THICK FILM CIRCUITS: PRESENT AND FUTURE DEVELOPMENT

Electrocomponent Science and Technology, 1981, Vol. 9, pp. 131-1370305-3091/81/0902-0131 $06.50/0

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

THICK FILM CIRCUITS: PRESENT STATE AND FUTUREDEVELOPMENT

J. NOVOTNY

ITT Components Group, Standard Elektrik Lorenz AG, Nuremberg, West Germany

(Received November 10, 1980; in final form May 11, 1981

Thick film technology has been known for about 20 years. The technique of inks, printing, firing and trimming hasgrown in this time to a well established state of the art. This paper gives an overview of the status today and an outlookinto the future, not only for thick film, but also for hybrids built out of thick film circuits with incorporation ofadditional components. The paper gives the main market figures and market segments for thick film applications andmentions the main reasons for this. New fields of applications for thick film will be found in the future whereconsiderations like reliability, space savings and cost advantage are of importance.

1. DEFINITION

The hybrid circuit based on thick film technologyrepresents only one of many possible solutions for thecombination of electronic components and assemblies.However, today this solution is of quite major marketsignificance in the field of hybrid technology. Weunderstand by this solution the combination of a planararrangement of passive components such as conductivelayers resistors, crossovers and capacitors which areapplied by a screen printing process, with passive oractive devices in various forms, usually in the chip form.Greatly simplified, we can describe it as very similar topc-board technology; and since this latter technologywas the starting point for the hybrid circuit, theresemblance is not surprising.

2. MARKET SIGNIFICANCE

In the last two years the market for thick film circuitshas been the subject of considerably detailed analysisby several investigators. 1,2,3,4

The results show that in 1980 the size of the worldmarket was somewhere in the region of 2 billiondollars, of which the US market alone accounted for70%. The European market for 1979 was estimated atapproximately 400 million dollars; Great Britain,France and West Germany each showed figures for1979 of roughly 60 to 90 million dollars. The estimates

’Paper originally given in the Technical Papers programmeof Electronica, Munich, November, 1980.

show a considerable variation because of the difficultyin ascertaining the infeed demand of severalcompanies.

3. MARKET SEGMENTS

Figure 1 shows a breakdown of sales in WesternEurope on the basis of the individual market segments,and highlights the major significance oftelecommunications in this market. This emphasis isalso reflected in the pattern of world sales. In secondposition we find applications in the military sphere,which is of very great importance in France and GreatBritain in particular. In West Germany this segmentdoes not have the same degree of significance.Industrial applications, the consumer andentertainments field, data processing, and automobileapplications account for roughly equal shares of themarket. Of considerable interest are the predictedgrowth rates, which show a continued bias towardstelecommunications and military technology, togetherwith above-average figures for the spheres of consumergoods and automobile technology. The marketbreakdown for West Germany is seen in Figure 2.Again the main emphasis is in the telecommunicationsfield.

Predictions as to growth rates over the next 5 to 10years range from 10 to 25 percent per year. In the fieldof electric components there are very few branches forwhich similar high figures are predicted.These growth rates are projected for the following

131

132 J. NOVOTNY

M. DM

198t, /,0% probability

80% probabitity

500

00

300

200

100

r

7////// F1//// v/ v

[’//

78 St, 78

FI F1 r,, II r-

II

78 84 78 8L,. 788 78 8/, 78fete- mitiry industry consumer computer automotive medicct

communications

FIGURE W. European Markets for Thick film Hybrid circuits 1978 and 1984.

M. DM.

100

50

//////////

_///////////

7885

Totals

1978 125 M.DM.I5 300 M.DM.

/

?885 78 85 7B85 78 85 78 85fetecomms, enter- mitifary, industry automotive, computers

inc. fctinmenf.rnitifary.

FIGURE 2 W. German Markets for Thick film-Hybrid circuits 1978 and 1985.

THICK FILM CIRCUITS FUTURE DEVELOPMENT 133

reasons:

-The general expansion of the microelectronicsfield.

-The fact that the technological and economiccharacteristics and advantages of hybrid circuits arebecoming more generally known, both to designersand sales staff.

-The effect of the fundamental changes becomingapparent in the telecommunications field (satellitetelevision, digital technology, glass-fibre opticaltransmission).

-The availability of hybrid-compatible components.

