Abstract – Production of modern microelectronic devices

needs advanced materials and packaging technologies oriented

towards the miniaturization and high reliability of the systems.

Nano sized silver can be treated as such advanced material and

it is used in modern packaging technologies, especially for

flexible electronics. The paper presents information about the

practical use of silver particles with the size from 4 nanometers

in materials for electronic packaging technologies.

The special ink containing nanosilver is use for ink-jet

printing electrically conductive structures. Such process can be

applied for flexible electronics even when structures are printed

on temperature sensitive materials. Ink-jet printing may be

used for creating the conductive layers of microvias joining

both sides of flexible substrates with acceptable level of

resistance. Nano sized silver is also used as a component of

hybrid filler for electrically and thermally conductive polymer

based composites, although the role of nanosilver particles in

transport of current or heat in such materials is limited.

Nanosilver plays an increasingly important role in joints

elements working at high temperatures. The sintering process is

done at temperatures significantly below 300 °C and the highest

shear strength of joined elements reached several dozens of

MPa.

I. INTRODUCTION

Silver nanoparticles have been widely used today, especially in two branches of science and application. The first one is connected with antibacterial or more general – antiseptic application. It is well known that silver ions have shown high toxicity to different species of bacteria with long-term antimicrobial properties. Silver has excellent resistance to many sterilization techniques as well as high biocompatibility. It can be used in contact with the human body because it does not induce skin irritation and has low toxicity to human cells. All these features cause that nanosilver is willingly utilized in the textile industry and medical engineering.

The nanotechnology is the second main area of nano sized silver using. According to the National Nanotechnology Initiative definition, the nanotechnology, or "nanoscale technologies", means the manipulation of matter particles with at least one dimension sized from 1 to 100 nanometers. In such scale quantum mechanical effects may play important role. But it is worth to notice that in many

J. Felba is with the Faculty of Microsystem Electronics and Photonics,

Wroclaw University of Technology, Wroclaw, Poland (phone: +48 71

3531053; fax: +48713531055; e-mail: jan.felba@ pwr.wroc.pl).

T. Fałat is with the Faculty of Microsystem Electronics and Photonics,

Wroclaw University of Technology, Wroclaw, Poland (e-mail:

tomasz.falat@ pwr.wroc.pl).

A. Mościcki is with the Amepox Microelectronics Ltd., Łódź, Poland (e-

mail: amepox@amepox.com.pl).

application nano sized silver is use only in first technological steps and then is transformed into micro sized or even bulk material, which is subjected to “traditional” mechanics laws rather. In both form nano sized silver is used as material for electronic packaging.

II. METHODS OF OBTAINING NANO SIZED SILVER

There are many methods of producing nano-Ag particles [1, 2]. The methods can be briefly described as chemical reaction process, metal vapor deposition, metal dissipation in plasma process, electrochemical process, thermal decomposition process of silver fatty acids under inert atmosphere, and others. In each of the foregoing methods, a chemical compound is administrated in an appropriate manner to produce on the surface of the forming metal particles a thin organic coating that effectively prevents coagulation of particles and their aggregation into bigger structures.

In our labs nano sized silver with different dimensions is produced. One type (named as „type A”) is characterized by very narrow size distribution (from 4 to 10 nm) [3, 4]. This silver powder is received from silver salts of fatty acids during their thermal decomposition in an oxygen-free atmosphere (according to process proposed by Nagasawa et al. [5]). In order to obtain highly divided powders, it is necessary to moderate their coagulation during the production process, e.g. by protective coating of fatty acid. The EDX analysis reveals this protection layer as carbon [6, 7] with mass of singular percent of the total product.

For more uniform dispersing nano silvers in composites, also bigger particles are produced with the size dimensions in the range between tens an more than 100 nanometers [8]. In this type of product (named as „type B”) can be distinguished carboxyl-coated, amino-coated and polymer-coated nanosilvers.

III. ELECTRICALLY CONDUCTIVE STRUCTURES

To form electrically conductive structures for electronic packaging, the ink-jet technology can be used, especially on flexible substrates (Fig. 1). The technology needs a special liquid, ink, which should be characterized by following features: very low viscosity, particles can not be separated during high acceleration (when printing), and certainly, ability to making electrically conductive structures. Additionally it should be stable at room temperature for weeks without any sedimentation.

