IMAPS Nordic 2017 Conference on Microelectronics Packaging
NordPac 2017
Improved Reliability of Electrically Conductive Adhesives Joints on Cu-Plated PCB Substrate Enhanced by Graphene Protection Barrier Shirong Huang1, Wei Ke1, Yiqun Yang1, Hui Ye1, Shujing Chen1, Bo Shan1, Jie Bao2, Qianlong Wang3, Guangjie Yuan1, Xiuzhen Lu1, Yan Zhang1, Yifeng Fu4, Lilei Ye5, Johan Liu1, 4 1)SMIT Center, School of Mechatronics Engineering and Automation, Shanghai University, Shanghai, P. R. China 2) School of Mechanical and Electrical Engineering, Huangshan University, Huangshan 245041 3) Shenzhen Institute of Advanced Graphene Application and Technology (SIAGAT), Shenzhen 518106, China 4) Electronics Materials and Systems Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, SE-412 96, Gothenburg, Sweden 5) SHT Smart High Tech AB, SE-411 33 Gothenburg, Sweden Corresponding author:
[email protected] Abstract—Graphene protection barrier was introduced to
applications of commercial ECAs’ in electronic packaging,
the interface between the ECAs and Cu-plated wire to enhance
among which one critical obstacle is that the contact resistance
the reliability of the ECAs joints on Cu-Plated PCB substrate due
between the ECAs and nonoble metal substrate increases
to its excellent properties of impermeability to all gases/salts as
significantly during high temperature and high humidity aging
well as its thermal/chemical stability. The results of shear test
test [4-6]. It has been reported that galvanic corrosion is the
indicated graphene protection barrier can improve the shear
mainly underlying mechanism for the contact resistance shift
strength of the ECAs joints on Cu-plated PCB substrate by
during high temperature and high humidity aging, which results
almost 22% after 500 hours high temperature and high humidity
from metal oxide formation between the ECAs and metal
cyclic test. Characterizations by optical microscope and XPS
substrate [7]. Therefore, it is of great necessity to modify the
were further performed to explain the mechanism. To sum up, it
contact between the ECAs and nonoble metal substrate by
can be believed that the graphene protection barrier can
introducing humid-proof covering or modified conductive
dramatically enhance the reliability of the ECAs joints on
adhesives to inhibit the galvanic corrosion and further improve
Cu-Plated PCB substrate.
the reliability of ECAs joints on nonoble metal substrate.
Keywords: Graphene protection barrier; Cu-Plated PCB; Reliability
Graphene, a novel two-dimensional nanomaterial with single atom layer thickness, has attracted immense attention recently due to its extraordinary physical and chemical
I. INTRODUCTION Electrically conductive adhesives (ECAs) possess many advantages outperforming the traditional interconnecting materials in electronic assemblies for surface mounting applications, such as the tin lead solders, including the lower processing temperature, finer pitch capability and reduced environmental impact, which have been considered as an environmentally friendly and promising alternative to solders [1-3]. However, there are still some limitations for the wide
ISBN: 978-1-5386-3054-9
properties, which has high mobility of charge carriers of 10 000 cm2 V-1 s-1 at room temperature, high thermal conductivity of 5000 Wm-1 K-1 (10 times higher than copper) and high optical transparency of around 97.7%, respectively [8-11]. In particular, the properties of both the impermeability to all gases/salts and the thermal/chemical stability make graphene an excellent candidate for a novel anti-corrosion protection barrier [12-13]. Recently, many publications on graphene anti-corrosion application for different metal have been
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IMAPS Nordic 2017 Conference on Microelectronics Packaging
NordPac 2017
reported, including copper foil, nickel, iron etc. [14-16]. In our
onto 40 oC heating platform to get the solvent evaporated. To
group, Hui Ye et al. has reported that graphene based barrier
ensure graphene protection barrier coated onto copper wire and
can prevent ECA from aging and keep the contact resistance
wire gap well, both the drop-casting and drying procedure
o
between ECA and nonoble metal wire stable under 85 C/85%
needed to be repeated several times. In the second step, the
RH tests [17]. To the best of our knowledge, the application of
ECAs were dispensed to fill in the wire gap as well as bridge
graphene utilized as anti-corrosion protection barrier to
the whole copper wire (see Fig 1(b)), which were then cured
improve the reliability of ECAs joints on nonoble metal
under required conditions. At the same time, the samples
substrate has not been reported yet.
without graphene protection barrier named bare group were
In this work, the reliability of ECAs joints on Cu-plated
also prepared in the purpose of comparison.
PCB substrate enhanced by graphene protection barrier was investigated. The graphene protection barrier was applied to the Cu-plated PCB substrate by drop-casting method and the ECAs were then dispensed onto the PCB substrate. After the high temperature and high humidity aging test (85 oC/85% RH, 500 h), the shear strength of the ECAs joints on Cu-Plated PCB substrate was measured. The results show that the shear strength of the ECAs joints on Cu-plated PCB substrate enhanced by graphene protection barrier can be improved by about 22% compared with the control group (without graphene protection barrier). Both X-ray photoelectron spectroscopy (XPS)
and
scanning
electron
microscope
(SEM)
characterizations of the shearing cross-sections were further carried out to analyze the morphology and explain the mechanism. II. EXPERIMENTAL Fig. 1 Schematic diagram of samples preparation including two steps (a)
A. Material Preparation
Step 1, graphene dispensed by drop-casting method (b) Step2, ECA
ECA, CT220HK-S1, 74.2% sliver filling, standard curing
dispensed onto graphene protection barrier
conditions, 1.5h @ 150 oC, Fairfield USA Inc.; graphene, XF019, 1mg/ml, water solvent with PVP dispersing agent, Nanjing XFNANO Materials Tech Co.,Ltd; PCB substrate, FR4 material with Cu-plated wire, Kunshan SuYuan Electronics Group Co.,Ltd.
