity nylon yarn [16]
U
... 23
U
Fig. 19: Stress relaxation of Nylon 6 in air at 65% r.h., 20 °C [19]
U
... 24
U
Fig. 20: Changes in length of drawn Nylon 6.6 yarn during treatment with water
U
... 25
U
Fig. 21: Cross-section view of the silver layer. (a) Explanation of the viewing angle of
(b) and (c). (b) Overview of the fiber with silver layer and platinum layer visible. (c)
Close-up view of `A'. Visible particles in the silver layer with measurements. Cosine
corrected scale values in y-direction.
U
... 29
U
Fig. 22: Electron microscope image of Shieldex surface
U
... 30
U
Fig. 23: Thread measurement tool (TMT)
U
... 32
U
Fig. 24: Test assembly for resistance-strain measurements
U
... 33
U
Fig. 25: Typical resistance versus strain curve of Shieldex yarn
U
... 34
U
Fig. 26: (a) Unstressed fiber; (b) Microcrack formation at silver layer under tension
U
... 35
U
Fig. 27: CTS climatic test chamber and in-situ measuring system
U
... 37
U
Fig. 28: Temperature profile of the climatic test chamber
U
... 38
U
Fig. 29: Temperature profile and Shieldex resistance in cyclic testing
U
... 40
XIV
U
Fig. 30: Cross-section view of copper metalized contact pad (left) and supporting FR4
board structure (right)
U
... 41
U
Fig. 31: Views of an embroidered contact: (a) stereomicroscopic side-view; (b) 3D X-
ray tilted cross-section view; (c) 3D X-ray side-view; (d) 3D X-ray top view
U
... 44
U
Fig. 32: Cross-section view of embroidered contact
U
... 45
U
Fig. 33: center: embroidered module and contact resistance measurement; top-left:
embroidered contact
U
... 46
U
Fig. 34: (a) Flat and rough surfaces prior to contact; (b) during contact; (c) visualization
of different contact areas during contact
U
... 47
U
Fig. 35: Schematic diagram of a bulk electrical interface
U
... 48
U
Fig. 36: Constriction resistance over number of a-spots
U
... 49
U
Fig. 37: Composition of measured contact resistance
U
... 50
U
Fig. 38: Schematic of fiber-to-fiber resistance
U
... 52
U
Fig. 39: Assembly of Contact Model; (a) cross-section view; (b) side view with
measurement
U
... 53
U
Fig. 40: Embroidered Contact Model; top left: assembly; top center: fixing Shieldex
with glue; top right: gold-Shieldex interconnection; bottom: model, mechanically fixed
and electrically connected for 4-wire measurement
U
... 55
U
Fig. 41: Embroidered test modules with different pad metallizations (fltr: Cu, Ag, Au)
U
56
U
Fig. 42: Contact resistance over time; contact #4 silver pad
U
... 59
U
Fig. 43: Contact resistance over temperature; contact #4 silver pad
U
... 59
U
Fig. 44: Contact resistance over time; contact #9 silver pad
U
... 60
U
Fig. 45: Contact resistance over temperature; contact #9 silver pad
U
... 60
U
Fig. 46: Detail A from Fig. 44; contact resistance and temperature over time; contact
#9 silver pad
U
... 61
U
Fig. 47: Yarn resistance and temperature over time
U
... 62
U
Fig. 48: Contact resistance over time of a ECM contact
U
... 65
U
Fig. 49: Contact resistance over temperature of a ECM contact
U
... 65
U
Fig. 50: Visualization of yarn loosening effect
U
... 66
U
Fig. 51: Gap formation at gold hot-melt interface of an embroidered TexFlexII contact
U
... 70
U
Fig. 52: Three different contact improvements
U
... 71
U
Fig. 53: (a) Resistance over time and (b) resistance over temperature of gold pad contact
#3 improved with conductive adhesive
U
... 73
U
Fig. 54: (a) Resistance over time and (b) resistance over temperature of silver pad
contact #11 improved with two-component adhesive
U
... 75
U
Fig. 55: Likely failure cause in laminated contact
U
... 77
U
Fig. 56: (a) Resistance over time and (b) resistance over temperature of polyurethane
laminated TexFlexII pad contact #10
U
... 77
1
Introduction
Merging electronics with textiles has become more and more serious in recent years.
