geometric and compaction dependence of printed polymer-based rfid tag antenna

7/27/2019 Geometric and Compaction Dependence of Printed Polymer-Based RFID Tag Antenna

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Geometric and Compaction Dependence of Printed Polymer-Based RFID Tag Antenna

Performance*

S Y Y Leung and D C C LamDepartment of Mechanical Engineering, Hong Kong University of Science and Technology

Clear Water Bay, Kowloon, Hong Kong SAR, ChinaE-mail: [email protected] , [email protected]

* Accepted for publication in the IEEE Trans. on Electronics Packaging Manufacturing

Abstract

Radio frequency identification (RFIDs) tag with printedantennas are lower in costs, but have lower performance thanthose with metal antennas. Printed antennas can replace metalones if the performance is increased without raising cost. The performance of printed antennas can be increased if the seriesresistance in the antennas is lowered. The resistance isdependent on the line thickness and the resistivity of theconductive ink. Printed antennas with different linethicknesses were fabricated to investigate the effect of compaction and thickness on the resistance. The resistance of the printed antenna coils was decreased by more than 40%after compaction, while the inductance and the parasiticcapacitance were unchanged. RFIDs with compacted printedantennas were found to have significantly increased the readrange. RFIDs with thick printed antennas were fabricated andtested. These RFIDs were shown to have read rangescomparable to the RFIDs with copper wire antennas.Moreover, a geometry-independent plateau for the read rangewas found. The presence of a plateau is valuable for thick-line printed antenna design since the plateau will enable the usageof lower precision high volume printing techniques to lower tag fabrication cost.

Introduction

A variety of radio frequency identification tags (RFIDs) isavailable for wireless tracking of objects large and small.Pallets parked in a spacious warehouse are widely spacedcompared to items on shelves, and so require RFIDs that can be read over long distances. RFIDs which operate at ultrahigh frequency (UHF) have a wide read range, but signals atthis frequency are relatively easily absorbed by fluids andmetals [1]. High frequency (HF) tags operating at 13.56MHzare better suited for tracking items in absorbing environments.Antennas for UHF tags can be small. The planar inductor coils serving as antennas for HF tags are typically larger andare built separately on organic substrates instead of on siliconchips. Such coil antennas are made from aluminum or copper,

and are relatively expensive compared with the tagsthemselves. Conductive ink based printed antennas on lowcost substrates are lower in cost, but normally have a lower quality factor (Q) [1, 2] and more limited read range becauseof the higher resistance of the printed antenna lines.

The read range of a tag is governed by the inductivecoupling behavior of the tag and the reader. The equivalentcircuit diagram of an induction-coupled RFID system is shown

in Figure 1. The voltage induced in the tag coil supplies power for the die circuit to function. An embedded capacitor in the die circuit is connected with the tag coil to form aresonant circuit. The relationship between the tag parametersand the maximum read range has been analyzed [3-5] as afunction of the electrical components. The ratio between the

voltage in the die circuit IC V and the voltage induced at the

tag coil ind V is

2 2

22 2

2 2 2 2

1

1

IC

ind

IC IC

V

V L R

R C L C R R

, (1)

where is the angular frequency of the signal,2

L is the

inductance of the tag coil,2

R is the series resistance of the

antenna coil; IC R is the circuit load resistance on the die;2

C

is the sum of the embedded capacitance in die IC C and the

parasitic capacitance of the tag coil pC . For simplicity, V IC

can be defined as

IC Q ind V S V (2)

where

2 2

22 2

2 2 2 2

1

1

Q

IC IC

S

L R R C L C

R R

. (3)

Die

L1 L2

R2

Cp CIC

Vind

RIC

M

I1 I2

VIC

Reader

AntennaTag

Coil

L2

R

Cp

Vi d

I2

Tag

Coil

Figure 1. Equivalent circuit diagram of an inductivecoupled RFID system

If the series resistance of the antenna coil is kept low, S Qcan be maintained high. On the other hand, the quality factor



Q at the resonant frequency ( 2 21 L C ) is dependent on

R2 as

2

2 2 2

2

1

IC

LQ

R L R

L R

for 2 IC R R , (4)

which means that increasing R2 will reduce Q.

The induced voltage ind V depends on the coil geometry

and the distance away from the reader antenna. The ind V

developed across the terminals of the transponder coil is

2 1ind V MI , (5)

where2

is the flux across the tag antenna coil, M is the

mutual inductance between the reader coil and the tag coil,

and1

I is the current through the reader coil.2

is the

magnetic induction over the surface area of the tag coil [6]

2

2 0 1 1 2 232 2 22r

N I N A

d

, (6)

where r is the relative permeability of the substrate,0

is

the permeability of air, is the radius of the reader coil, d is the distance between the reader antenna coil and the tag

coil,1

N is number of turns in the reader coil,2

N is number

of turns in the tag coil, and2

A is the area of the tag coil.

