considerations in formulation and manufacturing of...

Electrocomponent Science and Technology, 1981, Vol. 9, 43-490305-3091/81/0901-0043 $06.50/0

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

CONSIDERATIONS IN FORMULATION ANDMANUFACTURING OF THICK FILM INKS

ADRIAN ROSE

Engelhard Industries Division, Engelhard Minerals & Chemicals Corporation, East Newark,New Jersey 07029

This paper discusses considerations in formulation and manufacture of thick film inks. It deals with the components ofthick film inks, their properties, functions, and manufacturing methods. The vehicle is defined and its components arediscussed in terms of function, properties, and types. The various types of metals and their important properties aredescribed and related to ink properties. Glass functions, types and properties are reviewed. Both vitreous anddevitrifying glasses are examined. The usage of various types of oxides is discussed. Typical thick film inks aredescribed. They include conductor, resistor, dielectric inks, and protective coatings. The types of components used arerelated to specific property requirements of each type of material, ink manufacturing methods are described.Preparation of vehicle, metal powder, and glass frit is discussed. The process of incorporating these materials into anink is detailed from initial mixing to final product. Testing, both in-process and final Q.C., is briefly discussed. Theimportant considerations in both formulation and manufacturing are identified.

1. INTRODUCTION

Thick film engineers working in the design andfabrication of circuits usually have a reasonableunderstanding of the major ingredients in thick filminks. However, they often do not know in any detail theproperties and purposes of many of the componentsgoing into such compositions. A knowledge of thesecomponents, their subtle and obvious interactions,functions, idiosyncracies, and the methods by whichthey are combined to produce a product can be ofconsiderable value. This knowledge can contribute tothe understanding of what inks can and cannot do, howthey may best be processed, what each processing stepmay do, what can go wrong, and can assist in thesolution of problems.A thick film ink is a dispersion of very small particles

of inorganic powders in a viscous liquid known as avehicle. The inorganic powders may consist of one ormore of the following classes of materials depending onthe desired ink function and properties: metal, glass,oxides, and salts. In this paper, all of the components,their properties and functions, how they are used invarious types of inks, as well as the manufacturingtechniques used to produce thick film inks will beexamined.

2. INK FORMULATION

Inks are usually applied by silk screen printing toceramic substrates. They are then fired at hightemperatures to give precise mechanical and electricalproperties. Because of the necessity of printingextremely small circuits with good dimensional controlin large numbers, the flow or rheological properties ofthe ink must be well charffcterized. The effect of theseproperties on screen printing must be known. Thesecharacteristics should not change appreciably evenafter the ink has been on the printer for hours during aproduction run. Such properties must be reproduciblefrom batch to batch. Unlike the graphic arts, thick filmprinting requires a precise three dimensionaldeposition. Solids content must be constant from lot tolot. This must be achieved without varying rheology.Because all inks are fired at high temperatures, thermalcharacteristics must be known and controlled to allowsmooth and complete organic burn-off and filmmaturation during firing. Finally, the desired physical,chemical, and electrical properties of the fired filmmust be achieved even when processing conditionsdeviate within reasonable limits. Each component hasits contribution to these ends.

43

44 A. ROSE

3. VEHICLE

The vehicle usually consists of three main types ofmaterials: an organic binder, a solvent, and a wettingagent. While inorganic powders are responsible for theproperties of the fired film, the vehicle enables thesepowders to be applied properly to the substrate. Forthis reason the vehicle is vitally important althoughduring firing the vehicle is completely decomposed or"burned-off" leaving no trace of it in the final film.The binder is a resin or polymeric material. It serves

two main functions: a) to impart proper rheology to thecomposition, b) to hold the inorganic powders togetherafter the ink is printed and dried. Various materials areused as binders. Cellulosic polymers and terpene resinsare two classes of materials commonly used. Specialcare is taken in selecting the binder to insure that itsmolecular weight, chemical composition, moleculepolarity, and solubility will impart desired rheologicalproperties to the ink. In addition the binder mustburn-off during firing leaving no residue and should notimpart stringiness or tack to the vehicle. The bindershould also wet the powders involved.The solvent should have a low vapor pressure at

room temperature so that the ink will not dry or thickenby evaporation on the screen during a long printing run.It must have a relatively high vapor pressure at100-150C to facilitate rapid oven drying of theprinted parts. High boiling solvents, such as the longerchain esters, half alcohol esters, and terpenes arefrequently used. In addition to clean burn-off duringfiring, solvents should not possess objectionableodours, and should have low toxicity. The binder andsolvent are usually chosen as a compatible pair.The wetting agent allows the inorganic powder to be

thoroughly wetted and dispersed in the vehicle. It alsocauses the ink to better wet the substrate thusfacilitating printing. Since it frequently effects rheologythe wetting agent is selected carefully from an almostendless list of commercially available surfactants.

