In recent years, interfacial fracture becomes one of the most important
problems in the assessment of reliability of electronics packaging. Especially,
underfill resin is used with solder joints in flip chip packaging for
preventing the thermal fatigue fracture in solder joints. In general, the
interfacial strength has been evaluated on the basis of interfacial fracture
mechanics concept. However, as the size of devices decrease, it is difficult to
evaluate the interfacial strength quantitatively. Most of researches in the
interfacial fracture were conducted on the basis of the assumption of the
perfectly bonding condition though the interface has the micro-scale structure
and the bonding is often imperfect. In this study, the mechanical model of the
interfacial structure of resin in electronic components was proposed.
Bimaterial model with the imperfect bonding condition was examined by using a
finite element analysis (FEA). Stress field in the vicinity of interface
depends on the interfacial structure with the imperfect bonding. In the front
of interfacial crack tip, the behavior of process zone is affected by
interfacial structure. However, the instability of fracture for macroscopic
crack which means the fracture toughness is governed by the stress intensity
factor based on the fracture mechanics concept.Comment: Submitted on behalf of TIMA Editions
  (http://irevues.inist.fr/tima-editions

Matsumoto, T.

Matsuzaki, T.

Shibutani, T.

Shiratori, M.

Yu, Q.

English

arXiv

In recent years, advances in information technology are taking place with tremendous speed. Especially in electronic devices, pursuit of reduction in size and weight, sophistication, and increase in speed never seem to stop. Since the first computer chip was invented in the 1970¡¯s, it only took less than 30 years for the spread of computers to regular households. In order to support such rapid growth, electronic device technology has been put under great pressure, and numerous problems have been raised one after another. Uses not only in computers but also in cellular phones, portable music players, and recently in car technologies, electronic devices are in need to be produced in diversity in terms of forms, composing materials, and environment in which they are used. Also in order to improve in function, device structures are increasing in complexity. Along these stated above, there is absolute need to consider about environmental issues, such as reducing use of load, recycling, and long duration life designing. Other than gaining speed in advancing technology, reliance designing is an important issue as well. Recently, problems in mounting reliability include delamination and fatigue in adhesive jointing structures. One of the most significant technologies in terms of reliance improvement in mounting structures is the use of underfill materials between the chips and the substrate that lies underneath. First, using underfill reduces the impact caused by mismatches in the coefficient of thermal expansion (CTE) between the silicon chip and the attaching substrate when there are changes in the temperature of the environment, such as heating up of the circuit. Second, underfill reduces the impact of physical shocks and bending. In surface mounting structures where the chip is attached directly to the substrate with solder joints, the solder joints themselves represent the weakest points in the structure and therefore are most susceptible to stress failure¡ªobviously the most critical problems for a failure at any interconnect point leads to failure of the functionality of the whole circuit. By tightly adhering the chip, solder balls and substrate using underfill, stresses and strains are redistributed, and problems at weak points can be greatly reduced. Other than the advantages stated above, underfill materials can be used to protect structures from moisture and contamination. For these reasons, the use of underfill materials is a technology necessary in electronic device production. However, on the other hand, there are problems caused from using underfill. By underfilling the structure, in addition to the conventional problems such as the weakness of solder joints at both the chip side and the substrate side, stress is generated at the chips-underfilll interface and the undefill-substrate interface as well. This leads to concerns about interfacial delamination and is a critical issue in terms of reliability. Including the complex structure around interfaces expected to cause delamination, different destruction modes such as chips cracking and solder cracking effect each other and eventually result in the destruction of the whole structure. And this leaves necessity for unified assessment of the destruction modes, and for predicting the reliability of the structure. Structural materials are often assumed to be bond to each other perfectly. However, as shown in the bottom figures, bonded interfaces are constructed partly, and localized nonlinear deformations are often appears. To evaluate the interfacial strength, microstructure of interfaces should be comprehended. In this study, the strength evaluation technique on the resin field side is examined. In particular, microstructure of interface is¡¡the main factor. Then, observation of interfacial structure by using SEM during low cycle mechanical fatigue clarifies the microstructure and the interfacial model with complex structure is constructed. In addition, destruction behavior in micro interfacial area is clarified by doing the analysis that considers the destruction process

Yu, Qiang

I-Revues

MODELING WITH STRUCTURE OF RESINS IN ELECTRONIC 
COMPONENTS 
 
Qiang YU*, Tadahiro SHIBUTANI*, Masaki SHIRATORI*, Tomio MATSUZAKI* 
Tsubasa MATSUMOTO* 
 
