Download Space Charge Behavior in Multilayered Polyimide

VB19
Conference Proceedings of ISEIM 2014
Space Charge Behavior in
Multilayered Polyimide Films under
DC High Stress near Breakdown Strength
l'
l
Keigo Matsubara , Shohei Kawano I, Hiroaki Miyake and Yasuhiro Tanaka I
I Tokyo City University
*E-mail: [email protected]
Abstract-We
investigated the relationship between the space
charge accumulation characteristics and the breakdown in
various combination of multilayered polyimide samples under
dc stress using the pulsed electro acoustic (PEA) method.
field near breakdown strength. Furthermore, we also show the
characteristics of time to breakdown in the multilayered
samples.
In
recent years, there are many examples which a multilayered
II.
polyimide film is used as the flexible Printed-Circuit Board
(PCB) inside an advanced electronic devices or motor windings.
Therefore,
it
is
necessary
to
investigate
the
insulation
performance of the multilayered polyimide. In previous work, it
has been revealed that a space charge accumulated in polyimide
film under a high stress and it is closely related to the electrical
breakdown. Therefore, it is also necessary to investigate the
space charge accumulation characteristics of the multilayered
type sample. In this paper, we measured the space charge
accumulation
in
various
combinations
samples under dc high electric field at
of
80t
the
multilayered
by using the PEA
method. From the space charge measurement, we found that
the
multilayered
sample
has
more
superior
insulating
properties than the single layer sample. Furthermore, we found
that it is hard to have the breakdown by arranging a thin layer
putting next to the anode electrode.
Keywords: space charge, pulsed electro acoustic method,
multilayered poly imide films, breakdown, electronic device
I.
A.
EXPERIMENTAL AND SAMPLE CONDITION
Space Charge Measurement System
Fig. 1 shows the principle of the PEA method, which is a
technique for measuring the space charge distribution in
polymeric materials. When a pulse voltage is applied to the
sample, the Coulomb force acts on the charge and the force
generates a pulsive pressure wave. The pulsive pressure wave
propagates through the sample and the electrode, then it
reaches the piezo-device. By the piezo-device, the pressure
wave is transformed into a voltage signal. Since the pulsive
pressure wave and its traveling time to the piezo-device are
proportioned to the charge density and the distance between
the position of the charge and the piezo-device, the charge
distribution is obtained from the time dependent signal.
Details of the measurement principle are described elsewhere
[2].
----,
Protectivcl\1<ltcrial
__
0----1
INTRODUCTION
High Voltage
Amplil'olr
Coupling Capacitor
Recently, many attractive electronic devices like smart­
phones, tablet PC and so on have been developed and
improved every day. To make such devices, it is necessary to
use a so called flexible Printed-Circuit Board (PCB) to reduce
their sizes. As a flexible board, polyimide film is commonly
used, because it has enough flexibility and shows a good
insulating performance even at high temperature. Since a
demand to reduce the sizes of the device has been increasing,
we also need to reduce the total thickness of the combination
of the PCBs. To reduce the total thickness, the multilayered
polyimide films is often used. Therefore, in addition to the
investigation of the insulating performance for each layer, the
investigation of the multilayered polyimide films is also
important. Furthermore, since such a substrate are supposed
to be used in an environment at high temperature with high
humidity, it is required to show a good performance as a
insulator even under such severe condition. Therefore, the
investigation of the performance under such condition should
be carried out. On the other hand, it has been revealed that a
space charge accumulation in polyimide is closely related to a
dielectric breakdown under a high dc electric field [1].
However, the investigation for the performance of the
multilayered polyimide has been insufficient yet. In this
paper, we shows the results of the space charge behavior in
the multilayered polyimide films that were obtained using the
pulsed electro acoustic (PEA) method under dc high electric
-
(( (B))
(((B))((Et))J
(( (B))
(((B))
(((B))
Upper electrode
Sample
�
Lower electrode
Packing
material
Piezo Electric Device
Figure I. Principle of the PEA method
B.
