Nanotechnology
Institute of Physics and Nanotechnology
Skjernvej 4A, 9220 Aalborg Ø
Phone 99 40 92 15, Fax 99 40 92 35
www.nano.aau.dk
Title:
Failure Mechanisms in Metalized Film
Capacitors
Project Period:
8. Semester, Spring 2014
Project Group:
5.230
Group Member:
Anders Tang
Supervisors:
Kjeld Pedersen
Dennis Achton Nielsen
Special Thanks To:
Dorthe A. Rasmussen
Danfoss
Number Printet: 4
Number of Pages: 41
Closing Date: 31-05-2014
Abstract:
The aim of this report was to investigating the most
common failure mechanisms of metalized film capacitors. This was done by first investigating polypropylene, BOPP and the degradation here of. TThis was
then coupled to the metalized film capacitor and the
electrical and thermal induced degradation. The selfhealing process and the corrosion of electrodes were
investigated to understand the mechanisms leading to
different failures.
It was concluded that the two major graceful aging
mechanisms were self-healing and corrosion. The
capacitor investigated suffered a heavy loss in capacitance and it was concluded that this was due to
the heavy corrosion of the metalized film. The selfhealing was not detected insinuating that electrode
corrosion inhibits the self-healing process. IR measurements was used to examine the degradation of
the polypropylene film and it was concluded that the
polypropylene film had not undergone any degradation as a result of the accelerated aging process.
Preface
This report is written by group 5.230 which consists of a nanotechnology student at the 8th semester at
Aalborg University. The report is a project running from February to May, 2014.
Reading Guide
Throughout the report, references to different sources will be made; these will be found on the form [#]
where the number in the square brackets refers to a specific source in the bibliography at the end of the
report. In the bibliography the sources will be listed with its title, author, and other relevant information
depending on whether the source is a book, article, or web page. If the bibliographic references are listed
after a specific section it indicates that the reference applies to all of the above if nothing else is stated.
Tables and figures are numbered after the number of the chapter in which they are placed. Hence the first
figure in chapter 4 would be named ’Figure 4.1’ whereas the next one would be ’Figure 4.2’ etc.. Since
tables are numbered according to the same system, it is possible to find both ’Table 4.1’ and ’Figure 4.1’
in the same chapter. For each figure/table there is a short descriptive caption that will be made together
with a bibliographic reference where necessary.
There is a CD attached to the back cover of the report. This CD includes an electronic version of the
report, raw data and pictures.
In addition, some abbreviations are made consequently throughout the report; these constitute of the
following:
IR:
BOPP:
ESR:
Infrared
Biaxially Oriented Polypropylene
Equivalent Series Resistance
Aalborg University, May 2014
Anders Tang
v
Contents
1
Introduction
2
Basic Theory
2.1 Polypropylene . . . .
2.2 Film capacitors . . .
2.3 Failure mechanics . .
2.4 Infrared spectroscopy
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3
Materials and Methods
17
4
Results and Discussion
4.1 Polypropylene Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
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5
Conclusion
5.1 Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
28
Bibliography
29
A Appendix
33
vii
CONTENTS
viii
Introduction
1
The introduction will briefly investigate the different basic types of capacitors, such as the ceramic, the
electrolytic and the film/foil capacitor and establish an aim for this project.
The reliability of capacitors have always been an important issue ever since the first applications of capacitors. There is a natural desire to improve the device, since an increase in reliability and detection of
device failures and a thereby incease in reliability, would greatly reduce the cost. In recent years there
have been a great increase in reliability of power film capacitors. For example, the average failure rate
for power capacitors have dropped from 1000 failures in time (FITs1 ) in 1995 to 20 FITs in 2000 [1].
An industrial survey was done in 2011 by Yang et. al. [1]. Based on the survey they concluded that
capacitors were regarded as the devices which were most fragile, but of course the fragility is also application dependent. They also concluded that the main stresses to the capacitor comes from environmental
conditions such as temperatures, mechanical vibration, and moisture. They did, however, not investigate
the influence of voltage stress on the components, which has a high influence on the reliability of the capacitor. It is therefore of interest to investigate the reliability of the capacitor and how the voltage affects
the component. [1]
A capacitor is defined as a passive electrical component, that stores electrical charge for releasing it at
a specific time or rate. There are different kinds of capacitors. In this section the basics of electrolytic,
ceramic and film/foil capacitors will be described.
The electrolytic capacitor is a capacitor that utilizes an electrolyte to store charge and an oxide layer as
the dielectric material. It is used to achieve a large capacitance. Electrolytic capacitors are constructed
from two conducting foils, one which is coated with an insulating oxide layer, and a spacer soaked in
the electrolyte. The foil insulated by the insulating oxide layer is the anode while the liquid electrolyte
and the second foil acts as the cathode see figure 1.1(a). This capacitor type is polarity dependent, and
therefore primarily used in DC circuits. If the DC voltage is applied incorrectly the insulating oxide layer
is broken down and will damage the capacitor. Some of the drawback for this type of capacitor is the
large leakage current and the limited lifetime.
The ceramic capacitor uses a ceramic material as the dielectric. They are made by coating two sides of
a small ceramic disc with a metal and then stacking them together to make a capacitor see figure 1.1(b).
Ceramic capacitors are usually made with very small capacitance values. The ceramic capacitor is not
1
1 FIT = 1 failure per 109 device hours.
1
CHAPTER 1. INTRODUCTION
polarity dependent and can therefore handle an AC current. The ceramic capacitor is very stable regarding temperature.
Film/foil capacitors basically consist of two metal electrodes separated by a dielectric material such as
polypropylene or another insulating material see figure 1.1(b). Film/foil type capacitors are available in
capacitance ranges from as small as 5 pF to as large as 100 µF. They operate well under high temperature2 , have long service life and high reliability. The film/foil capacitors undergo what is referred to as
a graceful aging, where the capacitor does not have a single point failure but rather loses the capacitance
slowly due to different mechanisms. [2] [3]
Figure 1.1: (a) Illustration of an electrolytic capacitor. (b) Illustration of a ceramic/film capacitor.