Various companies have been working on this topic, and today some of the products are
already available on the market especially in the form of wearable electronics. However,
whether it is entertainment, navigation or medical electronics, state of the art technology
is still using conventional wiring techniques, i.e. cables, to interconnect single electronic
modules and components.
The Berlin Center of Advanced Packaging of the Technische Universität Berlin
(TU-Berlin) together with the Fraunhofer Institute for Reliability and Microintegration
IZM (FhG-IZM) has chosen a different approach. Instead of using conventional wiring,
actually integrating the electronics into the textile has become the objective. One way of
doing it, is laminating thin circuit boards onto fabrics and connecting them with woven
or embroidered conductors. Thereby, electronics and fabrics are being merged and
become an inseparable unit. The challenge, however, is to build high performance
electronics and conductors, which are small, light, and flexible, not restrain the textile
character and at the same time sustain the various conditions which act on textiles, such
as in wearing and washing.
Figure 1a is showing the Philips Emotions Vest, as an exemplary Smart Textile.
Conventional cables are used to connect the electronic modules. Compared to that,
Fig. 1b is showing the Fully Integrated EKG-Shirt which was developed at the
FhG-IZM
X
[1
X
]. Clearly visible are the embroidered wiring and embroidered
interconnections, which lead from the electronic module to the skin electrodes. These
are also made up of conductive embroidered yarn.
The technology of using conductive yarns for creating embroidered circuits
F
1
F
, and
moreover to establish an interconnection to an electronic module, was presented in
X
[2
X
].
The electronics were meant to become part of the textile that the person wearing the
clothes would benefit from the advantages of the electronics, not noticing their presence
while wearing them. In
X
[3
X
] the term overall textile character has been introduced. This
character needs to be preserved when integrating electronics into textiles. The biggest
advantage of this technology is to fabricate the textile wiring and the interconnection to
a contact pad of an electronics substrate on the same machine in the very same process
without any additional tools or materials. If the technology shows good results in terms
of reliability, performance and producibility, it will have the opportunity to become a
technology for cost-efficient large-scale production in the future.
1
Term was introduced in [2]
2
1 Introduction
(a)
(b)
Fig. 1: (a) Philips Emotions Vest
F
2
F
; (b) Fully Integrated EKG-Shirt
X
[1
X
]
2
Source: Philips Research Online
2
Motivation
The IEEE defines the term reliability as...
,,...the ability of a system or component to perform its required functions
under stated conditions for a specified period of time."
F
3
In embroidered circuitries, this function might be to transmit an electrocardiogram-
signal over an embroidered wire to an electronic module. If the connecting wire breaks,
the module fails or the electrical resistances rise to unacceptably high values then the
required function cannot be performed any longer.
To investigate the reliability of such new systems, a number of tests need to be
performed. Reliability testing of electronics is one of FhG-IZM's expertises for which
the guidelines are well known. For textile-integrated electronics, however, the
guidelines are different because these systems represent combination of electronics and
textiles. Therefore, reliability tests from both fields need to be included. The acting
forces are varying temperatures, humidity, water and mechanical forces as well as a
combination of them. Guidelines for reliability tests for embroidered circuits were
defined in
X
[3
X
] and shall not be explained here.
In previous research projects, the reliability of embroidered circuits has been
investigated. In temperature cycle tests, the relatively high failure rate of the
embroidered circuits was revealed. A failure occurred by definition if the contact
resistance increased more than a hundred percent from its original value. Since a
degradation of the yarn or failures of the electronic modules were not accountable for
the increase, the weak points are to be found in the interconnections themselves. These
are formed by two contact members namely an electrical conductive yarn and a
metalized contact pad. The yarn consists of 34 silver plated Nylon fibers. The contact is
made with a professional embroidery machine by stitching the needle through the
metalized pad on the circuit board and laying a loop of the conductive yarn around it.
X
Fig. 2
X
shows a detail of such an assembly. The green electronic module is fixed on a
blue fabric. The silver-colored yarn is forming loops by going through holes in the
copper metalized pads. The electrical contacts are formed at certain spots in the contact
area of yarns and pads.
