AN METAL CONDUCTOR SYSTEM FOR LOW COST THICK …

Electrocomponent Science and Technology, 1981, Vol. 8 pp. 61-650305-309.1/81/0802-0061 $06.50/0

(C) 1981 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

AN AIR FIRING BASE METAL RESISTOR AND CONDUCTORSYSTEM FOR LOW COST THICK FILM CIRCUIT

MANUFACTURE

E. K. BROWNE and B. WALTON

Electrofilm Products Division, ERA Technology Ltd., Leatherhead, Surrey England

A series of air-firing base metal resistor pastes which yield resistors with properties comparable to those of conven-tional precious metal thick film resistors has been developed. The pastes can be post-fired or co-fired with air-firingnickel conductors in a very fast firing cycle thus providing a base metal combination which can be processed usingconventional furnaces. This is concluded to be a significant contribution to the manufacture of thick film circuits of lowcost especially in view of recent large increases in the prices of precious metals.

1. INTRODUCTION

The thick film industry has been based on the use ofresistor and conductor pastes containing precious met-als and the use of these materials has become firmlyestablished. The outstanding electrical conductivity andchemical inertness of noble metals are attributes whichare well suited to the role of these elements in thick filmtechnology. Unfortunately noble metals are expensiveand their replacement by cheaper alternatives has longbeen a goal of paste technologists. Most attempts to pro-duce base metal conductor and resistor systems haverelied upon the use of reducing atmospheres in thethick film furnace to protect the metals from oxidationat high temperatures. This is a rather expensive andinconvenient procedure requiring specially designedfurnaces. Furthermore the characteristics of the firedfilms are inferior to those obtainable with conventionalmaterials fired in air. An air-firing base metal resistorsystem with promising properties which was developedin the authors’ laboratory was first described byLaurie. The system was originally intended mainly foruse with lower cost palladium-silver conductors con-taining relatively high proportions of silver. Now, withrecent further steep increases in the price of preciousmetals the use of even these silver-rich compositionscarries a significant cost penalty. There is therefore astrong incentive to introduce an all base-metal resistorand conductor system. The paper describes such a sys-tem which comprises an improved series of air firingresistor pastes fully compatible with air-firing nickelconductors.

61

2. BASIS OF THE RESISTOR SYSTEM

The original system described by Laurie consisted of acadmium alumino borate glass containing as a dissolvedcomponent, molybdenum trioxide. During the firingcycle, after decomposition and removal of the orga,nicvehicle, the glass melted and flowed together to form acontinuous film. On further increasing the temperature,an electrically conducting phase, molybdenum dioxide,was precipitated by the action on the trioxide of a care-fully chosen reducing agent. Thus the conducting oxidewas completely protected from re-oxidation in the fur-nace atmosphere by total encapsulation in the glassphase. A filamentary network of interlocking needle-shaped molybdenum dioxide crystals was formed andthis filamentary structure provided the high degree ofnon-uniformity in the distribution of the conductingphase which is necessary to produce high resistancevalues. A range of resistance values was obtainedessentially by varying the concentration of the conduct-ing phase in the surrounding non-conducting glassymatrix.

This basic system has been modified and refined toimprove the properties and to extend the range in orderto develop an attractive commercial system. (Electro-film Series R1000 resistor Pastes).

3. PROPERTIES OF THE FIRED RESISTORS

The basic characteristics of resistors fired on 96%alumina substrate with palladium silver terminations

62 E.K. BROWNE AND B. WALTON

Electrofilm

"gnation

Resistivity (--.25%) 30 /sq 100 /sqTCR (ppm/C) -55C to +150 50 +25 50+125"C

High temp. stability <0.5% <0.5%(10,000 hrs 150C) R

Load stability (10,000 hrs <0.5% <0.5%@ {25 W/in2} @ 70C) R

Moisture resistance (six <0.5% <0.5%damp head cycles) 6R

TABLE

R1025 R1030 R1035 R1040 R1045 R1050

300 f/sq 1Kf/sq 3Kf/sq 10 t2/sq 30Kt2/sq 100Kf/sq-50__.50 -75__50 -150_50 -200--50 -250+-50 -300+-.50

<0.5% <0.5% <0.5% <0.5% <0.5% <0.5%

<0.5% <0.5% <0.5% <0.5% <0.5% <0.5%

<0.5% <0.5% <0.5% <0.5% <0.5% <0.5%

are summarised in Table I. A more detailed picture ofthe rate of change in resistance at elevated tempera-tures and on load, for totally unprotected resistors overa period of 10,000 hours is given in Figures 1 and 2.The resistance values available (30f/sq-100KD/sq)

covers the useful range required for most applications.The temperature coefficient of resistance (TCR) is

within 250 ppm/C except at the top end of the rangeand the TCR tracking is particularly close being gen-erally within 10 ppm for resistors printed from the samebatch of paste. Laser trimming is readily accomplishedwith a clean kerr and the post-trim stability is littleaffected. Stability at high temperatures is outstanding,changes in resistance of unprotected resistors after1000 hours at 250C being less than 1%. Thus it isconcluded that the resistors conform to a highly satis-factory specification.

4. PASTE PROCESSING CHARACTERISTICS

A conventional vehicle system is used in the pastes butthe absence of any heavy precious metal compoundsleads to a low density material and the paste coverage(print area per gm of paste) is therefore high.

