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Mechanical and electrical cold bonding based on metallic nanowire surface fasteners

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2012 Nanotechnology 23 365202(http://iopscience.iop.org/0957-4484/23/36/365202)
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IOP PUBLISHINGNANOTECHNOLOGY

Nanotechnology 23 (2012) 365202 (6pp)

doi:10.1088/0957-4484/23/36/365202

Mechanical and electrical cold bonding
based on metallic nanowire surface
fasteners
Yang Ju, Masahiro Amano and Mingji Chen
Department of Mechanical Science and Engineering, Nagoya University, Nagoya 464-8603, Japan
E-mail: ju@mech.nagoya-u.ac.jp

Received 19 March 2012, in final form 6 July 2012
Published 21 August 2012
Online at stacks.iop.org/Nano/23/365202
Abstract
Mass production of surface mount devices (SMDs) relies heavily on reflow soldering and has
become the cornerstone of today’s electronic industry. However, the traditional reflow
soldering technique is characterized by high heating temperatures, toxic solder materials andlow recycling rate of SMDs. Here, we propose a new patterned structure of Au nanowire
arrays named a surface fastener through which cold bonding for surface mount technology canbe realized. The mechanical bonding enables normal and shear bonding strengths of more than
5 N cm−2 . Simultaneously, the parasitic resistance of a pair of surface fasteners is only
approximately 2 . The present technique can be performed at room temperature, thereby
improving the process compatibility and reliability of SMDs. Surface fasteners based on highmelting point metallic nanowires are temperature-resistant for many critical applications. In
addition, bonding without solder material is positive for the recycling of rare metals in SMDs.
(Some figures may appear in colour only in the online journal)

1. Introduction

With regard to developing a cold bonding technique forsurface mount technology (SMT), one possible approach is
to make use of cold welding. Cold welding of thin gold
films on elastomeric supports has been carried out underambient conditions and low loads [1]. However, only thelower limit of approximately 0.1 N cm−2 was reported for theadhesion strength. In recent years, researchers have succeededin joining individual low-dimensional nanostructures [2–6].
Although direct heating was not performed and large forceswere not applied in these nanoscale welding techniques,fine manipulation of an individual nanowire or nanotube byspecific equipment was always needed. Additionally, thesetechniques, in which the connection of two nanowires or
nanotubes was performed in one process, are sufficient fornanoscale connection but are inefficient for use in the mass
production of SMDs. On the other hand, the discovery of vander Waals interactions as the primary adhesive mechanism
in geckos [7, 8] has led to an explosion of efforts totry to replicate this adhesion mode by some well-arranged
micro-/nanostructure arrays [9]. Micro-/nanostructures ofsoft polymers [10–19] were cast or formed within porous

Mass production of surface mount devices (SMDs) relies
heavily on reflow soldering and has become the cornerstoneof today’s electronic industry. However, heating temperaturesof up to 220 ◦ C during reflow soldering may cause not only
energy consumption but also thermal damage to the surface
mount components. Additionally, the toxicity of traditional
Sn–Pb solder has led to a trend of worldwide legislationthat mandates the removal of lead from electronics. Althoughvarious types of lead-free solder have been proposed andadopted in the electronics industry, the melting points ofthese lead-free solders are always 5–20 ◦ C higher than Sn–Pb
solder. In addition, the recycling of rare metals in the surface
mount components and printed circuit boards is not easy due
to the difficulties in detaching the components, circuit boards
and solder materials. Therefore, there is an increasingly urgent
need for a nontoxic and low-temperature bonding technique toafford good mechanical support as well as electrical contact,especially for micro/nano-electronic circuits and flexible
electronic devices.
0957-4484/12/365202+06$33.00

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c 2012 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 23 (2012) 365202

Y Ju et al

Figure 1. Sketches and micrographs of the samples under test. (a) Sketch of samples I and II with specific patterns. (b) Micrographs ofsample I after the synthesis of Au nanowire arrays. (c) Connection of samples I and II, together with the electrical and mechanical testing
setup.