3
IEEE Standard Glossary of Software Engineering Terminology
4
2 Motivation
Fig. 2: Example of Embroidered Interconnections
Hence, the objective of this study is to give an understanding of the reasons, which lead
to a contact resistance change in the embroidered interconnections. This is done by first
investigating the single components involved. The physical and electrical properties of
the electrical conductive yarn have to be analyzed in detail. A comprehensive
understanding of the behavior of both contact members due to changing conditions is
indispensible before the assembled contact can be analyzed. The causes for contact
resistance changes and even failures can be reasoned then from in-situ measurements of
embroidered contacts together with the knowledge of the behaviors of the single contact
members. A theoretical model of the measured contact resistance will be presented.
Analyzing the contributions to the total resistance value will make it easier to find ways
to decrease the contact resistance and to increase the reliability of the embroidered
interconnection.
3
Components and Assembly of Test Structures
The challenge of integrating electronics into textiles is to select components and
materials, which comply with the claimed electronic functionality and concurrently can
sustain typical factors which clothes are exposed to, e.g. washing and wearing. In the
following, a short description of the utilized components is listed. Then the assembly of
the analyzed test structures is explained. Moreover the used machines and processing
parameters are given.
3.1
Components
Electrical Conductive Yarn
The most important component and focus of this study is the electrical conductive yarn.
As it is one of the contact members it needs to have an adequate conductivity for the
electrical aspects and at the same time needs to be suited for processing on conventional
embroidery machines. It is transmitting the electrical signals and establishing the
electrical as well as mechanical contact to the module. As it turned out the yarn is a
fairly complex component. The electrical characteristics are directly linked up to the
mechanical properties of a typical Nylon yarn. Chapter 5 is focusing on the properties of
the electrical conductive thread used.
Electronic Module
The used electronic substrate is a test module that has been designed especially for
reliability tests of embroidered circuits (
X
Fig. 3
X
a). The internal name is TexFlex 3.0, but
will just be termed electronic module. The module has test structures as well as 14
embroidery pads at its edges. The test parts are two underfilled CSPs and a number of
dummy components, which can be measured in a daisy-chain to investigate the
reliability of onboard structures. The board is made of FR4 and has a total thickness of
540
. An important requirement is the contact pad thickness, which must not be
thicker than
150
to be able to stitch the embroidery needle through it. Hence, a
laminated design of multiple layers had to be used. A detailed description of the module
and contact pad can be found in Chapter
X
7
X
. Important for the shown assembly process is
the necessity of the module to adhere well to polyurethane (PU) and to a polyamide-
based hot-melt, which is later be used as an encapsulant. Furthermore, the metalized pad
must also show good adhesion qualities, which constrains the selection of possible
metallizations, as will be shown later.
6
3 Components and Assembly of Test Structures
Encapsulation
Various factors like temperature, humidity, water and mechanical forces are acting on
electronic circuits. Hence, the electronics and interconnections of the yarn as well as the
module need to be protected. For this purpose, different molding compounds can be
used as encapsulants. In the beginning a very stiff epoxy-based transfer mold compound
has been used, which was promising a high reliability. However, its drawbacks were
revealed later in the project, as because the molding process is pre-damaging the
conductive yarn due to high clamping pressures and processing temperatures. A gentler
encapsulation process, i.e. hot-melt encapsulation, replaced it, eventually. The big
advantages of hot-melt are the low costs of the raw material, the machines and low
power consumption. Injection molding machines are known to be expensive pieces of
equipment because of their purchase price and their maintenance costs. Additionally,
tools for hot-melt molding, which are made of aluminum, are much cheaper and easier
to manufacture. A detailed description of the pros and cons of both technologies has
been given in
X
[3
X
]. A challenge, however, is to find a protective material, which provides
good adhesion on both, the circuit board and the metalized pad areas. Problems, which
may occur due to bad adhesion on the contact pad, will be shown in Chapter
X
9
X
.
3.2
Assembly
The first step in assembling the test structures is to fix the electronic modules to pieces
of fabric (
X
Fig. 4
X
b). An adhesive layer does the fixation onto the fabric. The utilized
aramid fabric is called Nomex NX120T produced by DuPont. It is a high enduring and
temperature resistant material, which is also used in protective clothes for firefighters
and racing drivers. A laser cut foil of
100 polyurethane (PU) is used to bond the
module to the fabric. This is done with a thermopress HP45S from SEFA (
X
Fig. 3
X
b). It
applies adjustable pressure, and temperatures from its upper and lower plate. However,
an additional embossing tool had to be used to achieve a uniform pressure distribution
and to avoid damaging the delicate components. This tool is an aluminum milled
negative of the module topography (
X
Fig. 4
X
a). First, the embossing tool is placed onto the
plate; on top the electronic module with the PU foil and finally the fabric. A separating
foil protects the upper plate from contamination. The parameters for achieving the best
results have been determined through various experiments and are listed as follows. The
module is laminated with
3
plate pressure for
180 seconds. Temperatures of
240 ° and 105 ° are applied to the upper and lower plates, respectively. Two
modules are always processed at the same time to achieve a more uniform pressure
distribution.