Firing cycles of widely varying length can be used.The firing cycle can be exceptionally fast, total firingcycles as short as 10 minutes have been successfullyused, and it is not essential to dry the prints beforefiring. A typical 15 minute firing profile is shown inFigure 3 with a peak at 750C. The profile is bell-shaped, there being no need to incorporate the usuallong flat zone at the peak temperature. This simplifiesfurnace profiling. The effect of changes in peak firingtemperature on resistivity is shown in Figure 4.

*0"50

/0" 25

7emperoture 150 eC

O0 I00 1,000

FIGURE Stability after high temperature exposure.

RIO20

RIO50

RO5

R030RIO40

Hours IoDoo

AIR FIRED BASE METAL RESISTOR PASTES 63

-0-5

Lood tobillty

1,000 Hour

RIO20

RIO30

RIO40

I0.000

RIOI5RIO50

FIGURE 2 Power level of 25 W/in at ambient of 70C (R1050 derated to 500 V/in level.

800

700

600

500

400

300

200

I00

750"C Otimum

Exit from furnoce

0 I0 Minutes 15

FIGURE 3 Firing profile.

The curves are of unusual form and show that theresistivity passes through a minimum near 750C forcompositions of all values. This means that providedthe firing conditions are set close to this minimum thefiring temperature sensitivity is very low. Small changesin temperature near the minimum do not affect theTCR appreciably.

5. CONDUCTOR TERMINATIONS

The resistors can be terminated with conductors of allof the conventional precious metal types: gold,platinum-gold, palladium-gold, palladium-silver andplatinum-silver. However, for economic reasons theemphasis in assessment has been on the use of pal-

64 E.K. BROWNE AND B, WALTON

#K

730 735 740

Furnace throughputtime 15 rains

RIOSO

.....- RI O 20

RIalS

750 75.5 760 765 70Peak lrinj temperature eC

FIGURE 4 Variation of sheet resistivity with firingtemperature C.

lOOK

IOK

I00

Resistor width O’O50 inch

RIO50

RIO40

RIO30

RIO20

RIOI5

o.oes o-os o.Length inch

FIGURE 5 Variation of resistivity with resistor length.

ladium silver conductors of relatively high silver con-tent. These proved to be particularly suitable. Theresistors are remarkably free from ’end effects’attributable either to the presence of interfacial layersat the conductor/resistor overlap causing a positive con-tact. resistance, or to diffusion of metal from the con-ductor into the resistor causing a reduction in value or’negative contact resistance’. These effects are com-monly observed, sometimes to a very marked degree, inruthenium-based resistor systems. This freedom fromserious end effects is evident from the data illustrated in

Figure 5 which shows the variation of apparent sheetresistance with resistor length. The only noticeableeffect is a very slight increase in apparent sheet resis-tance for very short resistors at the 100Kohm/squarelevel.The above data was obtained using palladium-silver

conductors. More recently assessment of terminationsprepared from air-firing nickel compositions has been

undertaken. Preliminary results are very encouraging.Table II shows the results of co-firing nickel conductorswith Electrofilm base metal resistors in a 15 minutefiring cycle, peak temperature of 750C, in air. Thetests were carried out using pastes of two nominalvalues 100f/square and 100KQ/square.The results show that over a wide range of resistor

length the apparent resistivity remains essentially con-stant indicating an almost-total absence of end effects.This is confirmed by visual examination which revealsno sign of undesirable interactions at the interface be-tween conductor and resistor. Similar results wereobtained with pre-fired conductors. Measured TCR’sare similar to those obtained using palladium-silverconductors. Short term load and dry heat exposure testsare highly satisfactory and the consistency of as-firedvalues and absence of ’end effects’ give grounds forexpecting long term behaviour at least as good as thatobtained with palladium silver terminations. Indeed the

AIR FIRED BASE METAL RESISTOR PASTES 65

TABLE II

Resistor size length x width 0.2 x 0.05 x 0.016 x 0.025 x(inches) 0.05 0.05 0.05 0.05

0.1 x 0.1 x 0.1 x0.05 0.1 0.13

Mean value of ten resistors 464f 105f 30.3fl 57f 230f 113f 149fprinted with R1020

Mean normalised value in 116 105 91 114 115 113 112

Mean value of ten resistors 456Kf 105Kf 23.3Kf 47.5Kf 218Kf ll0Kf 136Kfprinted with R1050

Mean normalised value in 114 107 70 95 109 110 102Kf/[]

absence of a highly mobile ionic species such as silver inthe nickel conductor gives further grounds for confi-dence in the absence of long-term degradation.The fast, relatively low temperature firing cycle used

for the base metal resistors is well suited to the air-firing of nickel conductors. The conductivity obtained isfairly high (<0.05 ohms/square) and the adhesion isgood. The conductors are readily solderable with nor-mal tin-lead solders after a mild cleaning operation toremove surface contamination.

6. CONCLUSION

An air-firing base metal resistor and conductor systemwhich has characteristics suitable for the manufacture

of thick film circuits has been developed. The savings inmaterial costs compared to precious metal alternativestogether with fast firing in conventional furnaces makethe system attractive for low cost high volume applica-tions.

ACKNOWLEDGEMENTS

It is a pleasure to thank colleagues N. Davey who contributedto the research programme, and C. Stringer who carried outmuch experimental work.

REFERENCE

1. A. S. Laurie, A High Quality Base Metal, Thick FilmResistor System, IEEE Electronic Components Confer-ence, Washington D.C. 137-139 (May, 1973).

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AN METAL CONDUCTOR SYSTEM FOR LOW COST THICK …