1.0.1
Project Goal
The aim of the report is to investigating the most common failure mechanisms of film capacitors and what
environmental factors influence the capacitor and how they affect it. It is of interest to investigate how the
film capacitor handles the stress that is applied to it, and what mechanisms are utilized to ensure a graceful
aging. The capacitors will be investigated using microscopy, optical spectroscopy, and IR spectroscopy.
2
Depending on the dielectric material.
2
Basic Theory
2
This section will look at polypropylene and the structure of polypropylene. It will also describe the degradation process of polypropylene due to electrical and thermal stress. This is done to better understand
the mechanisms for the graceful aging and to better understand the capacitor design. The fabrication
process of biaxially oriented polypropylene (BOPP) will also be described to better understand why this,
and not regular polypropylene, is used in capacitors.
2.1
Polypropylene
Polypropylene is a thermoplastic material, which is used in many different commercial applications including food packaging, textiles, plastic parts, reusable containers of various types, laboratory equipment
and as dielectric material in film capacitors.
Polypropylene can be found in three different configurations, the isotactic, the syndiotactic and the irregular atactic configuration. These configurations are defined by the position of the pendant methyl group
in relation to the backbone. In the isotactic configuration all methyl groups are on the same side of the
polymer backbone. In the syndiotactic configuration every second methyl group is on the opposite side
of the backbone, and in the atactic configuration the position of the methyl groups is random, as can be
seen in figure 2.1. [4] [5]
The isotactic configuration is a regular and repeating arrangement, which leads to a high degree of crystallinity. A high crystallinity makes the structure very stiff and gives it a high tensile strength. The
syndiotactic configuration is less stiff than the isotactic configuration, which gives the structure a better
clarity and higher impact strength. The atactic configuration is highly irregular, making the crystallinity
low and the material sticky. The three configurations can be mixed to change the properties of the material. If for example atactic polypropylene was added to a mixture with isotactic polypropylene, it would
result in an increase in impact strength and stretchability, and decrease stiffness and color quality. [4] [5]
When the polymer chain crystallizes it forms a helical arrangement, like a winding staircase, due to the
methyl group. The helix chain appears in four different chiralities. Depending on the turning direction
of the helix spiral and the direction of the methyl groups, two right handed and left handed helices in
which the methyl groups are pointing either upwards or downwards are found. The configuration of the
3
CHAPTER 2. BASIC THEORY
Figure 2.1: Illustration showing how the methyl groups are placed, in relation to each other and the
backbone, in each polypropylene configuration. [6]
polypropylene chain affects the crystallization and crystal form. The helix spiral folds back and forth to
form thin, ordered, plate-like structures called lamellae. These lamellae combines to form spherulites,
see figure 2.2. The spherulites vary in size, from 1-50 µm. [4] [7]
Figure 2.2: Illustration of polypropylene crystallinity and how the lamellae sits inside the crystal. It can
be seen how some of the regions are amorphous and some are crystalline. [8]
The atactic polymer can not be crystallized since the structure lacks regularity. The syndiotactic polypropylene is orthorhombic and the chains in the crystal lattice take a planar zigzag conformation. The isotactic
polypropylene has three different crystal forms, monoclinic α-, orthorhombic γ- and hexagonal β-forms.
These forms depend on temperature, pressure and mechanical stress state during crystallization. The αform is the most common form obtained under normal processing conditions. The β-form can be found
at low crystallization temperatures or in the presence of nucleations agents. The γ-form is only found
4
CHAPTER 2. BASIC THEORY
under high pressure in high molar mass polypropylene. Polypropylene samples with low molar mass or
low tacticity crystallize partially in the γ-form. [4] [5]
2.1.1
Degradation of polypropylene
A material is only good as long as it retains the properties of the material. If it unintentionally loses
its useful properties it is no longer of value. The degradation of most polymers manifests itself in a
deterioration of the mechanical characteristics, in the appearance and growth of cracks on the surface
and in discoloration. The most typical reason for degradation of polypropylene is caused by physical
or chemical stress. Polypropylene consists of long carbon chains, which gives it its unique properties.
When it degrades the chains are scissored or cross-linked and in this way the structure is altered, thereby
changing the properties and gradually leading to deterioration. [9]
High electric stress can cause degradation in polymer films [9]. When exposed to electrical stress, the
chains in polypropylene are bombarded or otherwise affected by electrons, which leads to randomly
scissored chains thereby lowering the molecular weight. The chemical process that leads to degradation
of polypropylene due to electrical stress is suggested by Liufu et. al. [10] to be as follows:
The first and second reaction are suggestions of how the first step of the trap creation reaction could take
place. In the first reaction the bond between the tertiary and secondary carbon are split, resulting in two
separate chains.
In the second reaction, which is another possible first step in the trap creation, the tertiary carbon may
dissociate to radicals, this is suggested since the tertiary carbon atom is more lose bound than the others.
The formation of radicals due to electrical stress was suggested by Kao [11].
The following reactions are suggestions to how the radicals could interact.
When the radicals have been formed, it may result in scissoring of the chain, due to disproportionation of
the free radical, which means that a double bond will be formed along with leaving a radical.
The radicals also have the possibility to bond to another short chain, thereby creating smaller chains.
Another problem is the electron in the terminal group and the unsaturated bond, which creates traps that
trap electrons.
