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High-definition c

aSchool of Chemical Engineering & Technolo

150001, China. E-mail: lininghit@263.netbDepartment of Chemical and Biological Eng

University of New York, Buffalo, NY 14260,

† Electronic supplementary informa10.1039/c5ra23694k

Cite this: RSC Adv., 2016, 6, 7582

Received 10th November 2015Accepted 10th January 2016

DOI: 10.1039/c5ra23694k

www.rsc.org/advances

7582 | RSC Adv., 2016, 6, 7582–7590

onductive silver patterns onpolyimide film via an ion exchange plating method†

Yanqing Wang,a Ning Li,*a Xianliang Wang,b Dana Havas,b Deyu Liab and Gang Wu*b

In this work, we developed a new ion exchange plating (IEP) method that is different to traditional

electrochemical plating or electroless plating techniques. A variety of silver patterns were prepared by

using the IEP technique as demonstrated in this work. The key factors to the definition and the

conductive performance of silver patterns were systematically studied. Mechanisms of the formation of

the conductive silver patterns was studied as well. A new model was established for elucidating the

deposition process. Importantly, using this IEP method coupled with mask technology, double-sided

interconnected conductive silver patterns on polyimide substrates were successfully fabricated with high

definition. This new technology will provide a unique capability to fabricate double-sided or multi-sided

interconnection for next generation printed circuit board (PCB).

1. Introduction

Conductive patterns have many applications including inte-grated circuits (IC), radio frequency identication (RFID) elec-tronic tags, and molded interconnect devices (MID). Owing tothese important applications, more attention has been drawnrecently in research to the preparation of high-denitionconductive patterns. Of all metals, silver has the highestconductivity, leading to the proposal of many methods offabricating silver patterns on exible substrates in the past fewyears. At rst, scientists chose to prepare silver patterns bydirect printing silver nanoparticle ink on exible substratesfollowed by a sintering process at high temperature.1–5 Thismethod has many disadvantages including the use of highsintering temperature and instability of silver nanoparticleinks. The high sintering temperature is required to remove thedispersants and the high boiling point solvents used for addinginto inks to stabilize Ag nanoparticles and to reduce the coffeering effect, respectively. To obtain sufficiently high conductivity,dispersants and solvents must be decomposed or volatilized. Toreduce the sintering temperature, many researchers attemptedto use different sintering methods, such as intense pulse light,6

microwave,7 argon plasma,8 and opposite charge effect,9

however, the instability of silver nanoparticles is still a bigproblem. Novel silver precursor ink methods have beenproposed in order to resolve this silver nanoparticle instability.Silver precursor inks include two types: organic silver precursor

gy, Harbin Institute of Technology, Harbin

ineering, University at Buffalo, The State

USA. E-mail: gangwu@buffalo.edu

tion (ESI) available. See DOI:

inks and inorganic silver precursor inks. Organic silverprecursor inks can be further divided into three categories: therst, organic silver decomposition ink, can be directly decom-posed to a conductive silver layer at a specic temperature.10,11

The second, fractional reduction method, uses organic silverprecursor ink pre-printed on the substrate followed by silverreduction via exposure to reduction conditions.12 Lastly thethird, direct inspired reduction ink, uses a potential reductantadded to the ink which inspires to a true reductant at particulartemperatures13–15 or by laser radiation.16

Inorganic silver precursor inks mainly consist of silvernitrate or silver oxide as a precursor. Silver nitrate decomposesat about 440 �C and silver ions must be reduced with a reduc-tant. Inorganic silver precursor inks can be divided into twotypes, similar to the rst two types mentioned with regard toorganic precursor inks: fractional reduction method and directinspired reduction ink. The fractional reduction method usessilver ion ink pre-printed on substrates, which is then reducedto silver by exposure to reduction conditions17 or printingreductant ink.18,19 Direct inspired reduction ink includesa reductant precursor that is added to the ink, which decom-poses to a true reductant such as aldehyde at the propertemperature.20,21

