J Mater Sci: Mater Electron (2014) 25:627–634
DOI 10.1007/s10854-013-1478-6
Advanced characterization of mechanical properties of
multilayer ceramic capacitors
Kun-Yen Chen • Chang-Wei Huang
Marklaw Wu • Wen-Cheng J. Wei •
Chun-Hway Hsueh
•
Received: 16 January 2013 / Accepted: 29 August 2013 / Published online: 11 September 2013
Ó Springer Science+Business Media New York 2013
Abstract Characterization of the mechanical properties
of small components is a significant issue. For the multilayer ceramic capacitor (MLCC), direct loading by conventional facilities is not suitable because of its small size.
To date, the standard method used to determine MLCC’s
mechanical properties is board flex test; i.e., mounting the
capacitor onto a printed circuit board (PCB) and applying
bending to the entire system. Failure is defined as cracking
or capacitance loss of the MLCC when the mounted PCB is
subjected to a specified deflection, and the measurements
are usually performed after the test. In this case, characterization of the mechanical properties of MLCCs is
qualitative. The purpose of the present study was to
quantitatively characterize the mechanical properties of
MLCCs. Specifically, the acoustic emission was used to
detect cracking of MLCCs during the board flex test. To
confirm cracking-induced acoustic emission, telemicroscope was used to perform the in situ observation of
cracking. Finite element analyses were also performed to
analyze the stress field resulting from the test to compare
with the observed cracking path. In addition, nanoindentation was used to explore the mechanical properties of the
constituents of MLCCs in the nanoscale. Our work not only
K.-Y. Chen W.-C. J. Wei C.-H. Hsueh (&)
Department of Materials Science and Engineering, National
Taiwan University, Taipei, Taiwan
e-mail: hsuehc@ntu.edu.tw
C.-W. Huang
Department of Civil Engineering, Chung Yuan Christian
University, Taoyuan, Taiwan
M. Wu
MLCC Specialty R&D Division, Yageo Corporation,
Kaohsiung, Taiwan
allows identification and understanding of the fracture
origin, but also provides guidelines in the material design.
1 Introduction
With the miniaturization of commercial electronic products
and devices, the sizes of electronic components have
become much smaller than ever. Multilayer ceramic
capacitor (MLCC) is now an indispensable electronic
component because of its characteristics of small volume
and high capacitance density [1, 2]. The system consists of
alternate layers of dielectric ceramics and metal electrodes
sandwiched between two ceramic cover layers and cosintered at high temperatures. After sintering, the component is cooled to room temperature and the end termination
electrodes are formed on both sides of the component. The
MLCCs are then mounted to the printed circuit board
(PCB) by the soldering process. A photo of half of the
mounted MLCC is shown in Fig. 1. This surface mounted
electronic component must withstand severe environment
with vibration and high temperatures [3, 4]. Based on the
analysis of Center for Advanced Life Cycle Engineering
(CALCE), the capacitors are more responsible for the
failure of the system than other components [5]. Therefore,
other than the investigation to increase MLCC’s capacitance density, the mechanical properties-related issues of
reliability and durability are of great concern [6, 7].
However, characterization of the mechanical properties of
small component is a daunting challenge. Because of its
small size, direct loading on MLCCs by conventional
facilities is infeasible.
Because failure of surface mounted MLCCs results
mainly from bending of the PCB during the handling and
assembly process such as solder reflow, or by the vibration
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628
Fig. 1 A photo of half of the surface mounted MLCC
and shocking in the function environment, the mechanical
strength of MLCC to survive a certain deflection that it
might experience under manufacturing and functioning is
Fig. 2 The schematic diagrams
of a board flex test under AECQ200-005 [9] and b four-point
bending test under IPC/JEDEC9702 [22]
123
J Mater Sci: Mater Electron (2014) 25:627–634
necessary [5]. The method used to date to determine
MLCCs’ mechanical properties is board flex test [8, 9]; i.e.,
mounting the capacitor onto a PCB that is subjected to
bending, and the failure criterion is defined as cracking or
the capacitance loss of the MLCC when the board is under
a specified deflection. A schematic drawing of the board
flex test is shown in Fig. 2a.