*Department of Mechanical Engineering and Materials Science 
Yokohama National University 
79-5, Tokiwadai, Hodogaya-ku, 240-8501, Japan 
 
 
ABSTRACT 
In recent years, interfacial fracture becomes one of the 
most important problems in the assessment of reliability 
of electronics packaging. Especially, underfill resin is 
used with solder joints in flip chip packaging for 
preventing the thermal fatigue fracture in solder joints. In 
general, the interfacial strength has been evaluated on the 
basis of interfacial fracture mechanics concept. However, 
as the size of devices decrease, it is difficult to evaluate 
the interfacial strength quantitatively. Most of researches 
in the interfacial fracture were conducted on the basis of 
the assumption of the perfectly bonding condition though 
the interface has the micro-scale structure and the bonding 
is often imperfect. In this study, the mechanical model of 
the interfacial structure of resin in electronic components 
was proposed. Bimaterial model with the imperfect 
bonding condition was examined by using a finite element 
analysis (FEA). Stress field in the vicinity of interface 
depends on the interfacial structure with the imperfect 
bonding. In the front of interfacial crack tip, the behavior 
of process zone is affected by interfacial structure. 
However, the instability of fracture for macroscopic crack 
which means the fracture toughness is governed by the 
stress intensity factor based on the fracture mechanics 
concept.  
 
1. INTRODUCTION 
 
In recent years everyday life has become more and more 
convenience through significant progress in 
computerization. On the other hand, there is great stress 
put upon the technology that supports these new 
technologies. There is no end to demanding smaller, 
lighter, more high tech and faster electronic devices, and 
the modern field of science is struggling with many 
problems that need to be overcome. These problems 
include expanding the variation of forms and materials 
used in the production, adaptation to a wider range of 
environment where these technologies may be used, 
making device forms more complicated, consideration of 
environmental issues (i.e. reduction of lead usage, 
recycling, long life expectancy designing), and foundation 
of a method for reliability designing. One of the 
significant methods to improve the strength of chip 
assemblies these days is the dispensing of resin between 
the spaces between the silicon chip and the substrate, 
where the circuits come in action through solder balls1),2). 
The most critical failures in chip assembles are the 
disconnections of the circuit, and the joints of different 
materials, such as the chips and the solder ball joints, are 
the most susceptible to failure. These failures occur when 
stress concentrates at the weak points in the structure 
often caused by shocks and differences in the coefficient 
of thermal expansion (CTE) in different materials. Resin 
materials are used for the purpose of redistributing the 
stress and reducing the effect of differences in CTE.  
Since electronic devices consist of various materials, 
there are many interfaces: metal/ceramic, metal/resin, 
ceramic/resin and so on. Therefore, interfacial cracking 
affects the reliability of devices. Interfacial strength has 
been often evaluated as the fracture toughness on the basis 
of fracture mechanics concept3),4). Perfect bonding was 
assumed and stress intensity factor is dominant parameter 
of interfacial failure. However, as the size of devices 
decreases, interfacial fracture behavior becomes more 
complicated. Figure 1 shows an example of interfacial 
cracking between resins. These materials are bonded 
partly and interfacial structure can be seen at the crack tip. 
As the devices size becomes smaller, the effect of 
interfacial structure appears. Additionally, in the 
interfacial facture mechanics, the failure process of 
interface has been ambiguous yet. Therefore, interfacial 
structure-based evaluation should be constructed. In this 
study, the simple partly bonded interface model was 
proposed. FEA for the proposed model was compared 
with the bimaterial interface model. The effect of 
interfacial structure was extracted. Considering the failure 
criteria of the interfacial structure, the behavior of 
interfacial fracture was examined.  
 
 
 
2. INTERFACIAL STRUCTURE BASED MODEL 
 
Figure 2 shows the bimaterial model with interfacial 
structure. Bonding parts of which width and height are a 
and h, are arrayed periodically with the interval s. FEA 
analysis was conducted under two boundary conditions: 
uniaxial tensile and thermal loading. In the former cases 
as shown in Fig.2, the bottom and right sides were fixed. 
Uniform tensile stress was applied on the upper of resin. 
In the latter case, temperature changed from 150 C to 20 
C. No stress is applied at 150 C. Five models were 
prepared where sizes of bonding structure, a and h range 
as shown in Table I. 4-node plain strain element was used 
and their numbers of elements and nodes are shown in 
Table I. Fine mesh was constructed near the interfacial 
zone. The solver is Marc 2005. Materials are assumed to 
be isotropic elastic body and each material property is 
listed in Table II. Additionally, by considering the fracture 
property of interfacial structure, the failure behavior at the 
crack tip was investigated. In this case, the interfacial 
structure and resin is assumed to be elasto-plastic body. In 
order to determine the criteria of fracture, the maximum 
plastic strain was employed. Using the subroutine 
“uactive”, elements which reach the criteria are 
deactivated.  
 
