conduction mechanisms in conductive adhesives with metal-coated polymer spheres

202 Pages·2016·29.21 MB·English

by  Sigurd Rolland Pettersen

Doctoral theses at NTNU, 2016:349
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ISBN 978-82-326-2040-1 (printed ver.)
ISBN 978-82-326-2041-8 (electronic ver.)
ISSN 1503-8181
Doctoral theses at NTNU, 2016 NTNUNorwegian University ofScience and TechnologyThesis for the Degree ofPhilosophiae Doctoreering Science and Technologyment of Structural Engineering
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Sigurd Rolland Pettersen
CONDUCTION MECHANISMS IN
CONDUCTIVE ADHESIVES WITH
METAL-COATED POLYMER
SPHERES
Thesis for the Degree of Philosophiae Doctor
Trondheim, December 2016
Norwegian University of Science and Technology
Faculty of Engineering Science and Technology
Department of Structural Engineering
NTNU
Norwegian University of Science and Technology
Thesis for the Degree of Philosophiae Doctor
Faculty of Engineering Science and Technology
Department of Structural Engineering
© Sigurd Rolland Pettersen
ISBN 978-82-326-2040-1 (printed ver.)
ISBN 978-82-326-2041-8 (electronic ver.)
ISSN 1503-8181
Doctoral theses at NTNU, 2016:349
Printed by NTNU Grafisk senter
Preface
This doctoral thesis is submitted to the Norwegian University of Science and Technology
(NTNU) in partial fulfillment of the requirements for the degree of philosophiae doctor (PhD).
The thesis comprises an introductory section, five journal papers (three published and two
submitted), and one international conference paper.
The work presented in this thesis was conducted in the period between August 2013 and
September 2016 at NTNU Nanomechanical Lab (NML), Department of Structural Engineering
(IKT), Faculty of Engineering Science and Technology (IVT), NTNU, Trondheim, Norway.
Associated Prof. Jianying He (NTNU NML) has been my main supervisor during this work.
Prof. Zhiliang Zhang (NTNU NML) and Adjunct Prof. Helge Kristiansen (NTNU NML, and
Conpart AS, Skjetten, Norway) have been co-supervisors.
I have conducted much of the experimental work of this thesis outside of our department and
university. Sample preparation, environmental testing, and bulk electrical characterization have
been conducted at Conpart AS and Lindberg & Lund AS (Vestby, Norway). Much of the single-
particle characterization was performed at NTNU NanoLab, the advanced clean-room facilities
at NTNU. Ion milling and scanning electron microscopy imaging have been conducted at the
Paper and Fibre Research Institute (PFI, Trondheim, Norway) and at the Electron Microscope
Laboratories, Department of Materials Science and Engineering (IMT), Faculty of Natural
Sciences and Technology (NT), NTNU. Some thermal measurements were conducted at the
Department of Energy and Process Engineering (EPT), IVT, NTNU. In February and March
2015 I went on a research stay at the Institute of Scientific and Industrial Research (ISIR),
Osaka University, Japan, which was a greatly rewarding experience, both academically and
culturally.
The research was performed within the user-driven research-based innovation (BIA) project
“Novel Conductive Adhesive Technology Platform for Solar Industry” (pSiCAT, Grant No.
225962/E20), in cooperation with Mosaic Solutions AS, Skjetten, Norway as well as other
industrial partners, with financial support from the Research Council of Norway.

