CA2052210A1

CA2052210A1 - Method for reducing shrinkage during firing of ceramic bodies

Info

Publication number: CA2052210A1
Application number: CA 2052210
Authority: CA
Inventors: Kurt R. Mikeska; Daniel T. Schaefer
Original assignee: Individual
Current assignee: EIDP Inc
Priority date: 1990-10-04
Filing date: 1991-09-25
Publication date: 1992-04-05
Also published as: CN1145350A; CN1035609C; DE69106830D1; US5254191A; KR920007950A; CN1061585A; CN1048969C; US5474741A; CN1125002C; US5387474A; MY107198A; EP0479219B1; JP2554415B2; JPH04243978A; KR940001653B1; CN1130606A; DE69106830T2; EP0479219A1

Abstract

TITLE
METHOD FOR REDUCING SHRINKAGE
DURING FIRING OF CERAMIC BODIESw Abstract A method for reducing X-Y shrinkage during firing of ceramic bodies in which a flexible constraining layer, which becomes porous during firing, is applied to the ceramic body such that the flexible constraining layer conforms closely to the surface of the unfired ceramic body as the assemblage is fired.

Description

20~2210 MET~OD FOR REDUCING SHRINKAGE
DURING FIRING OF CERAMIC BODIES
F~eld of T~nt ~ a~
The invention relates to a method for substantially reducing and controlllng planar shrinkage nnd reducing distortion of ceramic bodies during flring.
~L~ .
An interconnect clrcult board ls the physical realization of electronic circuits or subsystems from a number of extremely small circuit elements electrically and mechanically interconnected. It is frequen~ly desirable to combine these diverse electronic components in an arrangement so that they can be phys$cally isolated and mounted adjacent one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers.
The conductive layers are interconnected between levels by electrically conductive pathways through the dlelectric called vias.
One well known method for constructing a multilayer circuit is by co-f$ring a multiplicity of ceramic tape dielectrics on which conductors have been printed with metalllzed vias extending through the dielectrlc layers to interconnect the various conductor layers. (See Steinberg, U.S. 4,654,095.) The tape layers are stacked ~n reglstry and pressed together at a pre~elected temperature ~nd pressure to form a monolithic structure which is flred at an elevated temperature to drive off the organic blnder, sinter the conductive metal and densify the dielectric. Thls process has the advantage 20522~0 over classlcal ~thick f$1m~ methods since f~ring need only be performed once, savlng fabricating t~me and labor and limiting the dlffuQ~on of mobile metals which can cause shorting between the conductors. However, this process has the disadvantage that the amount of shrinkage w~ich occurs on flrlng may be difficult to control. This dimensional uncerta$nty 18 particularly undesirable in large, complex circults ~nd can result ln mlsregistration durlng subsequent ~ssembly operatlons.
Pressure slntering or hot presslng, the firing of a ceramic body with an externally applied load or weight, ls a well known method for both reduc$ng the poros~ty of and-controlling the shape (dimensions) of ceramic parts.
~See Takeda et al., U.S. 4,585,706; ~ingery et al., ~ Q.~ mi~ , p 502-503, Wiley, 1976.) Pressure sintering of ceramic circuits in simple molds is made difficult by the tendency for the part to adhere to the mold and/or for cross contamination to occur between the part and the mold. Further, application of a load or similar constraining force to the surface of a ceramic part durlng burnout of the organic binder may restrict the escape of volatiles, causlng lncomplete burnout and/or distortion.
Copending V.S. applicatlon, Serial No. ~7~466,937, discloses a method for constralned sintering that perm$ts escape of volatiles during burnout of the organic binder. A release layer ls applied to the surface of the unflred ceramic body. A weight is subsequently placed on the release layer to reduce shrinkage ln the X-Y direction. The release layer between the weight and ceramlc body provldes pathways for the volatlleQ to esc~pe. If a method were established whereby ceramic circuits could be constra$ned-sintered without need for ~ mold, without applylng external loads, and without restricting the escape of volatlles during burnout, and yet still largely ellm$nate dlmenslonal uncertalnty ln the flnal circult, processlng steps associated wlth firlng the circuitry with reduced shrinkage could be simplified or eliminated. The advantage would be greater yet if the method would permit co-flrlng of conductive metalllc pathways on the outer surfaces of the ceramic circuit.
Flaitz et al. ~European Patent Appllcatlon 0 243 858) describe three approaches to circumventlng the aforementioned difficulties. With the first ~pproach, constr~int is applled only to the outer edges (periphery) of the part, provlding àn open escape path for volatiles and an entry path for oxygen. With the second approach, a co-extensive force ls applled to the entlre surface of the piece to be sintered by either us$ng co-extensive porous platens or by application of an air-bearing force to the surface or surfaces of the piece to be sintered. With the third approach, a frictional force is applied to the sintering body by use of contact sheets comprised of a porous composition whlch does not sinter or shrlnk durlng the heating cycle and whlch prohiblt any shrinkage of the substrate. The compositlon of the contact ~heets is selected so that they remaln porous durlng flrlng, do not fuse to the ceramlc, are thermally stable so that they will not shrlnk or expand durlng the slnterlng cycle, and have contlnuous mechanlcal integrity/rlgidity. The contact sheets malntaln thelr dimenslons during the slnterlng cycle, thus restricting the ceramic part from shrlnklng.
After lamlnatlon of the contact sheets to the article to be ~lntered, slnterlng takes place wlthout use of additlonal welght~.

