Letter
www.acsami.org
Thermally Stable Siloxane Hybrid Matrix with Low Dielectric Loss for
Copper-Clad Laminates for High-Frequency Applications
Yong Ho Kim, Young-Woo Lim, Yun Hyeok Kim, and Byeong-Soo Bae*
Laboratory of Optical Materials and Coating (LOMC) Department of Materials Science and Engineering, Korea Advanced Institute
of Science and Technology (KAIST), Daejeon, Republic of Korea
S Supporting Information
*
ABSTRACT: We report vinyl-phenyl siloxane hybrid material (VPH)
that can be used as a matrix for copper-clad laminates (CCLs) for highfrequency applications. The CCLs, with a VPH matrix fabricated via
radical polymerization of resin blend consisting of sol−gel-derived linear
vinyl oligosiloxane and bulky siloxane monomer, phenyltris(trimethylsiloxy)silane, achieve low dielectric constant (Dk) and
dissipation factor (Df). The CCLs with the VPH matrix exhibit excellent
dielectric performance (Dk = 2.75, Df = 0.0015 at 1 GHz) with stability
in wide frequency range (1 MHz to 10 GHz) and at high temperature (up
to 275 °C). Also, the VPH shows good flame resistance without any
additives. These results suggest the potential of the VPH for use in highspeed IC boards.
KEYWORDS: copper clad laminates, low dielectric loss, low dielectric constant, siloxane hybrid material, thermal stability
T
thermal stability, are currently used as alternatives to conventional epoxy resins.10,11 Furthermore, epoxy based nanocomposite and modified glass fabric that can enhance the
performances of CCLs have been reported.12,13 Despite such
efforts, the dielectric properties of such epoxy resins are not
greatly improved because of the presence of polar hydroxyl
groups, which drastically increase the values of Dk and Df in
polymeric dielectrics via the curing of the epoxy resins.14−16
To address this issue, researchers use engineering plastics
(e.g., polyimide, poly(p-phenylene ether), bismaleimide, and
polytetrafluoroethylene) for HPCCLs.1,2,4 Despite their high
thermal stability and excellent dielectric properties, such
engineering plastics generally have poor processability. In an
attempt to improve their processing ability, engineering plastics
blended with other kinds of thermoplastic or thermoset
polymers have been reported. However, such polymer
composites lose their own excellent dielectric and thermal
stability because of the decreased cross-linking density of the
original polymer matrix.2,3
In this paper, we report, to the best our knowledge, the first
siloxane based matrix for CCLs to exhibit excellent dielectric
properties (low Dk ≈ 2.75, low Df ≈ 0.0013 @ 1 GHz) that are
maintained at high temperature (∼275 °C) and in a high
frequency range (up to 10 GHz), allowing the fabricatation of
HPCCLs based on vinyl-phenyl siloxane hybrid material
(VPH). In VPH, in order to decrease Dk and Df, a high
he integrated circuit (IC) board is one of the greatest
inventions in the electrical industry during the past
century and is a crucial component of most electrical devices in
the current era.1−4 The IC board, which is fabricated with
copper clad laminates (CCLs) and electronic components,
plays critical roles in serving as a structural basis and performing
electrical functions for electrical devices by interconnecting the
electronic components. CCLs are key base materials to
fabricate IC boards; CCLs consist of a polymeric matrix
reinforced with glass fabric and copper layers. Among these
components, the polymeric matrices mainly determine the
dielectric, thermal, and mechanical properties of CCLs.2,3
Because of their acceptable dielectric properties, thermal
stability, and good processability, the polymeric matrices
conventionally used for CCLs are epoxy resins.5−7 However,
modern electrical devices such as mobile phones, computers,
network routers, etc., are more integrated and require highspeed information transfer in a high-frequency range, so there
has been increasing demand for high performance CCLs
(HPCCLs) that have lower dielectric constants (Dk) and
dissipation factors (Df) and higher thermal stability that can be
maintained in high frequency range; these factors are not
achievable by conventional epoxy resins.1,8 It should be noted
that the signal propagation speed in IC boards is inversely
proportional to the square root of Dk; the signal propagation
loss is proportional to frequency, Df, and the square root of
Dk.9,10
Modified epoxy resins (e.g., dicyclopentadiene epoxy),
because of their enhanced thermal and dielectric properties
with good process ability by introducing dicyclopentadiene
(DCPD) groups that have excellent dielectric properties and
