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Chemical Engineering and Processing 50 (2011) 589–591

Contents lists available at ScienceDirect

Chemical Engineering and Processing:Process Intensification

journa l homepage: www.e lsev ier .com/ locate /cep

nkjet printed electronics: The wet on wet approach�

ernando Varela ∗, Estíbaliz Armendáriz, Cecilia WolluschekEMITEC Centro Multidisciplinar de Innovación y Tecnología de Navarra, Pza CEIN 4, E-31110 Noain (Navarra), Spain

r t i c l e i n f o

rticle history:eceived 31 March 2010

a b s t r a c t

Inkjet printing of functional materials has been in recent years a driving technology for the manufacturingof circuits and sensors. Inkjet printing provides a way for direct printing without the need of intermediate

eceived in revised form 13 January 2011ccepted 2 February 2011vailable online 1 April 2011

eywords:unctional printing

tools and without contact to the substrate. In the case of printed electronics, the wetting behavior of eachmaterial with the substrate, as well as the wetting properties between liquid phases, are key factorsthat must be well understood in order to achieve good results. In this work we present experimentalresults showing the interaction between liquid metallic inks printed simultaneously over polyimidefilms. The interface between liquid phase and the contact line dynamics observed during the process are

ed.

nkjet printinglectronic manufacturing

characterized and discuss

. Introduction

Inkjet printing of functional materials has been in recent yearsdriving technology for the manufacturing of circuits and sensors.

nkjet printing provides a way for direct printing without the needf intermediate tools and without contact to the substrate. For theseeasons, inkjet provides an alternative to well-established tech-iques such as screen printing or flexography. The use of piezorintheads provides also an advantage from the manufacturingoint of view: by using several printheads on a small foot print, it

s possible to achieve wet-on-wet printing capabilities, simplifyinghe overall process, increasing productivity and reducing energyonsumption.

Printed electronics manufacturing usually prints one layer at aime, providing curing before a second layer is applied. This impliesonger idle times specially on short-runs. Also register problems,s alignment between layers, are to be considered. Wet-on-wetrocess consists in the direct deposition of two or more liquidsimultaneously over a non-porous substrate, as it is usual in dec-rative digital printing. It is well known that color bleeding andmage quality is a central problem in digital printing, and is usually

olved by choosing the right media.

In the case of printed electronics, the wetting behavior of eachaterial with the substrate, as well as the wetting properties

etween liquid phases, are key factors that must be well understood

� This work was presented at the 8th European Coating Symposium, Septem-er 7–9, 2009, Karlsruhe. ECS shall not be responsible for statements or opinionsontained in papers or printed in its publications.∗ Corresponding author. Tel.: +34 848 420 800; fax: +34 948 317 754.

E-mail address: fvarela@cemitec.com (F. Varela).

255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2011.02.001

© 2011 Elsevier B.V. All rights reserved.

in order to achieve acceptable values of printing resolution andelectrical properties required. In this work, we present experimen-tal results obtained with 4 different ink layer deposition methods.The visual quality and the electrical resistance of the final printingare characterized and discussed.

2. Method

Conductive silver layers were deposited on polyamide substrateusing a commercial piezoelectric printing system. The printheadhas 16 nozzles. Each nozzle delivers drops of 1–10 pl. However,to provide better accuracy in drop volume and velocity, only onenozzle was used. The conductive ink is a Sunjet glycol dispersionwith high solids content (up to 25 wt%). The dispersion viscosityis 10–15 cps at 23 ◦C (manufacturer information). The equilibriumcontact angle between the silver dispersion and the polyamidesubstrate is about 30◦, providing a spot diameter between 25 and28 �m. A two step printing pattern has been designed for studyingthe wetting interaction between liquid droplets (see Fig. 1). Eachprinting step consists in an array of strategically located drops sothat the final printing, after all steps, form a completely filled pic-ture. If, like in the pattern, the distance between drop centers isabout 30 �m, once the two steps have been printed, a uniformsquare is obtained. If the ratio between spot diameter and print-ing resolution (distance between drop centers) is varied, squares ofdifferent sizes would be obtained (Fig. 2).

The present study was conducted with a distance between dropcenters of 20 �m, in order to achieve coalescence between neigh-boring drops [1–4]. The designed pattern provides a way to studythe effect of 4 different situations to print the second layer over thefirst layer:

590 F. Varela et al. / Chemical Engineering and Processing 50 (2011) 589–591

Fig. 1. Template for wet-on-wet printing p

(

(

Fig. 2. Schematic coalescence of two drops.

