The Effects of Silica Fillers on
the Properties of Encapsulation
Molding Compounds
Yowching Liaw
Department of Engineering Science,
National Cheng Kung University,
Tainan 70101, Taiwan
Jung-Hua Chou1
Department of Engineering Science,
National Cheng Kung University,
Tainan 70101, Taiwan
email: jungchou@mail.ncku.edu.tw
Encapsulation molding compounds (EMCs) are commonly used to protect integrated circuit (IC) chips. Their composition always contains fillers of a large amount (about 70%)
and will affect the properties of the compounds. Thus, in order to clarify the filler effects,
in this study, three types of silica fillers including crystal silica, edgeless silica, and fused
silica were studied experimentally to explore their effects on the compounds. The results
show that all of the flow spiral length, glass transition temperature (Tg), coefficient of
thermal expansion (CTE), and water absorption rate of the encapsulation molding compounds decrease as the filler amount increases, irrespective of the filler type. In contrast,
both thermal conductivity and flexural strength of the compounds increase as the filler
amount increases, but also irrespective of the filler type. For the three fillers, the edgeless
silica filler has the advantage of a large flow spiral length and can be molded better. It
also has a larger thermal conductivity, larger flexural strength, and lower water absorption rate. Hence, for low stress industrial applications, the edgeless silica should be
adopted as the filler of the encapsulation molding compounds. [DOI: 10.1115/1.4037145]
Keywords: encapsulation molding compound, silica fillers, flow spiral length, thermal
conductivity, moisture, flexural strength
1
Introduction
Currently, silicon is the most commonly used material in the
semiconductor industry to fabricate electronic components. As the
silicon material is fragile, electronic components made from it
require proper protection to prevent possible damages from the
dissipated heat and from the environmental factors such as moisture, chemicals, dust, etc. For consumer electronics, the cost per
performance is of great concern; hence, the protection is typically
accomplished by polymer encapsulation molding compounds
(EMCs) due to their relatively lower cost and suitability for mass
production and automatic sealing.
With the quest for portability of electronic devices, the integrated circuit (IC) integration density constantly increases as projected by Moore’s law. One of the consequences of this trend is
that ICs tend have higher heat dissipation which can affect the
performance and reliability of the electronic devices. Thus, an
urgent need is for EMCs to have higher heat transfer capabilities.
Polymer EMCs are composite materials which generally contain resins, hardeners, and inorganic fillers (mainly the natural
silica). The literature [1] and our field experience indicated that by
increasing the filler content of the EMC, the thermal conductivity
of the EMC can be increased. Therefore, fillers have two main
advantages: a better thermal conductivity than that of polymers as
just described and a lower cost as compared to that of the epoxy
resin. In other words, a large amount of fillers is more desirable
for practical applications. However, increasing the filler amount
can lead to a reduced flow spiral length. This latter situation may
result in voids or unfilled molding regions in the encapsulated
devices and has to be resolved if the filler amount were to be
increased in the EMCs. Therefore, for this purpose, edgeless silica
fillers are introduced in the industry by grinding off the sharp
edges of the crystalline silica with the goal of reducing flow resistance to resolve the reduced flow spiral length issue.
1
Corresponding author.
Contributed by the Electronic and Photonic Packaging Division of ASME for
publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received
November 5, 2016; final manuscript received June 13, 2017; published online July
10, 2017. Assoc. Editor: S. Ravi Annapragada.
Journal of Electronic Packaging
Qualitatively, because of edgeless, it is expected that edgeless
silica can flow better to achieve a larger filler amount in the EMC.
Thus, an EMC with a larger thermal conductivity can be made for
high power electronic components. This kind of larger thermal
conductivity cannot be accomplished by the spherical silica, even
though it also flows better [2], due to its fused nature which has a
lower thermal conductivity. Hence, edgeless silica can be the
important filler for EMCs. Therefore, it is investigated in this
study to examine its pros and cons for industrial applications.
In the literature, the effects of filler loading on the EMC properties were mainly focused on crystal silica [1,3,4], fused silica
[1,4], and spherical silica fillers [5], because they are the popular
fillers adopted in the industry. In contrast, the performance of
edgeless silica as EMC fillers was rarely discussed, probably due
to its recent introduction to the industry. Thus, this study tried to
fill in this gap by exploring the effects of edgeless silica fillers on
the properties of EMCs to better serve the packaging community
requiring EMCs with larger thermal conductivities. Hence, the
main contribution of this study is to provide the merits of edgeless
silica as the fillers of the molding compounds.