4. METALS

Gold, platinum, palladium, iridium, ruthenium,rhodium, silver, copper, nickel, and aluminum, in fineparticle size form either alone or in combination areused (see Figure 1). Combinations may be as alloys or inmixtures. The nature of the metal particles is extremelyimportant. They must be selected carefully for twomain reasons:

1) Thick films are heterophase composites in whichthe properties of the film are not only influenced by the

Product Composition Tap SurfaceNumber Density Area

gm gm

A 1570 Au 6.5 40 70"6408 Pd 0.9 12 17A.2270 Pd 1.1 1530A.2382 Pd 1.5 5.56.5A-3565 Pd 1.0 3.0 25A.3491 Pd 1.0

Pt 1.1 12 17A-2206 Ag 3.5A-3358 80 Ag 20 Pd 2.2

AverageParticleSize

2519201.7

152011

4015

A.3142 65 Ag 35 Pd

A 3498 40 Ag 60 Pd

A-3381 30 Ag 70 Pd 2.0

A. 2979 20 Ag/80 Pd

2399 75 Au 25 PdA2614 75 Au 25 PdE-303E 60 Au 20 Pd 20 PtA.2146 40 Au 20 Pd40 Pt 2.0 7090

2207 70 Au 20 Pd 10 PI 1.52216 70 Au 22.5 Pd Pt3361 Pd0

A-2810 Ru03366 Ru0

FIGURE Typical metal powders for thick filmformulations.

Form

Flake-SphereSphereSphereSphereSphereSphereSphereSphereSub McronMlxturoSub M(:ron

MxtureSt)l) M(ron

YlxturoSut)

S,b M.MxttJr,AI IoyAlloy

Alloy

AlloyAlloy

bulk properties of the metals, but also are stronglyinfluenced by their surface and other particulateproperties.2

2) The metals in some formulations form the majorportion of the inorganic powders and strongly influencethe rheological properties of the ink.

The most commonly measured properties of metalpowders are surface area, particle size, tap density, andpurity. Surface area is one of the most importantpowder properties. It is expressed in square meters ofsurface per gram of powder and measured by theamount of gas absorbed on the surface of a knownweight of powder (see Figure 2). In general, finerparticles have a greater surface area than coarser ones.Changes in surface area strongly influence viscosity andthe amount of metal loading that can be achieved. Anumber of physical and chemical reactions are alsoinfluenced. Platinum group metals exhibit catalyticactivity and can cause chemical changes in vehicles.Combustion of the film during the drying step can occurfrom a too high surface area or wrong vehicle choices.Capacitor internal electrode inks require very narrowranges of surface area powders.Particle size and particle size distribution affect both

ink behavior and fired film properties. They aremeasured by a number of methods including theCoulter Counter and Micromerograph (see Figure 3).Printing, film density, presence of voids, film surfacesmoothness, film conductivity, and adhesion are allinfluenced by particle size. Sintering is greatly affected

THICK FILM INKS FORMULATION AND MANUFACTURE 45

DIAMETER MICRONS

FIGURE 4 Typical particle size distribution curves.

FIGURE 2 Surface area measurement

important to control the precious metal percentageprecisely. This is accomplished by using metal powderswith the range of particle size distributions and surfaceareas held within very narrow limits. At Engelhard,precious metal content of inks is held within -0.1%,from lot to lot.Some formulators are interested in particle shape

when specifying powder on flake or a mixture of thetwo. It should be noted that only extremely malleablemetals, silver and gold, are likely to be used as flake.

It may be necessary to determine whether particlesare regular or long, irregular and/or feathery, since thedensity of the fired film depends not only on size but onshape. The presence or absence of agglomerates whichcan be quite hard and difficult to break up is also ofinterest. Shape can best be determined by electronmicroscopy although optical microscopy can determinethe presence of agglomerates (see Figures 5 and 6).

FIGURE 3 Particle size measuring equipment.

by both particle size and particle size distribution.Figure 4 shows particle size distribution of a typicalpowder.

It is desirable to maintain constant solids contentfrom batch to batch for dimensional control. It is FIGURE 5 Gold powder A-1179 10,000 .