In the past decades, a novel ion exchange self-metallizationmethod was employed to fabricate conductive silver coatingson polyimide substrates.22–27 The amide bond of polyimide lmcan be hydrolyzed to carboxyl potassium compounds in strongalkali solution. Silver ions are doped into the alkali modica-tion layer by ion exchange process. Finally, the conductive silverlayer is obtained by reduction process (reduction action,migration and assembly). A typical reduction procedureincludes chemical reductants, UV light, or high heat treatment.Owing to the excellent binding force between the silver layer

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and polyimide lm, many researchers have begun preparinga variety of silver patterns on the polymer lm.28–30 As for thestudiedmethods, a mask is necessary to ensure the denition ofconductive patterns when a variety of conductive patterns areprepared on polyimide lms via ion exchange and electrolessplating processes.31 Several problems that arise from this maskprocess, however, are difficult to solve. These difficultiesinclude stringent requirements for the mask, reversible andnon-residual removal of mask, and potential micro crackdamages to polyimide substrates. All of these issues are caused

Fig. 1 (a) A scheme of ion exchange resins. (b) A scheme of fabricatingzation method. (c) A scheme of repeatable IEP method.

This journal is © The Royal Society of Chemistry 2016

by strong corrosion modication and penetration of the alkalisolution.

Considering the aforementioned problems, we were inspiredby the idea that ion exchange resins can be regenerated (Fig. 1a).Arising from this, we report on a novel method to obtainconductive silver coatings on polyimide lms by continuous ionexchange processes. The new method named “Ion ExchangePlating (IEP)” is totally different from the traditional electro-chemical plating or electroless plating techniques. In addition,compared with the traditional ion exchange self-metallization

one conductive silver layer via a traditional ion exchange self-metalli-

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method (Fig. 1b), IEPmethod hasmany advantages such as onlyminor damage to polyimide lm, low requirements for themask (when used in the preparation of silver patterns), and thesilver layer thickness is not dependent on the degree of alkalimodication, but instead on IEP repetitions under particularconditions. The principle scheme of this method is shown inFig. 1c. It can be found that the ion exchange process's smoothprogression is due to the introduction of a neutralizing regen-eration process unlike the traditional ion exchange self-metallization method.

2. Experimental details2.1 Fabricating silver patterns by an IEP method via masktechnology

A typical preparation procedure of fabricating conducive silverpattern onto a polyimide lm was presented in Fig. 1c. In orderto prepare alkali modication solution, glycerin was rst mixedwith deionized water. Then potassium hydroxide (KOH) wasadded into the glycerin solution and the beaker was sealedimmediately. The mixture was stirred until potassiumhydroxide was completely dissolved and cool down to roomtemperature. On the other hand, the double-sided polyimidelm (TORAY Dupont, Kapton 500H, 50 � 50 � 0.125 mm3) wasrinsed with ethyl alcohol in order to remove the possible grease.The bright side of polyimide lm was covered with a polyvinylchloride (PVC) hollow mask (covered with no residual poly-acrylate pressure-sensitive adhesive and variety of patterns).Then the polyimide lm was immersed in the prepared alkalimodication solution at room temperature for 1 min followedby rinsing with deionized water. Aer peeling off the PVC mask,the pretreatment of polyimide with the alkali modicationpattern was completed. The above treated polyimide lm wasimmersed in 0.4 M silver nitrate solution at room temperaturefor 1 min, and then rinsed with deionized water. Next, thepolyimide lm was immersed in 0.25M ascorbic acid in order toreduce adsorbed Ag+ to metallic Ag. The solution pH wasadjusted between 3 and 4 by using ammonium hydroxideaqueous solution (volume ratio 1 : 1). Finally, a neutralizingregeneration process (abbreviation N) was carried out. Inparticular, the treated polyimide lm was immersed in 50 g L�1

potassium hydrogen phosphate solution at room temperaturefor 1 min, and was washed with deionized water. Above proce-dure was repeated until the conductivity of silver pattern layermet the requirements. Aer the reduction process, the entireIEP process was nished. Then the above polyimide lm wasrinsed with deionized water and blow-dried by the cold wind. Asa result, the polyimide lm with the silver pattern was preparedcompletely.

