3D Simulation of Three-Dimensional Filling Process for Stacked-Chip Scale Packages (S-CSPs)

Abstract

Encapsulation is one of the key processes in electronic packaging and to protect the integrated circuit chips from environmental and mechanical damages. The most obvious choice for the encapsulation process is transfer moulding due to its capability to mould small parts with complex features. An electronic package that employs transfer moulding is Stacked-Chip Scale Package (S-CSP). However, a computer simulation is one of the tools that could be used to simulate and predict the mould process. It is highly desirable in order to avoid the typical time-consuming procedure of mould design and process optimization by trial and error. In this paper, a fully three-dimensional (3-D) analysis to predict transfer moulding process of S-CSP encapsulation using finite volume method (FVM) based software, FLUENT is presented. The proposed FVM simulation model is built and meshed using GAMBIT. Some simplification is done for the simulation model due to time consumption and complicated geometry of actual S-CSP model. In the analysis, volume of fluid (VOF) technique was used to track the flow front of the encapsulant. The viscosity versus shear rate is plotted and void formation problem also discussed. The numerical results are compared with the previous experimental results and found good conformity. KEY WORDS: Stacked-Chip Scale Package (S-CSP); Finite volume method (FVM), Volume of Fluid (VOF), Front tracking, Void.

Full text

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3D Simulation of Three-Dimensional Filling
Process for
Stacked-Chip Scale Packages (S-CSPs)
M.F.M.A. Majida, M.Sabri Sidika,M Husaini A.Ba, K. Shahrila, Zainal Nazria, Shukri Zaina, M. Z.
Abdullahb
aUniversiti Kuala Lumpur, Malaysian Spanish Institute,
Kulim Hi-Tech Park, Kedah
bSchool of Mechanical and Aerospace Engineering,
Universiti Sains Malaysia, Engineering Campus,
Seri Ampangan, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia.
Abstract—Encapsulation is one of the key processes in electronic
packaging and to protect the integrated circuit chips from
environmental and mechanical damages. The most obvious
choice for the encapsulation process is transfer moulding due to
its capability to mould small parts with complex features. An
electronic package that employs transfer moulding is Stacked-
Chip Scale Package (S-CSP). However, a computer simulation is
one of the tools that could be used to simulate and predict the
mould process. It is highly desirable in order to avoid the typical
time-consuming procedure of mould design and process
optimization by trial and error. In this paper, a fully three-
dimensional (3-D) analysis to predict transfer moulding process
of S-CSP encapsulation using finite volume method (FVM) based
software, FLUENT is presented. The proposed FVM simulation
model is built and meshed using GAMBIT. Some simplification is
done for the simulation model due to time consumption and
complicated geometry of actual S-CSP model. In the analysis,
volume of fluid (VOF) technique was used to track the flow front
of the encapsulant. The viscosity versus shear rate is plotted and
void formation problem also discussed. The numerical results are
compared with the previous experimental results and found good
conformity.
KEY WORDS: Stacked-Chip Scale Package (S-CSP); Finite
volume method (FVM), Volume of Fluid (VOF), Front tracking,
Void.
I. INTRODUCTION
Electronic packaging is defined as a package to house a
silicon chip in an electronic system. The main functions of an
electronic package are to protect the electronic components
from adverse environmental and mechanical effects and to act
as a structural support and electrical insulation [1]. It also
provides heat dissipation, signal timing and power distribution
[2]. In electronic packaging, one of the key processes is
encapsulation. Generally, transfer moulding and liquid
encapsulation are the most common encapsulation techniques.
In transfer moulding, epoxy moulding compound (EMC) is
preheated before loading into transfer port. By applying
certain pressure, the heated molten moulding compound is
transferred from the transfer port through the runners and then
into the mould cavities which may consist of a single or many
dies [3].
An example of electronic package using transfer moulding
process is Chip Scale Package (CSP) which is a package
whose area is less than 1.2 times the area of the IC. CSP has
smaller, thinner and lighter characteristics and has been
developed to address the demands of modern electronics. The
pace of CSP technology development is accelerating rapidly.
The semiconductor industry is driven by the broad adoption of
CSP in wireless handsets and handheld electronic devices.
Looking into the modern life today, the market demand for
thin, small, light and user friendly electronic packaging that
can provide wider variety of functions is still on the increase
in recent years [4-6]. The Stacked-Chip Scale Packages (S-
CSP) can be a best option to meet the aforesaid demands to a
remarkable extend. It integrates Application Specific
Integrated Circuit (ASIC) and memories such as flash, Static
Random Access Memory (SRAM), and Double Data Rate
(DDR) into one package by stacking dies, interconnecting
them with wire bonding and moulding all into one package
based on Joint Electronic Device Engineering Council
(JEDEC) standard [7-8]. S-CSP is adopted widely in portable
multi media devices such as cellular telephones, digital
cameras, PDAs and audio players [9].
There are several factors that can affect the mould filling
yields, such as die thickness (gap clearance), size and array
arrangement of complicated stacking dies. They are defined
as critical factors at initial stage of product quality planning
[10]. As the S-SCP mould is the matrix array type with thin
space and wide filling area, the quality concern of filling
process become very significant. It involves complex non-
Newtonian fluid flow, coupling heat transfer and chemical