46 A. ROSE

FIGURE 6 Palladium powder A’2270 1000 .Tap density is also an important parameter. It is

related to particle shape and size distribution. Tapdensity is sometimes used where more sophisticatedparticle measuring equipment is not available.

5. GLASS AND GLASS FORMING OXIDES

During firing the organic vehicle burns off. It is thennecessary for some portion of the inorganic powder toundergo a physical or chemical change to develop filmadhesion. This function is frequently performed byglass which flows and forms a composite with metalpowders present while bonding to the substrate.

Glass may also be used as a diluent to modify theproperties of another component in the system. Inresistor inks, for example, the conductivity of metalsand semiconducting oxides is reduced by dilution withglass to obtain a variety of sheet resistivities.

Glass is a viscous liquid, not a solid, and has nomelting point. However, viscosity drops as temperatureis increased. Various terms are used to describe thebehavior of glass at elevated temperatures. Ideally, aglass should have enough flow over the firingtemperature range of interest to develop adequateadhesion and a stable compact structure. It should notbe so fluid as to flow out of the composite producing aporous or cracked film.

The thermal coefficient of expansion from the settingpoint of the glass (this is the temperature below which itcan be considered rigid and corresponds to a viscosityof 1014 poises) down to the lowest temperature towhich the circuit will be exposed should be close to thatof the substrate. The glass should be in compression

rather than tension since glasses are stronger incompression.

Glasses usually contain oxides of silicon and variousother elements such as lead, boron, aluminum, zinc,cadmium, and. sodium, to name a few. The selection ofthe ingredients in a glass is important since each affectsthermal coefficient of expansion, chemical activity, andelectrical properties. Some glasses may work well inone type of ink and poorly in another. For example,glasses with relatively high chemical activity wet boththe metal powders and the substrate well. This may bedesirable in a conductor formulation but such chemicalactivity might produce a rather unstable resistorcomposition. For resistor compositions glass isgenerally chosen for resistance to the effects of ambientmoisture, thermal stability, and compatibility with thesemiconducting metal oxides in the system. Alkalimetals are generally avoided in glass compositions.Many thick film inks use glasses of the lead borosilicatetype. These often contain other oxides to obtainspecific properties.The glasses chosen must wet the substrate and

generally the metals in the system as well. The tendencyof a glass to wet a surface may be determined bystudying the surface energies of all the phaseboundaries present. While this method may be usefulfor calculating initial wetting behavoir, the behavoirmay change as the surfaces irreversibly alter duringfiring.4 For example, metals may dissolve in the glasschanging its composition. Metal oxides often dissolveand change the surface of the metal. In practice glassesare often simply tested on a substrate by empiricalspreading tests.Both vitreous and devitrifying glasses are used. A

vitreous glass retains its fluid structure duringprocessing and subsequent cooling. A devitrifying glassdevelops one or more crystalline phasesduring heating.These phases usually develop after some fusion hasoccurred. Since their melting points are usuallyconsiderably higher than the processing temperaturethe glass, which is now more like a glass-ceramiccomposite, demonstrates more dimensional stabilityand less activity than vitreous glasses during subsequentfiring operations. The properties are sometimes usefulin crossover dielectrics. The devitrified glass is oftenstronger than a vitreous glass.

Various semiconducting oxides are used in thick filminks. These include ruthenium dioxide used in resistorcompositions, and non-stoichiometric oxides of iron,nickel, and cobalt in thermistors.

Modifiers are frequently added to ink compositionsto obtain specific properties. Bismuth compounds areused to increase solderability A120 may be added to a


glass in a dielectric formulation. When the glass startsto flow during firing it will dissolve the alumina rapidlyincreasing the melt viscosity and rendering thedielectric immobile and relatively nonreactive duringsubsequent firings.Sometimes a small quantity of an oxide which reacts

with the substrate is used to obtain adhesion bychemical rather than mechanical means. Copper oxide,for example, reacts with alumina substrates at highfiring temperatures to form a copper aluminate spinel.The fired film is more adhesive than a similar film inwhich only glass is used to adhere to the substrate.6

Occasionally, salts are used in place of oxides toincorporate the necessary elements in a convenientform. These may by either inorganic such as bismuthsubcarbonate, or metallo-organic such as copperocotate.