© XXXX American Chemical Society
Received: February 4, 2016
Accepted: March 15, 2016
A
DOI: 10.1021/acsami.6b01497
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Scheme 1. Fabrication of CCLs with Vinyl-Phenyl Siloxane Hybrid Matrix: (a) Synthesis of LVPO Resin from Sol−Gel
Condensation of VMDMS and DPSD; (b) Schematic Illustration of the Arrangement of VPH and Linear Vinyl−Phenyl Siloxane
Hybrid Material (LVPH); (c) Fabrication Procedure of CCLs with VPH Matrix; LVPO/PTTMSS Resin Blend Was
Impregnated in Glass Fabric, Then Sandwiched between Two Copper Foils; The Sample Was Cured by Hot Vacuum Pressing;
Inset Photographs are the LVPO/PTTMSS Resin Blend (upper left) and the CCLs with the VPH Matrix (lower right)
structure and mechanisms of the radical polymerization, ). 7628
woven E-glass fabric (a kind of standard woven fabric
developed by IPC), which is commercially available and
commonly used for printed circuit board, was used for
mechanical enhancement of the final product. First, four sheets
of woven glass fabrics were impregnated with the LVPO/
PTTMSS resin blend, which is a viscous, clear, and thermally
curable resin blend. The four drenched sheets were then
stacked on a copper layer perpendicular to each other (across
or along the fiber orientation). After the sample was covered
with another copper layer, the laminating process was
conducted in vacuum hot pressing, which is a typical fabrication
method of CCLs; subsequently the LVPO/PTTMSS resin
blend was cured via radical polymerization to form VPH-based
CCLs.
To optimize the dielectric properties of VPH, we compared
properties of various linear vinyl-phenyl siloxane hybrid
materials (LVPH, i.e., a PTTMSS-free version of VPH) with
varying Ph/Si ratios (Figure 1a). The assessment of the
amount of rigid aromatic moiety is introduced, along with bulky
siloxane monomer, which can increase the free volume.
As illustrated in Scheme 1, CCLs with the VPH matrix were
fabricated by impregnation and lamination of glass fabrics with
thermally curable resin blend consisting of linear vinyl-phenyl
oligosiloxane (LVPO) and phenyltris(trimethylsiloxy)silane
(PTTMSS) between two copper layers; this is the conventional
fabrication method of CCLs. The LVPO resin was synthesized
by mild nonhydrolytic sol−gel condensation between vinylmethyldimethoxysilane (VMDMS) and diphenylsilanediol
(DPSD) (Scheme 1a). The detailed synthesis method and
the expected structures of LVPO were previously reported (see
the Figures S1 and S2 for the FT-IR and 29Si NMR spectra).17
To decrease Dk and Df of VPH, we blended a bulky methyl
monomer, phenyltris(trimethylsiloxy)silane (PTTMSS), with
LVPO at a certain amount for increasing free volume of the
VPH (Figure 1b). 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane
was used as an initiator for the radical polymerization of the
LVPO/PTTMSS resin blend (see the Figures S3 and S4 for the
B
DOI: 10.1021/acsami.6b01497
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. Optimization of dielectric properties of the LVPH. (a) Dielectric properties of the LVPH depending on varying Ph/Si ratio. (b) The peak
value of the tan δ of the LVPH in DMA measurement. Note that the greater damping peak reflects the lower rigidity of the materials. (c) Dielectric
properties of VPH depending on varying amounts of PTTMSS. (d) Density of VPH depending on varying amounts of PTTMSS.
Figure 2. (a) Dielectric constant and (b) dissipation factor of VPH and DCPD epoxy according to 1 MHz to 10 GHz frequency range at room
temperature. VPH exhibits superior dielectric properties in the entire measured frequency range. In particular, the dissipation factor of VPH
maintains its superiority up to the 10 GHz range. The inset photograph shows a disk-like bulk sample of VPH (left) and the DCPD epoxy (right)
with 2.5 cm diameter and 2 mm thickness.
where ε∞ is the dielectric constant of infinite frequency, ε0 is
the static dielectric constant, ω is the angular frequency, and τ
is the relaxation time.19 The high rigidity leads to high
relaxation time, resulting in a decrease in the dielectric constant.
These results suggest that introducing phenyl groups in the
LVPH can have a greater contribution to increasing the rigidity
than to increasing polarizability. The increased rigidity of the
LVPH with higher amount of phenyl groups can be confirmed
by the low tan δ peak value in the DMA measurements (Figure
1b).20 The LVPO with Ph/Si ratio higher than 1 might have
better dielectric properties; however, they have poor storage
stability resulting in precipitation (see Figure S5).