3.2 Assembly
7
Once the circuit board is fixed to the fabric the embroidery can be done. For this
purpose, a professional embroidery machine (ZSK JCZ 01) has been used (
X
Fig. 4
X
c). The
layout is done with CAD embroidery software. Changes of the design can be made
easily as well as changing the number of yarn layers, which are embroidered. In the
research projects, the yarn was always embroidered three times. For most of the later
shown measurements, only one time embroidery has been chosen. An advancement of
embroidery, compared to weaving, is that any arbitrary two-dimensional design can be
made. An overview of the embroidery process and especially the stitch formation is
presented in the following chapter. As has already been said, the yarn is embroidered
through the metalized contact pads. The module is providing 14 of them. Measurement
pads on the module make 4-wire resistance measurements possible for each yarn-pad
interconnection.
At this point, the test structures are completely assembled for reliability tests of non-
encapsulated modules (
X
Fig. 4
X
d). Most of the experiments in this study are done in this
stage to investigate the direct behavior of yarn and module without any potential
influences of the encapsulant.
The optional step, which is necessary for application purposes, is the hot-melt
encapsulation. A special low-pressure molding machine (Optimel 550 by Optimel) is
used for this purpose (
X
Fig. 4
X
e). The molten hot-melt is injected in the cavity of an
aluminum tool through a nozzle. It is a thermoplastic polyamide, which can be re-
melted various times. As it cools to lower temperatures, its viscosity increases until it
becomes a rubbery solid. The final assembled module can be seen in
X
Fig. 4
X
f.
(a)
(b)
Fig. 3: (a) Electronic module with assembled components; (b) SEFA thermopress
8
3 Components and Assembly of Test Structures
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4: (a) embossing tool; (b) laminated module on fabric; (c) embroidery machine; (d) embroidered
module; (e) hot-melt molding machine; (f) encapsulated module
3.3 Embroidery Technology
9
3.3
Embroidery Technology
At this point reference shall be made to
X
[5
X
], which is explaining embroidery technology
in detail.
X
Fig. 5
X
is copied from the given reference and visualizes the stitch formation of
a sewing machine. However, the stitch formation in embroidery is the same, although
only one fabric layer is used. The upper layer can be thought of the contact pad, which
is embroidered to the fabric.
The stitch is formed by two threads, namely the needle and the under thread. In this
study, the electrical conductive yarn is used for both of the threads. Needle and under
threads are interlaced at each stitch. The position of the interlacing can be adjusted by
changing machine parameters. The stitch length can be defined in the design software.
As the needle punches a hole through the fabric (and contact pad) the thread is directed
by the needle. It is running through the needle hole and will follow the needle's
movements. At the lowest point, it is forming a loop, which is guided around the spool
case. When the take-up lever is moving upwards, the under thread is being pulled into
the fabric and the stitch is tightened. However, the tightening is only done by interlacing
both threads. Hence, they are still moveable to some extent. The pulling force is applied
by the take-up lever and the counterforce is applied by the brake of the under thread
spool. After that, the fabric is fed forward and the next stitch is made. When the
embroidered contact is formed, the fabric is not forwarded but it is fed backwards after
the stitch has been made. This means that a single-time embroidery actually is laying
two yarns on the top and two on the bottom side of the pad.
In this process, yarn is guided through the embroidery machine along different metal
parts. Some of them are fixed others are moving. Hence, strong bending forces are
acting onto the yarn. Then the yarn is fed through the needle hole. During embroidery,
the yarn is first pulled downwards by the needle and by the spool's hook and then
upwards by the take-up lever. All these different mechanical forces are acting onto the
yarn and may lead to alterations. The frictional forces may peel off parts of the
conductive silver layer and the strong bending of the yarn may pre-damage the silver.