The EMC properties are generally characterized by the flow
spiral length, dielectric constant, gel time, flammability, extracted
sodium, hydrolyzable chloride, volumetric resistivity, glass transition temperature (Tg), coefficient of thermal expansion (CTE),
water absorption rate, flexural strength, and thermal conductivity.
In this study, we focus on the thermal and mechanical characteristics of EMCs with the edgeless silica filler for its larger flow spiral
length. The characteristics pertinent to package reliabilities,
including the flow spiral length, thermal conductivity, CTE, Tg,
water absorption rate, and flexural strength, are compared to those
of fused and crystalline silica. Thus, their relative practical applicability can be delineated for the related industry. There are no
industrial standards for these characteristics as they depend on the
molding conditions, package sizes, and cost; they are always
determined by the customers’ specifications. For current industrial
applications, the flow spiral length is from 50 to 120 cm, Tg from
135 C to 170 C, CTE from 1.2 to 2.7 ppm/mK, moisture absorption from 0.2% to 0.35%, flexural strength from 15 to 20 kgf/cm2,
and thermal conductivity from 0.8 to 2.3 W/mK.
C 2017 by ASME
Copyright V
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2
Experimental Approach
2.1 Filler Properties. The properties of crystalline silica,
edgeless silica, and fused silica investigated in this study are as
follows:
(1) Crystalline silica: Crystalline silica is usually produced by
purification chemically and crushing mechanically natural
quartz. Its morphology has irregular shapes with sharp corners as shown in Fig. 1. The scale bar at the lower right corner of the scanning electron microscope picture is 100 lm
(likewise for Fig. 2). In the figure, the value of 40 lm of
D50 represents the median of the particle size distribution.
Because of the irregular shapes with sharp edges, crystalline silica has a larger flow resistance and tends to cause
adverse effects such as wearing the manufacturing equipment, and encapsulating and testing molds.
(2) Edgeless silica: Edgeless silica is made by grinding crystalline silica mechanically to turn the sharp edges into blunt
ones as shown in Fig. 2; therefore, it is commonly referred
to as blunt corner silica. Thus, it has a better flow spiral
characteristic due to its round edges and can also achieve a
higher filling speed during molding as compared to that of
crystalline silica. On the other hand, it costs more than that
of crystalline silica due to the additional grinding process.
(3) Fused silica: Fused silica is commonly made by melting
crystalline silica in high temperatures of 1900–2500 C
first. Then, after resolidification, the silica is crushed and
pulverized. Thus, fused silica is amorphous and transparent.
Because of the melting and resolidification processes, the
internal stress is released and also results in a smaller CTE
than that of crystalline silica [6].
Table 1 summarizes the specific area and particle size distributions of the three silica fillers. The sizes of crystalline and edgeless silica fillers are about the same and are larger than that of the
fused silica fillers.
2.2 Raw Material Compositions of Encapsulation Molding
Compounds. Table 2 lists the raw material compositions of the
EMCs used in this study. In the table, the weight percentages
(wt. %) of A, B and C are not given specifically due to commercial
constraints; only the equivalent molecular weights are shown. The
weight percentage of fillers was varied from 72% to 81% with an
increment of 3%.
2.3 Testing Methods. The EMCs had three different silica
fillers; each filler had four different loadings. That is, there was a
Fig. 2
Edgeless silica (D50 5 40 lm)
Table 1 Filler specific surface areas and particle size
distributions
Silica
Specific surface
area (m2/g)
D10 (lm)
D50 (lm)
D90 (lm)
0.618
0.707
0.696
3.697
3.169
3.231
26.781
23.178
18.129
86.071
87.034
74.751
Crystalline
Edgeless
Fused
total of 12 testing configurations. Five samples were tested for
each configuration. The methods of determining the EMC properties described above are as follows.
(1) Flow spiral length: For this test, the testing mold was
heated and pressurized between two plates first. Then, the
preheated molding compound was put into the cavity of the
testing mold and was further pressurized by the injection
cylinder for molding until the compound was solidified so
that the flow spiral length could be measured. Typically,
the recommended testing temperature and injection pressure [3] are 150 6 2 C and 1078 N/cm2, respectively. In
this study, the corresponding conditions were 175 6 2 C
and 1078 N/cm2.