Sample
In this investigation, a commercially available polyimide
®
film, named "Kapton type H" supplied from DU PONT­
TORAY CO., Ltd, is used for all experiments. Fig. 2 shows
®
the chemical structure of Kapton . It consists of an ether
bond and imide bond, which has a hydrophilic property. A
single-layer and several multilayered samples composed of
layers with thicknesses of 25, 50 and 75 /lm were prepared
for the experiments. In the case of a double layered sample,
one film was just put on the other film without any glue or
adhesive. Between layers, only a drop of silicone oil was put
to improve the acoustic signal propagation. Since the sample
was sandwiched between the upper and the lower electrodes
in the PEA system, the double or the triple layered sample
was stacked together by the pressure from the electrodes. In
the multilayered samples, the layers with the thickness of 25,
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Conference Proceedings of ISEIM 2014
50 and 75 /lm are used in combination. The total thicknesses
of all multilayered samples are about 100 /lm. The
multilayered samples are described using the thicknesses of
the layers. For example, a double layered sample composed
of a 25 /lm-thick layer in upper electrode side and a 75 /lm­
thick layer in lower electrode side, it is described as "25+75
/lm". In this experiment, all samples were su�iected to a
heating humidification treatment by keeping in a chamber at
80°C with humidity of 80 % for 1 hour to simulate the high
temperature and the high humidity environment.
Figure 2. Chemical structure of Kapton®
C.
Experimenrai Procedure
For the space charge measurement, the PEA system for
high temperature was used [3]. In the space charge
measurement, a dc voltage corresponding to an average
electronic field of 110 or 120 kV/mm was applied to the
samples at 80 °C. In the PEA system, a semi-con layer is
preferably used to improve an acoustic impedance between
the sample and the upper metal electrode. In this
measurement, a semi-con layer was used as the upper
electrode. In this measurement, when the positive high
voltage was applied to the sample through the upper electrode,
we call the polarity of the voltage as "positive". The lower
electrode is aluminium plate and it is always grounded.
The space charge measurements were carried out with
interval of 5 s for maximum 3 hours. When a breakdown
occurs in the sample, the circuit is automatically shut and
shuted, them the measurement is stopped.
III.
RESULTS AND DISCUSSION
Fig. 3 shows typical space charge distributions in (a)
single layer and (b) double layered(50+50 /lm) samples under
relatively low average electric field of 100 kVImm. As shown
in Fig. 3(a), a positive and a negative charges were observed
near the anode and the cathode, respectively, immediately
after the start of the measurement. It is found that the
distribution was stable through the measurement. In the case
of the double-layered sample as shown in Fig. 3(b), the
positive and the negative space charges were observed near
ChargeDensil:p(z)[Clnf]
- 100
120
0
'---..:..ri';';'"
Cathode
Anode
128
PositionZ[ftmJ
Charge accllmulationbehavior
( a) S ingle-layer sam p le
(I25ftm )
100
Cathode
120 r--
Anode
I
'-5
Position z[!!m]
100
Charge accumulation behavior
( b ) D oub le- l ayered sam p le
(50+50ftm )
Figure 3. Typical space charge distribution in (a) single-layer and (b)
double layered PI films at 80°C under dc 100 kV/mm
-
the interface of the anode and the cathode sides in each layer.
In the case under the relatively low stress, the combination of
the positive and the negative charges appears soon in each
layer and it is stable during the measurement.
Figs. 4 and 5 show time dependent space charge
distributions in various samples under the average electric
fields of 110 and 120 kV/mm, respectively. In these figures,
Fig. 4(a) shows the result observed in a single-layer sample
with thickness of 100 /lm. Figs. (b)-(t) shows the results
observed in various multilayered samples. In Figs. 4 and 5,
Figs. (b), (c), (d), (e) and (t) show the time dependent space
charge distributions in 75+25 /lm, 25+75 /lm, 50+25+25 /lm,
25+50+25 /lm and 25+25+50 /lm samples, respectively. In
these figures, the time dependent charge accumulation
behaviors are shown in the top row, the charge distribution
profiles are in the middle and the electric field distributions
are in the bottom. In the figures for the charge accumulation
behaviors, the charge density is described using color scale,
and the color bar on the top of figure shows the scale. The
horizontal and vertical axes in the figures indicate the position
in direction of thickness and the voltage application time,
respectively. Since the voltage application was stopped at the
breakdown, the length of the time axes of the Figures are
proportioned to the time to breakdown. In other words, the
shorter axes mean the faster breakdown.