 
3. RESULT AND DISCUSSION 
 
3.1. Uni-axial tensile 
Figure 3 (a) and (b) are distributions of σy and τxy 
along the interface, respectively. Each plotted data is 
taken at the middle of interfacial structure. The stress 
curves in bimaterial model with perfect bonding are also 
shown by solid lines. When s is small, the curve is close 
to the solid line. Since separated area is small, the 
structure is almost that in the perfect bonding. Other 
curves are higher than the solid line. When interfacial 
height h is low, the stress σy increases. The mismatch of 
deformation concentrates near the interface and the strain 
in the normal direction depends on the height of 
interfacial structure. On the other hands, in the case of τxy, 
when interfacial height h is low, the stress decreases. It 
suggests that the interfacial structure affects the mix mode 
condition at the edge of interface. The effect of interface’s 
height appears in the middle of model. Though the 
horizontal mismatch of displacement is generated at the 
edge of interface, the vertical mismatch affects the shear 
stress in the middle of model.  
 
3.2. Residual stress 
 
Figure 4 (a) and (b) shows the distribution of the residual 
stresses. When a=5 and s=5, (the fraction of bonded area 
is 50%), the residual stress, σy  increases. Not only the 
bonded area decreases, and but also the mismatch of 
deformation is changed due to the rigidity of interfacial 
structure. It implies that the decrease in the bonding area 
causes that the effect of interfacial structure appears. 
When the height of interfacial structure, h becomes high, 
the stress increases. The effective zone of localized 
deformation due to the mismatch depends on the bonding 
structure. When h is low, not only the interfacial structure 
but also bulk materials can be deformed by the mismatch 
of CTE. On the other hand, when h is high, the 
deformation by mismatch concentrates on the bonding 
area.  
Comparing with the uniaxial tensile state, the effect of 
interface height on the shear stress is not so big. CTE 
mismatch does not depend on the interfacial structure and 
subject each interfacial structure. Therefore, the width and 
interval of bonding part mainly affect the behavior of 
shear stress.  
 
3.3. Fracture of interface 
 
Figure 5 and 6 show the failure process, near the crack tip 
under uniform tensile stress state. The fracture in the 
interfacial structure occurred in the front of the tip. As the 
applied stress increase, failure process proceeds. Finally, 
instability of fracture takes place. Figure.7 and Figure.8 
are the stress distribution from the tip at the instability of 
fracture. When the stress singularity field near the crack 
tip reaches critical value, the instability begins. It implies 
that macroscopic stress field governs the interfacial 
fracture. On the other side, stress depends on the 
interfacial structure in r < 100 µm. Therefore, the process 
zone is affected by the interfacial model. In the case of 
electronics components, the size of failure is less than that 
of process zone. It suggests the importance of the 
interfacial structure model concept proposed in this paper. 
 
4. CONCLUSION  
 
1) Interfacial stress behavior in the state of elasticity of 
the proposed two dimension model was able to be 
confirmed. 
2) The stress field along the interface depends on the 
interfacial structure. At the edge of interface, mixed 
mode state is affected by the bonding structure.  
3) In the front of the crack, the process zone is affected 
by the interfacial structure. On the other hand, when 
the crack length is long, the fracture toughness obeys 
on the stress singularity field along the interface.  
4) In electronics components, the fracture behavior of 
process zone affects the reliability of devices. FEA 
results in this study suggest the importance of the 
interfacial structure. 
 
REFERENCES 
1) Hirohata, K., Kawamura, N., Mukai, M., 
Kawakami, T., Aoki, H., Takahashi, K., 
“Mechanical fatigue test method for chip/underfill 
delamination in flip-chip packages”, IEEE Trans. 
on Electronics Packaging Manufacturing, Vol. 25, 
No. 3, pp. 217-222, 2002. 
2) Dutta, I, Gopinath, A, Marshall, C, “Underfill 
constraint effects during thermomechanical cycling 
of flip-chip solder joints”, Journal of Electronic 
Materials, Vol. 31, No. 4, 2002. 
3) Timothy Ferguson and Jianmin Qu, “Effect of 
Moisture on the Interfacial Adhesion of the 
Underfill/Solder Mask Interface”, Journal of 
Electronic Packaging, Vol. 124, No. 2, pp. 106-110, 
2002.  
4) Zhong, Z.W. , Wong, K.W. , Shi, X.Q., “Interfacial 
behavior of a flip-chip structure under thermal 
testing”, IEEE Trans. on Electronics Packaging 
Manufacturing, Vol. 27, No. 1, pp. 43-48, 2004. 
 