5
CHAPTER 2. BASIC THEORY
Traps are interesting since the rate of degradation, of the stressed polypropylene, is dependent on the
concentration of traps. The traps already created can lead to a breaking of carbon chains thus creating
more traps which accelerates the trap creation. As more traps are created the polypropylene structure is
degraded, thereby lowering the molecular weight. This also results in a decrease in dielectric breakdown
strength [12]. [10]
The thermal degradation of polypropylene follows a simple basic pattern, where the thermal degradation
is a auto-oxidation process. When polypropylene is thermally degraded, the same step happens as above:
A hydrogen atom is removed from the backbone, typically from the tertiary carbon atom. This creates a
free radical and a free hydrogen. The free radical may then react with oxygen to form a peroxy radical,
which can then remove a hydrogen atom from another polymer chain, to form a hydroperoxide and then
a new free radical is created. The hydroperoxide can then form new free radicals and since it is easy
to remove hydrogen from the backbone, due to the tertiary carbon, polypropylene is very sensitive to
thermal degradation. [13] [14]
2.1.2
Biaxially Oriented Polypropylene
The polymer described in the previous section is normally referred to as homopolymer polypropylene.
It is not strong enough to handle the thermal and electrical stress, which it would experience in normal
film capacitors. Therefore a film production process has been developed which is capable of changing
the properties of the capacitor film. The two most used processes are a tenter frame process and a bubble
process. These two processes have the same basic steps. The first step is extrusion of a several millimeters thick tube (bubble line) or sheet (tenter frame line). In the next process step this tube or sheet is
heated above the softening but below the melting temperature. The polymer is then exposed to biaxial
orientation, in the machine (rolling) and the cross direction of the film. At this step the final thickness of
the film is adjusted. The final film is cooled and cut to the roll size used for the application. [15]
The tenter and bubble process are different processes, and the difference depends on how the stretching
is done. In the bubble process, the polypropylene tube is heated and warm air is blown into the tube. The
pressure of the air forces the tube to expand. At the same time the speed of the rolls in the production line
is adjusted so that an orientation in the machine direction is achieved. In the tenter process, the orientation
of the film in the machine direction is obtained in the same way. The orientation in the cross direction is
caused by tension clips that grip the film edges. The clips are running along divergent baths and the film
is progressively stretched along the line when the clips diverge. The obtained polypropylene film from
both lines is biaxially oriented and often called BOPP film. [15]
The stretching during crystallization changes the morphology of the film as can clearly be seen in figure
2.3. It goes from the normal spherulitic form to a fibrillar structure. The fact that the film changes
morphology brings several changes to the film, such as higher tensile strength for some thicknesses,
6
CHAPTER 2. BASIC THEORY
Figure 2.3: Pictures showing the morphology difference between the two BOPP films. Film A is fabricated
using the bubble process and film B is fabricated using the tenter process. [16]
improved optical properties, making the film harder to stretch, and better water and gas resistance. These
properties can be controlled by the fabrication process. [9] [15]
7
CHAPTER 2. BASIC THEORY
2.2
Film capacitors
This section will be focusing on metalized film capacitors. It will describe the design and fabrication
process. It will also explain some of the concepts of lifetime for a metalized film capacitor.
Metalized film capacitors are capacitors which are generally made out of a dielectric material covered with
a thin metallic layer, which is then wound into a cylindrical shaped winding, it then gets terminals attached
to the ends and is encapsulated. Metalized film capacitors are used in many applications because of their
stability, low inductance and low cost. There are many types of metalized film capacitors, eg. polyester
film, polypropylene film and PTFE film capacitors. The fundamental difference in these capacitor types
is the dielectric material, and it is changed based on the application, eg. PTFE film capacitors are mainly
used in military and in aerospace equipment, due to the films heat-resistant capabilities and low leakage
current. This section will look at metalized film capacitors that uses polypropylene for the dielectric
material.
Polypropylene is one of the most used materials in film capacitors. This is due to the higher dielectric
breakdown strength, than other materials. Examples of other materials can be found in table 2.1. [17]
Capacitor Type
ε
Polypropylene (PP)
Polycarbonate (PC)
Polyester (PET)
Polyvinylidenefluoride (PVDF)
Polytetrafluoroethylene (PTFE)
2.2
2.8
3.3
12
2.1
Dielectric
Strength
(V/µm)
500
350
400
200
250
Max Oper.
Temp (◦ C)
105
150
125
105
250
Table 2.1: Examples of other dielectric materials used in film capacitors. [17]
Polypropylene and polyester can both operate at high voltages, are very reliable and have low moisture
absorption. They both have good self-healing capabilities and have high dielectric strength. PVDF has
a much higher dielectric constant than both of them but it has some drawbacks including, very poor
insulation resistance, poor self-healing ability, higher leakage current, and in the end it is very expensive.
The reason to choose between different dielectric materials depend on the applications. If the working
environment for the capacitor exceeds the max operating temperature for the chosen material, then another
dielectric material, with better temperature withstand capabilities, should be chosen. [17]
2.2.1
Design
To understand why the capacitor is shaped like it is we first need to look at what it does. A capacitor
is designed to store a charge. The capacitance (the capacitors amount of charge per potential difference,
measured in Farads) is given by
C=
Q εε0 A
=
V
d
8
(2.1)
CHAPTER 2. BASIC THEORY
where ε0 is the dielectric permittivity; A is the electrode surface area; and d is the distance between the
electrodes. So to achieve a large capacitance a thin dielectric material with a high ε, and metal with a
large surface is needed.