The observation of flex cracking is a non-trivial problem. Unless the MLCC is sectioned and polished, the crack
may not be apparent on the external surface of the MLCC
[10]. Also, it has been reported that while cracking causes
separation along its path and disconnection of one or more
of the electrode plates, the separation may be eliminated
and allow the disconnected electrodes to return and connect
again upon load removal [8, 11]. Because of this return, it
is difficult to test a cracked MLCC for capacitance and
insulation resistance after removal of the flex force. To
resolve this problem, the purpose of the present study was
J Mater Sci: Mater Electron (2014) 25:627–634
629
to perform the in situ detection of flex cracking. Specifically, the telemicroscope and acoustic emission [12–14]
were used to perform in situ inspection of cracking when
the MLCC was subjected to the board flex test. The crack
initiation site was identified and compared to the stress
field obtained from the finite element analysis (FEA), and
the observed cracking event was correlated to the acoustic
emission signal. Furthermore, with the identification of the
cracking-induced acoustic emission, cracking of MLCC
can be readily detected without in situ observation of
cracking for which cutting and polishing of the specimen
are required prior to testing. In addition, nanoindentation
[15, 16] was performed to explore the mechanical properties of the constituents of MLCCs in the nanoscale. Our
work not only allows identification and understanding of
the fracture origin in MLCCs, but also provides guidelines
in the material design.
1,140 GPa and mi is 0.07. With the Poisson’s ratio of
material obtained from the literature, Young’s modulus of
the test material can be calculated from Eq. (1) and is listed
in Table 1. Young’s moduli obtained elsewhere [17–19]
are also listed for comparison.
It is worth noting that the PCB is FR4 material, which is
a glass fiber-reinforced epoxy composite. As a result,
Young’s modulus of PCB obtained from nanoindentation is
the surface property; i.e., the property of epoxy matrix. To
obtain Young’s modulus of the PCB, three-point bending
was performed and Young’s modulus can be calculated
from the following equation [20]
E¼
I¼
The MLCCs were provided by YAGEO with X7R dielectric type and 1210 size that have a length of 0.12 inch and a
width of 0.10 inch. Hysitron TI 950 TriboIndenter Nanomechanical Test Instrument was used to perform nanoindentation in order to obtain the mechanical properties of
each constituent of MLCC. Specifically, ten indentations
each were performed on the MLCC’s dielectric, inner
electrode, end termination, solder, and copper pad,
respectively. From the slope of the curve upon unloading,
the reduced Young’s modulus, Er, could be obtained. This
reduced Young’s modulus is related to Young’s modulus of
the test specimen by the following equation
1
1 m2i 1 m2s
¼
þ
Er
Ei
Es
ð1Þ
where E and v are Young’s modulus and Poisson ratio, and
the subscripts i and s denote the indenter tip and the
specimen, respectively. For a diamond indenter tip, Ei is
Table 1 Material properties of
MLCC mounted on PCB
ð2Þ
where P is the applied force during bending, l is the loading
span, d is the bending deflection, and I is the moment of
inertia of the cross-sectional area which is given by the
following equation [21]
2 Experimental procedure
2.1 Material properties and nanoindentation
Pl3
48dI
bh3
12
ð3Þ
where b and h are, respectively, the width and the height of
the rectangular cross section of the PCB.
2.2 Acoustic emission and telemicroscope
A lead-free solder, Sn95.5Ag3.8Cu0.7, was used to mount the
MLCC at the center of PCB. After assembling, the PCB
was subjected to deflection based on IPC/JEDEC standard
9702 by four-point bending [22] and MTS Criterion
tabletop test system was used to perform this test. The
inner load span was 4 cm and the outer support span was
9 cm. The loading condition used in this study was displacement control with the crosshead moving to a specified
displacement at the speed of 0.1 mm/s, holding at the
specified displacement for 5 s, and then unloading to origin
at the same speed. A schematic drawing of this four-point
bending is shown in Fig. 2b. The acoustic emission
detector was attached to the PCB during the test. The
acoustic emission analog filter frequency was set between
Young’s modulus (GPa)
This study
Franken et al. [17]
Park et al. [18]
Poisson ratio
[17, 19]
X7R dielectric
200
91
175
0.25
Internal electrode
167
110
208
0.33
Termination
105
207
128
0.33
Solder
45.3 (Sn95.5Ag3.8Cu0.7)
26.5 (Sn63Pb37)
26.5 (Sn62Pb36Ag2)
0.36
Copper pad
100
131
128
0.33
PCB
17.2
23
–
0.25
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630
20 kHz and 1 MHz, and the threshold was set at different
levels. The contact between the bending fixture and the
PCB was lubricated to minimize the frictional stress.
Optem Zoom 125 telemicroscope, which has a working
distance of 10 cm, was used to perform the in situ observation of cracking of MLCC during the test. A photograph
of the experimental setup for in situ cracking observation is
shown in Fig. 3. In order to facilitate the observation, the
mounted PCB was cut and polished (see Fig. 1 as an
example) prior to the test. The observed cracking event was
then synchronized with the acoustic emission signals to
identify the cracking-induced acoustic emission. To eliminate the acoustic noises from PCB itself during the test,
four-point bending was also performed on the PCB without
the MLCC.