4. REASONS FOR THE USE OF HYBRIDCIRCUITS

The chief arguments for the use of hybrids haveremained unchanged for years. These are:

-Saving of space by concentrating the circuitfunctions.

-High reliability as a consequence of the reducednumber of connections and the improvedproperties of the materials employed.

Simplified handling.

-The possibilities of reliable functional trimming.

-Cost advantages resulting from all the abovefactors.

5. MATERIALS AND COMPONENTS INHYBRID TECHNOLOGY

5.1. Substrates

Alumina oxide ceramic (A1203) has been established asthe main substrate for hybrid thick film circuits formany years and will remain so in the immediate future.This material has so many merits that the disadvantageof a certain fragility can be tolerated in mostapplications. The enamelled steel substrates, thesubject of much discussion over the last few years,without doubt have their place in certain applications; acost benefit can be expected, for example, whereparticularly large areas are required, or special shapes,or particularly great mechanical stability. These factorsmay well become important in the automobile field.However, there are still some technical problems notfinally solved, so that a broad utilisation of thissubstrate has not yet occurred. Steel substrates cannot

be broken into smaller sections. Conventional hybridcircuits are generally produced by multiple printing; onaverage 3 to 5 circuits are made from a 2" 2"substrate. For this reason a certain minimum size isnecessary in order to exploit cost benefits.

5.2. Inks

At the beginning of 1980 the dependence of price ofthe thick film on the precious metal market againbecame particularly pronounced. The conductivepastes essentially contain precious metals, silver,palladium, gold and platinum; there will be no basicchange in this situation. In the immediate future,endeavours will concentrate on achieving the sameproperties as hitherto with a smaller proportion ofprecious metals. Trials with non-precious metals willcertainly be carried on with vigour by all users;however, apart from one or two special developmentsin resistance networks, at the moment no breakthroughis in sight. Nevertheless, it is possible that in the longterm several new materials will find their way into thickfilm technology. At present, copper seems to be themost likely candidate.The main criteria for a good conductive path are

strong adhesion to the substrate, solder leachresistance, and bondability when semiconductor chipsare attached. Nowadays the resistive pastes arenormally based on the bismuth-ruthenium system. Thefiring temperatures vary from 850C, for conductorsand resistors to 500C for overglazes. The properties ofresistance layers produced in this manner are describedin terms of their stability in long-term tests:

Typical stability values for tests over 10 000 hours at125C are zlR/R equals -+0.5% for resistances between10 ohms and 5 Mohms. (Figure 3, Figure 4) There areapplications for which even narrower limits arerequired, and maintained. Other tests, such asresistance to moisture or rapid temperature cyclingresult in similar stability values. The specification forthe temperature coefficient of resistors is usually-+250 ppm/C or -+ 100 ppm/C; (Figure 5) tighterlimits, such as +-25 ppm/C, are required only inexceptional cases. Pastes with low TC values often havenarrowed values for such data as voltage sensitivity orpower loading capability.A further improvement of resistance layers for

technical reasons seems unnecessary at the moment,except for special applications (high voltage, highloading, high resistance to abrasion). Furtherdevelopment should be seen in terms of cost-reductionmeasures and fundamental changes in combinationwith new materials for conductive paths.

134 J. NOVOTNY

Resistor- ink 13/+1/1351 with overgtoze

Rn 30 Kn

Number of resistors 1/+0

+2-o [owed change occ. toDIN /+18/+8

Termin(afi(ns;, AuPd,Pd,Ag.500 1,000 2,000 5,000 10,000 20,000

Time (h)

FIGURE 3 Stability of Thick film resistor at 125C storage, no load (30KD/sq with overglaze).

Resistor ink 1361 without overgtaze

Rm Mn/sc

Number of resistors 36

0

-2-Termination PdAg

At towed chant oct. to

DIN /+18/+8

500 1,000 2,000 5000 10,000 20,000

Time (h)

FIGURE 4 Stability of Thick Film resistors at 125C storage, no lead (1MD/sq without overglaze).


+2

+I

+/-0

-I

-2

-3

+250 ppm/C

40 -25 0 25 50 75 100 +125Temperctfure C

FIGURE 5 Temperature coefficient of resistance of resistors (100K/sq -40 samples).

Dielectric pastes are of significance as glazes, asdielectric layers for crossovers, and as dielectrics forprinted capacitors. With the price decrease in ceramicmultilayer capacitors over the last few years, theimportance of printed capacitors has not grown to thesame extent as that of general film technology.