In lts primary aspect, the lnvention is directed to a method for substant~ally reducing X-Y shrinkage durlng firing of ceramlc ~odies comprlsing the sequential ~teps of a. Provlding an unfired ceramic body comprising an admixture of finely divided particle-q of ceramic solids and sinterable lnorganlc blnder dlspersed ln a volatillzable solld polymeric binder;
b. Applying to a surface of the unfired ceramlc body a flexlble constraining layer such ~hat the constra$ning layer conforms closely to the surface of the unflred ceramic body, the flexlble 16 constraining layer comprislng flnely divided particles of non-metallic inorganlc sollds dispersed ln a volatlllzable polymeric blnder, the Penetration of the sinterable inorganlc binder into the constraining layer being no more than 50 ~m;
c. Firing the assemblage at a temperature and for a time sufficient to effect volatilization of the polymeric binders from both the cer~mic body and the constralning layer, forming interconnected porosity in the constralning layer and sintering of the inorganlc blnder in the ceramlc body without incurrlng radlal bulk flow of the sintered body;
d. Cooling the flred a~semblage; and e. Removlng the porous con~tralnlng layer from the ~urface of the slntered ceramlc body.
In a second aspect, the inventlon i8 dlrected to a composlte unfired ceramic body comprlslng an admlxture of flnely dlvlded part~cles of ceramlc sollds ~nd slnterable lnorgan$c blnder dispersed in a volatlllzable ~olid polymerlc blnder havlng afflxed and closely conformed to a surface thereof a constraining layer 20~2210 comprlslng finely dlvided particles of non-metallic inorganlc ~olids disper~ed in a volatllizable solid polymeric binder.
In a still further aspect, the lnventlon ls directed to a method for maklng the compo~lte unflred ceramic tape comprising the sequentlal Qteps of applying to at least one surface of an unfired ceramlc tape ~
constraining layer comprislng finely divided particles of non-metallic $norganic solids di~per~ed in a volatilizable organic medium comprising solid polymeric binder dissolved in a volatile organlc solvent, and removing the organic ~olvent by evaporation.
pr~ or ~t EP0 87 105 868.1, Flaitz et al.
The patent is dlrected to a constralned slntering method which uses a restrainlng force ln the Z-dlrectlon to prohib~t X-Y distortlon, camber and shrln~age durlng firing of a ceramlc MIC substrate. Prlor to firing, porous, rigld unfired ceramlc, thermally stable contact sheets are lamlnated to the surfaces of the ceramic article ln order physlcally to restrlct the ceramic from shrlnklng wlthout the appllcatlon of additional pressure. ~he contact sheet~ malntaln thelr mechanlcal integrity and dimensional stability throughout the sintering cycle and the fired sheets are removed from the substrate ~urface by pollshlng or scraplng.
U.S. 4,521,449, Arnold et al.
The patent teaches the use of a dielectric layer of ceramic material to facll~tate sintering green ceramic sheet~ that contain surface vias and pad areas that are ~oined by lndented llnes and fllled wlth a conductlve metal paqte. After flrlng, the components are coated w$th a su$table metal to make them solder-wettable for lead attachment. ~he lnventorq recognlze the need for post-metallizatlon to accommodate the signiflcant (17%~