membranes. The specific shapes of these polymer micro/nanostructures, such as mushroom-shaped fibers [11–14],
wedge-shaped fibers [15], bent fibers [16, 17], and fibers withflared tips [18] or spatula tips [19], led to considerable shear
and normal adhesion strengths but also more complicatedmanufacturing processes. Biomimetic adhesives fabricated bygas phase methods were always made of stiff and high aspectratio fibers [20–27]. Compared with the casting method, more
expensive equipment and higher processing temperatures
are needed in the gas phase fabrication method [9], andthe normal adhesion strength contributed by these stiff
nanostructures is always much smaller than the shear adhesion
strength. Until now, most existing biomimetic adhesives haveaimed to realize strong shear binding-on and easy normal
lifting-off [20] for special applications such as climbing
robots [18], and almost all of them have been designed to
implement universal binding with large surfaces, such as
walls and glass plates. Additionally, electrical connection has
rarely been realized in the aforementioned work. Althoughelectrical connection between a pair of hybrid core–multishell
nanowire forests has been reported, normal adhesion was notrealized [26]. Data for adhesion and electrical resistance wereacquired in adhesion between carbon nanotubes (CNTs) andflat surfaces [27]. However, the contact electrical resistance
was as high as several hundred ohms after the pressure wasremoved, which cannot be adopted for electrical connectionin SMT. Therefore, to the best of our knowledge, there is noreport in the literature of a biomimetic adhesive which canmeet the requirements of simultaneous high normal and shear
adhesion strengths and good electrical connections for SMT.
Here, we propose a new patterned structure of Au
nanowire arrays named a surface fastener through which cold
bonding for SMT can be realized. The mechanical bonding
enables normal and shear bonding strengths of more than
5 N cm−2 . Simultaneously, the parasitic resistance of a pairof surface fasteners is only approximately 2 . This technique
can be performed at room temperature, thereby improving
the process compatibility and reliability of SMDs. Besides,
bonding without solder material is positive for the recyclingof rare metals in SMDs.

2. Experiment2.1. Pattern design and fabrication
Samples I and II, which have specific patterns for the fastenerareas and printed wires, as shown in figure 1(a), were designed
to facilitate the testing of the mechanical bonding strengthand the parasitic resistance of the electrical bonding. The
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Nanotechnology 23 (2012) 365202

Y Ju et al

Figure 2. Microstructures and components of electrodeposited Au nanowires: (a) SEM image, (b) EDXS pattern and TEM images ofindividual nanowires with diameters of (c) 100 nm and (d) 200 nm.

diameter of each of the four fastener areas was 2 mm. Au
films approximately 100 nm thick were deposited onto thefastener and wire areas on Si substrates with a 30 nm thick Cr
adhesive layer to improve the adhesion between the substrate
and the Au film. Both the adhesive layer and the Au film were
deposited by electron beam (EB) evaporation.

The parasitic resistance was measured by the four-pointprobe method after the preload was completely removed(figure 1(c)), and was quite different from that reported
in [27]. During the measurement, electrical current from 0 to
20 mA was applied by the current source to the four-pointprobe measuring circuits, and U–I curves were generated. The
normal and shear bonding strengths were tested by measuring
the maximum forces that the surface fasteners could afford(figure 1(c)). The weights of a balance ranging from 200 mg
to 100 g were used to fulfil this testing.

2.2. Electrodeposition of Au nanowires
Porous alumina (PA) membranes serving as the templates
for synthesis of Au nanowires were fixed right above thefastener areas by the insulation holders. Three types ofcommercial-grade PA membrane with nominal pore diametersof 20, 100 and 200 nm were utilized in the experiments. Au
nanowire arrays were then synthesized in the fastener areas byelectrodeposition under a constant current of approximately
0.001 A in 40 g l−1 KAu(CN)2 and 100 g l−1 KH2 PO4
aqueous solution at room temperature. After etching in 3 M
NaOH aqueous solution to remove the PA membranes, the Au
nanowires were observed by scanning electron microscopy(SEM), as shown in figure 1(b). It should be mentioned
that, although uniform growth of Au nanowires can be
implemented over a large-scale area, cabinet fastener areas
which were 2 mm in diameter were adopted in our design
according to the normal size of current solder pads in SMDs.