5.3. Trimming

It is hardly possible to conceive of a modern thick filmproduction without the laser as a tool for trimmingresistors. Since it has so far not proved possible toachieve high limits of accuracy of value after printingand firing of resistors, this instrument will continue tohold a position of major importance in the future. Infact, it is expected that the use of lasers in connectionwith the trimming of functions will increase. Modernlaser systems offer many possibilities, either in theproduction of passively trimmed resistors, or whenequipped with software, for production of qualitycontrol.

5.4. Components

Most of the components mounted on hybrid circuits aresemi-conductors. The developments of thesesemiconductors has followed different ways in the USAand Europe, and this has led to a situation in which theprocessing of non-encapsulated semiconductors ismuch more widespread in the USA than in Europe.

Since both of these procedures offer advantages, butalso have their drawbacks, work has long beenunderway on developing new production techniqueswhich will combine all the advantages. From ourpresent point of view the answers are chip-carriertechnology and tape automated bonding (TAB).The advantages in processing encapsulated

semiconductors are obvious: The available componentsare tested, robust against handling, and do not requirehighly skilled personnel for their processing(mounting). The move from hand mounting towardsautomated operations is well underway. There is verylittle difference in the technology from mounting onp.c. boards, and this is the reason why many users withprevious experience of p.c. board technology haveintroduced hybrid technology for their ownrequirements The necessary investments in the area ofmounting are relatively low, at least as far as manualprocedures are concerned.The semiconductors employed in this technology are

single transistors or diodes. If function complexityincreases and integrated circuits are used, the smallforms (SO-packages) are often not available, or thenumber of leads is too large for this form.However, since the functional density of hybrid

circuits has shown in the last few years, a similarcontinuous increase to that seen with integratedcircuits, the need for space-saving solutions for thesemiconductors is becoming more important. The idealsolution would be a space requirement not larger than

136 J. NOVOTNY

the chip area, so that no additional space is wasted forthe leads. The chip must be mounted upper sidedownward (towards the substrate). Although thistechnique is employed for some special cases, for avariety of reasons it has not become generally accepted,and it is probable that this will continue to be the casein the future.The classical bonding technique obviously requires

more space, and the time needed for the learning phasecan often be rather long. On the other hand, since thesemiconductor chips are usually delivered not fullytested, the reject rate is higher. In addition there arethe rejects from the chip mounting and bondingoperations, as well as the costs for possible rework. Onesolution is the chip carrier. Basically this is a new formof package, which is designed for maximum use ofspace and is suitable for processing by the reflowsoldering technique. The assembly repackaged by thistechnique exhibits corresponding advantages: ease oftesting and handling, and hermetic sealing. For thesereasons the use of such assemblies will become of majorimportance when this package is fully available andwhen benefits become apparent as regards overallcosts. However, at the moment it is often up to themanufacturer of hybrids to install the semiconductorsin the chip carrier himself; and, in this case, there is nogreat advantage.The utilisation of chip carriers will also become of

interest for small production runs when these parts aredelivered fully assembled from the manufacturer ofsemiconductors. On the other hand, for large runs theuse of TAB will continue to increase. In this process,which requires relatively large investments, thesemiconductors are bonded onto a film which has beenlaminated with copper foil and etched according to awiring pattern (inner lead bonding). This film permitsthe testing of the IC as a component; in a furtheroperation the IC with its attached leads is stamped outof the film and mounted on the substrate (outer leadbonding). This method needs approximately the sameamount of space as the chip carrier, and allowsextremely fast mounting of the semiconductors. Forlarge series, and when the necessary limiting conditionsare observed, this technique offers the mostcost-beneficial method of mouting semiconductors. Inmost cases a protection for the semiconductor chips inthe form of an organic covering is required, even if onlyas a mechanical protection for the bond connections.

5.5. Coatings

The most satisfactory coating of hybrid circuitsrepresents a complex question. Depending on theapplication, the most varied requirements are set. Little

needs to be said about hermetically sealed hybrids astheir use is generally determined by the intendedapplication or by the existing standards. Accordingly,the structural forms in this area are extremely varied.