substrate shrinkage and distortion that is typical for fired ceram~c material.
U.S. 4,340,436, Dubetsky et al.
The patent discloses superlmposlng an lnert, S coextensive nonadherent, removable, light welght, planar platen onto a green glass ceramlc lamlnate to restrict lateral X-Y shrlnkage and dlstortion when the glass has reached coalescent temperature during firing. The lnventors reported that platen pre~sures of about 0.012 to about 0.058 lbs/ln2 over the lamlnate produced enhanced planarity aDd lateral d$menslonal integr~ty.
~rief Desc-~t ~ or of the Draw~ no The Drawing consists of six figures. Figure 1 is a schematlc representation of the arrangement of the various components of the lnvention prior to firing in which a constralning layer is affixed to both sides of a substrate. Figure 2 is a schematlc representatlon of the arrangement of the varlous components of the inventlon prlor to flrlng in whlch a constralnlng layer is affixed to one side of a substrate and a rigid substrate i8 adhered to the opposlte slde of the substrate. Flgure 3 is a schematlc representation of the arrangement of the varlou~ components of the inventlon prior to flring in which multiple ceramic parts are assembled into a monolith wherein each part has a constrainlng layer adhered to opposlte sldes.
Figure 4 is a schematic representation of delamination at the ceramlc/constraining layer interface wlthout buckling of the constraining layer. Figure S is a ~chematlc representatlon of delaminatlon at the ceramlc/constraining layer lnterface with buckllng of the constralnlng layer. Figure 6 i~ a graphical correlatlon of inorganic blnder Penetratlon wlth binder viscosity and wetting angle.

20~2210 ~ ta~led nescr~ptlon of the Tnvent~on Gerle ra 1 The general purpose of the lnventlon ls to provlde a new and lmproved method for reduclng X-Y shrlnkage durlng the flrlng of ceramlc bodles. A preferred appllcatlon of the lnventlon 1~ for fabrlcatlng ceramlc multllayer clrcults using conventlonal conductlve metalllzatlons, lncludlng conductors, reslstors and the llke, and dlelectrlc tapes ln ~uch a manner that the clrcult feature dlmenslons establlshed durlng via punchlng and printing are ~ubstantially maintained durlng flrlng. The method of the inventlon ls therefore more economlcal ln by-paQsinq many of the sources of dimensional uncertainty ln ceramic part~ and by eliminating many of the circult development and manufactur$ng steps necessary to avold dimensional errors and misreg$stratlon.
Durlng the flrlng cycle, sfter volatillzation of the organlc blnders, the lnorganlc components of the tape undergo slntering when heated to a ~ufficlent temperature. Durlng ~intering~ the particulate-porous tape undergoes changes ln lts structure whlch Are common to porous fine-gralned crystalline and non crystalllne materials. There ls an lncrease ln graln size, there ls a change ln pore shape, and there ls change ln pore size and number. Slnterlng u~ually produces a decrea~e ln poroslty and results ln densiflcation of the particulate compact.
Central to the inventlon ls the use of a flexible ceramlc constralnlng layer whlch i8 applled to the ~urface(s) of the ceramlc clrcult layers. ~he constralning layer serves several functlons: (1) it provide~ a uniform hlgh frictlon contact layer which ~ubstantially reduces Qhrlnkage ln the plane of the 3~ slnterlng part; and (2) lt provldes an escape pathway 20~2210 for the volatile components of the ceramlc tape prior to sintering. In certain cases, it facilltates co-firing of top surface metalllzation wlthout lncurring damage thereto.
In order for the conQtralning layer to effectlvely reduce shrinkage in the plane of the slnterlng part, lt ls applied as a flexlble layer to the surface(q) of the unfired ceramic circuit layer ~8) . The flexibility of the constrainlng layer enables the layer to conform closely to the topography of the unflred ceramic surface(s). Lamination of the flexible constraining layer to the unfired ceramic surface(s) may be used to force the constraining layer into even closer conformance, depending upon the mode of application of ~5 the constraining layer. For example, the constraining layer may be spray coated, dip-coated or rolled onto the unfired ceramic ln the form of a dispersion, or lt may be formulated as a flexible sheet and laminated onto the unfired ceramic. Laminatlon ls partlcularly effective ln reducing the size of any gaps (flaws) between a constraining layer and surface(s) of ceramic body.
Close conformance of the constraining layer to the ceramic part ls necessary to prevent the constraining layer from delaminating and buckllng away the from the ceramic part during sintering. During firing, as the dlelectric substrate beglns to shrink, the constraining layer ls put lnto biaxlal compression by the in-plane slnterlng strain of the dielectric part. If the compresslve stress ln the constraining layer reaches a critlcal polnt, the constralnlng layer delaminates and buckles away from the sintering dlelectric substrate.
The buckllng problem germane to the lnventlon can be examined by analyzlng elastic lamlnated plates and shells after partial debonding that are sub~ected to compressive loads parallel to the laminated layers.