3. Results and discussion
After electrodeposition and removal of the PA membranes,
it was easy to distinguish the fastener area from the printed
wire by their surfaces because the former became dark incolor (figure 1(b)). SEM observation indicated that mostof the Au nanowires were grown vertically on the fastenerarea but with random orientation (figure 2(a)). The average
length of these Au nanowires was approximately 10 µm.
The nanowires were demonstrated to be polycrystalline Au
nanowires by energy dispersive x-ray spectroscopy (EDXS)
and transmission electron microscopy (TEM), as shown infigures 2(b)–(d), respectively.
The voltage–current (U–I) curves of one pair of fasteners
are shown in figure 3. The solid lines in figure 3 were obtained
through linear fitting of the measured values. Figure 3(a)shows the U–I curve corresponding to a pair of fasteners
based on Au nanowires with nominal diameters of 200 nm
under different preloads. It can be seen from this figure thatbonding under larger preloading results in superior electricalconduction performance (with smaller parasitic resistance).

2.3. Testing of parasitic resistance and bonding strength
Samples I and II, which had patterned Au nanowire arrayson the fastener areas, were connected to each other under
different magnitudes of preload (4.9, 9.8 and 19 N).
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Nanotechnology 23 (2012) 365202

Y Ju et al

Figure 3. Measured U–I curves of the bonding. (a) U–I curves
under different magnitudes of preload. (b) U–I curves based on
nanowires with different diameters.

The reason for this relationship is that the electrical bondingis implemented by the connection between two groups ofAu nanowires, and a relatively large preload can make theconnection of the nanowire arrays stable enough to enable
electrical conduction. Taking the total area of the four fastenerareas into account, the preload pressures resulting from loads
of 4.9, 9.8 and 19 N are 0.39, 0.78 and 1.51 MPa, respectively.
It is noted from this figure that preload pressures of 0.78
and 1.51 MPa give rise to similar parasitic resistances ofabout 1.89 and 1.71 , respectively. Therefore, cost-effective
electrical bonding may occur under a preload pressure of
approximately 1 MPa. The U–I curves of bonding basedon Au nanowires with different diameters of 20, 100 and
200 nm under a preload of 19 N are presented in figure 3(b).
Typical Ohmic contact performance was observed for eachof these three bonding conditions. Parasitic resistances of
2.20, 1.75 and 1.71  can be extracted from the slopesof the fitted lines of the three groups of nanowires withdiameters of 20, 100 and 200 nm, respectively. It should be
mentioned that the parasitic resistance of bonding based on
20 nm thick nanowires is approximately one quarter larger
than those of 100 and 200 nm thick nanowires. The causeof this relationship is that the parasitic resistance consistsof the intrinsic resistances of the nanowires and the contact
resistance, while nanowires with lateral dimensions down to

Figure 4. Testing of bonding strengths. (a), (b) A pair of surface
fasteners sustaining a weight of 50 g in the shear and normal
directions, respectively. (c) Bonding strengths of samples under
different preloads. (d) Bonding strengths of samples with different
diameters of nanowires.

100 nm or even smaller are characterized by higher intrinsic
resistivity than the bulk material [28]. However, it is possible
to use a resistance of approximately 2  in a 2 mm diameterbonding area for electrical bonding in SMT.
Figures 4(a) and (b) show photographs of surface
fasteners sustaining a weight of 50 g in both the shear andnormal directions. Considering the 2 mm diameter of eachpad, the total contact area for each sample is 12.56 mm2 .
The bonding strength is determined by dividing the maximum
load which can be sustained by the fastener by the total
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Nanotechnology 23 (2012) 365202

Y Ju et al

Figure 5. Micrographs of the surface fasteners before and after bonding and debonding. Surface fasteners with ((a), (b)) 200 nm, ((c), (d))
100 nm and ((e), (f)) 20 nm Au nanowires ((a), (c), (e)) before and ((b), (d), (f)) after bonding and debonding. The scale bar is 5 µm.