Another term used in this report is Equivalent Series Resistance (ESR). It can be described as the total
resistance that contributes to power loss in the capacitor. It can be represented by a single resistance in
series with the ideal capacitor. This term is used since the capacitor it is not ideal and will experience
loss. The resistance can come from things like the end spray and the electrode. [18]
2.2.2
Fabrication
The capacitors investigated in this project are all fabricated from the same basic winding technology. A
example of how the capacitors are made is described by KEMET Corporation like: At first BOPP film is
fabricated and coated with a thin metallic layer, usually between 10-50 nm thick [19], and is rolled onto
a, so called, mother roll. This mother roll is then cut into small film strips with widths according to the
width of the required capacitor. Two film strips are then rolled together into a cylindrical winding. The
two metalized films that make up the capacitor are wound slightly shifted from each other. This is done to
ensure that a significant amount of electrodes are exposed to the “end spray,” see figure 2.4. The winding
is round, but in order to save space, the capacitor is flattened to an oval shape, which ensures a more
optimal use of the casing. After the capacitor has been compressed, metallic contacts are sprayed onto
the ends. These contacts are normally referred to as "end sprays". When this is done tabs are soldered to
the end sprays. Then the capacitor is encased in an insulating material, sometimes resin, and encapsulated
in a protective casing, to protect it from the environment, and a series of tests are run to secure a more
stable product. The capacitors made by KEMET Corporation are tested for capacitance, dissipation factor
and isolation resistance [20]. This is just a basic example of how a capacitor could be made. [2] [20]
2.2.3
Capacitor Lifetime
A metalized film capacitor undergoes an aging process. The aging process of a capacitor can be visualized
by considering a water dam with a small leak. Over time, the small water leak grows. The movement
of the water through the dam causes deterioration within the structure of the dam. But although the leak
grows, the amount of water leaking is still small and the dam still functions correctly. But at some point
the amount of water leaking is so great it compromises the structure of the dam. When this happens the
probability for a terminal failure of the dam becomes very high and the dam needs to be repaired or taken
out of commission.
Capacitor aging has two basic mechanisms which leads to aging of the component. The first, the chemical
reactions, is a combination of heat and different chemical contaminates such as oxygen and moisture.
They lead to an deterioration in the dielectric material which reduces the voltage withstand capability in
small localized areas. These chemical reactions lead to an increase in the probability that the dielectric
material will not withstand the applied voltage, thus a failure in the capacitor. The second mechanism
is an insulation mechanic. The dielectric material will always conduct a vary small current whenever
a voltage is applied. This current is usually called a leakage current. Even though the current is very
small it still gives rise to localized heating and material electron interactions, since it moves the electron.
9
CHAPTER 2. BASIC THEORY
Figure 2.4: A schematic view of a metalized polypropylene film capacitor. The shift referred to in the text
is the margin in the figure. [21]
As the mechanism deteriorates the capacitor, the capacitance slowly decreases and the resistance slowly
increases ultimately leading to a short circuit and capacitor failure. [22]
The speed of the aging process is accelerated by temperature, applied voltage and humidity. By determining the speed of the aging process the lifetime for the capacitor can be estimated.
A capacitors lifetime has two components, there is the expected lifetime and reliability. The expected lifetime is, as the name suggests, the expected lifetime for the capacitor, it is to be understood as a guideline
to the lifetime of the capacitor. If e.g. the expected lifetime of the capacitor is 10 years, it does not mean
that the capacitor will fail after 10 years, but only that it will fail within these 10 years. The reliability is
a probabilistic statement of the failure probability for a specific set of operating conditions and a specific
operating time. It is an assessment of when the capacitor will actually fail. The metalized film capacitor
is considered to have failed when it loses 5 % or more of its capacitance [23]. One way of improving
the reliability of the capacitor is to design the metalized electrode by either having high electrode resistance or arranging the elctrodes in a segmented pattern. Arranging the electrodes in a pattern also have a
downside, since more air is introduced to the capacitor. [24]
In the ideal world each capacitor manufactured with the same model number and operated in the identical
rated conditions, would all reach the expected end of life at the same time and fail after the expected
lifetime. But this is not the case. In the real world a group of capacitors all manufactured with the same
model number and operated under the same max rated conditions will all fail at different times. This leads
to the lifetime expectancy curve known as the "bathtub curve" shown in figure 2.5. It shows that the life
cycle of a capacitor has three failure modes or three different areas of failure intensity. The first area is
called the early failures, these are failures that happen during the first year of use and are usually induced
by major faults in the capacitor. The second area is the random failure mode, these failures are produced
by chance or operating conditions or lightning. The third area is the wear-out failures, these are generated
10
CHAPTER 2. BASIC THEORY
by general degradation of the capacitors internal structure. It is the wear-out failures that can be induced
using accelerated aging processes. [22]
Figure 2.5: Illustration of a capacitors life cycle. It is divided into three regions: a early failures region,
a random failures region and a wear-out failures region. [21]
2.3
Failure mechanics
This section will investigate the causes of capacitor failures and the different reasons for failures. This
section will discuss subjects such as dielectric breakdown, electrode corrosion and the self-healing processes, found in metalized film capacitors.
2.3.1
Dielectric breakdown
The phenomenon known as dielectric breakdown is when a dielectric material, that is normally insulating,
begins to conduct a current. This usually happens, when a high electric field, greater than the breakdown
strength of the material, is applied. The dielectric breakdown can best be explained using the band theory. The dielectric material is an insulator, meaning the valance band will be filled and the conduction
band will be empty, under normal conditions. When a high electric field is applied to the material it will
excite a lot of electrons to the conduction band, and since electrons in the conduction band acts as charge
carriers, the material conducts charges, rather than storing it.
For each material a different field strength is needed to cause dielectric breakdown, also known as the dielectric strength. Typical values for the dielectric strength can be seen in table 2.1. The dielectric strength
depends not only on the bandgap but also on things like the thickness, the temperature, internal structure
(defects), the environmental conditions and the time the material experiences the field. This means that
the maximum energy that can be stored in a metalized film capacitor is limited by the breakdown voltage.
In metalized film capacitors the dielectric material is normally very thin.