As shown in Fig. 4(a), the dielectric breakdown occurred
in the single-layer sample at about 15 minutes after the
voltage application. It is not obvious from the figure for the
behavior, but the space charge accumulation under this
condition was not stable. The positive charge accumulation
was spread into the bulk and a small amount of positive
charge was observed near the cathode .iust before the
breakdown as it was reported in our previous work rn On
the other hand, the electric field distribution was relatively
stable from the start of the measurement. While the electric
field at the middle in the bulk was enhanced by the
accumulation of the space charges, any obvious charge of the
distribution was not observed between those at the start and
the end of the measurement as shown in Fig. 4(a).
On the other hand, as shown in Fig. 4(b), it took a longer time
to breakdown in the multilayered (75+25 /lm) sample than
that in the single layer. Judging from the other results of
multilayered sample shown in Fig. 4(b), (c) and (d), the all
times to breakdown in them were longer than that in the
single layer. In the case of double multilayered samples
shown in Fig. 4(e) and (f), the breakdown was not observed
for 3 hours. Therefore, it can be said that the multilayered
sample has high insulation performance than the single-layer
sample. From the measurement results shown in Fig.4 (b)
and (c), it is found that the negative and the positive charges
were accumulated near the cathode and the anode sides,
respectively, in each sample and they continuously
accumulated until the end of the measurement. In the (75+25
/..lm) sample, since larger amounts of the positive and the
negative charges were accumulated in the 75 /..lm-thick than
those in the 25 /..lm-thick layer as shown in Fig. 4(b), the
electric fields in the 75 and 25 /..lm-thick layers were larger
and lower than the average applied electric field of 110
kVImm from the start of voltage application. With increase of
the voltage application time, a positive packet like charge was
observed in the 75 /..lm-thick layer and it moved towards the
cathode side, then the breakdown occurred in the sample. On
the other hand, in the case of the (25+75 /lm) sample, while
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Conference Proceedings of ISEIM 2014
ChargeDensilyp(z)[C/m'1
100
0
! :�
!
Anode
Cathode
77
66
the larger amounts of the positive and the negative charges
were observed in the 75 !--tm-thick layer, no positive packet­
like charge was observed in the layer as shown in Fig. 4(c). A
particular feature in this sample is the negative charge
appearance in the 25 !--tm thick layer that was observed from
about 70 minutes later as shown in Fig. 4(c). After that, the
breakdown was observed in this sample at 157 min later,
which was longer than that was observed in the (25+75 /.-tm)
sample. The times to breakdown in both double layered
samples were longer than that in the single layer. Therefore, it
can be said that the property against the dielectric breakdown
in double layered sample is better than that in the single layer,
and the property in (25+75 !--tm) sample is also better than that
in the (75+25 !--tm) sample. As shown in Figs. 4(b) and (c), the
trigger for the breakdown is assumed as the change of the
space charge distribution in the layer mounted near the anode
side. If it is true, the thinner layer must have the better
property against the breakdown.
From the measurement results in the triple layered
samples, as shown in Fig. 4(d), (e) and (f), it is found that the
negative and the positive charge combinations are observed
near the cathode and anode sides, respectively, in each layers,
and they continuously accumulated until the end of the
measurement. In the result in the (50+25+25 !--tm) sample, it is
found that a lot of negative charge accumulation was
suddenly observed in the layer of 50 !--tm thick on the top
electrode side, and the breakdown occurred at 89 min later,
while the other charge combinations were stable during the
voltage application. On the other hand, in the results in the
(25+50+25 !--tm) and the (25+25+50 /.-tm) samples as shown in
Fig. 4 (e) and (f), while large amounts of the negative charge
accumulations and little increases of the electric field were
observed in the 25 !--tm layer on the top electrode side, no
dielectric breakdown occurred in them.