(2) Tg and CTE: The specimen was heated in a thermomechanical analyzer (TMA, HITACHI_TMA7100) apparatus with
a temperature increasing rate of 5 C/min from 25 C to
245 C to measure Tg and CTE as illustrated in Fig. 3.
From the two temperature zones with linear increase in
temperature: one at the lower temperature and the other at
the higher temperature, two tangents were drawn and their
intersection was the Tg deduced. The corresponding CTEs
below and above Tg were calculated by the two formula
shown on the right-hand side of the figure.
(3) Water absorption rate: Samples of 50 mm in diameter and
3 mm in height were placed in a cooker and boiled in water
at 100 C for 24 h. Then, the weight difference before and
after boiling was measured to obtain the water absorption
rate. Namely, water absorption rate ¼ [(w2 w1)/w1]*100%,
where w1 and w2 represent the sample weights before and
after boiling, respectively.
Table 2 Raw material compositions of EMCs
Materials
Epoxy resin
Phenol hardener
Filler
Others
Fig. 1
Wt. %
Remark
A
B
72/75/78/81
C
Epoxy equivalent MW: 200
Phenol equivalent MW: 105
Crystalline, edgeless, and fused silica
Wax, coupling agent, etc.
Irregular polygons with sharp edges (D50 5 40 lm)
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Fig. 3 Specimen size and the determination of Tg and CTE
(4) Flexural strength: The flexural strength was measured by a
Tensile Strength Tester (HT-2402, Hung Ta Instrument)
under room temperature. The specimen size was 4 mm in
height, 9.8 mm in width, and 60 mm in length.
(5) Thermal conductivity: The thermal conductivity was measured by a QTM-500 thermal conductivity instrument of
Kyoto Electronics.
3
Results and Discussion
3.1 Flow Spiral Length (cm). The test results are shown in
Fig. 4 where the flow spiral length is expressed in centimeter with
a standard deviation of about 0.577–1.155 cm. It can be observed
that the flow spiral length of edgeless silica is the largest; whereas
that of fused silica is the smallest. The former can be twice that of
the latter, fairly independent of filler percentages. That is, the
EMC filled with edgeless silica flows better in the molding process due to its blunt edges which reduce flow resistance. On the
other hand, increasing the filler amount can reduce the flow spiral
length due to the solid nature of fillers, irrespective of filler types,
even though the extent of reduction is different for different
fillers.
The Mooney’s equation [3] given in the following equation can
help explaining the above observations:
ln ðg=g1 Þ ¼ KE w2 =ð1 w2 =wm Þ
(1)
In the above equation, wm and w2 represent the maximum filler
content and the filler content of the EMC, respectively; g andg1
are the viscosities of the resin composite and the resin matrix,
respectively. The value of the Einstein constant, KE, is 2.5 for
spherical particles and increases as the particle becomes longer
and thinner [7]. That is, KE reduces as the sharp-edged polygon
Fig. 5 Trends of viscosity versus filler amount in terms of KE
silica is ground into a blunt shape and results in a smaller g as
shown in Fig. 5. Moreover, the effect of filler content on KE is
more evident as the filler content increases. Therefore, if we want
to reduce KE while keeping g the same, then we can increase w2/
(1 w2/wm). As shown in Fig. 6, by keeping g constant, a smaller
KE will result in a larger filler amount, and the effect of filler
amount is larger when g is smaller. Because the viscosity is proportional to the filler amount [8], one can increase the filler percentage while keeping g constant by reducing KE. In other words,
by converting the polygon silica into edgeless silica, the result of
either reducing the viscosity to increase the flow spiral length or
increasing the filler amount while keeping g the same can be
achieved. For the latter situation, a filler amount of 10% more can
be added if edgeless silica is used instead of crystal silica; thus,
reducing the EMC cost as silica is cheaper than epoxy [9].
On the other hand, the densities of crystal silica and fused silica
are 2.64 g/cm3 and 2.2 g/cm3, respectively [4]. For the same
weight, the amount of filler added to EMCs of crystal silica is
smaller than that of fused silica. That is, the volumetric filling
ratio of crystal silica is smaller than that of fused silica. Therefore,
the flow spiral length of crystal silica is larger than that of fused
silica.