The breakage of fibers can also not be excluded. The applied tensions will also lead to
crack formation in the silver layer, as shown in chapter
X
6.1
X
. Another crucial part is the
needle, which can pierce whole fibers and thus impair or destroy their conductivity.
10
3 Components and Assembly of Test Structures
Fig. 5: Stitch formation in embroidery
F
4
The forces that are applied to form the loop can be adjusted by breaks. Two breaks act
on the needle thread and one on the under thread, respectively. Stronger breaks may
lead to a tighter loop but the forces acting on the yarn are simultaneously higher and
may even lead to yarn breakage. By adjusting the breaks one can decide at which
location the interlacing should happen. Additionally, the subjective appearance of the
embroidery can be tuned. However, quantitative adjustments and measurements of the
forces of an embroidered loop are difficult to make. Further investigations in this field
need to be done. The later mentioned effects of stress-relaxation should also be
considered.
4
modified from [5], chapter 7.2.10, "Lockstitch (1)"
4
Introduction to the Electrical Conductive Yarn
As has been said, the wiring as well as the interconnection has been made by an
electrical conductive yarn. Hence, it is one of the contact members of the
interconnection, which is investigated in this study. The first part of this chapter focuses
on the physical behavior of the yarn due to the changing environmental conditions. The
coefficient of thermal expansion (CTE) is of particular interest. Furthermore,
technologies for making fibers conductive as well as their applications are presented.
Then the conductivity of the particular yarn used is investigated. Changes of
conductivity by influencing variables are explained as well. Of particular importance is
the linkage between physical and electrical behaviors. The effects of one another are
investigated as well as the relation to a changing contact resistance. However, primarily
a short introduction to fibers in general and definitions of the used terms shall be made.
4.1
A Short Introduction to Fibers and Yarns
The word fiber (or British English: fibre) which is often used interchangeably with
filament is defined by the Textile Institute
X
[6
X
] as
,,a unit of matter characterized by flexibility, fineness and a high ratio of
length to thickness."
A yarn or a thread is defined as
,,a textile product of substantial length and relatively small cross-section
and that consists of fibers or filament(s) (or both) with or without twist."
Fibers in general are divided into two groups. Natural fibers are made of plants, animal
hair or minerals. Man-made or synthetic fibers on the other hand are made of natural or
synthetic polymers or even inorganic materials.
F
5
F
Since the 1950s synthetic fibers have
been in major use for general textiles
X
[7
X
]. They can be tuned for specific requirements
of the application. Well known have been fibers under the name of Nylon, which is
made of the polymers Polyamide 6 or Polyamide 6.6. The fibers of the herein used
conductive yarn are based on a Polyamide 6.6 core material. Because literature exists of
that particular material, some of the known properties will be used. However, fibers
consisting of the same material but from different manufacturers may vary in some of
the properties due to different processing parameters. For extensive information about
Nylon and other synthetic fibers
X
[7
X
] shall be suggested.
5
in [5], chapter 1.1, "Overview"
12
4 Introduction to the Electrical Conductive Yarn
4.2
Fiber and Yarn Manufacturing
Synthetic fibers are formed from molten polymer solutions. These are made through
two processes namely 1) synthesis of reactive precursors and 2) polymerization.
F
6
F
To
form the fibers, the fluid polymer is being pressed through a special nozzle, the so-
called spinneret. Due to the simultaneous cooling, the fibers become a solid of more or
less disordered molecules. The drawing process, however, orders the microfibrils in the
direction of the filament axis. Thereby the fibers become thinned and a mixture of
crystalline and amorphous regions is formed. Drawing is the process that gives the
filament its strengths. By using differently shaped spinnerets, different fiber cross-
sections can be formed, e.g. circular, triangular or quadratic.
Because a single fiber might not have the required strength or elasticity, yarns can be
formed from a number of fibers. The so-called open yarns consist of a number of fibers
in parallel. In this case, the total strength is equal to the sum of the strengths of every
single fiber. The twisting process, however, increases the strength of the yarn. A twisted
yarn will have a higher strength than the sum of strengths of the single fibers.
F
7
F
Weak
places in single fibers can be supported by neighboring fibers due to transverse
compressive forces. Twisting can be done either in the Z-direction, which is
synonymous to right-handed or in the S-direction which is left-handed. The twist level
adjusts the flexibility and is defined as the turns of twist per unit of length, e.g. turns per
meter.