11
CHAPTER 2. BASIC THEORY
When a breakdown occur in a capacitor, the area in the film may become permanently conductive, due
to the carbon compounds, which are formed during the breakdown. The part of the capacitor where the
breakdown occurred is normally a short circuit and unable to store a charge. But this problem where the
capacitor is short circuited, due to breakdown, is a problem in foil capacitors. In metalized film capacitors
when a breakdown occur the self-healing properties comes into play.
2.3.2
Self-healing process
Self-healing is a mechanism found in metalized film capacitors. It is what happens when a defect in
the capacitor experiences a dielectric breakdown. During a self-healing process, the process where the
capacitor experiences a dielectric breakdown over a weakness in the film, current will flow from one end,
through one electrode, through the weakness, through the opposite electrode and out the other end. The
current in the area of the weakness will be attempting to go through a metal conductor that is so thin that it
is optically translucent. The amount of current that can go through the carbon compound is very limited.
The immediate area of the weakness will be blown away, acting much like a fuse, and the current will not
short circuit the capacitor, for a visual understanding see figure 2.6. When the self-healing process is done
it will be at the expense of capacitance and not at the expense of the entire capacitor. The self-healing
process takes around 1-5 µs and removes 2-8 mm2 of the area, of course this all depends on the size of
the contaminant, the thickness of the dielectric material, and the thickness of the electrodes. [9] [25]
Figure 2.6: Illustration of the self-healing process. A) Metalized film with a weakness. B) Voltage applied
between the electrodes causes breakdown. C) Heat from fault current vaporizes electrode and isolates
the defect area. [26]
According to Shaw et. al. [26] the energy required to do a successful self-healing process is very much
dependent on the thickness of the electrode. They found the energy required to do a self-heal was proportional to the thickness to the power of 2. This relation was found at a voltage of 400 V and a zero
interlayer pressure. This makes sense since the self-healing process must vaporize the metal in order for
the process to have been completed successfully, and this step is also dependent on the electrodes capacity
to quickly supply electrical energy to do this. [26]
According to Reed et. al. [9] the following happens when polypropylene experiences a dielectric breakdown: the electrical arc, created between the electrodes, heats the material to somewhere between 3000
K to 5000 K, but only for a short period, meaning the C atoms all are in the gas phase. When the arc is
extinguished most of the C atoms decay to products such as: CO, H2 , CH4 , C2 H, and solid residues of
graphite. The solid graphite was measured to be around 50% of the material, the rest is gasous products.
[9]
12
CHAPTER 2. BASIC THEORY
It is however not only defects in the film that causes breakdowns, also partial discharges are a main
cause for dielectric breakdown. A partial discharge is a localized dielectric breakdown of a small portion
of the dielectric material. Partial discharges occur more frequently in the outer layers of the metalized
film capacitor, because the outer layers are looser than the inner ones. The looser ones have more air
in them and have heavier partial discharge activity. Partial discharges weakens the dielectric film and
will eventually cause localized breakdown followed by the self-healing process. The self-healing process
also tends to require more energy and take longer, in the more loose bound outer layers than in the inner
layers. This is due to the pressure in the capacitor changes the required energy for at successful clearing.
Reed et. al. [9] discovered that the required energy could be 2-10 times as high in the outer layer than in
the inner layers. [24] [27]
Self-healing can be used as an indicator to see when the the capacitor needs replacing. When self-healing
has occurred enough times, the pressure inside the capacitor can release a mechanism which makes the
casing pop up. It is then clearly visible to the user which capacitors have failed. However when one
capacitor has failed it puts extra stress on the remainder of the capacitors, so if one goes early in the
lifetime it should be replaced, but if on fails after e.g. 8 years (expectancy of 10 years) then the entire
block should be replaced to reset the lifetime clock. If no such mechanism is found in the capacitor a high
pressure can result in a blowout of the capacitor resulting in critical failure. [9] [22] [28]
2.3.3
Electrode corrosion
Corrosion is a degradation of materials, normally metals, due to a chemical reaction with its environment.
In the case of aluminum corrosion, this means an oxidation of aluminum into Al2 O3 , which was proven
by Taylor et. al. [29]. They also suggested that the self-healing process is slowed or altogether stopped
by increase in oxidation area. The corrosion has an impact on the capacitance, according to [9] it was
shown that when accelerated tests had been carried out the capacitance had dropped around 5 %. The
corrosion visualised itself as transparent dots on the aluminum film in sizes from 1-3 mm. Taylor et. al.
[29] proved that the corrosion of aluminum layers requires a presence of water. However the moisture
does not have to come from the air, it can come from within the polypropylene. Meanwhile as mentioned
in section 2.3.2, the outer windings are more loosely bound and therefore more air is present in the outer
windings, leading to heavier corrosion. [9] [24] [29]
When corrosion occurs in the electrode, the metalization breaks down, which results in a thinning of the
metal layer and ultimately loss of electrode surface area with corresponding loss of capacitance. Since
the electrodes are thin, corrosion has a high impact on the structure of the electrode. The electrode is
heavily influenced by humidity, this may lead to a correspondingly high loss of electrode surface area and
loss of capacitance and increase of ESR. But this thinning of the electrode by corrosion can be helped by
thickening the electrode, however this would hinder the success rate of the self-healing process. Since the
self-healing process requires to burn away the electrode at the faulty area, if the electrode is thick more
heat would be required to vaporize the electrode. The use of this much heat could damage the dielectric
even further and reduce the dielectric strength. [24] [29]
13
CHAPTER 2. BASIC THEORY
2.3.4
Summary
Figure 2.7 show a graphical approach to understand the link between the mechanisms upon which the
capacitor undergo a graceful or single point aging process.
Figure 2.7: Flowchart linking the individual failure mechanisms. Both ESR and loss in capacitance lead
to graceful aging.
14
CHAPTER 2. BASIC THEORY
2.4
Infrared spectroscopy
This section will explain the theory behind and purpose of infrared spectroscopy technique and investigate the expected outcome to be observed during the IR investigation of the capacitors. This is done to
understand and interpret the measurements carried out as a result of the capacitor investigation.