Generating free volume is the one effective way to decrease
the dielectric constant and the dielectric loss.21,22 PTTMSS, a
siloxane monomer with methyl groups, was introduced in an
dielectric properties was conducted using pure bulk samples in
order to exclude the effects of the other components of the
CCLs such as glass fabric or copper layers. Interestingly, at
higher amounts of phenyl groups, both the dielectric constant
and dissipation factor decreased. From point of view in polymer
dielectric, the phenyl groups are like a double-edged sword
because introducing phenyl groups can increase the dielectric
constant because of their high polarizability but can also
decrease the dielectric constant because of their high rigidity.18
According to the Debye relaxation model, the real and
imaginary parts of the dielectric constant can be expressed by
the following equation
ε′ = ε∞ +
ε0 − ε∞
1 + ω 2τ 2
, ε″ =
ε0 − ε∞
1 + ω 2τ 2
ωτ
C
DOI: 10.1021/acsami.6b01497
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
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Figure 3. Thermal resistance tests of VPH and DCPD epoxy. (a) The plot of tan δ of VPH and DCPD epoxy measured by DMA. Note that low
intensity of tan δ reflects high rigidity of materials. (b) TGA measurement of the VPH and DCPD epoxy with 5 °C/min ramping rate in nitrogen
atmosphere. Solid lines indicate the weight % and dashed lines indicate derivative of weight %. (c) Dk and (d) Df of the VPH and DCPD epoxy
according to the temperature from 25 to 275 °C at intervals of 50 °C at 10 GHz frequency. DCPD epoxy shows significant increases of Dk and Df at
275 °C, whereas VPH shows stable performance up to 275 °C.
attempt to increase the free volume of the VPH. As illustrated
in Scheme 1b, VPH with PTTMSS could have higher free
volume because of its steric hindrance, which originates from its
bulky structure, as evidenced by the lower density of the VPH
compared to its PTTMSS-free version (Figure 1d); 4.85 wt %
PTTMSS is the most effective amount for decreasing the values
of Dk and Df (Figure 1c, d). Above 4.85 wt % PTTMSS, the
decreasing rate of the density was gradually dropped and the
values of Dk and Df increased. This is thought to be due to the
excess amount of the PTTMSS filling the free volume in the
VPH rather effectively increasing the free volume of VPH.To
determine the best dielectric performance of VPH as a matrix
for CCLs, we blended LVPO, whose Ph/Si ratio is 1, with 4.85
wt % PTTMSS because that resin blend exhibits the best
dielectric properties.
To evaluate the dielectric performance reliability in terms of
the stability over a wide frequency range, we compared the
dielectric properties of the VPH and DCPD epoxy (a
commercial high performance epoxy) at 1 MHz, 1 GHz, and
10 GHz (Figure 2). The DCPD epoxy was used as reference for
comparison. The dielectric properties of the VPH (Dk = 2.75
and Df = 0.0015 at 1 GHz) are superior to those of DCPD
epoxy (Dk = 3.08 and Df = 0.0167 at 1 GHz) in the 1 MHz to
10 GHz frequency range. In particular, the Df of the VPH
shows stable performance up to 10 GHz (Df = 0.0033 @ 10
GHz). This result confirms the excellent dielectric performance
stability of the VPH over a wide frequency range.
Because the fabrication process of electronic devices applies
heat during soldering and because the operation of electronic
devices inevitably generates heat, thermal resistance is another
important factor for CCLs in terms of both thermal
decomposition and dielectric properties. Because of the
degradation of dielectric properties and the drastic dimensional
change of the epoxy matrix at the glass transition temperature
(Tg), followed by subsequent delamination of the copper layer,
which can be confirmed by the peak temperature of tan δ in
DMA measurement,23 Tg is generally considered an indicator of
the thermal resistance of epoxy-based CCLs.24 Tg, however, is
not an appropriate indicator for comparing the thermal
resistance between DCPD epoxy and VPH, as evidenced by
the TGA measurement (Figure 3a, b). It should be noted that
VPH, which has a lower Tg value than that of the DCPD epoxy,
has higher thermal decomposition temperature than DCPD
epoxy, which confirms the higher thermal resistance of VPH
(Figure 3b).
To evaluate the thermal stability in terms of the dielectric
properties of the VPH, we assessed the dielectric properties in a
temperature range of 25 to 275 °C at intervals of 50 °C at 10
GHz frequency (Figure 3c, d). As the temperature increases,
the values of Dk of both VPH and DCPD epoxy increase
because of the reduced relaxation time that result from thermal
agitation.18 Above the Tg value of the DCPD epoxy, significant
increases of Dk and Df were observed, whereas VPH maintains
its excellent dielectric properties (Dk = 2.73, Df = 0.0039 at 275
°C) up to 275 °C (Figure 3a, c, d). These results suggest that
the VPH maintains its structural rigidity at high temperature,
suppressing the chain mobility, as further evidenced by the
small value of the maximum peak of tan δ.