Substituting equation (6) into (5) and rearranging gives themaximum read range for the inductive RFID system, d, interms of the coil’s area and the electrical parameters as

2

2 30 1 1 2 2 2

2

r Q

IC

N I N A S d

V

. (7)

The influence of the electrical parameters are related to QS ,

which is shown in equation (3).

Printed antennas are made from conductive inks withhigher resistivity than bulk copper, and their read ranges arelower than those of copper antennas. In this study, the effectof geometry was investigated to determine if the read range

behavior follows the predictions of equation (7). Since theseries resistance of a printed coil depends on its geometry, theoperating frequency, and the resistivity of the ink, low

resistance coil geometries were examined to determine if printed coils can be designed with read range performancecomparable to that of copper coils. In addition, a compactiontechnique for reducing the resistivity of the ink wasinvestigated to determine whether it could increase the readrange of a printed antenna.

Experimental Method

A. Fabrication of tag coils

Octagonal planar coils (Figure 2) were fabricated todetermine the effects of materials and geometries (Figure 3

and Table I) on the performance of the tags. In order tominimize the printing tolerance, coil line design with >1mmline widths were used [7]. A polyester mesh screen with196T/inch was used in this study. Silver-filled conductive

polymer paste (Coates ZX250) was screen printed on 100 µmthick polyethylene terephthalate (PET) substrates with ascreen printer (DEK model 260) and then cured using themanufacturer’s recommend curing procedure. Samples were

printed repeatedly to increase line thickness as required. Thedrying time between each run was about 15 minutes. The coilthicknesses after curing were measured using a digitalmicrometer (Mitutoyo 293-721-30). Copper coils (15µm linethickness) on PCB with were also prepared for comparison.

Figure 2. An RFID tag coil fabricated by screen printing aconductive paste onto a flexible PET substrate

Figure 3 Schematic of the antenna coils listed inTable 1



Table 1. Test coil parameters*

Layout Inner

diameter

din (mm)

Outer

diameter

dout

(mm)

Line

width

w

(mm)

Line

space

s

(mm)

#

turns

N2

Line

length

(mm)

A 39.5 50.5 1.0 0.5 4 608

B 20 44 1.5 0.5 6 637

C 38.5 54.5 1.5 0.5 4.25 645

D 19.5 37.5 1.0 0.5 6 564

* Printed coil line thickness range from 10 to 78µm after cured.

B. Pressure compaction of conductive ink

A composite of conductive ink with metal fillers was usedto print the antennas. Electrical current is carried in such acomposite from one filler particle to another through pointcontacts in a network of contacting particles. The

conductivity of the network can be increased by increasing theinter-particle contact area. Heat treating the filler at hightemperature can cause them to sinter, which increases thecontact area [8]. To avoid breakdown of the organiccomponents, low temperature sinterable nano-silver or nano-copper filled inks with sintering temperatures <150oC arerequired [9].

The inter-particle contact area of the micron-sized filler particles can also be increased by compaction. The effect of compaction on coil resistance was investigated using a cured printed coil. The coil was pressed at 980psi using a hot press(Technical Machine Product - model HVP) at 160oC for 10minutes. The electrical behavior of the compacted coil was

then tested.

C. Electrical characterization

A subminiature version A (SMA) connector was mountedon each individual test coil and connected to a network analyzer (Agilent E5071B). One port S-parameter of the coilwas scanned from 300kHz to 15MHz. The coil inductance,the series resistance, and the parasitic capacitance weredelineated with an equivalent circuit model (Figure 4). Inaddition, the DC series resistance was measured using amultimeter (UNI-T DT830E).

D. Maximum read range measurement of the fabricated RFID

tagsAn ISO 15693-compliant RFID die (EM Microelectronic4135) embedded with a 90pF capacitor was used to examinethe read range. The RFID tags were fabricated by connectingthe coils with the RFID die bonded inlays. The maximumread range of the RFID tag was measured with a 13.56MHzRFID reader (EM Microelectronic 4094). The reader antennawas a planar circular coil with a diameter of 12cm and 2 turns(Figure 5). The RFID tag was flat mounted parallel facing thereader antenna and tested. Twenty detection trials were performed for each d setting, and the number of successfuldetections was recorded and plotted as a probability function.

(For details of this data reduction procedure, please refer tothe Appendix.) The maximum read range of each tag wasdefined as the distance (d ) at which the probability function predicted a 90% success rate for detection.