2.2 Physical characterization

During the preparation of conductive silver patterns, scanningelectron microscopy (SEM) was used to monitor theirmorphology changes. In addition, elemental contents in thepatterns were measured by using coupled energy dispersivespectrometer (EDS). The line width of the silver pattern was

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measured using metallographic microscope. The thickness ofthe silver layer of the silver pattern was determined using theCMI900 & 950 X-ray Fluorescence Spectrometer (OxfordInstruments). Denition of the silver patterns was observed byusing USB electronic microscope. Conductivity of the silverpattern was tested with the ST2258C multifunction digital four-probe tester (Suzhou Jingge Electronic Co., Ltd).

3. Results and discussion3.1 Fabricating silver patterns on polyimide lm

Feasibility of an IEP method via mask technology to fabricatesilver patterns on polyimide lms was rst investigated. Beforethe neutralizing regeneration process (Fig. S1a†), elementalcontents determined by EDS indicated a silver pattern iscomposed of C (5.93 wt%), N (15.66 wt%), O (41.45 wt%), and Ag(36.95 wt%). However, aer the neutralizing regenerationprocess (Fig. S1b†), it is composed of C (6.95 wt%), N (9.01 wt%),O (46.41 wt%), K (9.81 wt%) and Ag (27.82 wt%). The detectedpotassium conrms that an alkali modication layer on thepolyimide lm can be regenerated aer the neutralizingregeneration process. Regeneration of the modication layerindicates that it is feasible using the IEP method with repeat-able steps to fabricate silver patterns to obtain sufficientthickness on polyimide lms.

Furthermore, circular silver patterns on polyimide lm werefabricated with 15 repetitions by using the IEP method (Fig. 2a).We also prepared a circular silver pattern by using only onerepetition as a comparison (Fig. 2b). Their surface morphology,adhesion, conductivity and denition of the obtained circularsilver pattern were also characterized.

The appearance of circular silver pattern prepared by 15 IEPrepetitions is white color (Fig. 2a), while the one prepared byonly one IEP repetition is a light blue-white color (Fig. 2b). Thethickness of the pattern layer prepared by 15 IEP repetitions is0.23 mm that is much thicker than the 0.02 mm of the one ob-tained by one repetition. Moreover, the circular silver patternlayer prepared by 15 IEP repetitions has a more uniform surface(Fig. 2c), smaller sheet resistance value 0.281 U cm�2 (TableS1†), and smaller resistivity value 6.46 mU cm (the conductivityis 24.61% of the bulk silver value), relative to the one by onerepletion. The results of denition measurements (Fig. 2e and f)show that sufficient denition of silver patterns can be fabri-cated by the IEP method. The results of a cross-cut test and the3M 600 tape test (Fig. 2g), along with the clear scratch obser-vation test (Fig. 2h) indicated a good adhesion between thesilver layer and polyimide lm.

3.2 Factors to affect silver patterns

Aer conrming the feasibility, we further studied key factorsthat control the eventual properties of the silver patterns (e.g.,including denition and conductivity) fabricated by the IEPmethod. We rst studied the denition of silver patterns asfunctions of the mass ratios of glycerin to deionized water, andthe time of alkali modication process t(A). With increasingmass ratios of glycerin to deionized water, the permeability and

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Fig. 2 Silver patterns on polyimide film fabricated by using the IEP method. Digital photographs of circular silver pattern (diameter of the circlepattern: 3 cm) prepared by (a) 15 IEP repetitions and (b) one IEP repetition with conditions: immersing time t(I)¼ 120min and reducing time t(R)¼15 min. (c) and (d) Microscope images for (a) and (b), respectively. (e) and (f) The corresponding definition images for (a) and (b). (g) Digitalphotograph of (a) crossed by Cross-Cut Tester (2 mm space) and after the 3M 600 tape test. (h) Microscope image of (g). Scale bars in (a)–(e) andtheir inserts and (h) are 0.5 and 0.123 mm, respectively.