-100
Br�akdoI\'n
C athode
Anode
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'---'�L-
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l'ositionz[)U1l1
Chargc ac<:II1lluI<lIion behavior
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Charg�accumulatiollbehavior
-,--- -,
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100
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-200'------::---c:,--�---'
l'ositiollz[)un1
Posilionz[)l1ll1
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(b) Sample (75 +25 11m)
Elcctrieficlddislribution
Electric fidddistribUlion
Anode
105 Cathode
89
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52
Positionz[fU1l1
I'ositionz[)un1
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200
,----,-,------,
-- -j�tstah !
100
-jhtb<SoreBD
j�tar�BDi
Therefore, it can be also said in the triple layered sample
that the multilayered sample has the better property than the
single layer against the breakdown and the sample with the 25
!--tm thick layer mounted next to the anode is the better than
others.
-200
'----':-0
-- ----:26
---::
::52
'------=\02
-'------"
-Positionz[)u1l1
l'ositionz[)m11
(d) Samp1e (50+25+25 11m)
(c) Sample (25+75 11m)
Ekctriefidddislribution
E1ectriefidddistribulion
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o
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l'ositionz[)un1
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!
To confirm the above characteristics, the sample
experiments under higher applied electric field of 120 kVfmm
were carried out. As shown in Fig. 5 (a), the dielectric
breakdown occurred about 8 minutes later in the single-layer
sample. As shown in Figs.5 (b) and (c), the dielectric
breakdowns were observed within shorter times that were
observed in them under 110 kVfmm. However, under higher
applied electric field, it is found that the space charge
accumulation processes to the breakdown seems to be very
similar to those observed under 110 kVfmm except for the
shorter times to breakdown.
50 76102
l'ositionz[)un1
Positionz[)un1
Charg�accumulationbehavior
From the above results, in the double and triple layered
samples, a drastic change in the charge distribution in a layer
put next to the anode electrode seems to lead the breakdown.
Furthennore, the thinner layer seems to have the superior
characteristic against the breakdown.
Anode
Chargcdistribulion
100
-.iuststan i
-JuslbeforeBp
-lus'afterBDi
For the triple layered sample, as shown in Fig. 5(d), (e)
and (f), the observed properties of the space charge
distributions were very similar to those observed under 110
kVfmm except for the shorter times to breakdown, and the
breakdown occurred I n the samples of (25+50+25 !--tm) and
(25+25+50 !--tm) were not observed under 110 kVfmm.
Therefore, even under higher applied the electric field, it is
considered that the breakdown in multilayered samples
50 76102
l'ositionz[)un1
(e) Samp1e (25+50+25 11m)
E1ectriefidddislfibo.llion
Posilionz[)un1
(f) Sample (25+25+50 11m)
Elcctric lidddistribution
Figure 4_ Space charge distribution in (a) single-layer and (b)-(f)
multilayered PI films at 80°C under dc 110 kV/mm
-
435
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Conference Proceedings of ISEIM 2014
successively occurred after the drastic changes of the space
charge accumulations in the layers put next to the anode
electrode. Furthermore the thinner layer may prevent the
breakdown for long time when it is put next to the anode
electrode.
ChargeDensity p(z) [Clml]
[00
�
Breakdown
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/
L
Cathode
: t ..
o
Finally, we would like to discuss the reason of the above
properties in the multilayered samples. The positive and the
negative charge combinations observed near the interface of
the anode and the cathode sides in each layer may be the
injected charges from the electrodes or the layered next to the
layers. In general, since the moisture in the polymer enhances
the charge injection into the bulk, the humidified layers must
be affected by the moisture. After the accwnulation of the
combination charges, the drastic changes were observed in
the layer put next to the anode. Since it was observed in the
anode side layer, the space charge accumulation must be
affected by the anode electrode. In these layers, spreads of the
positive charges into the bulk were observed in many cases. If
a large amount of positive charge is injected from the anode
of the semi-con layer, the electric field near the opposite
interface must be higher. Under such condition, larger
negative charge must be injected from the next layer, and it
might be observed as the negative charge accumulation near
the anode in some cases before the breakdown. Anyway, we
need to wait for the further investigation to make the
mechanism clear. It is also unknown at this moment about the
reason why the thinner layer shows the better performance
against the breakdown. We need to reveal the reasons by
some experimental works in near future.