contact area. The test results for the shear and normal bonding
strengths for the samples with 200 nm thick nanowires under
different preloads are shown in figure 4(c). It is indicatedfrom this figure that the bonding strengths rely heavily on themagnitude of the preload. A larger preload can give rise tohigher shear and normal bonding strengths. The main reason
is that a larger preload can make the two groups of nanowires
insert deeper into each other (with larger h), as shownin the inset of figure 4(c), thereby increasing the contact
area and bonding strength. Large preload can also cause
plastic deformations to nanowires to form hook-and-loop
structures, thereby introducing mechanical forces in addition
to the van der Waals interactions, which further enhance thebonding strength. For bonding strength testing of samples
with different diameters of nanowires, a preload of 19 N was
adopted; the testing results are shown in figure 4(d). It canbe seen from this figure that both the shear and the normal
bonding strengths become much higher when the diameter ofthe nanowires decreases from 200 to 20 nm. Since the bonding
strength is contributed by the interaction between two groupsof nanowires through their contact area [25], it is reasonable
to assume that the bonding strength, S, is proportional tothe contact area, i.e. S ∝ nπdh, where h is the insertion
depth while n and d denote the number density and diameterof nanowires, respectively. This relation contains anotherassumption that the contact area is proportional to the total
surface area of the nanowires. For fasteners with nanowires ofdifferent diameters, if the total volume of nanowires, i.e. V ∝nπd2 h, is the same, the total surface area of the nanowires, andthus the bonding strength, will be inversely proportional to thenanowire diameter, i.e. S ∝ 1/d. The solid line in figure 4(d)illustrates the estimated relationship between the bonding
strength and the nanowire diameter, in which 5 N cm−2 isadopted as the bonding strength provided by fasteners with20 nm thick nanowires according to our measured values.
The trend of the measured values corresponds well with theestimated values. It is also noted from figures 4(c) and (d)that the normal bonding strengths of the present samples arealmost the same in magnitude as the shear bonding strengths;
this is not common for adhesives based on nanowires or
nanotubes with simple shapes [20, 25]. The reason for this isthat preloads can change the original shapes of the nanowires,
forming many randomly curling structures that may serve
as nanoscale hook-and-loop fasteners, as shown in figure 5.
As a result, the mechanical forces provided by these curling
structures together with the van der Waals interactions cancontribute to the enhancement of normal bonding strength. It
should be mentioned that the diameters used here are nominal
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Nanotechnology 23 (2012) 365202

Y Ju et alvalues of the pore diameters of membranes. We observed thenanowires by SEM and measured their diameters by ImageJ.
For wires growing from 200 and 100 nm pores, the measured
diameters were 230 ± 25 and 166 ± 33 nm, respectively,
while for wires growing from 20 nm pores, the measured
diameters were distributed between 60 and 120 nm. The actual
diameters of the nanowires were determined by the actual
values of the pore diameters of the membranes.
It should be mentioned that the maximum bonding
strength of the present samples (around 5 N cm−2 ) is smaller
than that of adhesives based on long CNT arrays [20].
However, these CNT array adhesives were designed for strong
binding-on and easy normal lifting-off from large surfaces
for use as clamps or adhesives; therefore, no electricalconnection was realized. In contrast, the purpose of ourresearch is to develop a novel surface fastener for electricaland mechanical cold bonding for SMT. In SMT, there aresome design guidelines regarding the ratio between the weightof the component and the bonding area. If we consider thisweight–area ratio to be R = 50 g cm−2 , which is morethan ten times as large as the standard value for secondside reflow mounting (30 g in−2 ) [29], the bonding strengthof S = 5 N cm−2 can withstand an acceleration of a =S/R = 100 N kg−1 , which is ten times larger than theacceleration of gravity (g = 9.8 N kg−1 ). This result meansthat SMDs assembled using the present surface fasteners
based on metallic nanowire arrays can be used in all manned
vehicles, including airplanes and spacecraft. For the surface
fasteners with 20 nm nanowires, mechanical testing was also
performed after several cycles of bonding and debonding
under a preload of 19 N. No obvious change of the bonding
strengths was observed within 3–5 cycles of bonding anddebonding.

solder enables easier detachment of the surface mountcomponent from the circuit board, by which recycling ofrare metals becomes significantly more convenient. Finally,mechanical bonding based on high melting point metallic
nanowires is temperature-resistant, which is necessary formany critical applications.

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4. Conclusions
In conclusion, a new kind of surface fastener composedof metallic nanowire arrays has been proposed throughwhich electrical and mechanical bonding can be realized
at room temperature. Electrical testing indicates that theparasitic resistance of one fastener is around 2  undera preload pressure of approximately 1 MPa. Mechanicaltesting indicates that larger preloads and smaller diametersof the nanowires are helpful for achieving higher shear andnormal bonding strengths. A maximum bonding strength of
more than 5 N cm−2 is demonstrated to exist in the present
fastener samples with 20 nm thick nanowires. Therefore,
the present metallic nanowire surface fasteners are suitablefor electrical and mechanical cold bonding for SMT. In
comparison to conventional reflow soldering, the present cold
bonding technique can be performed at room temperature,
which improves the process compatibility and component
reliability. Furthermore, a type of surface fastener without

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