 
 
Fig.1 Interfacial structure between resins in the front of 
crack tip with imperfect bonding. 
 
 
 
h1：Height of resin
h2：Height of silicon
a:Interfacial width
s:Interfacial
interval width
h:Interfacial height
w:Width of substrate
ac:Width of initial 
crackw
Si
resin a s
h
x
y
ac
h1
h2
 
Fig.2. Proposed model 
 
 
Table I Each parameter of model [µm] 
 
837276262551000500500
837376260.5551000500500
11154102301151000500500
1045995321351000500500
837276261551000500500
numbers 
of 
nodes
numbers 
of 
elements
hsawh2h1
 
 
 
 
 
Table II Material properties 
 
-3.60.26130silicon
60190.310interface
60190.310resin
Initial yield 
stress
(MPa)
CTE
(*10-6/K)
Poisson's 
ratio
(-)
Young's 
modulus
(GPa)
 
 
 
 
0.00E+00
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
3.00E+01
3.50E+01
4.00E+01
0 100 200 300 400 500 600
ｘ(µm)
σy
(M
Pa
)
Complete Interface
a=5 s=5 h=1
a=5 s=3 h=1
a=5 s=1 h=1
a=5 s=5 h=0.5
a=5 s=5 h=2
 
Fig.3.(a) Distributions of  σy  when uniform tensile 
stress was applied on the upper of resin  
(h1=h2=500 w=1000 µm) 
 
0.00E+00
2.00E+00
4.00E+00
6.00E+00
8.00E+00
1.00E+01
1.20E+01
1.40E+01
0 100 200 300 400 500 600
x(µm)
τｘ
ｙ
(M
Pa
)
Coplete Interface
a=5 s=5 h=1
a=5 s=3 h=1
a=5 s=1 h=1
a=5 s=5 h=0.5
a=5 s=5 h=2
 
Fig.3. (b) Distribution of τxy when uniform tensile 
stress was applied on the upper of resin 
(h1=h2=500µm w=1000µm) 
 
 
 
-2.00E+01
0.00E+00
2.00E+01
4.00E+01
6.00E+01
8.00E+01
1.00E+02
1.20E+02
1.40E+02
0 100 200 300 400 500 600
x(µm)
σｙ
(M
Pa
)
Complete Interface
a=5 s=5 h=1
a=5 s=3 h=1
a=5 s=1 h=1
a=5 s=5 h=0.5
a=5 s=5 h=2
 
Fig.4. (a) Distribution of σy 
(20C from 150C h1=h2=500 w=1000µm) 
 
 
 
0.00E+00
1.00E+01
2.00E+01
3.00E+01
4.00E+01
5.00E+01
6.00E+01
7.00E+01
0 100 200 300 400 500 600
x(µm)
τｘ
ｙ
（ M
Pa
)
Complete Interface
a=5 s=5 h=1
a=5 s=3 h=1
a=5 s=1 h=1
a=5 s=5 h=0.5
a=5 s=5 h=2
 
Fig.4. (b) Distribution of τxy 
(20C from 150C h1=h2=500µm w=1000µm) 
 
10µm
σy of stress  
 
Fig.5. Crack growth from the crack tip 
 
 
10µm
σy of stress  
 
    Fig.6. Crack growth from the crack tip 
 
 0.00E+00
2.00E+01
4.00E+01
6.00E+01
8.00E+01
1.00E+02
1.20E+02
1.40E+02
0 100 200 300 400 500 600 700
Distance from crack point(µm)
σｙ
（ M
Pa
)
a=5 s=5 h=1
a=5 s=1 h=1
a=5 s=5 h=0.5
 
    Fig.7. Distribution of σy from the crack tip at the 
instability of fracture 
(ac=100µm h1=h2=500µm w=1000µm ) 
 
 
0.00E+00
2.00E+01
4.00E+01
6.00E+01
8.00E+01
1.00E+02
1.20E+02
0 100 200 300 400 500 600 700
Distance from crack point(µm)
σｙ
（ M
Pa
)
a=5 s=5 h=1
a=5 s=1 h==1
a=5 s=5 h=0.5
 
Fig.8. Distribution of σy from the crack tip at the 
instability of fracture 
 (ac=300µm h1=h2=500µm w=1000µm ) 
 
 
 


Modeling with structure of resins in electonic compornents

Abstract

Similar works

Full text

Available Versions

I-Revues