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causticity of the alkali modication solution become weakerand the denition of silver patterns become higher (Fig. 3).However, further increasing amounts of glycerin leads to poor

Fig. 3 The influence of the mass ratios of glycerin to deionized wateron the definition of circular (F 3 cm) silver pattern on polyimide filmprepared (note: IEP repetitions n¼ 5, rt� 1 min). The definition imagesof circular (F 3 cm) silver pattern observed by USB electronic micro-scope (a) mass (glycerin) : mass (water) ¼ 1 : 3. (b) Mass (glycerin)-: mass (water)¼ 1 : 2. (c) Mass (glycerin) : mass (water)¼ 1 : 1. (d) Mass(glycerin) : mass (water) ¼ 2 : 1. Scale bars in (a)–(d) and their insertsare 0.5 and 0.123 mm, respectively.

This journal is © The Royal Society of Chemistry 2016

dissolving performance of potassium hydroxide, therebyresulting in a reduced denition. Therefore, the optimal massratio of glycerin to deionized water was determined to be 1 : 1.

Another factor that affects denition of silver patterns is thetime of alkali modication process t(A). With increasing time ofthe alkali modication process t(A), permeability and causticityof the alkali modication solution are enhanced, and thereforethe denition of obtained silver patterns becomes deteriorated(Fig. 4). In addition, we systematically investigated theconductivity of silver patterns as functions of alkali modica-tion process t(A), the mass ratio of glycerin to deionized water,IEP repetitions (n), and the concentration of AgNO3, C6H8O6,and K2HPO4. As listed in Table S1,† longer alkali modicationtime and more IEP repetitions result in thicker silver patternlayers and smaller sheet resistance. However, it should be notedthat smaller sheet resistance does not mean lower resistivity.There is a close relationship between thickness and conduc-tivity of silver pattern layers. To get good electrical conductivity,a certain thickness of the silver layer is necessary. Thus, wefurther determined the correlations between these factors (i.e.alkali modication process t(A), the mass proportion of glycerinto water, IEP repetitions and the concentration of AgNO3,C6H8O6, and K2HPO4) and thickness of silver pattern layers.

As is shown in Fig. 5a, under the same alkali modicationcondition with an increasing mass ratios of glycerin to deion-ized water, permeability and causticity of the alkali modica-tion solution becomes weaker, leading to a drop in thehydrolysis degree of polyimide lm. This decreases silver ionexchange capacity, leading to a signicant drop in the thickness

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Fig. 4 The influence of the time of alkali modification process t(A) on the definition of circular (F 3 cm) silver pattern on polyimide film preparedby an IEP method via mask technology (note: IEP repetitions n ¼ 5). The definition images of circular (F 3 cm) silver patterns observed by USBelectronicmicroscope. (a) t(A)¼ 1min. (b) t(A)¼ 2min. (c) t(A)¼ 3min. (d) t(A)¼ 4min. (e) t(A)¼ 5min. Scale bars in (a)–(e) and their inserts are 0.5and 0.123 mm, respectively.

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of the silver pattern layer. Similarly, with increasing the time ofthe alkali modication process t(A), stronger permeability andcausticity of the alkali modication lead to a rise of hydrolysis

Fig. 5 The influence of the mass proportion of glycerin to deionized wat(repetitions n ¼ 5) (b) and IEP repetitions n (c) on the thickness d of circmethod via mask technology (note: alkali modification condition: mass (repetitions n on the sheet resistance R, and resistivity r of circle (F 3 cmmask technology.