Infrared (IR) spectroscopy is a technique utilizing the vibration of atoms of a molecule. Infrared refers to
the region between the visible and microwave regions in the electromagnetic spectrum (700–1 mm). The
IR spectrum is commonly obtained by shining IR radiation on a sample and then detecting the amount
of IR absorbed in the sample, depending on the energies. The specific energies absorbed by the sample
corresponds to a specific vibration between atoms in the sample. The atoms in the molecule can move
relative to one another. This is described as the stretching and bending, that is referred to as vibrations
in the molecule. Stretching is when two atoms changes bond length and bending is a change in bond
angle. Some bonds can do symmetrical stretching or asymmetrical stretching. The atoms vibrate with a
frequency that depends on its mass and the length and strength of the bonds. However, not all vibrations
can be detected using IR. Take for instance CO2 , if it experiences symmetrical stretching, it is not detected
because this vibration does not lead to a change in the dipole moment in the molecule. In order for IR to
be detected an electric dipole moment has to be changed during the vibration.
CO2 has a spectra that is relatively easy to interpret. However, as the complexity of the sample increases
it becomes more difficult to assign each absorption to a particular vibrational mode. This is not a great
problem because although the exact description of the vibration is difficult, it is possible to assign particular peaks to the vibrations of functional groups. An example of this is the C=O bond stretching in
organic molecules, which always occurs in the range 1640–1815 cm−1 [30]
Figure 2.8: IR spectrum of butanal. It can clearly be seen that butanal does not have any triple bonds.
However the C=O can clearly be seen as the peak at 1715 cm−1 . [30]
An IR spectrum, extending from 4000-400 cm−1 , can be divided into four regions: hydrogen bond stretching region (4000-2500 cm−1 ), the triple bond region (2500-2000 cm−1 ), the double bond region (20001500 cm−1 ) and the fingerprint region (1500-400 cm−1 ). An example of a spectrum can be seen in figure
2.8.
15
CHAPTER 2. BASIC THEORY
The hydrogen bond stretching region is used to locate bond vibrations relating to hydrogen. These are
generally O-H, C-H and N-H stretching. O-H creates a broad band in the 3700-3600 cm−1 . N-H stretching occur in the 3400 to 3300 cm−1 range. C-H occur in the range 3000-2850 cm−1 . The high frequency
is related to the low mass of the hydrogen atom.
The triple bond stretching region is within 2500-2000 cm−1 since the strong bonds in the triple bond
require a high frequency to be vibrated. The C≡C bonds absorb radiation from 2300 to 2050 cm−1 , but
the C≡N absorbs radiation between 2300 and 2200 cm−1 . These two may lie within the same frequency
but since the C≡C is very weak and the C≡N is of medium intensity they are distinguishable.
The double bond region is normally due to C=C and C=O stretching. The stretching of the C=O bond
is very easy to identify since it has a high intensity and occur in the 1830-1650 cm−1 region. The C=C
bond occur in the same area (∼1650 cm−1 ) and has a lower intensity so it can be hard to observe if C=O
bonds are present.
The fingerprint region contains a complicated series of absorptions. These are mainly due to all manner
of vibrations within the molecule. Because not all vibrations are well behaved and my vary with hundreds
of wavenumbers, even for at simple molecule. This makes it more difficult to pick out individual bonds
in this region than it is in the "cleaner" region at higher wavenumbers. The importance of the fingerprint
region is that each different compound produces a different pattern. [30]
IR spectroscopy can be used to characterize polymer, since most polymers contain some of the easily
identifiable regions. The method can be used for things such as characterization of the polymer and
investigation of the degradation of the polymer. [30]
Figure 2.9 show a IR spectra obtained by analyzing polypropylene. As can be seen polypropylene has
peaks in the hydrogen bond region. The fingerprint region has a few peaks which will be assigned
vibrations; the peak at 1453 cm−1 is due to -CH3 asymmetric, inplane bending and the 1375 peak is
due to symmetric C-H bending. The peaks at 1166, 997 and 972 cm−1 are related to the C-C stretching
in crystalline polypropylene, in α crystalline and in the amorphous polypropylene, respectively.
According to Liufu et. al. [10] the degradation of polypropylene caused by electrical stress is observed
by changes in the 809, 841, 975, and 998 peaks. If the degradation is induced by thermal stress, C=O
would be formed, the peaks would show around the 1830-1650 cm−1 region. [30] [31]
Figure 2.9: IR spectrum of polypropylene. [32]
16
Materials and Methods
3
Sample Preparation
Supplied by Danfoss was 17 capacitors, numbered 18-34, which had undergone an accelerated aging
process. From this batch five capacitors were selected, number 24, 25, 32, 33, and 34. These were chosen
with the intent on getting capacitors with both a large capacitance loss and a relative small loss.
Slices were cut from sample 25, 32, 33, and 34 using a process similar to the micro-sectioning procedure
used by Pedersen et. al. [33]. The sectioning was done by first encasing the capacitors in epoxy and then
cutting a thin slice from each. The slices were then ground and polished. They were to be used for IR
spectroscopy, absorption measurements and optical investigations.
Sample 24 was prepared by separating the capacitor from the casing and then investigating the metalization depending on layer depth.
Measurements
The capacitors, used in this project, had experienced an accelerated aging processes. The aging process
was done at controlled temperature, humidity and voltage. Each of the capacitors investigated in this
project were exposed to 85 ◦ C, 85% relative humidity and 230 V AC for 541 hours. The capacitance and
ESR was measured using a E4980A LCR Meter. The capacitance and ESR of the individual sampels can
be found in appendix A.
The absorption measurements was carried out using a LAMBDA 1050 UV/Vis/NIR Spectrophotometer
to measure transmittance and reflectivity. These were then used to determine the absorption.