6. TYPICAL INKS

6.1 Conductor

These contain a large amount of metal and a smallamount of glass, reactive oxides or mixtures of these,and possibly modifiers or salts. The metal glass ratio isimportant. Too much glass will give poor solderabilityand wire bonding and not enough will cause pooradhesion. Superior films result when glass compositionis carefully optimized to give the best combination ofproperties.The vehicle must wet the powders well. This is a

problem with some base metal inks which fire in neutralor reducing atmospheres since binders and wettingagents which impart the needed wetting action areusually difficult to burn off. Others which give easierburn off have poor wetting action. For this reason manyearly base metal inks had poor rheology.The fine line definition required of many conductive

inks imposes stringent rheology requirements andmuch research is directed toward the formulation ofnewer and better vehicle systems.

6.2 Resistor

These contain semiconducting oxides and sometimesnoble metals as well as glass. Resistivity is varied byvarying the amount and type of semiconducting oxidesand the amount of glass present. TCR is also affectedby this and frequently small amounts of "dopants"(elements or compounds that have a strong effect onTCR) are introduced. Since each member of a series ofresistor inks has a different composition to obtain the

desired resistivities and TCRs, it is possible to blendtwo inks to achieve intermediate resistor values andhave the resulting resistivity or TCR outside the rangeof the blending members. Such nonlinear blendingcharacteristics are avoided if the formulator makes thecomposition of adjacent members of a series as similaras possible.

6.3 Dielectric

These inks are of two main types, capacitor andmultilayer. The capacitor formulations contain powdersfound in ceramic capacitor bodies, barium titanate,various Curie Point modifiers, and a small amount ofglass. Since even small amounts of glass lower thedielectric constant appreciably, the minimum amountnecessary is used in high "K" inks.There are two types of multilayer dielectrics. One,

called a glass-ceramic or crystallizing dielectric,contains a devitrifying glass. The second, known as afilled glass type, contains a combination of glasses and ahigh melting oxide such as alumina. The glass-ceramictype must be formulated so that crystallizationproceeds rapidly at the firing temperature of interest.However, too rapid or too much crystallization canresult in excessive porosity. With insufficientcrystallization the dielectric may be too "glassy". Thisresults in top conductor "swimming" and conducttrdielectric reaction. Filled-glass dielectrics offer greaterflexibility in formulation, since the number of suitableglasses far exceeds the number of compositions thatmay be induced to crystallize at the temperature rangeof interest. The formulation must be balanced so thatit is neither too glassy nor too porous. The expansion ofthe glass should match the substrate. Metallic diffusionrates in the dielectric should be as low as possible.Silver displays high diffusion rates in most glasses.

Multilayer dielectrics frequently are dyed. Organicdyes which burn off in firing are used for identificationand ease of inspection of the unfired film since a lightcolored dielectric on a white substrate is difficult to see.These are referred to as fugitive dyes. Permanent dyesmay be incorporated in the glass to retain color afterfiring. These are selected carefully since some glasscolorants can effect dissipation factor.

6.4 Protective coatings

Resistor overglazes are the most commonly usedprotective coating for hybrid circuitry. These consist ofa glass which matches the thermal expansion coefficientof the resistor, and which does not adversely affect it.Because the coating overlaps conductor and bare

48 A. ROSE

substrate area, each with different expansioncoefficients, some stresses in the overglaze areinevitable. Particle size distribution is optimized andthe solids content is adjusted so that a relatively thinfilm is deposited. This minimizes cracking. To avoidresistor shifts during overglaze firing low temperaturefiring glasses are used. Frequently a green colorant isadded to absorb YAG laser energy during resistortrimming.

7. INK MANUFACTURE

Methods of manufacture are similar for most inks.Typical methods are as follows.

7.1 Vehicle

The solvent and wetting agents are mixed and while theliquid is stirred the binder is added. The process may beaugmented by additional heating to obtain rapidsolution and keep the viscosity low. When solution iscomplete the vehicle is allowed to cool. During theoperation the temperature is kept below the point atwhich the components might begin to decompose.

7.2 Metal powder

Most powder used in thick film inks is produced bychemical precipitation under carefully controlledconditions although some mechanical processing isused (see Figures 7 and 8).

7.3 Glass

The glass forming oxides are mixed in a V-coneblender, placed in ceramic or platinum and fused attemperatures of normally between 1000C and1500C, then poured into cold water. This instantlyquenches the molten glass shattering it into a coarsepowder. The process is known as fritting, producing aglass powder known as a frit. The term "frit" if appliedto every glass powder used in thick film inks isincorrect. The powder is then ball milled and dried.

