a user made thick film system - hindawi

Electrocomponent Science and Technology 1980, Vol. 6, pp. 151-1580305-3091/80/0604-0151 $04.50/0

(C) 1980 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

A USER MADE THICK FILM SYSTEM

M. HONORE and R. GOVAEARTSK. U. Leuven, Departement Elektrotechniek (Afdeling ESA T), Belgium;

D. HOSTENS, B. BOUW, B. MATTON and V. JANSOONEBarco Electronic NV, Noordlaan 5, 8720 Kuurne, Belgium

(Received April 11, 19 79)

This paper describes the state of the art in the development of a polymer based printed circuit board (PCB)compatible resistor ink system. The subject is treated from the standpoint of the manufacturer who, up to now,supplied himself with PCB circuitry.

1. INTRODUCTION

The advent of thick film systems and hybridsinevitably leads to applications where the outstandingtechnical performance is balanced by an importantmaterials cost. Most paste suppliers counter this trendby developing systems with economical features(non-noble metal conductors, lower and shorterfiring protfles, enamelled steel substrates, high speedprinting etc.). In general one can say that thesedevelopments tend to duplicate the usual cermetcharacteristics with cheaper processing costs.

In this paper however, we will focus on highvolume applications in electronics where thesesuperior cermet specifications are not essential butwhere the profit margin is small and unflexible.Television-set manufacturing is a good example forsuch an application. In this kind of electroniccircuitry it is quite straightforward to develop athick film technology compatible with printed circuitboard technology.

In doing so, we can easily integrate both discreteand film components on one substrate. Moreover, wecan rely on the serigraphic know-how which isavailable in several printed circuit board manufacturingunits.Up to now this idea was propagated by two kinds ofsuppliers:

1) electronic circuitry subcontractors. 1,2

2) printed circuit board compatible ink manufac-turers.

f This research was sponsored by Barco Electronic NV,through IWONL grants 2429 and 2831 and was commissionedin part to the ESAT laboratory (director: Prof. R. VanOverstraeten) at the K.U. Leuven.

In this paper we will take the standpoint of yetanother party: the manufacturer of the equipmentwho, up to now, supplied himself with the printedboard circuitry. Apparently he is the one mostdirectly involved with this new development.

2. PROBLEM DESCRIPTION

The technology to be developed should show thefollowing features: use of additive serigraphicprocesses, ink curable on printed circuit boardmaterials, laser trimmable, compatible with all printedcircuit board processing manipulations (e.g. wavesoldering, ultrasonic cleaning).

With respect to cermet film technology weprobably will have to sacrifice on the performanceof the following characteristics: temperaturecoefficient of resistance, long term stability, powerdissipation, current noise.We further avoid multiple layers and metallised

holes. In short, we started the development for f’timresistors printed on the back side (the copper side)of a single cladded, printed circuit board.

Starting from all these assumptions, we made thefollowing options for the development of the pastesystems:

a) binder: two step thermosetting polymer of thesame kind as those used in printed circuit boardsubstrates;

b) conductive agent: carbon.In the next section, we will deal with some selectedresearch topics for the development of this system.

Out of the great variety of available printedcircuit board material, we selected Hp 2062.8

151

152 M. HONORE et al.

(according to DIN 773 5) in 1.5 mm sheet form. Weused the same material in the single cladded form.It is simply called PC board hereafter.

3. SELECTED RESEARCH TOPICS

In theory it is possible to infer the molecular massdistribution function from a GPC curve (if a cali-bration over the entire molecular weight range isavailable). This study is far from a trivial problemas can be concluded from recent bibliographies.4

3.1. Binder Material

We buy the linearised polymer as a solid, butsoluble, material from the specialised supplier. Thispolymer is produced in large quantities for a numberof industrial applications (e.g. injection molding).

In Table I we compare the results for the sheetresistivity (R) of three series of our pastes that wereprocessed in the same way. If we compare the firstand the second line of Table I, we can see that overa broadR range all the sheet resistivity values drop.The only difference between these two series is thebatch number of the basic polymer material. Ifhowever the viscosity of the polymer solution of thesecond batch is adjusted to the one of the first batch,the Ro approach their original values.

Gel permeation chromatography (GPC) is anappropriate method to gain more information aboutthe composition of the linear polymer. These GPCcurves prove indeed that there are a lot of differencesbetween the two batches. The dispersion factor forbatch A is 2.60 and for B 2.89. A more detailedstudy shows not only a marked difference in themolecular weight distribution on the high polymerside (see Figure 1) but also some striking points onthe lower polymer side.

The details in the dimer peak are interesting;indeed it is known that the isomers of this fractionhave quite different reaction constants.3 Thedifference in the monomer peak could be the resultof a reaction process divergence that occurred duringthe production of the batches.