(a) Wet-on-dry: The first layer is completely cured and sintered,keeping the printed substrate 20 min at 150 ◦C in a forced con-vection oven before printing the second layer. Then, for thecompleted printing cure, a second sintered operation of 20 minis necessary (this second operation is also required in the fol-lowing methods). This process prevents loose details in images,but it has the disadvantage that it needs twice as much curingand sintering time than single pass printing. Besides, the two-step sintering process requires that substrates were in thermalstress longer time, consequently, they degrade faster.

b) Two pass wet-on-wet: In this case, the second layer is directlyprinted after the first layer. Drops arriving from this secondstage impact the existing droplet arrangement and rearrangerandomly due to the mobility of both patterns.

(c) Single pass: This process consists in printing the whole imageat once.

d) Pinned wet-on-wet: In this case, the substrate is heated upto 60 ◦C. Second layer is printed 3 min after first layer, with-out moving substrate or performing any curing procedure. Theheating plate accelerates the evaporation of solvents present inthe ink.

Fig. 3. Effect of printing resolution on coalescen

rocess study, using conductive inks.

3. Results

In this part, we compare 4 methods visual quality printing:

(a) Wet-on-dry case. In this way, the second and complemen-tary image deposited later cannot spread and coalesce, but with aprinting resolution of 20 �m the holes are filled uniformly (Fig. 3a).(b,c) Two pass wet-on-wet and single pass. Results with both tech-niques are similar (Fig. 4): where both complementary images areprinted at once, the printed pattern looses definition. The spread-ing and coalescence of single drops produce uneven layers and,presumably, loss of conductivity.(d) Pinned wet-on-wet. The final curing provides a graph printingsimilar to the obtained using wet-on-dry procedure, but with asingle curing stage in the process (Fig. 3b).

From visual inspection, we can conclude that both, wet on dryand pinned wet on wet, processes produce a fill image. In orderto obtain a measurable physical parameter, we printed a line of1.5 cm × 3 cm of conductive silver using all the nozzles, as an usualprinting, with these two methods (Fig. 5). In this case, electricalresistance was measured using a multimeter Fluke 117, 0.2 ohmaccuracy. In the next table, results are summarized:

Wet-on-dry Pinned wet-on-wet

Total processes time 40 min (first layerprint, 20 min sintered,

23 min (3 min for curefirst layer and 20 min

second layer print andfinal 20 min sintered)

for final sintered)

Graph result Acceptable (fill image) Acceptable (fill image)Electrical resistance 1.9 ± 0.2 ohm 2.1 ± 0.2 ohmInk layer thickness 10.3 ± 0.1 �m 10.3 ± 0.1 �m

ce. From left to right: 30, 25, and 20 �m.

F. Varela et al. / Chemical Engineering and Processing 50 (2011) 589–591 591

Fig. 4. Wet-on-dry (left) vs. pinned wet-on-wet (right) processes. First layer (top), second layer printed (center) and final curing (bottom).

ing an

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Fig. 5. Two-pass wet-on-wet (left) vs. single pass (right) processes. The spread

. Conclusions

In this study, we tested 4 different approaches for printing thinayers of conductive ink using a two step digital printing process.

hile wet-on-dry procedure guarantees that that second layer willroceed as expected and provide an even surface, the overall pro-ess is inefficient. Energy consumption and time are higher andhe translation of the substrate between printing and curing stageseduces mechanical precision. On the other hand, wet-on-wet pro-edures with pinning provide a good alternative since it does noteed curing between layers. In this case, is necessary to control theobility (coalescence) of single droplets before curing and sinter-

ng stage. Moreover, the final wet on wet printed layer presentssimilar electrical resistance result compared to the wet on dry

rinting layer.These printing enhancements, together with mechanical preci-

ion and accurate driving of piezo actuators, make digital printed

[

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d coalescence of single drops produce uneven layers and loss of conductivity.

electronics an interesting option to standard manufacturing pro-cesses such as screen printing and lithography.

Acknowledgement

This work is partially supported by the EUROINNOVA program(Gobierno de Navarra).

References

1] D.A. Beysens, Phase transition, contact line dynamics and drop coalescence,in: International Workshop on Dynamics of Complex Fluids, Yukawa Institute,Kyoto University, Kyoto, Japan, March 8–10, 2004.

2] C. Andrieu, D.A. Beysens, V.S. Nikolayev, Y. Pomeau, Coalescence of sessile drops,

J. Fluid. Mech. 453 (2002) 427–438.

3] W.D. Ristenpart, P.M. McCalla, R.V. Roy, Coalescence of spreading droplets on awettable substrate, Phys. Rev. Lett. 97 (2006) 064501.

4] R. Narhe, D. Beysens, V.S. Nikolayev, Dynamics of drop coalescence on a surface:the role of initial conditions and surface properties, Int. J. Thermophys. 26 (2005)1743.