Figure 4. Printed coil with a SMA connector to beattached to a network analyzer for measurement (left) andthe equivalent circuit model (right) to delineate theelectrical parameters. L is the coil inductance; R is theseries resistance; and C is the parasitic capacitance

Figure 5. The set up for RFID tag reading test

Results and Analysis

A. Inductance and resistance of the printed coils

The measured parasitic capacitance of the coils was5.0±0.4pF for the sampled geometries. The inductance wasconstant and independent of line thickness (Figure 6). In theabsence of skin effect, the series resistance of the coil, R2, is

2

l R

wT (8)

where is the electrical resistivity of the conductor material,

l is the line length of the coil, w is the line width, and T is

the line thickness.2

R at 13.56MHz as a function of

ReadDistance (d )

Reader

antenna

RFID tag

mounted on

flat plate



reciprocal line thickness are shown in Figure 7. The data varylinearly with the reciprocal line thickness. The behavior issimilar to the DC resistance’s dependence on the thickness,which confirmed that the skin effect is negligible for the

printed coils at 13.56MHz. The skin effect may becomesignificant if operating frequency is increased [10]. Since theinductance and the parasitic capacitance were independent of the line thickness, changes in the read range as a function of

line thickness directly attributed to the coil resistance’s dependence in S Q.

1.25

1.3

1.35

1.4

1.45

1.5

1.55

0 20 40 60 8080

A B C D

I n d u c t

a n c e ( H )

Line Thickness (m)

Figure 6. Measured coil inductance as a function of coilline thickness. The layout of the coils refer to

Table 1

0

0.01

0.02

0.03

0.04

0.05

0 4 104

8 104

1.2 105

DC13.56MHz

R 2

w / l

1/T (m-1

)

Figure 7. Geometry normalized series resistance as afunction of reciprocal line thickness

B. Maximum read range of the printed tags

The maximum read ranges of the fabricated tags are plotted against the series resistance of the coil in Figure 8.Copper samples (indicated with arrows) were included for comparison. The maximum read range decreased withincreasing series resistance. Furthermore, the maximum readranges of the tags with larger coil areas (configurations A andC) were higher than those of tags with smaller coil areas(configurations B and D) at comparable series resistance.

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 5 10 15 20 25 30 35

A B C D

M a x

R e a d R a n g e ( m m )

R 2

(Ohm)

Figure 8. Measured maximum read range as a function of series resistance accompany with model predictions (lines).Data points indicated with arrows are copper samples



0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 0.04 0.08 0.12

A B C D

M a x R e a d R a n g e ( m m )

N 2 A

2 S Q

Figure 9. Measured maximum read range as a function of tag parameters. Data points indicated with arrows arecopper samples

The influence of coil area and the electrical parameters can be observed by plotting the maximum read ranges of the tags

against the tag parameters2 2 Q N A S , as shown in Figure 9.

The copper samples (indicated with arrows) are also includedfor comparison. The maximum read range of the tags

increased with2 2 Q N A S in accordance with the read range

equation (7). More importantly, the plot shows that themaximum read range of the copper tags was comparable withthat of the printed tags.

C. Measured parameters of the pressure compacted tag

The measured tag parameters of the pressure compactedsample (Layout D) before and after hot pressing are shown inTable 2. The resistivity decreased by more than 64% after compaction, while the line thickness decreased by more than40% and the line width increased slightly by 4.8%. At thesame time, the measured coil resistance decreased by morethan 40% with negligible change in inductance. Clearly, whilean increase in resistance from area reduction (equation (8)) isexpected, the coil resistance change is dominated by thereduction in the coil material resistivity from compaction.

This resulted in an increase of the maximum read range of thetag by more than 16%, and is in line with the theoretical predictions. If the resistivity was unaffected by compaction, a printed line thickness of about 21µm will be required toachieve the same coil resistance reduction. In other words,more than 42% (by volume) of conductive ink was saved bythe compaction process to achieve the same maximum readrange.

Table 2. Comparison of tag parameters before and afterpress treatment (Layout D coil)

w T (µm) L2 R2 (Ω) N 2 A2S Q d (mm)

(mm) (µH)

Notreatment

1.05 12 1.44 23.2 9.31x10-3 38.4

After Pressed

1.10 7 1.43 13.5 15.62x10-3 44.8

% Change 4.8% -41.7% -0.69% -41.8% 67.8% 16.7%

Discussion

The maximum read range of the fabricated RFID tagsvaried according to equation (7). Increasing the tag coil area

2 2 N A and QS increased the maximum read range.

However, the increase was limited by the power requirementsof the system. The limits of the model can be demonstrated by

plotting the read range data in normalized form (Figure 10)with the maximum read range and reader antenna parametersare grouped in the abscissa. The maximum read range

followed the model prediction up to2 2 Q N A S values of about

0.05, then reached a plateau. Further increases beyond this plateau would require increased power transfer from thereader [11]. This read range plateau can be smartly exploited

by designers concerned about fabrication. Instead of designingRFIDs in the increasing regime, tags designed within to the

plateau regime is more tolerant of fabrication imprecision, andlow cost, high throughput printing can be used to fabricate theantennas. This will help to reduce tag cost, enabling itemlevel tag applications to become more popular.