3. 2. Carbon

The first thing to realise is that carbon in anindustrial environment appears in two forms:amorphous (carbon black) and crystalline (graphite).The influence of these two carbon morphologies isillustrated by the data in Table II. Obviously theinfluence on the temperature coefficient of resistance(TCR) is greater than one could expect from the useof the unmixed carbon forms. The optimal ratiohowever is as far as TCR is concerned not afunction of the composition alone, because the TCRis also influenced by the substrate material.

Table III gives an example of this phenomenon.In general one can say that the TCR rises with400 ppm/degree just by switching from alumina toPC board.

Perhaps this is the moment to say something abouta resistor model. Although the SEM photographs of aresistor section are not particularly relevant, we willassume that there exists a polymer matrix filled withgrains of given mean electrical conductivity. Onecould guess that the termal expansivity of this matrixis better matched to PC board (expansivity around30 10 -6 ) than to alumina (expansivity 8.7 10-6 ) andthus attribute the substrate effect on the TCR tolatent stress in the resistor on alumina after curing.For the morphological effect, one should fall on thedifference in temperature behaviour for the meangrain conductivity. Differences in TCR betweencarbon black and graphite were reported beforealthough in another temperature range,

TABLEInfluence of binder material on sheet resistivity. The f’fller composition isconstant while the polymer batch and its solution viscosity are parameters.Mean values (m) and standard deviations (s.d.) on R are given in kohm for

n observations

comp. comp. II comp. III

n m s.d. m s.d. m s.d.

Batch A (6.5 Pa.s) 36 9.4 1.3 112 22 880 193Batch B (9.0 Pa.s) 24 4.4 1.0 30 21 81 163Batch C (6.5 Pa.s) 36 8.7 0.9 81 12 500 63

THICK FILM SYSTEM 15 3

rII

44 40 36 32 28 24 20 16 12 mlELUTIEVOLUME

FIGURE GPC for the batches A and B discussed in Section 3.1. The peak height (UV detection) is given as afunction of elutionvolume (V). The degree of polymerisation of some fractions is indicated with n.

TABLE IIInfluence of carbon morphologies. Some typical values for the sheetresistivity (Rn) and the temperature coefficient of resistance (TCR)

Carbon black/graphite TCR (ppm/degree) inratio RD(kohm) the 25C to 75C interval

100]0 1.22 -39080/20 1.24 -33060/40 1.34 -20040/60 1.02 -15020/80 0.55 400/100 0.32 -330


TABLE IIIInfluence of substrate material. Mean values (m) and standard deviations (s.d.)

are given for n observations

Alumina PC board

n m s.d. m s.d.

R(ohm) 36 1150 90 732TCR (ppm/degree)

25C-85C interval 12 -80 25 310relative R D change

for 10% to 90% RH jump 12 6.6% 0.2% 3.3%

59

27

0.5%

3.3. Relative Humidity (RH)Next we consider the sheet resistivity change withthe relative humidity. This is illustrated in the thirdrow of Table III.

The effect is rather pronounced in our ink system.The fact that alumina substrates enhance RHsensitivity is a general observation. We suggest thatthis behaviour has to do with the fact that alumina isless hygroscopic than PC board

At this point we want to make some comments onthe mutual influence of TCR and RH sensitivity.From an industrial standpoint one can live with thisphenomenon and eventually consider a worst casetreatment, but from a research standpoint it is betterto split both effects.

Our unloaded drift measurements (not presentedhere) showed an unexplained fluctuation until werealised that we were measuring the environmentalhumidity situation.

As a matter of illustration of the effect underdiscussion consider the following TCR measurementsin the 25C to 75C interval for resistors of the samerun (Ro . 500 ohm):

1) TCR -290 ppm/degree for a resistorencapsulated in a TO5 package under N2 atmosphere.

2) TCR 100 ppm/degree for a resistormanipulated in the same way as above but exposedto atmospheric air.

3) TCR 240 ppm/degree for the usualmeasurement.We conclude that it is necessary to take special

precautions if the temperature coefficient ofresistance and not some sort of climatic coefficientof resistance is under investigation. We furthersuggest that the IEC 115[ 1 prescription is perhapsnot the best way to do the TCR measurement forthis type of uncoated f’flm resistors (24 hours dryingat 55C will not help that much if moisturecondensates on the resistance at the first measurementon a lower temperature).

3. 4. Rheology

In Section 3.1. we have already mentioned animportant aspect of the systems rheology: theviscosity of the polymer solution. Figure 2 gives anexample of the ink viscosity as a function of shearstress.