For the microscopic investigations a Leica DMI3000M was used to investigate both the slices of capacitor
and the metalized thin film.
17
Results and Discussion
4
This section will present the results and discuss them. The degradation of polypropylene will be discussed
first. Then the metalized film capacitor will be discussed, which will be divided into two sub categories,
one regarding the capacitor, and a second that will discuss the surface structure of the polypropylene.
4.1
Polypropylene Degradation
Figure 4.1: IR spectra of the reference sample and of sample 25 and 33. The data has been shifted in the
y direction to make differentiate easier.
The IR measurements were carried out on sample 25, 32, 33, 34 and the reference. As can be seen in
figure 4.2 and 4.1 they all show peaks at approximately the same wavenumbers. For analysis in the fingerprint region see figure 4.3, which is a closer look at fingerprint region from figure 4.1. The first peaks
at 3000-2800 cm−1 are related to the hydrogen stretching. The peak at 1450 cm−1 is due to the bending
19
CHAPTER 4. RESULTS AND DISCUSSION
Figure 4.2: IR spectra of the reference sample and of sample 32 and 34. The data has been shifted in the
y direction to make differentiate easier.
of methylene groups (CH2 ) and the peak at 1375 cm−1 is caused by methyl group (CH3 ). According to
[31] the peak at 1167 cm−1 is due to the C-C stretching in the crystalline polypropylene. The peaks at
1303 cm−1 and 1255 cm−1 are due to C-H wagging and bending respectively [31].
According to Liufu et. al. [10] the peak at 973 cm−1 is due to the vibration of tertiary methyl groups
(-C-CH). The peak at 809 cm−1 is due to movements of irregular parts of the structure, the peak at 841
cm−1 is due to the vibration of double bond vinyl groups (R1 R2 C=CH–R3 ). The peak at 903 cm−1 is due
to an -CH3 rocking.
Section 2.4 relates the peaks at 1167, 997 and 972 to C-C vibrations depending on the individual crystalline vibrations. The presence of the peaks show how the structure contains both crystalline and amorphous regions. The 997 peak indicates that one of the most common crystalline structures is present,
namely the α form.
The intensity of the 841 peak is strong both in the samples and the reference, indicating that the peak
is not caused by electrical stress. The double bond vinyl groups are speculated to be created during the
BOPP manufacturing. The result for the reference sample show a constant baseline with only the peaks
to deviate from the base line. Whereas the other samples show a bending of the curve after 1500 cm−1 ,
the reason for this bending is unknown. The results show that the peaks and shapes of the curves are
almost the same. Indicating no electrical or thermal stress have caused degradation of the polypropylene,
during the accelerated aging process. If the samples had undergone thermal degradation, a peak at the
1830-1650 cm−1 region was expected, due to the creation of C=O. The electrical stress was investigated
by Liufu et. al. [10] by applying ∼1000 V to for 500 hours to a a 12 µm thick polypropylene sample.
Compared to the accelerated aging process made by Danfoss, Liufu et. al. [10] used a current four times
larger, in the same amount of time, and they only observed small changes in the peaks after more than
300 hours. So it is reasonable to assume that the accelerated aging process did not lead to any thermal or
electrical induced degradation, that was detected using IR spectroscopy.
20
CHAPTER 4. RESULTS AND DISCUSSION
Figure 4.3: IR spectra of the reference sample and of sample 25 and 33, shown for the fingerprint region.
The data has been shifted in the y direction to easier compare the peaks.
The results presented in figure 4.4 are used to determine if the four samples are similar. They all have
the same shape and all have a bend at 375 nm corresponding to the absorption of ZnO (3.3 eV). ZnO
is present if the corroded electrode is made from zinc. The first peaks at 280-290 nm is suspected to be
caused by some defects in the film, however they can not be related to anything specific.
Figure 4.4: Absorption measurements of all the samples.
4.2
Corrosion
Figure 4.5 show an overview of the capacitor slice, cut from sample 33. It clearly shows that the capacitor
has been compressed. The spacings in the layers are created from the grounding process and so are the
other impurities. During the investigation of the samples no trace of self-healing was found. A clear
contrast difference was observed between the reference and the other samples. This contrast difference
was suspected to be as a result of electrode corrosion due to electrical stress and humidity. To further
21
CHAPTER 4. RESULTS AND DISCUSSION
Figure 4.5: Overview of the bottom part of sample 33.
investigate if the lack of self-healing and loss in contrast was due to corrosion, sample 24 was removed
from the casing and examined.
Shown in figure 4.6 and 4.7 are the metalized film from sample 24 at 1 and 7 m into the capacitor
winding, respectively. These show that the heavy metalized edge helps prevent corrosion. Indicated by
the corrosion gap (B), separating the heavy metal edge and the remaining metalization. The heavy metal
edge does not prevent corrosion completely, in figure 4.7A the edge is clearly corroded to some extent.
The corrosion gap shows that a thinner metal layer is corroded faster. The formation of the gap was
explained by Brown et. al. [27] as the following phenomena: the thinner metal next to the heavy edge
is corroded first this result in a gap between the heavy edge and a strip of the metalization. Since the
current can not flow across the gap it has to go around it, this leads to a higher current density in the still
connected metal areas. This increases the rate of corrosion and creates the gap as seen in figure 4.6A.
The double layer shown in figure 4.6D is as a result of the incomplete separation of the metalized layer.
When separating the layers, some of the metal layer from the other layer might stick to the polypropylene
and not become separated but stick and show as a darker area on the film.
The results also show the corrosion is worse in the outer windings than in the inner windings. This can
be explained by the outer windings being more loosely bound than the inner one, which would increase
the amount of air present in the outer windings and thereby increase corrosion. The amount of corrosion
22
CHAPTER 4. RESULTS AND DISCUSSION
Figure 4.6: Picture of the metalized film 1 m into the winding. (A) Heavy metal edge, (B) Corrosion gap,
(C) Corrosion, (D) "Double" metal layer.