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degree of polyimide lms, thereby increasing the silver ionexchange capacity as well as the thickness of the silver patternlayers (Fig. 5b). Signicantly different from the above results,

er (IEP repetitions n ¼ 5) (a), the time of alkali modification process t(A)ular (F 3 cm) silver pattern layer on polyimide film prepared by an IEPKOH) : mass (H2O) : mass (C3H8O3) ¼ 1 : 1 : 1). (d) The influence of IEP) silver pattern layer on polyimide film prepared by an IEP method via

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a wide variety of concentration of AgNO3 (0.4–1.0 M), C6H8O6

(0.25–1.0 M), and K2HPO4 (50–200 g L�1) have insignicanteffects on the thickness.

IEP repetitions (n) is a vital factor in ion exchange platingmethod. Generally, a continuous supply of metal ions on thesurface of the substrate is necessary for continuous growth ofthe metal coating for both the electrochemical plating andelectroless plating methods. The ion exchange plating methodrelies on IEP repetitions to attain a continuous supply of silverions and the continuous growth of a silver coating under certainconditions. It can be seen that the thickness of the silver patternlayer shows a linear growth with an increase in IEP repetitions(Fig. 5c). Here we dene the plating rate V of the ion exchangeplating as follows:

V ¼ Dd/Dn

Fig. 6 AFM three-dimensional height images of circular silver patternsrepetitions n. (a) n ¼ 15, (b) n ¼ 17, (c) n ¼ 19, (d) n ¼ 21, and (e) n ¼ 23

This journal is © The Royal Society of Chemistry 2016

where Dd is the thickness increment of silver pattern layer, andDn is the increment of IEP repetitions. According to the abovedescription, the plating rate V of the ion exchange plating isapproximately equal to the slope of the linear t line 0.01371 mmper times (Fig. 5c).

Conductivity of the silver pattern layer as a function of IEPrepetitions was investigated. With an increase of IEP repeti-tions, the silver pattern layers transited from insulating, partlyconductive, completely conductive but not uniformly conduc-tive, to uniformly conductive (Fig. 5d). When IEP repetitions isless than three, the layer is very thin and insulating. IncreasingIEP repetitions to 11, the obtained silver layer becomes partlyconductive. When IEP repetitions reaches 13, the layer appearscompletely conductive, but the conductivity is not uniform.Until IEP repetitions is more than 15, the layer eventually ach-ieves even conductivity. Further increasing IEP repetitions lead

fabricated by the IEP method via mask technology under different IEP. (f) Surface roughness Ra of silver patterns as function of repetition n.

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to insignicant change of conductivity. The sheet resistance ofthe silver pattern layer with sufficient thickness is in the rangeof 0.2 to 0.5 U cm�2. The resistivity of silver pattern can reach 6–10 mU cm, which is 15.9–26.5% of the bulk silver value at 20 �C.

Furthermore, with an increase in IEP repetitions, the color ofthe silver pattern layer gradually changes from light blue-whiteto white (Fig. S2†) and the surface of silver pattern layer grows

Fig. 7 A possible model for the formation process of a silver pattern lay

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more uniform. This can be attributed to the silver pattern layerbecoming thicker with more IEP repetitions. The above resultsare conrmed by SEM images (Fig. S3†). In addition, largerparticles of silver are getting more numerous (Fig. S3†). Theemergence of large particles makes the surface prole of thesilver pattern layer rougher with more IEP repetitions. Theroughness of silver layers were studied using AFM, showing an

er fabricated by IEP method on polyimide film.

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Fig. 8 Preparation of a variety of silver patterns on polyimide films by the develop IEPmethod viamask technology. The digital photograph of 0.5mm (a) and 1.0 mm (b) line silver patterns (ion exchange plating repetitions n¼ 15). (c) The digital photograph of the “HIT” silver pattern (note: ionexchange plating repetitions n ¼ 21, HIT is the abbreviation of Harbin Institute of Technology). (d) The digital photograph of the conductivity testof double-sided interconnection conductive circular (F 3 cm) silver pattern on a polyimide film (note: an interconnection small hole (F 1 mm) liesin the center of silver pattern, ion exchange plating repetitions n ¼ 30). (e) and (f) The silver line width measurement of (a) and (b) by metal-lographic microscope, respectively.