3 Finite element analysis
A simple analytical model has been developed previously
to analyze the thermal stress in the MLCC [23]. The stress
development in the MLCC during the board flex test has
been simulated using the FEA elsewhere [18, 24–28]. Also,
a recent finite element study showed that the thermal stress
was much smaller than the flexural stress during the board
flex test [29]. For simplicity, a two-dimensional planestress FEA model of MLCC was used in the present study
and the deformation behavior of the mounted PCB system
was assumed to be elastic to study the essential trend of the
distribution of the flexural stress resulting from the board
flex test. Only half of the system was modeled because of
J Mater Sci: Mater Electron (2014) 25:627–634
the symmetric geometry. The material properties listed in
Table 1 were adopted in the simulation. Because the nickel
electrode is thin compared with other constituents in the
MLCC, it is difficult to construct a model with many thin
layer electrodes within the dielectric ceramic. Therefore,
the rule-of-mixtures was used to calculate the effective
elastic properties of the dielectric ceramic/electrode composite, such that [18, 30]
E ¼ fd Ed þ fe Ee
ð4aÞ
m ¼ fd md þ fe me
ð4bÞ
where f is the volume fraction and the subscripts d and
e denote the dielectric ceramic and the electrode,
respectively.
It should be noted that the dimension of PCB (length
10 cm 9 width 4 cm 9 height 1.6 mm) is much larger than
MLCC (0.3048 cm 9 0.254 cm 9 1.085 mm) and it is
impractical to simulate the entire system to obtain the accurate
stress distribution in the MLCC. Hence, the following methodology was used in the present simulation. First, the twodimensional simulation was performed within the inner
loading span that is subjected to a constant moment and has a
dimension of 4 cm 9 2.685 mm. In this case, the simulated
result in the MLCC is expected to be not sufficiently accurate
because of the coarse mesh-size. In the absence of MLCC, the
analytical solution for the stress distribution within the inner
span of PCB can be obtained, such that
rx ¼
My
I
ð5Þ
where rx is the normal stress along the beam length direction,
M is the applied bending moment, and y is the distance from
the neutral axis. The stress distribution, rx, through the
thickness of PCB is linear. In the presence of MLCC, the
stress field within PCB is expected to be perturbed in the
region surrounding MLCC and, hence, deviates from linearity. Then, a section encompassing the perturbed area was
selected and the stress distribution described by Eq. (5) was
defined as the loading condition at the edges of the selected
section. In this case, the area in simulation is much smaller
than the entire system and more accurate simulation result
can be obtained for the MLCC. The first principal stress in
MLCC obtained from FEA during the board flex test is then
compared with the observed cracking.
4 Results and discussion
4.1 Acoustic emission
Fig. 3 A photo of the experimental setup for in situ cracking
observation
123
With the aid of telemicroscope to perform in situ observation, the crack initiation and its propagation can be
inspected. It provides a means to correlate the acoustic
J Mater Sci: Mater Electron (2014) 25:627–634
signal to cracking. First, the board flex test was performed
on PCB only, the acoustic threshold was set at 28 db, and
the acoustic signals were detected. These signals are
induced by bending of PCB. The characteristic acoustic
waveform of PCB bending is shown in Fig. 4a. The
amplitude of the corresponding acoustic signal wave
decays in a relatively short time (about 100 microseconds),
and its initial amplitude was up to *6 mV.
Then, the sample of MLCC mounted on PCB was cut
and polished to facilitate the in situ observation of cracking
using telemicroscope. The acoustic threshold was set at
28 db initially. In this case, several hits were detected
during the test, and the waveforms could be categorized
into three kinds. The first has the characteristic acoustic
waveform pattern of PCB bending (Fig. 4a). The second is
the cracking-induced acoustic signal, which could be
identified by the moment of cracking observed by telemicroscope. This cracking-induced acoustic wave has a relatively large amplitude (*10000 mV) as shown in Fig. 4b.
The third kind of acoustic wave has an amplitude of
*2 mV that is smaller than the previous two, and the
associated pattern of the waveform is also very different
and is shown in Fig. 4c. The source of this acoustic wave is
currently unknown; however, it might result from the
deformation and void growth in the solder during bending.
Since only the acoustic signal resulting from cracking of
MLCC is of significance, other signals could be treated as
background noises and should be removed for better
characterization of cracking. Therefore, the acoustic
threshold was raised to 45 db, and all of the solder-contributed acoustic signals were removed; however, most of
the PCB bending-contributed acoustic signals still existed.
Therefore, the threshold was then raised to 60 db, and
nearly all of the noises were removed. After raising the
threshold to 70 db, only the acoustic emission resulting
from cracking of MLCC was detected.