3.2 Glass Transition Temperature. Tg is critical to the process window of molding. As EMCs are a resin-based thermosetting material, their stress–strain visco-elastic behavior depends on
the degree of curing, temperature, and time [10,11]. In the preparation of the EMCs, the hardening of epoxy resin is accomplished
by phenol resin through cross-linking of the functional groups of
hardeners and resins to obtain a specific Tg.
The result of Tg versus filler amount is illustrated in Fig. 7,
showing that Tg decreases as the filler amount increases. Moreover, CTE values shown in Fig. 8 (to be described later) also
decrease as the filler amount increases. A larger CTE implies a
larger shrinkage of EMCs during curing. The extent of shrinkage
during curing is proportional to the degree of curing; more curing,
larger shrinkage [12]. The large shrinkage needs to be avoided
because it induces internal residual stresses which greatly affect
EMC reliabilities. Hence, adding fillers can reduce both thermal
and curing related shrinkage [13] as fillers can reduce the degree
of curing.
The degree of curing (a) is related to Tg as indicated by the following DiBenedetto equation [14]:
TgðaÞ ¼ Tg0 þ ½ððTg1 Tg0 ÞkaÞ=ð1 ð1 kÞaÞ
Fig. 4 EMC flow spiral length versus filler contents
Journal of Electronic Packaging
(2)
where Tg0 and Tg1 denote the glass transition temperatures corresponding to a ¼ 0 and 1, respectively; k is a shape parameter with
values between 0.2 and 0.6. The DiBenedetto equation shows that
the higher the degree of curing, the larger the glass transition
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Fig. 8 The variation of CTE versus filler amount
Fig. 6 Filler amount versus KE
temperature, consistent with the present result of decreasing Tg
with increasing filler amount illustrated in Fig. 7.
3.3 Coefficient of Thermal Expansion. In the encapsulating
molding process, internal residual stresses can be induced by two
mechanisms. One is the CTE mismatch of different materials used
in the electronic components. This could cause thermal stresses
and lead to interface cracks [15]. The other is the volumetric
change from the liquid state to solid state of EMCs during curing,
commonly referred to as “chemical cure shrinkage” [16]. These
stresses can lead to undesired warpage and require proper treatment [17]. Furthermore, high density packages for high temperature performance are getting popular recently [18]. That is, the
issue of thermal expansion needs to be addressed for reliability.
As mentioned above, by increasing the filler amount, CTEs
decrease as shown in Fig. 8, an observation consistent with that of
Bujard et al. [4] due to a larger CTE of epoxy than that of silica.
Because the CTE of EMCs is larger than those of leadframes and
IC chips, a reduced CTE of EMCs can reduce the thermally
induced stresses. As depicted in Fig. 8, the CTE of EMCs with
crystal silica is the same as that with edgeless silica. This is
expected because edgeless silica is made by grinding off the corners of crystal silica. In contrast, the CTE of EMCs with fused
silica is about 59% of EMCs with crystal silica because fused
silica is prepared by pulverizing the solidified crystal silica in
1900–2500 C Compared to the findings of Tan et al. [8] for which
the CTEs are 14 ppm/mK and 0.5 ppm/mK, respectively, for
EMCs with crystal silica and fused silica, our corresponding values are about 2.5 ppm/mK and 1.5 ppm/mK which are not that different as those given in Ref. [8].
3.4 Water Absorption Rate. Epoxy-based EMCs are not
hermetic and can absorb water through diffusion which is
Fig. 7 Variations of Tg versus filler amount (data standard
deviation between 1.0–4.3 C)
031007-4 / Vol. 139, SEPTEMBER 2017
temperature dependent [19]. Water can be absorbed by raw EMCs
in either powder or ingot form, especially in a wet environment,
consequently affecting the rheology of EMCs [20]. In addition,
the water absorbed by EMCs after molding reduces the adhesion
strength of the interface between the EMC and the leadframe [21]
and can lead to delamination easily [22,23]. Moreover, under the
same humidity condition, EMCs absorb more water at higher temperatures because of the presence of porosity [24]. Also, water
absorption leads to expansion and results in warpage. The critical
interface fracture toughness between the EMC and Cu (part of the
leadframe alloy) reduces by 20% with 85/85RH humidity at room
temperature [25]. Water absorption by EMCs is the main cause of
popcorn and delamination problems in the component packaging
[26] and should be avoided.