Sigurd Rolland Pettersen
Trondheim, October 2016
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Abstract
Isotropic conductive adhesives (ICAs), i.e. polymer adhesives with a percolating network of
filler particles embedded for conductivity, have been used as alternatives to soldering in
electronic devices for decades. More recently, the photovoltaic (PV) industry have begun to
show major interest in ICAs, as this is a low-temperature alternative to lead-free solders. Lower
processing temperatures decrease solar cell damage and allows for the use of thinner wafers,
which can be a significant contribution to the cost-reduction of PV modules. However,
conventional ICAs contain large amounts (> 25 vol%) of silver, and are thus expensive.
The goal of this thesis has been to investigate the mechanisms controlling electrical and thermal
conduction in a new type of ICAs, developed by our industrial partners Conpart AS and Mosaic
Solutions AS. By replacing the micron-sized silver flakes and solid particles of conventional
ICAs with micron-sized polymer spheres coated with silver thin films (AgPS), a conductive
network can be formed throughout the adhesive matrix at silver contents in the range of ~ 0.5
– 5 vol%. The similar thermomechanical properties of the polymer cores and adhesive matrix,
which together comprise > 95 vol% of the ICA, can improve the reliability of the interconnects.
A major disadvantage with these ICAs is that they typically yield higher resistivities than
conventional ICAs, although the silver is more efficiently used in the former. By understanding
the factors limiting the electrical and thermal conductivities of the ICAs, strategies for
improving these can be devised.
In this work, we have developed two distinct methods for investigating the electromechanical
properties of individual AgPS. A major finding has been that for films thinner than ~ 100 nm
the electrical resistivity of the silver films increase drastically. We have discovered that when
incorporated into epoxy adhesive, the AgPS can form metallic contacts even when cured at
room temperature, and that these are sintered into metallurgical contacts at a curing temperature
of 150 °C. We found that the grains of the silver films partake in an electrochemical, size-
related growth process when the ICAs are exposed to elevated heat and humidity. This growth
process has a positive impact on conductivity of the ICAs in the absence of a bias voltage, and
is amplified by lower cross-linking of the epoxy matrix. We also investigated the thermal
properties of AgPS-based ICAs, and found that the obtained values were heavily dependent on
the choice of measurement method. The work presented in this thesis is expected to have a large
impact on the further development of these ICAs, especially for use in PV modules.
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Acknowledgements
I would not have gotten through the many ups and downs that are an inherent part of taking a
PhD without the massive support of good colleagues, friends, and family.
I would first like to thank my main supervisor, Jianying, for her enthusiastic support,
friendliness and professional guidance, which have been of great value for me in my
development as a scientific researcher. Zhiliang’s support and advice, combined with his solid
and fair leadership of our group have been very important for my work. Helge has been a never-
ending source of information and interpretation, and without his guidance and deep knowledge
of the field, the work in this PhD would not have been feasible. I am not a man of few written
words, and I sincerely appreciate the many hours my supervisors have spent reading through
long emails and manuscripts, always replying with good advice and important input.
I would also like to thank my colleagues and friends in our group and at the Department of
Structural Engineering for interesting and enlightening conversations during lunch and coffee
breaks, group meetings, salary beers, and trips, which have given me both social and academic
support to get through my PhD. Of special importance are Molly and Emil, who have been great
research partners in addition to friends. Without their contributions, this PhD thesis would have
been two papers shorter.
The last three years have lead me outside my home department on many occasions, and many
people have given vital contributions to the work in this PhD. Keith, Susanne, Tore, Erik, and
John have all helped with sample preparation, characterization, analysis, and advice, and are
gratefully recognized for their contributions. I thank Per Olav, Gary, Ken Roger, Yingda, and
Ignat for giving me access to and professional training on many of the advanced
characterization instruments I have used in this work. I would give a special thanks to Shijo,
who gave me the opportunity of a lifetime by inviting me to Osaka and finding time in his busy
schedule to be a warm and welcoming host.
I also give my heartfelt gratitude to my family; my mother, father and brothers for carrying me
to the point in my life where I was able to pursue a PhD. My family-in-law have been a great
contribution to my life over the last soon-to-be decade. At last, but most important, I thank my
fiancée Silje for all the love, support, frustrations and joy she has given and continues to give
me. You are a highly loved reminder that there are more important things in life than work.
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List of papers
The following publications are included in this thesis. Papers I and II are describing
experimental methods developed for investigating electrical and mechanical properties of
individual metal-coated polymer spheres (MPS). In papers III-VI we are investigating the
electrical and thermal conduction mechanisms of bulk isotropic conductive adhesives (ICAs)
with silver-coated polymer spheres (AgPS).
(I) Electromechanical characterization of individual micron-sized metal coated
polymer particles
Molly Bazilchuk, Sigurd R. Pettersen, Helge Kristiansen, Zhiliang Zhang, and Jianying
He.
Journal of Applied Physics, 2016, 119(24), 245102.
I developed the experimental setup presented in this study from an earlier setup in which
the electrical and mechanical measurements were conducted separately by different
instruments and software. M.B. performed the experiments and wrote the paper. All
authors contributed in revision and discussion of the paper.
(II) Electrical four-point probing of spherical metallic thin films coated onto micron
sized polymer particles
Sigurd R. Pettersen, August Emil Stokkeland, Helge Kristiansen, John Njagi, Keith
Redford, Dan V. Goia, Zhiliang Zhang, and Jianying He.
Applied Physics Letters, 2016, 109(4), 043103.
I had the initial idea of using the miBots to conduct four-point measurements on MPS,
and conducted proof of concept studies. A.E.S. conducted the miBots measurements in
the paper as a part of his master thesis. I created the finite element (FE) model,
performed the analysis, and wrote the paper. H.K., Z.Z., and J.H. helped to develop the
method and came with input on the analysis. J.N., K.R., and D.V.G. prepared the AgPS
and performed the x-ray diffraction (XRD) measurements. All authors contributed in
revision and discussion of the paper.

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