In cases where unencapsulated semiconductors areprocessed, i.e. for all bonding techniques, an additionalprotection is absolutely indispensible, both formechanical reasons and- because of the possibleeffects of moisture- on electrical grounds. The usualmethod is the covering of the semiconductor chips,including the bond wires, with a passivating material,generally silicone materials which are applied bydispensing units. These materials are mechanically notvery robust, so that a further coating is normallyrequired. This can be coverings of various materialssuch as plastic, metal or ceramic; very often, however,synthetic resin materials are used as a so-calledconformal coating. It is not often that the processingmethods, compositions or techniques are released forpublication. The investigative methods required beforequality approval are often quite protracted.The situation is relatively simple when the

semiconductors are already encased. Because of theinsensitivity of the actual film elements to temperature,moisture and touch, a coating can often be dispensedwith. We see here a parallel with pc boards, which arelikewise seldom coated. Nevertheless protectivecoatings are frequently required even if only forcosmetic reasons. However, such coatings are onlyrarely required to withstand particularly environmentaltest conditions.For this reason the use of thin dip-coatings is

widespread. A coating which is too thick can entaildisadvanges as a result of the differing thermalexpansion coefficients of substrate, components andcoating. In extreme cases this can lead to electricalfailure due to cracking of soldered connections. In allprobability the packaging of hybrid circuits willcontinue to be characterised by its presentmultiformity.

5.6 Testing

The importance of a flexible test technology cannot beoverestimated. This is especially true with regard tocomplex circuit functions, for which hybrids are beingincreasingly used. A considerable expenditure of timeand effort is called for in test planning, so that costs inthis area can be kept under control. Admittedly, inpractice it is still often the case that a special test deviceis produced for each product; and this certainly makessense for simple functions or functional sequences.However, it is also necessary to make timely provisionsfor the testing of complex functions. In this we find


ourselves facing the same problems which confront themanufacturers of integrated circuits. Far-sightedplanning and a flexible conception of test devices arecentral tasks for hybrid technology in the coming years.

6. THE SHAPE OF FUTURE TRENDS

The previous remarks have shown that thick filmtechnology is in a phase of diversification. In this phasethe fundamental technological problems in this fieldcan be regarded as solved, while application to agrowing number of spheres constitutes the main drivingforce for market penetration. One or two key areasremain to be mentioned.With the continuing growth of integration in all areas

of the technology, the hybrid circuit will, more thanever, take over the task of linking together integratedcircuits. Recently an American author coined the term"mechanical transformer" to characterise thisfunction. The hybrid circuit is conceived astransforming many leads on several components to afew leads on one assembly. The essential aim is less a

technology for the production of thick film elementsthan a highly reliable interconnection technology.Examples of this are in the increasing use of thick

film multilayer circuits, for instance by several largecomputer firms; and the heavy emphasis on bondingtechnology as one which can be automated for hybridcircuits.Two special areas of application are particularly

relevant in this context: telecommunications and use inmotorcars.The telecommunications field has been preparing

itself for some time for a technological revolutioninvolving the transition from analog to digitaltransmission of information. This entails a majorincrease in the use of digital IC’s requiringinterconnection. This function, together with the

accompanying need for high reliability andspace-savings, are best served by hybrid circuits.The application of electronics to the motorcar has

long been a topic of discussion. Already there are largenumbers of thick film applications in discrete electronicunits; however, the main significance must be seen inthe area of central electronics modules. There arealready products on the market, with designations suchas "on-board computer", which contain a large numberof IC’s to be connected to each other. In the context offurther developments the significance of hybridtechnology as an interconnection medium underconditions of limited space will become increasinglyapparent.

Finally, it has been shown that not all systemfunctions are amenable to digital solution. At suchinterfaces the development engineer is oftenconfronted with demands for particularly tighttolerances. With its possibilities for function trimming,thick film technology is particularly suitable forovercoming such interface .problems.The future of thick film hybrid technology may be

sought in all areas where a premium is placed on savingspace, increasing reliability, and- last but notleast- on reducing costs. And in which area is this notthe case?

REFERENCES

1. Thick Film Hybrids and Networks Western Europe, BPA(Management Services), 1979.

2. The hybrid circuit and materials market in Western Europe(Frost & Sullivan), 1980.

3. E. Effenberger, Electrocomponent Science & Technology,6, p. 205, 1979.

4. D. S. Campbell & D. Boswell, Electrocomponent Science &Technology, 7, pp. 3-17, 1980.

5. C. T. Goddard, Proc. 29th Electronics ComponentsConference (Cherry Hill, N. J., May 1979), pp. 80-84,1979.

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