In order to identify the moment of cracking and to
correlate the acoustic signal to cracking, the MLCC samples used for in situ cracking observation using the telemicroscope were all cut and polished. However, for the
board flex test in the industrial application, cutting and
polishing the sample represent extra steps. Therefore, it
would be fruitful if cracking of MLCC during the board
flex test could be characterized without cutting and polishing of the specimen. Hence, the board flex test was
performed on the pristine MLCC specimen mounted on
PCB, the maximum deflection of PCB was set at 4 mm,
and the acoustic threshold was set at 70 db. When a hit was
detected, the corresponding acoustic waveform was found
to have the characteristics of Fig. 4b. The MLCC specimen
was then cut and polished and the crack was observed (as
shown in Fig. 5 where the crack initiation site is indicated
by an arrow). On the other hand, when no acoustic signal
631
Fig. 4 The characteristic acoustic waveform resulting from a PCB
bending with a fast decay, b cracking in MLCC with a large
amplitude, and c the deformation and void growth in the solder with a
small amplitude
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632
was detected, no crack was observed after cutting and
polishing of the MLCC specimen. This confirms that
cracking of MLCC during the board flex test could be
identified by acoustic emission without cutting and polishing of the specimen.
J Mater Sci: Mater Electron (2014) 25:627–634
which the stress field is perturbed in the presence of MLCC
can be identified readily. For example, Fig. 6b shows that the
stress field in the PCB is unperturbed when its position is
4 mm away from the symmetric axis which is denoted by an
arrow. The FEA was then re-run by setting the simulation
domain much smaller than the entire system but larger than the
4.2 Finite element analysis
The stress fields, rx, of PCB without and with MLCC
mounting subjected to a constant bending moment of 3.5 N
per unit width are showed, respectively, in Fig. 6a, b. It should
be noted that a thin layer (*40 lm) of copper pad is present
on the top surface of the PCB which, in turn, results in the
redistribution of the stresses at the left edge. The region in
Fig. 5 The crack pattern in MLCC subjected to board flex test and
the crack initiation site is indicated by an arrow
Fig. 6 The stress field, rx, of bending of PCB a in the absence and
b in the presence of MLCC mounting
123
Fig. 7 The simulation results for a rx in MLCC/PCB subjected to the
board flex test, b detailed principal stress field in MLCC, and c the
orientation of the principal stress in MLCC
J Mater Sci: Mater Electron (2014) 25:627–634
633
confirmed by the in situ cracking observation using a telemicroscope. Nanoindentation is used to obtain the
mechanical properties of MLCC in the nanoscale. Using
the FEA, the location of the simulated maximum principal
stress agrees with the crack initiation site and the orientation of the first principal stress is also consistent with the
observed crack path in MLCC during the board flex test.
Also, it was found that reduced stiffness of the PCB shifted
the location of the maximum principal stress in MLCC and
decreased the magnitude of the maximum principal stress
at a fixed deflection of PCB in the test.
Acknowledgments This work was supported by National Science
Council, Taiwan under Contract No. NSC100-2221-E-002-129 and
Yageo Corporation under Contract No. 99-S-A74.
Fig. 8 The simulated principal stress contour in MLCC/PCB subjected to the board flex test with underestimated Young’s modulus for
the PCB
perturbed region to improve the efficiency and accuracy of the
simulation results.
Adopting the new simulation domain and finer mesh, the
simulated rx is shown in Fig. 7a with the details of the first
principal stress in the MLCC shown in Fig. 7b when cracking
occurs at the deflection of 2.99 mm. The maximum principal
stress in the MLCC shown in Fig. 7b is 128 MPa that agrees
with Park et al.’s [18] result of *130 MPa. The location of the
maximum principal stress in MLCC agrees with the crack
initiation site, which is located in the dielectric ceramics at the
bottom end of the termination (see Fig. 5) and is indicated by an
arrow in Fig. 7b. The orientation of the principal stress is also
consistent with the observed crack path as shown in Fig. 7c.
It is worth mentioning that Young’s modulus of PCB
obtained from nanoindentation is 3.95 GPa, which is much
smaller than that obtained from three-point bending,
17.2 GPa. This is because of the glass fiber-reinforced
epoxy structure of the PCB and the surface property (i.e.,
the epoxy matrix) is measured by nanoindentation. Using
this underestimated Young’s modulus for PCB in the
simulation, the simulated first principal stress is shown in
Fig. 8. The maximum first principal stress location is no
longer at the bottom end of the termination. Instead, it is
located at the side face of MLCC and its magnitude is also
smaller (115 MPa in Fig. 8 vs. 128 MPa in Fig. 7b). The
effects of other parameters (e.g., electrode number/thickness, solder material/geometry, and lateral margin length,
etc.) on the stress distribution in the MLCC during board
flex test can also be examined using FEA.
5 Conclusions
The acoustic emission system is a powerful tool to detect
cracking of MLCC during the board flex test. This is
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