Thick-film and LTCC microvaristors

Proc. 30th Int. Spring Seminar on Electronics Technology, Cluj-Napoca (Romania), May 2007, p.53-58 Digital Object Identifier: 10.1109/ISSE.2007.4432820

Thick-film and LTCC microvaristors Edward Mis1), Andrzej Dziedzic1), Witold Mielcarek2) 1)

Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland 2) Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 61, 50-369 Wroclaw, Poland e-mail: [email protected]

Abstract: ZnO-based varistors are frequently used for electronic circuits overvoltage protection. Such material requires sintering at temperatures of 1150 oC-1300 oC. Lowering it to the typical range 850 oC-950 oC used in thick -film and LTCC technology is desirable. ZnO-based paste was prepared and exploited for varistors fabrication on alumina and LTCC substrate. Different component topologies, materials for electrodes and firing profiles were used. I-V characteristics of varistors as well as long term stability and durability to high voltage pulses were investigated. Satisfactory results were achieved.

1. INTRODUCTION ZnO varistors give effective protection against overvoltages. Ceramic varistors and discrete thick-film ones are fired at relative high temperature in order to ensure proper material sintering [1,2,3,4]. Components fabricated in firing profiles typical for thick-film technology exhibit weak I-V characteristic nonlinearity [5,6]. Additionally stability of such components has not been described yet. Test structures were designed and fabricated from ZnO-based ink with various technologies [7]. Their basic electrical properties were examined. Next works involving more complex stability and pulse durability determination were carried out.

2. TEST STRUCTURE PREPARATION ZnO-based powder was used for varistor ink preparation. 2% wt. amount of Bi2O3 was added to facilitate sintering process. DP6146 (PdAg), DP9894 (Pt) and ESL8880-H (Au) conductive compositions were applied for electrodes. Alumina and fired LTCC tape (DP951) were used as substrate. Varistors were designed in two configurations. Planar structure has finger-like electrodes with 0.25 mm spacing and 2 mm

wide varistor layer (Fig. 1). Sandwich samples had various dimensions: 0.5×0.5, 1×1, 2×2 and 3×3 mm2 (Fig. 2).

Fig. 1. Planar configuration

Fig. 2. Sandwich structures

Bottom terminations were screen-printed and fired at different temperatures: 950°C (Au and Pt on alumina), 850°C (PdAg on alumina), 900°C (LTCC structures). Next varistor film was printed: two times for planar components, three or six for sandwich ones on alumina and six for LTCC ones. After each print film was dried at 120 °C for 10 min. Another drying at 170°C for 10 min was carried out after whole film deposition. Part of planar samples was subjected to lamination at 200 bars/70°C for 10 min. Then structures were fired in typical 10 min/l h profiles with two peak temperatures - 850°C and 950°C. Next top


Varistor I-V characteristics were measured with pulse generator (0.1 ms pulse duration, 1 s interval). Resultant curves were fitted using / = kVa dependency and the nonlinearity coefficient a and characteristic voltage V1mA were calculated. Tab.1-3 shows their mean values for various technology variants, while Figs. 3-6 present I-V characteristics. o

dim.

PdAg Au Pt

mm 0.5×0.5 5.7 1×1 5.0 2×2 5.1 3×3 7.2 0.5×0.5 6.6 1×1 6.5 2×2 5.2 3×3 7.6 0.5×0.5 12.3 1×1 13.5 2×2 8.8 3×3 10.9

214 163 145 155 70 58 69 64 105 96 78 70

11.1 7.4 16.9 10.1 6.1 11.9

LTCC V1mA [V] 112 232 138 101 461 127

Tab. 3. Parameters of planar varistors Au850 Au950 Pt850 Pt950 PdAg850 PdAg950

o

850 C V1mA [V]

2

Au PdAg Pt Au PdAg Pt

alumina V1mA [V] 6.0 310 5.5 170 6.2 165 5.8 185 5.0 140 9.5 100

850oC

3. ELECTRICAL PROPERTIES

Tab. 2. Parameters of sandwich varistors on alumina

950oC

electrodes were printed and fired at 850 °C for all applied conductive compositions.