A suspension saturated with nano sized silver particles as a filler can be used for this purpose. The ink formulated by

Nano sized silver for electronic packaging

J. Felba, T. Fałat, and A. Mościcki

Proceedings of the 13thIEEE International Conference on NanotechnologyBeijing, China, August 5-8, 2013

978-1-4799-0676-5/13/$31.00 ©2013 IEEE 30

Amepox Microelectronics [32], which contains 40 ÷ 60 wt% of type A nanosilver has viscosity lower than 16 mPa·s. It was stated, that for this ink the sedimentation in room temperature takes place with the speed of 0,83·10

-3 %/h. It

means, that after 50 days, about 1 % of whole silver nano particles precipitates out of solution [9].

Figure 1. Examples of electrically conductive structures jet printed on foils

As it was mentioned earlier, during the production process metal nanoparticles are usually covered by outer organic layer which is electrically insulating. Therefore for obtaining the electrical conductivity, the ink-jet printed structures requires additional technological steps to remove this layer. Usually it is done be overall thermal heating, but there are other technological processes which can lead to obtain low electrical resistance of printed structures. The protective layer can be removed by:

- local heating; energy is absorbed only by printed structure

when it is heated by laser beam [10, 11], by pulsed light

[12], or by UV light [13],

- delivering microwave energy [14],

- electrical excitation [15],

- chemical reaction in low temperature [16].

During the overall thermal heating, at the relatively low temperature a solvent is evaporated from printed structures and then in the higher temperature the nanoparticles are sintered. Firstly the protective layer is removed and then the recrystallization of silver particles take place [17]. The sintering process can be observed as changing of the grain size [18].

Figure 2. The changes printed samples resistance during heating in

different temperatures.

The sintering process strongly depends on time and

temperature [9, 19]. When ink with silver type A is used, the

temperature is usually higher than 200 °C (Fig. 2 [20]). It

can be seen that after a sufficiently long heating time, the

resistance of printed structures decreases over seven orders

of magnitude. The time required to achieve the final value of

resistance is closely correlated with the temperature. The

process temperature is higher, the faster changes in

resistance is observed. In such case, the structures can be

only printed on thermal resistant substrates like glass,

ceramic or polyimide foil. Much lower sintering temperature

is needed when ink with type B nanosilver is applied. It

allows to print structures on cheap transparent foils (e.g.

PET) which thermal resistance is at the level of 150 °C.

IV. ELECTRICALLY CONDUCTIVE ADHESIVES

In electronic packaging, the using of electrically conductive adhesives (ECA) is an alternative to soldering, especially for surface mounting of electronic elements on printed circuit boards. ECAs consist of polymer base material and a conductive filler, mostly silver, which provide the composite with electrical conductivity through contact between randomly dispersed conductive particles. The filler concentration has to be above a critical concentration called the percolation threshold. It is believed that at this concentration all conductive particles contact each other and form a three-dimensional network for current transport.

The most common morphology of conductive fillers used for ECAs is a flake with particle size ranges from 1 to 20 or more μm. The total resistance in “a three-dimensional network” of conducting filler particles can be summed using the concepts of serial and parallel resistances as in an electrical circuit. The resistance Rcp of the singular contact path between joining surfaces can be expressed by a simple

Rcp = Rp1 + Rp2 +… Rpn + Rc1 + Rc2 +…+Rc(n-1) (1)

where Rp is the resistance of a filler particle (bulk resistance), Rc is the contact resistance between particles making chain in direction of potential gradient and n is the number of particles in the chain (in this simple model one does not distinguish between constriction resistance and the tunneling resistance). Equation (1) points that the less contacts between particles the lower resistivity of the adhesives. It means that using of nano silver particles as an ECA filler seems to be senseless. In fact, such formulation is nonconductive [21]. However, in hybrid filler containing both the micro sized and nano sized silver particles, the nano Ag powder may reduce the electrical resistivity of the composite. Ukita et al. [22] describes the curing process of ECA with such filler and presents results as SEM image in which it is seen that nano sized particles improve contact between mico sized flakes. Other experiments with hybrid nano + micro sized silver filler did not confirm this observation as the nano Ag particles tend to form

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agglomerates located as “isolated islands” which do not improve the composite electrical conductivity [21, 23].