The ultimate test for the rheology however, issimply good screening and levelling. We experiencedthat, in general, it is good practice to use thixotropicadditives. Unlike the cermet pastes, we can forgetabout additives that disappear under curing. So wehave chosen a chemical inert additive also used in thepaint industry. It is simply SiO2 with a great specificsurface. This product helps to solve screeningproblems (e.g. small bubble generation after printing).For a given composition there is however an upperlimit to the amount of this particular additive. Abovethis threshold cratedike eruptions occur during thecure process.

An example for thixotropic behaviour is given inFigure 3. Viscosity and thixotropy were measuredaccording to Heinz et al.9

In this section we compare two curing methods"thermal curing (boxoven) and infrared (IR) curing.An infrared absorption spectrum of PC board materialshows a minimum in the 2000 cm -1 to 1800 cm-1

region while the wet spectrum is flat.6 So we havedone all our IR curing with a 5 micron lamp assumingthat the PC board is maximal transparent for thiswavelengths.

We also considered another parameter: the heatup velocity. The first two rows of Table IV give theresults. In this table slow heat up for thermal curingmeans oven stabilisation at the curing temperatureafter insertion of the wet resistors while fast heat upmeans stabilisation before the resistors are inserted.For IR curing slow heat up is approximately 15degrees per minute while fast heat up means some

THICK FILM SYSTEM 155

9

3

102

8

1010 50 100 150

v(s-)

FIGURE 2 Example of a viscosity (r/) versus shear rate () plot for one of our pastes.

45 degrees per minute. The latter data were derivedfrom a temperature profile of the stabilised oven. Toobtain the data in the third row of Table IV, werepeated the IR cure on samples with a longer curetime (= time at maximum temperature). From thefirst two experiments one could conclude that a fastheat up is better from a point of view of spread inthe final product. The third experiment howeverindicates that this difference disappears afterextended curing.

There remains of course an enormous point infavor of IR curing: the curing time is at least 5 timesshorter.

Coming back to our resistor model mentioned inSection 3.2. we can try to understand these results.If the heat up time is slow the three dimensionalpolymerisation process starts slower (less cross linksper unit time for a given chain length) and the riffler

material gets a chance to cluster. Consequently thereis a bigger spread in grain size and thus in resistorvalues. Continued curing eventually splits up thesebigger clusters into smaller grains of a more com-parable size.

The lowering ofRD as a function of curing timeis a well known effect in cermet technology.

3. 6 Thermal Shock

In this section we concentrate on the effect of wavesoldering on PCB compatible resistors screened at thebottom of the board. Table V shows the influence ofthis kind of thermal shock on TCR and RH sensitivity(relative change in RD if the RH changes from 10% to90%).

For the influence onR it is possible to perform acorrelated test. It turns out that for anR 10kohm


150-

5O

FIGURE 3stress

10 20 30 40 "(Pa)

Thixotropic behaviour of the same paste as in Figure 2 with shear rate (/) as a function of shear

TABLE IVInfluence of curing method and cure time. R[] values(in ohm) for the mean (m) and standard deviation (s.d.) for

n observations

Fast heat up Slow heat up

n m s.d. m s.d.

Thermal cure 108 905 45 1178 82IR cure (10min) 72 928 54 1328 194IR cure (20 min) 120 752 38 867 38

this relative change in Ro has a mean value of 0.9%with a standard deviation of 3.4% for 36 observations.

We have other data that confirm this trend. Duringthe wave soldering process the sheet resistivity of theunprotected resistor screened at the bottom of thePC board will increase, the TCR will lower somewhatand the RH sensitivity becomes slightly worse. How-ever, the results reported in this section were onlyobtained after thorough preheating before thethermal shock was applied.

TABLE VInfluence of thermal shock on TCR and RH sensitivity. Mean values (m)

and standard deviations (s.d.) are given for n observations

Before (n 12) After (n 8)m s.d. m s.d.

TCR (ppm/degree)25 C-85 C interval

Relative Ro changefor 10% to 90% RH jump

540 100 410 100

4.9% 1.5% 5.5% 1.8%

THICK FILM SYSTEM 15 7

4. FURTHER RESEARCH

Further research should be directed towards thedevdopment of the very simple model put forwardin Section 3. We need to know more about thestructure of the resistor and of the intermediatestages.

The results reported for GPC give a first indicationon how to proceed. In a first approach one couldtry to correlate the dispersion factor with thecharacteristics of the finished product. Clearly themolecular distribution function of the linear polymerhas a relationship with the final matrix structure (ifit exists) in the resistor.