950 C V1mA [V] 5.7 5.7 4.5 5.1 6.9 6.1 7.6 8.6 15.9 21.0 22.7 15.2

211 202 204 190 54 43 52 54 132 110 92 150

0.01

I [A]

1E-3

1E-4

1E-5

V [V] 50

100

150

200 250 300

Fig. 3. I-V characteristics of 0.5×0.5 mm2 sandwich varistors on LTCC

Tab. 1. Parameters of sandwich varistors on LTCC Pt8503 Pt9503 PdAg9503 Pt8506 Pt9506 PdAg8506 PdAg9506

I [A]

dim.

PdAg V1mA [V]

o

950 C print ×6

o

850 C print ×6

o

950 C print ×3

o

850 C print ×3

2

mm 0.5×0.5 1×1 2×2 3×3 0.5×0.5 1×1 2×2 3×3 0.5×0.5 1×1 2×2 3×3 0.5×0.5 1×1 2×2 3×3

3.3 3.9 2.4 4.9 5.8 4.2 3.2 4.2 4.8 5.0 -

28 22 23 96 61 47 20 51 46 41 -

Pt V1mA [V] 5.5 4.3 7.8 4.5 3.9 6.9 5.1 8.8 8.0 6.6 6.0 11.7 13.0 9.4 5.9

11 7.0 14 7.0 7.5 10 7.0 20 14 11 11 20 23 16 8.0

0.1

0.01

1E-3

1E-4

V [V]

1E-5 10

100

Fig. 4. I-V characteristics of 0.5×0.5 mm2 sandwich varistors on alumina


0.01

Au850 Au950 Pt850 Pt950 PdAg850 PdAg950

I [A]

1E-3

1E-4

1E-5

V [V] 100

Fig. 5. I-V characteristics of planar varistors on LTCC

I [A]

Au850 Au950 Pt850 Pt950 PdAg850 PdAg950

0.01

1E-3

1E-4

1E-5

V [V] 100

1000

Fig. 6. I-V characteristics of planar varistors on alumina

Electrical properties of varistors are strongly affected by technology. Electrode material has the most significant influence on nonlinearity. Value of a up to 20 was achieved for Pt, while for Au it is from the range 3-8 and 3-11 for PdAg. Higher values were found for LTCC substrate, especially in the case of Pt terminations. Firing temperature slightly affected a value for structures with Au and PdAg electrodes. Samples with Pt-based electrodes exhibited nonlinearity increase with higher firing temperature, except for planar LTCC structures, where the effect was opposite. The distribution of a values was about 25% for sandwich varistors and 10% for planar ones.

Three times printed sandwich varistors had a values noticeably smaller than six times printed ones. Moreover their I-V characteristics often diverged from straight line (in log-log scale). They were also more likely to break during tests. Sandwich structures dimensions influenced a value in some degree, but there was no clear dependence. V1mA varied in a wide range - between 10 and 200 V for sandwich varistors and from 100 to 460 V for planar ones. Its correlation with a seemed to be weak. Characteristic voltage was distinctly affected by electrodes metallurgy. Application of different conductive inks caused voltage change by several times. Sandwich varistors on LTCC substrate showed larger V1mA in comparison with those on alumina. Such relation was not found for planar samples. In the case of sandwich structures V1mA distribution is smaller than for planar ones. Firing temperature has various influences. Higher temperature resulted in either increase or decrease V1mA, dependently on technology variants. V1mA of varistors significantly depended on kind of substrate. Replacing alumina with LTCC reduced it by 50-80 % for sandwich samples. Particularly strong influence of firing profiles and substrate was observed for Au and PdAg planar structures. Obviously, influence of varistor layer thickness was seen for sandwich structures – V1mA was approximately twice larger for six prints in comparison with three times printed samples. Lamination process did not affect a and V1mA parameters. Component construction clearly affected its reliability. The numerous shorts occurred in the case of sandwich varistors, especially for three times printed ones, probably due to insufficient layer density. Therefore some technological variants of varistors were not examined. Additionally planar configuration makes it easier to control V1mA by strictly component length change. It should be also noticed that samples construction also affects firing process - in planar structures varistor layer was fired once, but two times for sandwich configuration.