7.4 Mixing and dispersing

In order to combine the powders and vehicle to form asmooth homogeneous ink, powder must be mixed withvehicle then milled to disperse the powder.A rough mix is first made. The powder is mixed in a

planetary mixer which acts similarly to a householdmixmaster. This wets the powder with the vehicle and

FIGURE 7 Gold powder production.

FIGURE 8 Platinum powder production.


Di recti onof ro] Is

back front

FIGURE 9 Roll action with three roll mill.

breaks up the larger agglomerates. The process iscontinued until all the powder is wet and large lumpsare broken. It is important to emphasize that if this isnot accomplished prior to roll milling the mill may bedamaged. Malleable metal insufficiently mixed has atendency to form large flakes or sheets during milling.The mixture is next transferred to a three roll mill

(see Figure 9). This is comprised of three in-line rollsseparated by small adjustable gaps which are rotated indifferent directions at different speeds relative to eachother. The rear roll is the slowest. The middle roll isfaster and the front roll the fastest. A takeoff bladescrapes the ink off the front roll. From here it goesdown a chute and is collected.The tremendous shearing action further breaks down

powder agglomerates and serves to completely wet thepowder. The ink is cycled repeatedly through the mill.The roll gaps are gradually decreased until the desiredamount of dispersion is obtained. Great care must betaken during the processing of the inks to avoid flakingthe metal particles. Formulations such as dielectrics andoverglazes must be milled carefully to avoidcontamination of metal from the rolls. At this point theink, now a smooth homogeneous material, is finished.Storage, testing, and packaging follows.

7.5 Testing

Thick film ink manufacturers measure physical andelectrical properties of interest to the user. Themanufacturer also tests for a number of otherproperties important for quality control, but which arenot of interest to every user.

Fineness of grind is a measure of both particle sizeand how well the ink dispersed in milling. It is used asboth an in-process test and a Q.C. test. Ink isdoctorbladed across a metal block in which a calibratedgroove with decreasing depth has been machined. The

resultant film has scratches in it which begin at thepoint where a particle larger than the depth of thegroove is picked up by the blade. The number ofscratches of different sizes is indicative of the sizedistribution of particles in the ink.Rheology is one of the most important properties

controlled by the manufacturer to assure that the inkwill perform properly in the users application. This ismeasured most often with both the Brookfieldviscometer and those of the cone-plated type such asFerranti-Shirley. Information can range from simpleprocess control data to sophisticated data which may becorrelated with screen printing behavior.

Product stability is an important consideration in theformulation of thick film inks. Stability at roomtemperature, and at high and low temperatures likely tobe encountered during shipment, is measuredwhenever a new system is formulated.

CONCLUSION

Formulation of thick film inks involves careful controlof raw materials as well as selection of materials tooptimize these complex heterophase products for theirintended purpose. Manufacturing is accomplished bystraightforward techniques, requiring only care inprocedure, and suitable and sufficient testing uponcompletion.

REFERENCES

1. M.L. Topfer, Thick Film Microelectronics, Van NostrandReinhold Co., N.Y., p. 43 (1971).

2. C. Schaeffer, and Sergent, "Tile Effect of Particle SizeDistribution on the Electrical Properties of RuO2 ThickFilm Resistors", Proceedings, International Micro-electronics Conference (ISHM, US), 1977.

3. S.S. Cole, "The Sintering Mechanism is a Silver PalladiumFilm", Proceedings, International MicroelectronicsConference (ISHM, US), 1972.

4. W. D. Kingery, H. K. Bowen, and D. R. Uhlmann,Introduction to Ceramics, John Wiley and Sons, N.Y.,p. 211 (1960).

5. S. S. Cole and G. Wellfair, "High Temperature ViscosityControl in Multilayer Glasses a New Concept",Proceedings, International Microelectronics Conference(ISHM, US), 1974.

6. P. Says, "A Discussion of Molecular Bonding ConductiveFilms", presented at IEEE Region 3 Conference, Orlando,Florida, 9/30/74.

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttp://www.hindawi.com Volume 2010

RoboticsJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation http://www.hindawi.com

Journal ofEngineeringVolume 2014

Submit your manuscripts athttp://www.hindawi.com

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation http://www.hindawi.com

Volume 2014

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

SensorsJournal of


Modelling & Simulation in EngineeringHindawi Publishing Corporation http://www.hindawi.com Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks


considerations in formulation and manufacturing of...

Documents