The properties of the VPH were further confirmed by
assessing the dielectric properties and SEM analysis of the
interfaces of the CCLs fabricated with the VPH matrix (Figure
4a and Table 1, see the Figure S6 for the SEM image of the
D
DOI: 10.1021/acsami.6b01497
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
frequency range (up to 10 GHz) and in a wide temperature
range (up to 275 °C); they exhibited excellent and stable
dielectric performance. The CCLs with the VPH matrix also
shows good thermal decomposition and flame resistance
without any additives.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b01497.
Experimental section, FT-IR and 29Si NMR spectra of
LVPO resin, structure of 2,5-bis(tert-butylperoxy)-2,5dimethylhexane, mechanisms for radical polymerization
between vinyl-functionalized and methyl-functionalized
siloxane resin, a photograph of precipitated LVPO resin
with 1.5 Ph/Si ratio, and SEM image of the cross-section
of the fabricated CCLs (PDF)
Video of the simple flame resistance test with conventional lighter (AVI)
Figure 4. (a) Fabricated CCLs based on VPH. The CCLs were
fabricated with 4 sheets of 7628 E-glass fabric between 2 layers of
copper layer via vacuum hot pressing. (b) Bent CCLs based on VPH
after thermal aging at 260 °C for 2 h; lower photograph is a 50×
magnification of the cross-section. There is no delamination of any
layers even after thermal aging.
Table 1. Comparison of Properties of CCLs with Varying
Matrices
a
dielectric constant
dissipation factora
T260
flame resistance
a
VPH
DCPD epoxy
FR-4b
3.56
0.002
> 60 min
V-0
4.0
0.012
∼30 min
X
4.0−4.5
∼0.02
■
V-0
*E-mail: bsbae@kaist.ac.kr.
AUTHOR INFORMATION
Corresponding Author
b
Notes
Dielectric properties measured at 1 GHz frequency. From ref 25.
The authors declare no competing financial interest.
■
cross-section of the CCLs). The fabrication procedure of the
CCLs with the VPH matrix was conducted almost same
condition with the procedure for the fabrication of the CCLS
with epoxy matrix except for curing temperature, pressure and
time, which confirms high processability of the VPH matrix
(see the Supporting Information for the experiment details).
The CCLs with the VPH matrix still exhibit superior dielectric
properties compared to those of DCPD epoxy or the
conventional FR-4 CCLs (Table 1). T260, which specifies the
time to delamination of the layers, is considered as a useful
indicator for comparing the maximum service temperature. The
CCLs with the VPH matrix maintains their laminated state after
an hour of thermal aging at 260 °C, even in a bended state,
whereas the CCLs with the DCPD epoxy matrix shows
delamination after 30 min, which indicates a tendency
consistent with that demonstrated by the TGA measurement
(Figure 4b).
Flame resistance is another important issue for CCLs;
however, petroleum-based polymers such as epoxy resins are
easily ignitable.26 To achieve flame resistance, halogenated or
phosphorus compounds are generally added to epoxy resins,
but the additives can lead to negative health or environmental
issues.27 On the other hand, VPH, a siloxane-based material, is
not ignitable. The CCLs with the VPH matrix display good fire
resistance without any additives; on these substances, a flame
does not self-propagate (Table 1 and Video S1).
In summary, we report a low dielectric constant (Dk = 2.75
at 1 GHz), low dissipation factor (Df = 0.0013 at 1 GHz)
siloxane matrix for CCLs using vinyl-phenyl siloxane hybrid
material (VPH). CCLs with VPH matrix were fabricated via
radical polymerization of a resin blend consisting of sol−gel
derived linear vinyl-phenyl oligosiloxane and bulky monomer,
phenyltris(trimethylsiloxy)silane. The addition of the bulky
monomer effectively enhances the dielectric properties by
generating free volume in the VPH. The dielectric properties of
the CCLs with the VPH matrix were maintained in a wide
ACKNOWLEDGMENTS
This work was supported by KOLON Industries Inc. and
National Research Foundation of Korea (NRF) grant funded
by the Korea government (MSIP) (NRF2015R1A2A1A15056057). This work was also supported by a
grant from the Korea Evaluation Institute of Industrial
Technology (Project 10051337). We gratefully thank to the
KOLON Industries Inc. for fabricating and measuring the
characteristics of the CCLs and Korea Basic Science Institute
(KBSI) for 29Si NMR spectra measurement.
■
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