Figure 4.7: Picture of the metalized film 7 m into the winding. (A) Heavy metal edge, (B) Corrosion gap,
(C) Corrosion, (D) Metal film.
23
CHAPTER 4. RESULTS AND DISCUSSION
also indicates that the corrosion happened faster than the self-healing process. 7 meters into the winding
the corrosion is significantly reduced but the film did not show any sign of self-healing. This can either
be due to the corrosion gap or the estimate that once the area is oxidized, the self-healing process cannot
take place.
Figure 4.8: Picture of the metalized film around 40 cm into the winding. Left part of the picture is the
undamaged metalized film.
Around 40 cm into the winding a sharp transition between the undamaged metalized film and the corroded
area was observed, this can be seen in figure 4.8. The transistion indicates that the first 40 cm in the
winding is not connected to the end spray and therefore a current does not flow through it. This clearly
shows that in order for a corrosion to take place, during the accelerated test, a current has to be affecting
the electrode. This suggests that the corrosion of the electrode is an electrochemical oxidation.
Sample 24 had experienced a 99.3 % drop in capacitance and an increase in ERS. This matches the theory
since corrosion results in a capacitance drop and an increase in ERS. The increase in ESR was suggested
by Brown et. al. [27]. They suggested that the corrosion gap is first formed in the outer windings, the
remaining metalization is usually still connected to the end spray at some point further into the winding.
The longer path means losses due to resistance are much higher. The observation of the corrosion gap
7 m into the winding correlates with the link between a longer current path length and the increase in
resistance.
4.2.1
BOPP Morphology
Figure 4.9 show the surface of sample 24 at 7 m into the winding. The normal polypropylene structure is
formed of unoriented spherulites which gives the surface a structure as seen in [34]. But as can be seen in
figure 4.9 the BOPP does not have the same morphology. This indicate that the BOPP fabrication changes
the morphology of the polypropylene from a spherulite structure to fibrillar structure, as presented in
24
CHAPTER 4. RESULTS AND DISCUSSION
Figure 4.9: Surface of sample 24.
section 2.1.2. Based on this it can be reasoned that the spherulite structure is obtained at the first stage
of the BOPP manufacturing process. When the film is stretched the spherulite structure is pulled which
leads to an elongation of the spherulite structure and an eventual creation of the fibrillar structure.
By comparing the surface structure of sample 24 and the surface structure shown in figure 2.3, it can be
concluded that the BOPP used in sample 24 was fabricated by the bubble process. The bubble process
lead to, in comparison to the tenter process, a rough surface. The roughness of the BOPP can lead to a
higher spacing between the layers, thereby introducing more air to the electrodes.
25
Conclusion
5
Since the capacitor is one of the more fragile electronic components, it is of great interest to understand the
mechanisms which leads to capacitor failure. Once the mechanisms are properly understood, investigation
into improvement of the capacitor can be initiated.
The aim of this report was to investigate the most common failure mechanisms of metalized film capacitors. This was done by first investigating polypropylene, BOPP and the degradation here of. This was
then coupled to the metalized film capacitor and the electrical and thermal induced degradation. The selfhealing process and the corrosion of electrodes were investigated to understand the mechanisms leading
to different failures.
A series of capacitors which had experienced an accelerated aging process were investigated. It was concluded that the two major graceful aging mechanisms are self-healing and corrosion. One of the capacitors
investigated did experience a loss in capacitance of 99.3%. This loss in capacitance was investigated. The
electrode was discovered to have suffered extencive damage due to corrosion, the corrosion was related
to the loss in capactitance. Since no self-healing was deteceted up to 7 m into the capacitor winding it
was concluded that the electrode corrosion inhibits the self-healing process, and that the corrosion of the
electrodes happened faster than the breakdown of the polypropylene film. The capacitor experienced an
increase in ESR, this was contributed to the extensive corrosion done to the electrodes.
Based on the IR measurements it can be concluded that the dielectric material did not undergo any significant degradation, detectable by IR spectroscopy, as a result of the thermal and electrical stress applied by
the accelerated aging process. This indicates that an accelerated aging test conducted at 85% humidity,
85◦ C and 230 V is not enough to degrade the polypropylene in the capacitors.
The surface of the polypropylene film was investigated. It can be concluded that the BOPP film was
fabricated using the bubble process. This process leads to a more rough surface and would perhaps
introduce more air to the electrodes.
It can be concluded that the capacitors in this series have a problem regarding corrosion. All of the
capacitors investigated experienced a loss in capacitance of 90 %, or higher, after a stress periode of
541 hours. The main loss in capacitance regarding this batch of capacitors is contributed to a corrosion
problem.
27
CHAPTER 5. CONCLUSION
5.1
Prospects
The corrosion is a serious problem in the capacitors, making it interesting to investigate the corrosion
effect in the capacitor even further. Investigation into origin of the moisture and possible weak point in
the capacitor casing, could lead to improvement of the capacitor.
Investigation into capacitors from an other batch to examine the self-healing capabilities of a film capacitor and perhaps a linkage between micro fractures in the polypropylene film and the dielectric breakdown
could be examined.
28
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Appendix
A
Table A.1 are the capacitance and the ESR measured for the individual samples. These were measured
using a E4980A LCR Meter. The testing conditions were, Temp: 22◦ C, AC Voltage: 1V, Frequency:
1000 Hz.
All the samples are Kemet F861 Mini 1500 nF with part nr. F861DP155M310Z.
Sample
24
25
32
33
34
Capacitance (nF)
10.309
132.012
10.288
99.89
8.115
Table A.1:
33
ESR (Ω)
92.25
14.73
29.595
17.146
250.488