20~210 Buckling has been analyzed extensively in S. P.
Timoshenko and G. M. Geere, Theory of Elastlc Stabillty, 2nd Edn., McGraw Hill, New York (1961). Speciflc problems of buckllng ln compressed fllms have been analyzed ln A. G. Evans and J. W. Nutchln~on, On the Mechanics of Delamlnation and Spalling ln Compressed Films, Int. J. Sollds Structures, Vol. 20, No. 5, pp.
455-466, (1984) The buckling problem can be solved for one dimension (beam), two dimension~ (rectangular or square geometry), and for a circular geometry. A circular geometry is the most appropriate for the instant configuration and is presented here. The problem concerns a single lnterface crack or flaw parallel to the free surfaces as shown in Figure 3. The flaw ls represented by a circular delamination of radius, a, whlch is ln biaxial compression ~O. If the crack or flaw is of sufficient size, the film above the crack is susceptible to buckling. An lnterfacial flaw or delamination parallel to the free surface does not disturb the stre~s fleld since the stress fleld also acts parallel to the surface. Thus, a stress concentration at the flaw or crack edge i5 not lnduced.
If the film buckles away from the substrate as shown in Flgure 4, the Yeparation redi~tributes (i.e.
concentrates) the stre-~s at the perlmeter of the lnterface crack which induces crack extension and failure by buckling. Conditions at the interface crack involve a combination of opening (Mode I) and shearing (Mode II) stresses. In our situation, where the film ls a compressed powder, once buckling occurs, the shear force~ at the crack tip wlll easlly cau~e the powder fllm to fail since the powder i~ ~ery weak ln shear and tenslon.