There are methods available to gain informationon what happens during the curing process. 7 As anexample we give here a differential scanning calori-metry (DSC) plot of one of our inks. In DSC onemeasures continuously the heat capacity of a sampleas a function of a programmable heating cycle. If thisheating cycle is linear in time the DSC is a straightline; it can be modulated by positive or negativedeviations if exothermic and endothermic processesrespectively take place. In more sophisticatedstudies one can use this heating rate as a parameterto derive the most favorable curing time, or simulatea real cure process by making full use of theprogramming function. The latter simulation is of

course only valid if the heating agent is the same as inthe real process.

In our example (Figure 4) we detect an exothermicand an endothermic process which both practicallydisappear when the sample is rerun after coolingdown. The exotherm is the polymerisation reaction.

The endotherm however could be a simpleevaporation of solvent material. The latter conjecturecould be checked by a thermogravimetric analysis.The intriguing point is that the evaporation occursafter polymerisation; this could indicate that thereexist solvent inclusions in the resistor.

To learn more about the structure of the tmalproduct, we can use noise measurements. Twosamples of the same run were given, a differentpreheating treatment before they experienced thesame thermal shock. The results are given in Table VI.

From the data in the last two columns of Table VIwe can calculate from

(Ru /Ru :) (C,/C )resulting in 2.1. In the Vandamme picture ofnoises in film resistors this would indicate anintermediate stage between constriction and interfacedominated grain conductivity. These results are onlygiven as an example. It would for instance be morerelevant to compare 1/f noise data of the samesample before and after thermal shock. These dataare not available to date.

50 100 150 200 250 300 350 400 450 5OO

T(C)

FIGURE 4 An example of DSC plot for one of our inks. Sample temperature is given in the abscissa in C. Themeaning of the ordinate is explained in Section 4. Heating rate was 20 C/min under N atmosphere. The brokenline is a plot for the same sample after the first experiment (full curve) was finished.


TABLE VISheet resistivity and current noise as a function of thermal shock. R

values are in ohms; C is the relative 1/f noise intensity

RD before R before R afterpreheating thermal shock thermal shock C

Sample 692 706 694 0.12 10Sample II 723 723 1323 0.50 10-’

5. EXAMPLE

In order to assess the possibilities of this technologyin a less theoretical way we produced prototypes ofan existing audio module for colour TV in thisalternative technology.

In this design 26 resistors were produced withthree different pastes. They were IR cured and lasertrimmed. The remaining discrete dements were handsoldered.

We launched a reliability lrogram on a set of 10modules in an ambient of 50vC at 90% RH producingmaximal power (6.5 Wff). From a quality standpointthere were some noise problems at minimal volume.The problem is located at two specific resistors in thefirst stage of the amplifier circuit. This impairmentcan be solved by using discrete components in thispart of the circuit. We could however try to solve itfrom a technological standpoint: either by increasingthe surface of this resistance or using conductingadditives.

6. CONCLUSIONS

The development of a PCB compatible thick filmresistor system is not entirely finished but we thinkthat the prospects are good. In order to develop aconsistent resistor model a strong multidisciplinarycooperation is necessary.From an industrial standpoint we conclude that:

1) this kind of technology has a chance only if theuser has a thorough understanding of the inkproduction and stabilisation process.

2) an apparatus manufacturer with a flexible anddiversified product range should be his own principalink supplier.

ACKNOWLEDGEMENTS

We thank L. Frisson, A. Meeus, J. Degraeuwe(K. U. Leuven) and A. Cardon (R. U. Gent) for their helpduring this research. The DSC was supplied by Prof.E. Goethals and the noise measurements by Prof. H. Pauwels(both at R. U. Gent).

REFERENCES

1. Electronics, July 25 1974 p. 23, April 27 1978 p. 114,January 18 1979 p. 70.

2. K, Kojima et al., Resistor Printed Circuits (RPC), NationalTechnical Report 18 (1972) 370-391 (in Japanese).

3. P. Montague, F. Peaker, P. Bosworth and P. Lemon, Aninvestigation of novolac resins by GPC, Br. Polym J. 3(1971) 93-74.

4. ASTM, Bibliography on liquid exclusion chromatography1972-1975, AMD 40-S1, Philadelphia 1977.

5. P. B. Coursey, Fixed resistors for use in communicationequipment, Proc. lEE 96 (1949) 169-186.

6. A. Meeus, Infraroodopwarming bij LTTF sehakelingen,Intern ESAT verslag, Dec. 76 (in Dutch).

7. A. Siegman and M. Narkis, Thermal analysis ofthermosetting phenolic compounds for injection molding,Z ofAppl. Polym. Sci. 21 (1977) 2311-2318.

8. L. Vandamme, Criteria of low-noise thick film resistors,Electrocomp. Sci. and Techn. 4 (1977) 171-177.

9. W. Heinz, H. Mewes, P. Haake, Rheologie und Rheometriemit Rotationsviskosimetern, third edition, Firma Haake(907-0107), Karlsruhe (in German).

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