The results shown in Fig. 9 reveal that water absorption
decreases as the filler amount increases, consistent with those of
Refs. [5], [18], [26], and [27], due to poor water absorption of fillers and the reduced volume of epoxy in the EMC. In comparison,
water absorption of crystal silica is smaller than that of fused
silica as the latter was fused and pulverized from the former. In
contrast, there is no significant difference in water absorption
between edgeless and crystalline silica as they only differ slightly
in geometry; their small difference may be related to the microstructural arrangement inside the EMC composite [1] and requires
further investigation for validation.
3.5 Thermal Conductivity. The results portrayed in Fig. 10
illustrate that thermal conductivities increase as the filler amount
increases. This is expected because the thermal conductivities of
crystal silica and fused silica are 14 W/mK and 1.4 W/mK, respectively [4], much larger than that of epoxy of 0.02–0.04 W/mK.
Thus, fillers can enhance the thermal conductivity of EMCs [3],
irrespective of their types. The thermal conductivity of edgeless
silica is the same as that of crystal silica as expected. On the other
hand, the thermal conductivity of fused silica is smaller than that
of crystalline silica. It should be noted that other factors such as
surface roughness, surface hardness, surface impurity, etc., could
also affect the thermal conductivity [28], in addition to the filler
Fig. 9
Water absorption versus filler amount
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Fig. 12 The trend of flexural strength versus filler amount
(data standard deviation around 0.020–0.035)
Fig. 10 The variation of thermal conductivity versus filler
amount
type examined in this study. The effect of fillers on thermal conductivity can be explained by the semi-empirical model of
Lewis–Nielsen as given in the following equation:
Kc =Km ¼ ð1 þ ABVf Þ=ð1 BVf WÞ
(3)
In Eq. (3), A ¼ KE 1 is shape related, KE is the Einstein constant
(KE ¼ 2.5 for spheres as described earlier), B ¼ ½ðKf =Km Þ–1=
2
Vf ; Vmax , and Vf are
½ðKf =Km Þ þ A; W ¼ 1 þ ½ð1 Vmax Þ=Vmax
the filler maximum volumetric ratio and volumetric ratio, respectively; Kc, Km, and Kf are the thermal conductivities of the composite, matrix, and filler, respectively. Thus, as shown in Fig. 11
for illustrating the relationship between Kc/Km and the shape factor, based on Vmax ¼ 0.9 and Kf/Km ¼ 700, a larger KE implies a
larger thermal conductivity, and the effect is more evident with a
larger amount of fillers as demonstrated by the experimental
results.
3.6 Flexural Strength. Flexural strength is a load bearing
indicator which is a function of material stiffness; a larger stiffness implying a smaller bending deformation. The results shown
in Fig. 12 indicate that EMCs with a larger amount of fillers also
have a larger flexural strength, increasing almost linearly as Ref.
[3]. This is due to the stiffness provided by the fillers as the levels
of hardness of crystal silica and fused silica are Hv10 (GPa) and
Hv6 (GPa), respectively [8]. Hence, the flexural strength of EMCs
with crystal silica is larger than that with fused silica by about
3.8% as expected. In comparison, the flexural strength of EMCs
with edgeless silica is about 1.4% larger than that with crystal
silica. This latter difference is most likely due to the smaller particles of the edgeless silica. As edgeless silica was made by simply
grinding off the sharp edges of the crystal silica mechanically, its
volume per particle is smaller. That is, for the same wt. %, the
EMC will contain more edgeless silica particles than crystal silica
particles. Therefore, the edgeless silica particles will be packed
closer to each other in the EMC and result in a slightly larger flexural strength. Further experiments are needed for confirmation.
4
Conclusions
The effects of filler types and amounts on the properties of
encapsulation molding compounds are experimentally investigated in this study for three silica fillers. Key findings are as
follows:
(1) Irrespective of filler types, all of the flow spiral length, Tg,
CTE, and moisture absorption rate decrease as the filler
amount increases; whereas both thermal conductivity and
flexural strength increase.
(2) The flow spiral length of EMCs of different fillers in
descending order is edgeless silica > crystal silica > fused
silica.
(3) The thermal conductivity of both EMCs with edgeless
silica and crystal silica is the same and is much larger than
those with fused silica.
(4) Water absorption rate by EMCs with both edgeless silica
and crystal silica is about the same and is lower than those
with fused silica.
(5) The flexural strength of EMCs with different fillers in
descending order is edgeless silica > crystal silica > fused
silica.
Overall comparison reveals that the EMCs with edgeless silica
are better for low residual stress and/or high thermal conductivity
applications.
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