20~2210 The f~lm (constraining l~yer) will undergo buckling lf the compresslve 8tres8 exceeds the critical buckling stress for the film. The appropri~te circular solution for the present case a8sume8 flxed or cl~mped film edges. The crltlcal buc~ling stress, ~c,i~ expre~ed as a _ [ kE ] (t) 2 (1) where t is the th~ckness of the con-Qtraining layer, a is the radius of the crack or flaw, k - 14.68 for a clamped edge (k is a numerical factor determined from the Bessel function used to solve the initial differential equation appropriate for the circular geometry), E is the Young's Modulus of the constrainlng layer ~nd Vis Poisson's ratio. Equation (1) shows that buckling will occur during the process if a crack or flaw of a cr$tical size is present at the interface between the constraining layer and the part being slntered. Equation (1) also shows that the thickness t and Modulus E of the constraining layer ~re lmportant in determning the critical buckling stress.
In practice, flaws can be generated during the appllcation of the constraining lsyer to the ceramic body substrate and during heat-up. If the constraining layer is not flexible enough to conform closely to the topography of the ceramic circuit layer(s), or if application methods are not optimized to ensure close conformance of the constrainlng layer to the topography of the ceramlc clrcult layer(~), then a flaw or crack may be created ~t the con~training l~yer/ceramic circuit interface. Durlng heat-up, flAws can be generated by thermal expanslon mi~matche8 between the constralnlng layer and the cer~mic clrcuit 8ub8trate. Thermal expanslon flaws that are not parallel to the constralnlng layer/sub3trate lnterface act as additional str~ss concentrators. Thermal expans~on effects 2~2210 (cracklng, etc.) can ~ometlmes be ellmlnated or reduced by uslng a constralnlng layer which has a coefficient of thermal expanslon hlgher than the substrate, thus, puttlng the constralnlng layer ln planar compression durlnq heat-up.
In order to facllltate removal of the constralnlng layer ~fter flrlng, the gl~ss from the ceramic part whlch is belng flred must not Jub~tantlally Penetrate or lnteract w$th the con~tralnlng layer durlng the process.
Exces~lve Penetration of the glass lnto the constraln$ng layer ls likely to lnhibit the removal of the constralnlng layer from the part belng fired and posslbly adver~ely affect the propertles of the ceramic substrate lf a large quantity of constralnlnq material were to adhere to the flnal fired part. When selectlng a glass composltlon for the dlelectrlc, two general reguirements should be considered. Flrst, the glass ln the dielectric substrate should meet the requirements of the dielectric ~l.e., dlelectrlc con~tant, hermeticlty, slnterabillty, etc.) and second, the composltlon of the glass should be such as to inhibit glass Penetration lnto the constralnlng layer. Penetratlon lnhibitlon ls controlled ln part by ad~ustlng variables such aQ glass vlscoslty, wettlng angle, etc. as will be discussed below.
An analysls of the flow of a liguid lnto porous medla can be u~ed to examlne the gla~s Penetratlon phenomena and glve lnslght lnto the process. The analysls can be u~ed a8 a guldellnc ln selecting both glass compo~ltion and con~tralnlng layer composltlon ln con~unctlon wlth the glass requlrements ~peclfled for the dleiectrlc a8 dlscussed ~bove. In the followlng analysls, the porous medium ls the conQtralnlng layer and the llquld 1~ the glas~ ln the ceramic being flred.

20522~0 The analysis was developed based on Darcy's Law to predlct the Penetration of viscous flulds lnto porous beds and particularly within the context of the invention, the rate of Penetration dl/dt of inorganic binder into the constraining layer defined by:

LU ' r~ (2) Ll where D is the permeability of the porous medium, ~P is the driving pressure for Penetration, l ls the length of Penetration of the liquid into the medium at time t, and ~L iS the viscosity of the liquid.
Equation (2) is valid if we ~ssume the gradiant of pressure with respect to the Penetration direction, VP, is closely approximated by the change in pressure over the Penetration distance, or 1.
Taking into consideratlon the radius, r, of the pore channels in the porous medium, Rozeny and Carmen show in A. E. Scheidegger, ~he p~yc~cs of Fl ow T~r~ug~
ESL915~ , The MacMillan Co. ~1960) pp 68-90, that permeability, D, can be expreqsed as:

D - r2(1 - p)/20 (3) where P - PB/PT is the relative density of the porous media, PB is the bulk density and PT is the theoretical - density.

~P is the driving pressure acting to force the liquld lnto the porous medlum and ~s defined as:

~p 2~LVCOS~ + p where 2~Lvcos~/ris the capillary pressure, P~ls any external pressure dlfference (l.e., externally applled load), ~Lv iS the l$quld vapor ~urface energy and cos~is S the solld liquld contact angle.
Substltutlng equatlons (3) and (4) lnto eguatlon (2) and integratlng the ~ubstltuted equatlon glves:

2 t r ~1 - p) (2~Lycos~P~r) 10 ~lL

Slnce no externally applled load, Pa~ i9 used ln the invention, equation (5) can be expressed as:

2 c t r (l-p)2~Lvcos~ ~6) 10 llL
For a glven body under a constant driving pressure, the depth of Penetration ls proportlonal to the square root of tlme. Several methods for derivlng equatlon (6) are presented ln the llterature. In the present lnventlon, the porous medlum is the constralnlng layer and the viscouq llquld ls the glass ln the substrate belng fired. In practlce, the vlscosity of the glass, contact angle of the gla~s on the constralning layer materlal, poroslty and pore radlus of the constraining 26 layer, along with tlme, can be ad~usted to give a desired degree of Penetration. It can also be appreclated that the llquld/vapor surface energy can be modlfled by slnterlng ln more or less reactlve atmosphere~. Flgure 6 ~ 8 a plot of Penetratlon a~ a function of glass liquid vlscoslty ~L) for varlou~
contact angles for t - 30 mln. Radlus (r), porous l~yer density (l-p) and liquid/vapor surface energy (~LV) can also be used to influence Penetration as mentioned above.
As shown by equation ~6) and by the correlation given in Flgure 6, Penetratlon can be predicted from the vlscosity and contact angle of the lnorganlc blnder and thus can be controlled by the ad~ustment of these two vari~bles. As used herein, tbe term ~Penetratlon"
refers to the Penetratlon value of the ~lnterable - inorganlc binder component of the unflred ceramic body as determlned by the above-de~cribed correlatlon method.
The constralning layer comprises flnely dlvided partlcles of non-metallic inorganlc solids di~persed in volatllizable organic medium prepared by standard ceramic tape casting methods. The low ~intering rate and/or high slntering temperature of the inorgan~c solids in the constralnlng layer pre~erves the interconnected porosity in the layer as a pathway for volatlles and other gases to escape from both the ceramic part belng flred and the constralning layer. A
slnterlng temperature differential of at least 50C is adequate. The assemblage is f~red at a temperature and for a tlme sufficient to volatlllze the organic blnders from both the constraining layer and the ceramlc tape and to slnter the inorganlc b~nder in the tape.
Constrained sinterln~ where~n external pressure is applled to a ceramlc body during firing vla load bearlng rams, cannot be achleved in conventional belt furnaces.
In contrast, slnce external pressure ls omltted in the present proces~, conventlonal flring equlpment such as belt furnaces can be used. After compleSe sinterlng of the ceramlc tape layers, the assemblage ls cooled. The constralnlng layer can be subQequently removed from the surface of the flnlshed part by a dustlng or by ultrasonlc treatment wlthout aff~ct~ng or damaglng 205221~

either the ceramic surface of the part, or the conductive pathways.
During the sintering process, following volatillzation of the organic binders from the constrainlng layer and the ceramic body to be sintered, the constraining layer exigts as a layer of lnorganic powder. Applicatlon of the constrainlng layer in the form of a flexible tape prior to firing ensures that the loose layer of powder wlll be evenly distrlbuted 1~ over the surface of the ceramic part and that the constraining layer can conform closely to the surface of the body being fired.
~m~5~
Dielectric substrates typically comprise sintering (binder) and nonsintering (ceramic solid) phases. The composition of the ceramic solids in a dielectric body which can be used in the lnventlon is not ltself directly critical so long as the solids are chemically inert with respect to the other materlals in the system and have the approprlate physlcal properties relative to the inorganic binder component Gf the dielectric body.
The nonsintering sollds are added essentially as a filler to ad~ust propert$es such as thermal expansion and d$electric constant.
The basic physical properties that are essential to the ceramlc solids in the dielectric body sre (1) that they have sintering temperatures above the sintering temperatures of the lnorganlc binder, and (2) that they do not undergo slntering durlng the flrlng step of the lnvention. Thus, in the context of thls invention, the term ~ceram~c solids~ refers to inorganlc materials, usually oxides, which undergo essentially no sintering under the conditlons of firlng to which they are sub~ected ln the practice of the invention.