V. THERMAL INTERFACE MATERIALS

One of the main problem of miniaturized electronics is removing of heat generated by microelectronic elements. For this purpose the thermal interface materials (TIM) are used. The main requirement of such materials is their high thermal conductivity and also acceptable mechanical strength. One of the commonly used materials that meet the requirements of TIM are thermally conductive adhesives. Such materials are formulated similarly to the ECAs, thus there is no justification for the use of nano sized silver particles as filler [24]. However, it was shown that the addition of nanosilver as component of hybrid filler manufactured using electrospinning significantly improves the heat transfer in such TIM [25].

VI. INTERCONNECTIONS THROUGH THE MICROVIAS

In miniaturized electronics multi-levels printed circuits board are increasingly being used. Some points located on different levels of such board are joined by so-called microvias – via holes diameter range from 50 to 300 μm. Microvias are usually formed by laser drilling and filled with electrically conductive material to make the connections between the built layers and pads to which electronic elements are attached.

The conductive material which contains nano sized silver particles can be deposited by screen, aerosol-jet [18] or by ink-jet printing [26, 27]. In our experiments [26, 28] both sides of polyimide foil was joined with use o ink-jet printing technology. Inks for printing contained nano sized silver either type A or type B. The example of results is presented in Fig. 3. The obtained value of interconnections were acceptable. The best results - the lowest resistance and the lowest deviation – were obtained for the smallest holes with diameter of 30 µm for ink with type A nanosilver. For nano sized particles type B the resistances were higher, but this printed layers were sintered in lower temperature.

Figure 3. SEM picture of 60 µm diameter microvia filled with ink with

nanosilver type A.

The interconnection through the microvias made by ink-jet printing seems to be technologically very promising because of two reasons: (1) the same technology can be used for making planar as well as vertical connections in flexible

substrates, and (2) the first experiments show, that acceptable values of microvias resistance can be achieved.

VII. CONNECTIONS BY SINTERING

For high temperature electronics the commonly used electrically and thermally conductive adhesives can not be applied. The requirement of high temperature resistant materials for interconnections led to interest in sintering technology. Nano sized silver proved to be a good material for such connections. As an example – Lei et al. [29] used paste with nanosilver to bond small chips at temperatures similar to soldering temperatures with pressure up to 5 MPa. When copper plates (plated with nickel) were joined by sintering at 275 °C, the bonding strength was calculated on the level of 30 MPa. In other experiment [30], when plates with Ti/Pd/Au metallization were joined (at 200 - 250 °C), the highest shear strength (on the level of 40 MPa) was achieved when mixed nano + micro sized silver powder was used.

Mei et al. [31] proposed current-assisted sintering technology which was able to sinter nanosilver in a time less than one second. The dummy die was bonded with the copper substrate by sintering the nanosilver at a couple of electrodes that are treated by the alternating current (5.5 kA to 8.25 kA) in time from 50 ms to 1000 ms, and loading pressure from 5 MPa to 10 MPa. The highest shear strength of joined elements reached 90 MPa, but it depends on many factors like metallization, current, time.

VIII. CONCLUSION

Miniaturized and reliable electronic needs advanced materials and packaging technologies. Nano sized silver is material which meets the requirements of modern electronics and will be wider used in electronic packaging. It can be expected that nanosilver will increasingly be used to create electrically conductive structures especially on flexible substrates. The ink-jet printing now is applied in roll-to-roll production lines and further lowering the temperature necessary to obtain acceptable resistance of printed structures will make this technology more common, especially that in the process also microvias can be filled up.

Current positive experience with the use of nano sized silver for joining elements working in high temperatures points, that industrial technological equipment for sintering with low pressure will be used soon. This means the development production of nanosilver with the required particle size.

ACKNOWLEDGMENT

The presented work was partially performed within the

NANOTHERM project co-funded by the European

Commission under the “Information and Communication

Technologies” Seven Framework Program under the Grant

Agreement No 318117.

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