4. LONG-TERM THERMAL AGEING


Long-term stability of sandwich varistors was described previously [7]. The same test was carried out on planar samples. Varistors were kept at 150°C for 250 h. Their I-V characteristics before and after ageing are shown in Figs. 7-10. Tab. 4 presents calculated parameters. before V1mA [V]

after V1mA [V]

850

10.4

7.7

107

94

950

9.6

8.4

100

91

850

7.2

6.0

233

222

950

6.5

5.5

500

431

850

16.8

9.7

141

118

950

13.9

10.7

126

109

850

5.7

4.8

327

306

950

5.8

4.5

175

151

850

5.3

4.4

187

169

950

4.5

3.9

129

106

850

7.6

5.6

180

150

950

8.2

7.2

96

84

o

Pt

PdAg

Au

Pt

PdAg

Au

C

0.01

Au850 Au850aged PdAg850 PdAg850aged Pt850 Pt850aged

1E-3

1E-4

1E-5

V [V] 100

LTCC

after

Fig. 8. I-V characteristics of planar varistors on LTCC before and after thermal ageing (850oC firing) I [A] 0.01

alumina

before

I [A]

Au950 Au950aged PdAg950 PdAg950aged Pt950 Pt950aged V [V]

1E-3

1E-4

1E-5 100

Tab. 4. Long-torn thermal ageing results Au950 Au950aged PdAg950 PdAg950aged Pt950 Pt950aged

I [A] 0.01

1E-3

1E-4

Fig. 9. I-V characteristics of planar varistors on alumina before and after thermal ageing (950oC firing) 0.01

I [A] 1E-3

Au850 Au850aged PdAg850 PdAg850aged Pt850 Pt850aged

1E-4

1E-5

V [V]

1E-5

V [V]

100 100

Fig. 7. I-V characteristics of planar varistors on LTCC before and after thermal ageing (950oC firing)

Fig. 10. I-V characteristics of planar varistors on alumina before and after thermal ageing (850oC firing)

Generally thermal ageing slightly deteriorated varistors properties. All structures exhibited a small


drop in a value, except Pt ones on LTCC fired at 850°C, where the decrease was bigger. However their a parameter still remained high in comparison with other samples. Characteristic voltage shifted towards lower values by about 10-15 % in most cases. Manufacturing method did not affect stability of planar varistors too much. Structure configuration influenced V1mA stability, which was better for sandwich varistors. Nonlinearity changes were approximately in the same range.

0.1

Au950 Au950pulsed Au850 Au850pulsed PdAg850 PdAg850pulsed Pt950 Pt950pulsed

I [A] 0.01

1E-3

1E-4

1E-5

V [V] 100

5. PULSE DURABILITY

Pt

PdAg

Au

Pt

after V1mA [V]

Au950 Au950pulsed Au850 Au850pulsed PdAg950 PdAg950pulsed Pt950 Pt950pulsed

0.01

1E-3

850

10.6

10.6

106

104

950

10.4

9.9

104

102

850

6.8

6.1

234

245

950

-

-

-

-

850

-

-

-

-

950

11.9

11.9

122

124

850

5.8

5.7

306

306

950

5.1

4.7

200

197

850

-

-

-

-

950

4.3

4.1

142

145

850

-

-

-

-

950

7.7

7.8

88

87

Tab. 5. Pulse durability test results

1E-5

V [V] 100

LTCC

PdAg

Au

C

before V1mA [V]

alumina

o

400

0.1 I [A]

1E-4

after

300

Fig. 11. I-V characteristics of planar varistors on LTCC subjected to voltage pulses

Varistors durability to high voltage pulses was examined, similarly as for sandwich ones [7]. Components were subjected to series of 1000 pulses of 10 mA amplitude and 5 ms duration each. Test was done at room temperature. Results are included in Tab. 5, while Figs. 11-12 show changes in I-V characteristics of varistors. before

200

1000

Fig. 12. I-V characteristics of planar varistors on alumina subjected to voltage pulses

Part of structures set did not pass the test. Samples with Pt-based terminations fired at 850oC broke during the exposure to voltage pulses. On the other hand the same samples but fired at higher temperature showed very good durability. Structures with Au electrodes were the most reliable. Generally changes in a and V1mA were found to be very small, similarly to result obtained for sandwich varistors. While I-V characteristic nonlinearity mainly decreased little, except Pt/950 oC samples, V1mA went either up or down.

6. CONCLUSIONS Technology affected varistors properties significantly. Pt-based permitted to achieve relatively high nonlinearity coefficient – about 15-20. Characteristic voltage values from wide range could


be obtained. Varistors exhibited satisfactory durability to high voltage pulses and long-term stability. Promising results for both planar and sandwich components were found. Further works, including microstructure investigations will be done to complete structures characterization.

ACKNOWLEDGMENTS This work was supported by the Polish Ministry of Science and Higher Education, Grant No 3 T11B 075 29

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