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reaction. As a result, the problems like incomplete mould,
void formation, unbalanced flow and wire sweeping are
common. Moreover, these phenomena in the complex mould
geometry make it difficult to analyze the process and further
optimize the design [11].
Although transfer moulding is a mature technology, it is still
difficult to optimize and the mould design is a costly and
lengthy process. Prototype often requires numerous
modification and revision. To minimize the impact of these
problems and for better mould design and optimization,
numerical flow analysis during the encapsulation process is
needed [12]. Turng and Wang [13], Han and Wang [14], and
Nguyen et al [15-18] are the pioneers in numerical simulation
of the flow during encapsulation. Their numerical formulation
was mostly based on finite element method (FEM) coupled
with the volume of fluid (VOF) technique. Generalized Hele-
Shaw approximation was made for the fluid field. Basically,
the Hele-Shaw cell consists of two flat plates that are parallel
to each other and separated by a small distance. The Hele-
Shaw approximation uses gapwise-averaged mass and
momentum-conservation equations ignoring the gapwise
component of the flow. Moreover, the thickness of the model
is relatively small as compared to its width and length, and
viscous effect dominates the flow. Thus, the inertia effect is
negligible [19]. Abdullah et al. [20-21] presented flow
visualization and EMC rheology on S-CSP encapsulation
studies using finite difference method.
An alternative FVM-based three dimensional (3-D) mould
filling analysis using incompressible Navier-Stokes equation
is introduced in the current study. The epoxy moulding
compound (EMC) is modelled as a non-Newtonian fluid and
the EMC is treated as a generalized Newtonian fluid (GNF).
Accordingly, in our previous work [22], 3D simulation of
pressurized under-filling of flip chip package has been
presented. The simulations were done on the computational
fluid dynamic code FLUENT 6.3. The volume of fluid (VOF)
technique is used to track the flow front during calculation. In
the present study, we adopt the technique to investigate the
flow visualization and encapsulant filling in microchip
encapsulation process. Numerical results of flow front profiles
are compared with previous experimental results. In
additional, void formation are observed for the studies.
II. NUMERICAL MODEL
The three-dimensional incompressible flow namely
conservation of mass, Navier-Stokes equation and
conservation of energy for non-isothermal, generalized
Newtonian fluids (GNF) are given below:
i. Continuity equation:
0








z
w
y
v
x
u (1)
ii. Navier-Stokes Equation:
x-direction































2
2
2
2
2
2
1
z
u
y
u
x
u
x
p
z
u
w
y
u
v
x
u
u
t
u


(2)
y-direction































2
2
2
2
2
2
1
z
v
y
v
x
v
y
p
z
v
w
y
v
v
x
v
u
t
v


(3)
z-direction































2
2
2
2
2
2
1
z
w
y
w
x
w
w
p
z
w
w
y
w
v
x
w
u
t
w


(4)
iii. Energy equation:









TkTu
t
T
Cp

(5)
However, a modification in the conservation of energy has
been made by inserting energy source term. The energy
source term is as follows:





(6)
For predicting the relationship between viscosity and the
degree of polymerization that are given accordingly as:
The Cross rheology model:
 















o
T
T1
,0 (7)
where







T
T
BT b
exp
0

(8)
n is the power law index, 0

the zero shear rate viscosity,


is the parameter that describes the transition region
between zero shear rates and the power law region of the
viscosity curve,

 is the shear rate, B is an exponential-fitted
constant and Tb is a temperature fitted-constant, and T is the
absolute temperature.
In a three-dimensional filling simulation, accurate tracking
of the melt fronts as well as the representation and evolution
of the complex topology are very important. In the VOF
method the melt front can be tracked by solving the transport
equation of the fractional volume function. The transport