0
-100
Anode
ell
102
Positionz[)lm]
Positionz[!!m]
Chargcaccumulationbchavior
Charge�umulationbcIHw·or
400 ,-�------ �-- ,
: j::���D
�JUS\S\.rt
200
·200
400L-�------�102��
�O�
6
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2-- �
Positionz[)lmJ
Positionz[)tm]
Charged istribulion
Charged istribution
·100
-100
-200'---::-- -----cI""--'
O,
·200 �-O'---'26L----CCIO'-=-2--�
Positionz[)lm]
Positionz[)tmJ
Electrictield d istribution
Elcctricfie1d d istribution
(b) Sample (75 +25 ).J.m)
(a) Sample ( 100 11m)
Brc.1kdoll' n
Positionz[)lm]
Positionz[)lIn]
Charge accumulation bchavior
Chargeac<:lImlllnlionbehavior
400 ,--,-----,-,--,
200
·200
IV.
CONCLUSION
4OO ��----�7�6�I�02--�
Positionz[)lm]
In this paper, we investigated the relationship between the
space charge accwnulation behavior and the time to
breakdown in various multilayered sample of polyimide films
using the PEA method. Followings are obtained as the results.
•
•
•
Chargedislriblltion
200 ,--.,-----,-.,--,
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-"'l just afterBD
100
·100
OO
- -- . ' ��---- ;7
,
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Positionz[)un]
[2]
[3]
.
200
-----� 'O:-2:;---'
---! 6 52= COI -=
' '--�
PosiliollZ[)lInj
Eleclrictield d istribution
Electrict1e1d d islribl
l lioll
(c) Sample (25+75 ).lm)
In the multilayered sample, the dielectric breakdown
occurs after a drastic change of the charge
distribution in a sample put next to the anode
electrode.
(d) Sample (50+25+25 11m)
Cathode
143
13 If--
87
78
In the multilayered sample, the dielectric breakdown
is hard to occur by arranging a comparatively thin
layer putting next to the anode electrode.
Anode
Cathode
L.....
Anode
I
;
'111
50 76 102
Positionz[)tm]
PosiliollZ[)lInj
Charge accutllu1ationbehavior
Charge accl
l lt lUl alion behavior
200
REFERENCES
[1]
� i J
:: �re D
100 -ijust;nerBI�
·100
In the multilayered sample, the breakdown is less
likely to occur compared with that in the single layer
sample under high dc stress.
Positionz[)tm]
Chargcd istrib1
l lion
200,--- ,---,---,----- ,------,
.
.
Y. Kishi, T. Hashimoto, H. Miyake, Y. Tanaka and T. Takada:
"Breakdown and Space charge Formation in Polyimide Film under DC
High Stress at Various Temperature", 2009 lounal of Physics
Conference Series VoU83, 012005, 2009
·200
PosiliollZ[)lInj
Y. Li, M. Yasuda and T. Takada: "Pulsed Electroacoustic Method for
Measurement of Charge Accumulation in Solid Dielectrics", iEEE
Trans. DEI. YoU, No.2, pp.188-198, 1994
!
1. Taima, K. Inaoka, T. Maezawa, Y. Tanaka, T. Takada and Y.
�
Murata: "Observation of Space Charge Formation in LDPE/MgO
Nano-Composite Under DC Stress at High Temperature", Annu. Rep.
2006 CEfDP, pp.302-305, 2006
iil
E
�
i
200
100
Chargcd istrib1
l tion
=1�::t{;��D
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,--'!just',la-"
4OO L.....r---�SO�76�1�02---'
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Charge d istribution
200 ,--.,----,-,-.,---,
100
0r-��=-+s��
-100
i::��!D
·100
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Positionz[)lm]
-200L.......r--�SO�76-,I�O':----'
Positionz[)lm]
Electrict1cld d istrib1
l tioll
Eleclrict1e1d d istriblltion
(e) Sample (25+50+25 ).lm)
(f) Sample (25+25+50 ).lm)
Figure 5. Space charge distribution in (a) single-layer and (b)-(f)
multilayered PI films at 80°C under dc 120 kV/mm
-
43 6
-