C. High Temperature and High Humidity Cyclic Test (85 o
C/85% RH)
To investigate the anticorrosion effect of graphene protection barrier on the ECAs joints on Cu-Plated PCB
B. Samples Fabrication
substrate, both the samples with graphene protection barrier
The whole process of samples fabrication was divided into
and the bare group were carried out the aging test of high
two steps, just as illustrated in Fig. 1. The PCB substrates were
temperature and high humidity cyclic test (85 oC/85% RH) for
cleaned by acetone and dried in the air at first. The graphene
500 hours. After that, the shear strength of the ECAs joints on
solution was diluted from 1 mg/ml to 0.01 mg/ml before
Cu-Plated PCB substrate was evaluated by shear tester (DAGE
dispensing onto the copper wire of the PCB substrates and the
4000).
wire gap (gap distance is 1 mm) by drop-casting method, as shown in Fig.1 (a). The PCB substrates were then transferred
ISBN: 978-1-5386-3054-9
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IMAPS Nordic 2017 Conference on Microelectronics Packaging
III. RESULTS AND DISCUSSIONS
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To further verify the chemical composition of the black
SEM and AFM characterization were employed to
dots on the shear cross-sections in Fig. 3, XPS were utilized to
characterize the layer number of the graphene, as shown in
characterize these black dots, as illustrated in Fig. 4. It should
Fig.2. It can be found that the thickness of graphene is about
be mentioned that the presence of Cu2+ usually results in the
0.8 nm, which means that the layer number of graphene applied
appearance of a satellite peak around the binding energy of 942
here is 2-3 layers.
eV [18-19]. In the case of the spectrum presented in Fig. 4(b), no evidence for such a satellite peak exists and subsequently the Cu2+ is not observed. This can confirm that the Cu-plated wire has already been oxidized by galvanic corrosion in bare group without graphene protection barrier during the high temperature and high humidity cyclic test while the copper wire with graphene protection barrier hasn’t been oxidized by galvanic corrosion due to the existence of the graphene protection barrier.
Fig. 2 SEM and AFM characterization of graphene (a) SEM (b) AFM
Fig.3 shows the shear strength of ECA joints on Cu-Plated PCB substrate after the high temperature and high humidity cyclic test (85 oC/85% RH) for 500 hours. From the results, it indicates that the shear strength of ECA joints on Cu-Plated PCB substrate can be enhanced by graphene protection barrier and improved by up to 22%. Optical microscope (OM)
Fig. 4 XPS characterization of shear cross-sections for ECA joints (a)
characterization of the shearing cross-sections of ECA joints on
samples without graphene protection barrier (b) samples with graphene
Cu-Plated PCB substrate were taken at the same time (inset
protection barrier
picture in Fig 3). From the OM results, some black dots appeared on the ECA joints shear cross-sections of the samples
Meanwhile,
SEM
characterizations
of
the
shear
without graphene protection barrier while no black dots were
cross-sections for ECA joints were also implemented, as shown
found from the samples with graphene protection barrier.
in Fig.5.
Fig. 5 SEM images of shear cross-sections for ECA joints (a) samples with graphene protection barrier (b) samples without graphene protection barrier
Fig. 3 Shear strength of ECA joints on Cu-Plated PCB substrate for samples both with graphene protection barrier and without, inset pictures show the OM characterization
In Fig. 5, it can be found that there are some graphene flakes on the cross-sections surface from graphene group whilst there are cube particles on the cross-sections from the bare group, these cube particle maybe copper oxide particle caused
ISBN: 978-1-5386-3054-9
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IMAPS Nordic 2017 Conference on Microelectronics Packaging
NordPac 2017
by the galvanic corrosion during the high temperature and high humidity cyclic test.
[4].
IV. CONCLUSIONS
[5].
In this work, graphene protection barrier was introduced to the interface between the ECAs and Cu-plated wire to enhance the reliability of the ECAs joints on Cu-Plated PCB substrate.
[6].
The graphene protection barrier can improve the shear strength of the ECAs joints on Cu-Plated PCB substrate by up to 22%
[7].
even after the high temperature and high humidity cyclic test. Characterization by optical microscope as well as XPS was
[8].
further performed to explain the mechanism. The results show that the graphene protection barrier can enhance the reliability
[9].
of the ECAs joints on the Cu-Plated PCB substrate. [10].
ACKNOWLEDGEMENT This work is supported by the National Science Foundation
[11].
of China (Project No: U1537104, 11672171) and Shanghai Municipal Education Commission (Shanghai University High
[12].
Education Peak Discipline Program). We acknowledge the support from the Swedish Foundation for Strategic Research
[13].
(SSF) (contract No:SE13-0061 and E13-0061), Swedish
[14].
National
Board
for
Innovation
(Vinnova)
Graphene
SIO-Agenda Program, the Production Area of Advance at
[15].
Chalmers University of Technology, Sweden. We also thank the support from Anhui International Technology Cooperation
[16].
Program (No: 1704e1002208) and Anhui University Excellent [17].
Young Talents Support Program (No: gxyq2017074). REFERENCES
[18].
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