(a)
(b)
Fig. 6: (a) Melt spinning of Nylon from a polymer melt; (b) orientation of microfibrils by drawing
F
8
6
in [5], chapter 1.3.2, "Fibre-forming Materials"
7
in [7], chapter 14.4, "Composite-specimen effects"
8
copied from [5], chapter 1.3, "Man-made Fibres"
4.3 Fiber Formation and Molecular Fiber Models
13
If an improved strength and regularity is required, two or more yarns can be folded (or
plied) together. This is basically the same process as the twisting of fibers unless yarns
are used. Combinations of differently twisted yarns can be made, which changes the
physical properties and appearance of the final yarn. The combination of the very low
thickness together with their high flexibility and strength make man-made fibers an
outstanding material.
4.3
Fiber Formation and Molecular Fiber Models
The particular ratio of fiber-length to thickness is achieved by drawing. However, this
process does change the internal structure of the core material. Hence, synthetic fibers
will show different behaviors compared to their undrawn bulk materials. Furthermore, it
is worth noting that fibers from the same base material but drawn with different draw
ratios will show wide differences in their physical properties, as shown in
X
[8
X
].
4.3.1
39B
Basic requirements for fiber formation
Looking at the set-up of natural yarns is making it easier to understand the basic
requirements for forming a fiber from linear polymers. Cotton fibers generally have
lengths between
20
and
40
.
F
9
F
A cotton ball does not have any substantial
strength because the very fine fibers are unarranged and tangled up, similar to the
disordered arrangement of long-chain molecules in rubber (
X
Fig. 7
X
a). To form a yarn of
continuous strength out of it, the spinning process is an important step. Spinning is
giving the fibers a more or less parallel order and inserts twist (
X
Fig. 7
X
b). That gives
lateral compression to the fibers and results in frictional forces between them, which
then lead to a binding of fibers. From this knowledge, the following requirements can
be derived to form man-made fibers. The fiber must consist of more or less parallel
(a)
(b)
Fig. 7: (a) Disordered arrangement of long-chain molecules in rubber.
(b) Oriented arrangement of molecules in stretched rubber.
F
10
9
in [5], chapter 1.2.1, "Vegetable Fibres: Cotton (2)"
10
in [7], chapter 1.1.2, "Intermediate bonds: hydrogen bonding"
14
4 Introduction to the Electrical Conductive Yarn
arranged long-chain molecules. Forces must exist between these molecules to give
cohesion to the fiber. However, the long-chain molecules must be moveable to a certain
extent to ensure the flexibility of the fiber.
F
11
4.3.2
40B
Molecular Models of Drawn Fibers
To describe the exceptional physical behavior and especially the thermomechanical
responses of Nylon fibers one has to focus on the manufacturing process. Drawing the
fibers is the crucial process that leads to a completely different behavior compared to
bulk polymers. To understand what is happening during the drawing process, it is
important to consider the molecular fiber structure. X-ray diffraction analyses have
shown that fibers consist of partially oriented, partially crystalline, linear polymers.
From these results, a large number of models evolved for describing their molecular
structure
X
[10
X
]. The crystalline regions are thought of as rigid, space-filling parts,
whereas the non-crystalline regions account for the particular physical behavior of
fibers. In that anisotropic model, crystals would thus represent rigid node elements that
are interconnected by non-crystalline chain segments.
X
Fig. 8
X
represents an early fringed-
micelle model after crystallization from a melt (a), and after it has been drawn (b) to an
oriented fiber
X
[11
X
]. As was investigated later, this model is more relevant for stiffer
polymers than Nylon, but it does suffice to understand the orientation process while
drawing. Working models for Nylon 6.6 and Nylon 6 were proposed by Hearle and
Greer
X
[12
X
] and Murthy et al.
X
[13
X
], respectively, which take into account the findings of
folded chain molecules.
(a)
(b)
Fig. 8: Fringed micelle structures: (a) in undrawn fiber; (b) in drawn fiber
X
[11
X
]
11
in [7], chapter 1.3.1, "Requirements for fibre formation from linear polymers"
Ende der Leseprobe aus 112 Seiten
- nach oben
Details
- Arbeit zitieren
- , 2009, Analysis of Contact Resistance Change of Embroidered Interconnections, Hamburg, Bedey Media GmbH, https://www.diplom.de/document/227390
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