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equation can be solved by either in the geometrical approach
or the algebraic approach. The VOF equation is given as:
0
2
2
2
2
2
2






























z
F
y
F
x
F
t
F
w
t
F
v
t
F
u
t
F
dt
dF (9)
where F is defined either it is equal to one (F = 1) for fluid
region or equal to zero
(F = 0) for empty region and partially full if F has value in
between one and zero at the melt front (0<F<1).
III. SIMULATION SETUP
The volume of fluid (VOF) model in FLUENT 6.3.26 is
utilized to simulate the
S-CSP mold filling process. In the VOF model, a single set of
momentum equations is shared by the fluids, and the volume
fraction of each of the fluids in each computational cell is
tracked throughout the domain. Air and encapsulant material
Hitachi CEL-9200 XU (LF) [23] are defined as the phases in
the analysis and the mould temperature is set as 175oC.
Implicit solution and time dependent formulation are applied
for the volume fraction in every time step. The volume
fraction of the encapsulant material is defined as one and zero
value for air phase. Besides, viscosity cross model and VOF
techniques are applied to track the melt front. The model is
created by using GAMBIT software and total 94196
tetrahedral elements are generated for simulation. The
simulation took about seven hours to complete for a single
case. The S-CSP package model used in the present study and
its simplified model and the meshed model are shown in Fig. 1
and 2 respectively. Some simplifications have been made to
actual model such as replacement of chamfered corners by 90o
corners and removal of vents for simulation model. The
material properties [23] for the current study are summarized
in Table 1. The boundary and initial conditions are used in the
calculation are as follows:
a) On mould wall: 0


wvu ; w
TT  ; 0


n
p
b) On melt front: 0

p
c) On inlet:
g
ivenwvu



;

tzyxpp in ,,,; in
TT

Table 1 Material Properties [23]
Parameter Value
n 0.7938
7.264E-4
B 5.558E-4
7.166E3
2.000E3
1079
0.97
a) Actual model b) Simplified model
Fig. 1. The actual and simplified S-CSP models.
Fig. 2. 3D meshed model

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IV. RESULTS AND DISCUSSION
The experiments have been done to investigate the flow
behaviour of epoxy moulding compound (EMC) inside the
actual mould of Stacked-Chip Scale Package (S-CSP) with
twelve arrays of six stacking dies. The EMC used in this
investigation is HITACHI CEL-9200-XU (LF). The mould
temperature is set at 175oC and the package pressure is 70
kg/cm2. Short-shot results have been performed to observe the
melt front advancement at different times step. The short-shot
samples can be obtained by setting certain stroke length of the
plunger. The package will be incomplete if the sets less stroke
length. Thus the front profile can be attained. Fig. 3 illustrates
the gate, air vents, stacking dies in matrix array of 4 × 3 and
the flow direction.
Fig. 4 demonstrates respectively the experiment and
simulation (short shot) results of melt front advancement with
inlet velocity of 4 mm/s in the S-CSP. Hitachi CEL-9200-XU
(LF) is used as an encapsulant in the simulation. The
encapsulation process shows the good agreement of flow front
profile at 1.5s and 2s for experimental and simulation result.
The mould compound flows around the dies and moves
quickly before it starts to cover the dies. However, the flow
above the dies covers more area in experiments compared to
that of simulation. The effect of dies on the flow fronts is
clearly visible. It restricted the flow along the edges and over
the dies. As a result, the flow around the dies is accelerated.
Fig. 3. Schematic of flow in the cavity
Experiment Simulation Experiment Simulation
1.5 s 2.0 s
2.5 s 3.0 s
Gate
Free passage flow
Backside flow
Die
Top die flow
Vent

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3.5 s 4.0 s
5.0 s 6.0s
Fig. 4. Short shot results of melt front advancement at distinct time steps with velocity inlet of 4 mm/s
However, at 2.5s the simulation flow front profile is found
more slowly in filling compared to experiments. This
phenomenon is caused by the simplification of the simulation
model. The simplification on the stacked chip may be a factor
for affecting the encapsulant during the process. The
simplified chips act as a larger obstruction to the flow and
caused the encapsulant to fill the free region. At 6s of filling
stage, void is found in the mold filling process. Fig. 5 shows
the void formation of S-CSP filling process.
Fig. 5. Void formation at circled region.
Fig.6 shows viscosity variation versus shear rate. The curves
show as a power law viscosity variation where the viscosity
reduces with the shear rate.
Fig. 6. Viscosity versus Shear rate
V. CONCLUSION
A three-dimensional non-isothermal incompressible analysis
model based on finite difference method (FVM) for the
transfer moulding is presented and compared with the
experimental results. The three dimensional S-CSP package is
simulated to study the flow visualization in the process. The
encapsulant material used is Hitachi Chemical CEL-9200-XU
(LF). Cross-viscosity model and volume of fluid (VOF)
technique are used to track the flow front in the numerical
simulation. Navier-Stokes equations are solved by finite
volume method and SIMPLE segregated algorithm. It is found
that simulation results of melt fronts are in good agreement
with the experimental results, thus proving the strength of the
model and the fluent software in handling stacked chip
encapsulant problems. The present study may be extended

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further for more actual type of stacked chip and different
parameter on different S-CSP packages.
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