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increasing trend with more IEP repetitions (from n ¼ 15 to 23)(Fig. 6).

To further explain the formation process of the conductivesilver patterns fabricated by the IEP method, a possible modelwas proposed as shown in Fig. 7. At rst, potassium carboxylategroups are generated on polyimide lms via a hydrolysisprocess under alkali modication solution. Then, silver ionscan be adsorbed on polyimide lms via an ion exchangeprocess. A thin silver layer can then be prepared on polyimidelms via a reduction process (including reduction, migrationand assembly stages). Aer the reduction process, carboxylgroups are generated at the surface of polyimide lms. Aer oneIPE repetition, only little silver particles formed on the surface.Conductivity is very poor. Subsequently, repeating IEP proce-dures, including alkaline modication, adsorption of silverions, reduction, and neutralizing regeneration, lead to moredeposited silver particles, thereby resulting in graduallyincreased thickness and conductivity. Eventually, polyimidelms are completely covered by uniform silver layers.

3.3 Preparation of high-denition silver patterns onpolyimide lm

In order to further demonstrate the practical application of thenewly develop IEP method to prepare silver patterns, we fabri-cated a variety of silver patterns on polyimide lms using the

This journal is © The Royal Society of Chemistry 2016

IEP method viamask technology including 0.5 mm line pattern(Fig. 8a), 1.0 mm line pattern (Fig. 8b) and “HIT” characterspattern (Fig. 8c). The silver line width measurement of 0.5 mmand 1.0 mm line patterns by metallographic microscope is507.34 mm (Fig. 8e) and 947.20 mm (Fig. 8f) respectively. Theyshow a small deviation (The relative error is 1.47% and �5.28%respectively) compared with the theoretical value. There ispotential in preparing a much higher denition and conductivesilver patterns in the future. Moreover, we successfully prepareda double-sided interconnected circular silver pattern (Fig. 8d).The resistance was measured at 116.3 U, revealing thata double-sided circular silver pattern achieved conductiveinterconnection in one step for the rst time by a small hole (F1.0 mm) in the center of silver pattern. This manufacturingprocess is completely different from the traditional preparationprocess of interconnected PCB, whose pattern metallizationprocess and hole metallization process are separated.

4. Conclusions

In this work, we developed a novel ion exchange plating (IEP)method to prepare high-denition and conductive silverpatterns on polyimide lms potentially for printed circuit board(PCB) fabrication. Compared to traditional ion exchange self-metallization methods, the IEP leads to negligible damage to

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polyimide lms and doesn't require masks during lm fabri-cation. Importantly, thickness of the silver layers is not depen-dent on the degree of alkali modication, and only controlled bythe repetition times of ion exchange plating process. By usingthe IEP to prepare silver patterns, higher mass ratio of glycerinto deionized water and less alkali modication time canimprove the denition of the silver patterns. Thickness of silverpattern layers show a linear growth with an increase of IEPrepetitions. The optimal IEP repetitions were determined interms of achieving sufficient conductivity and denition. Thekey factors to lm quality was systematically studied to maxi-mize silver coverage and minimize the agglomerated silverparticles. Finally, we successfully fabricated various silverpatterns on polyimide lms by the IEPmethod. Resistivity of thefabricated silver patterns can be reduced to 6–10 mU cm, whichis 15.9–26.5% of the bulk silver value at 20 �C. Importantly, forthe rst time, a double-sided interconnected conductive silverpattern on polyimide substrate was fabricated by the IEPmethod couple with mask technology via one step. The newmethod to prepare double-sided or multi-sided interconnectionwill lay a solid foundation for PCB application. In addition, theIEP method may be applied in preparing other metals or metaloxides on polyimide lms or other potential substrates fora variety of applications.

Acknowledgements

We acknowledge nancial support by Highnic Group inGuangdong. G. W. thank the nancial support from start-upfund from the University at Buffalo, SUNY.

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