$hus, ~ub~ect to the above cr~ter~a, v~rtually any high melting inorganic solid can be u~ed as the ceramic sollds component of dielectr~c tape. For example, such materials ~s ~aTiO3, CaTiO3, SrT103, PbT103, CaZrO3, ~aZrO3, CaSnO3, ~aSnO3, A1203, metal carbides such as silicon carbide, metal nitrldes such as aluminum nitrlde, minerals such as mullite and kyanite, zirconia and var$ous forms of sllica. Even hlgh softening point glaQses can be used as the ceramic component provlding they have sufficiently high softening points.
Furthermore, mixtures of such materials may be used in order to match the thermal expansion characteristics of any substrate to which they are applied.
I~or~7an~ C 1~ nder The composition of the lnorganic b$nder which can be used in the ceramic bodies for use in the invention is also not itself directly critical so long as it is chemically inert with respect to the other materials ln the system and it has the appropriate physical properties relative to the ceramic solids in the ceramic body and the non-metallic solids in the constraining layer.
In particular, lt i5 es~ential that the Penetration of the inorganic blnder component of the ceramic body lnto the constraining layer during the firing not exceed 50 ~m and preferably not exceed 25 ~m. If the Penetration exceeds about 50 ~m, removal of the constraining layer $s likely to become difficult.
Though the lnventlon 18 not limited to the~e temperatures, flring wlll usually be conducted at a peak temperature of 800-950C and held at least 10 minutes at the peak temperature.
The baslc physlcal propertles that are preferred for the inorganlc binder ln the ceramic body for use in the method of the lnvention are tl) that it have a 20~210 sintering temperature below that of the ceramic sollds in the body, (2~ that lt undergo vlscous phase sintering at the firlng temperatures uqed, and (3) that the wettlng angle and vlscosity of the inorganic blnder are such that it will not penetrate appreciably lnto the constraining layer durlng firing.
The wetting characteristics of the lnorganlc binder, usually a glass, are determined by measurlng the contact angle of the slntered lnorganic blnder on a smooth planar surface of the $norganic ~olids contained in the constra$ninq layer. This procedure ls descrlbed hereinbelow.
It has been determined that if the lnorganic binder has a contact angle of at least 60, lt ls sufflciently non-wetting for use in the lnvention. It is nevertheless preferred that the contact angle of the glass be at least 70. In the context of the method of the invention, the hlgher the contact angle, the better are the release properties of the constrainlng layer.
Wben, as is usual, the inorganic binder component of the ceramic unfired tape is a glass, lt may be either a crystallizing or non-crystallizlng glass at the firing conditions.
Tbe particle size and particle size distribution of the inorganic binder are likewise not narrowly critical, and the particles will usually be between 0.5 and 20 microns ln size. It is, however, preferred that the 50%
point of tbe lnorganic binder, which ls defined as equal parts by weight of both larger and smaller particles, be equal to or less than that of the ceram$c solids.
Sinterlng rate is related directly to the ratio of inorganic binder to ceramic solids and lnversely to the gla8s tranqition temperature (Tg) and particle slze of the lnorganic binder.

20~22~0 pol y~e-lc R~nder The organic medium in which the glass and refractory $norganlc solids are dispersed is comprlsed of the polymeric bfnder, optlonally having dissolved therein other materials such as plasticizers, release agents, dispersing agents, strlpplng agents, antlfouling agents and wettlng agents.
To obtain better blnding efficlency, lt is preferred to use at least 5~ wt. polymer blnder for 95%
wt. ceramic ~olids. However, lt is further preferred to use no more than 204 wt. polymer binder in 80% wt.
ceramic solids. Within these limlts, lt ls desirable to use the least possible amount of binder vis-à-vis solids in order to reduce the amount of organics which must be 16 removed by pyrolysis and to obtain better particle packing which gives reduced shrinkage upon firing.
In the past, various polymeric materials have been employed as the binder for cerAmic tapes, e.g., poly(vinyl butyral), poly(vinyl acetate), poly(vinyl alcohol), cellulosic polymers such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, methylhydroxyethyl cellulose, atactic polypropylene, polyethylene, silicon polymers such as poly(methyl siloxane), poly(methylphenyl siloxane), polystyrene, butadiene/styrene copolymer, polystyrene, poly(vinyl pyrollidone), polyamides, high molecular weight polyethers, copolymer~ of ethylene oxide and propylene oxide, polyacrylamides, and varlous acrylic polymers such as sodium polyacrylate, poly(lower ~lkyl acrylates), poly(lower alkyl methacrylates) and various copolymers and multlpolymer~ of lower alkyl acrylates and methacrylate~. Copolymers of ethyl methacrylate and methyl acrylate and terpolymers of ethyl acryl~te, methyl methacrylate and methacrylic acid have been 36 previously used as binders for slip casting materials.