0

0.04

0.08

0.12

0 0.04 0.08 0.12

A B C D

2 V

I C ( d 2 + 2 ) 1 . 5 / 2 N

1 I 1

N 2 A2S Q Figure 10. Normalized maximum read range as a functionof tag parameters. Data points marked with arrows werecopper samples

Conclusions

The electrical behavior at high frequency of RFID tagswith printed coil antennas and their maximum read rangeswere experimentally measured in this study. The coil antennaswere fabricated by screen printing using a polymer based,



silver filled, conductive ink. Printed tags with maximum readranges comparable to those with metal line antennas werefabricated by increasing the line thickness to minimize theseries resistance of the coil. Pressure compaction of the

printed coil to enhance contact among the filler particles wasshown to be effective in reducing the printed ink’s resistivity

and extend the maximum read range of the tag withoutadditional conductive ink cost. No significant change in

inductance was observed before and after compaction, or as aresult of increasing line thickness. A dimensionlessrepresentation of the maximum read range was found to

plateau. Antennas printed for commercial applicationsdesigned in the plateau regime will have high geometrictolerance and allow high throughput printing be used for the

production of antennas.

Acknowledgments

This work was supported by the Research Grants Councilof the Hong Kong Special Administrative Region, China,under projects RGC HKUST6190/03E, 615007, and 615505.

Appendix

At each d setting, twenty detection trials were performedand the number of successful detections was recorded. Atypical data plot of a detection performance versus d is shownin Figure A1. The probability of detection at each distancewas defined as the number of successes divided by the totalnumber of trials at that distance. The detection probabilitiesfor the data shown in Figure A1 are shown in Figure A2. The

probability data were fitted by logistic regression [12]. Thefunctional form of the logistic curve is

1( )

1 exp

P d d m

b

, (A1)

where m and b are the logistic mean and shape factor,

respectively. The maximum read range of a tag was taken asthe d with a 90% success probability.

References

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[2] S. Cichos, J. Haberland, and H. Reichl, "Performanceanalysis of polymer based antenna-coils for RFID,"

in Polymers and Adhesives in Microelectronics and Photonics, 2002. POLYTRONIC 2002. 2nd International IEEE Conference on, 2002, pp. 120-124.

[3] K. Finkenzeller, RFID handbook : fundamentals and applications in contactless smart cards and identification, 2nd ed. Chichester, England: Wiley,2003.

[4] D. Paret, RFID and contactless smart card applications. Chichester, England: Wiley, 2005.

[5] L. Setian, Practical Communication Antennas withWireless Applications. Upper Saddle River, NJ:Prentice Hall PTR, 1998.

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[7] P. L. Cheng, S. Y. Y. Leung, T. W. Law, C. K. Liu, J.I. T. Chong, and D. C. C. Lam, "QuantitativeAnalysis of Resistance Tolerance of Polymer Thick

Film Printed Resistors," Components and Packaging Technologies, IEEE Transactions on, vol. 30, pp.269-274, 2007.

[8] M. Gabl, N. Memmel, and E. Bertel, "Analysis of compacted and sintered metal powders bytemperature-dependent resistivity measurements," Applied Physics Letters, vol. 86, p. 042114, 2005.

[9] S. K. Volkman, S. Yin, and V. Subramanian, "Ink- jetted silver/copper conductors for printed RFIDapplications," Proc. 2004 Materials Research Soc.Spring Meeting, vol. 814, pp. 151-156, 2004.

[10] H. W. Johnson and M. Graham, High-speed digital design: a handbook of black magic . Upper Saddle

River, NJ, USA: Prentice-Hall, 1993.[11] I. Kirschenbaum and A. Wool, "How to Build a Low-Cost, Extended-Range RFID Skimmer," IACR eprint architecture, vol. 54, 2006.

[12] E. W. Weisstein, "Logistic Distribution," in MathWorld--A Wolfram Web Resource.

0

5

10

15

20

-100 -80 -60 -40 -20 0

N o . o f s u c c e s s f u l r e a d

Read Distance (d )

Figure A1. A typical result of the number of successfulreads versus read distance



0

0.2

0.4

0.6

0.8

1

-100 -80 -60 -40 -20 0

P r o b a b i l i t y o f s u c c e s s f u

l r e a d ( P )

Read Distance (d )

Figure A2. A probability plot of the data shown in FigureA1

geometric and compaction dependence of printed polymer-based rfid tag antenna

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