20~2210 More recently, Usala, ln U.S. Patent 4,536,535, has disclosed an organic binder whlch is a mixture of compatible multlpolymers of 0-100~ wt. Cl_g alkyl methacrylate, 100-0% wt. Cl-8 alkyl acrylate and 0-54 wt. ethylenically unsaturated carboxyllc acid of amine.
Because the polymers permit the use of minimum amountq of blnder and maximum mounts of dlelectrlc solld~, thelr use ls preferred wlth the dlelectrlc composltion of this lnventlon. For thls reason, the disclosure of the above-referred Usala patent 18 incorporated by reference herein.
Frequently, the polymeric binder wlll also contain a small amount, relative to the binder polymer, of a plasticizer which serves to lower the glass transltion temperature (Tg) of the binder polymer. The choice of plasticizers is, of course, determined primarily by the polymer which must be modified. Amonq the plasticizers which have been used in various binder systems are diethyl phthalate, dibut~l phthalate, dioctyl phthalate, butyl benzyl phthalate, alkyl phosphates, polyalkylene qlycols, glycerol, poly(ethylene oxides), hydroxyethylated alkyl phenol, dialkyldlthiophosphonate and poly~isobutylene). Of the-~e, butyl benzyl phthalate is most frequently used ln acrylic polymer systems because it can be used effectively in relatively small concentrations.

Vnflred tapes are prepared by casting a slurry of the dielectrlc particles and inorganic binder dispersed ln a solutlon of blnder polymer, plastlcizer and solvent onto a carrler ~uch ~8 polypropylene, Mylar~ polyester fllm or stalnle~s ~teel and then ad~usting the thickness of the ca~t film by passing the cast ~lurry under a doctor blade. Thus, tapes which are used in the lnventlon can be made by such conventional methods, 20~2210 which are described in greater detail ln U.S. 4,536,535 to Usala.
It will be understood that the unfired tapes used ln the method of the lnventlon will frequently contaln vias for electrical lnterconnectlon of layers, reglstratlon holes and other perforatlons to accommodate devlces and chlp attachment. It has nevertheless been found that the method remalns effectlve to reduce X-Y
~hrinkage even when the tape does contain such perforatlons.
In some instances, the tape may contain fillers such as ceramic ffbers to provide special propertles such as thermal conductivity or tensile strength to the fired tape. Though the invention was developed and is described above primarily in the context of firing ceramic bodies made from layers of ceramic tape, it will be realized that the lnvention can also be used to reduce X-Y shrlnkage during firing of odd-shaped non-planar artlcles such a~ cast or molded cersmic parts.
~c~ .L_:,c~
The constraining layer for use ln the method of the lnventlon is comprised of non-metallic partlcles dlspersed in a solld organic polymer binder. As mentioned above, it is preferred that the non-metallic particles in the constralning layer have a lower sintering rate than the inorganic binder of the substrate being fired at the firing conditions and that the wetting angle of the inorganic binder on the constraining material and the visco~ity of the lnorganic binder be such that blnder Penetration into the constralnlng layer ls wlthin the bounds stated prevlously. Thus, the composltlon of the lnorganic ~olids component of the constralnlng layer ls likewise not critlcal aQ long as the above-mentloned crlterla are met. Any non-metallic lnorganlc material can be used as 20~2210 i long as lt does not undergo sinterlng during flring and as long as the wetting angle of the inorganic binder ln the ceramic body (part) being fired on the constraining tape and the viscosity of the inorganic binder ln the ceramic body are wlthin the preferred bounds of lnorganlc binder Penetration into the constraining layer as the lnorganlc blnder undergoes slnterlng during the flrlng process. Although the lnorganlc non-metalllc ~ollds used ln the constralnlng layer may be the same as those used in the ceramic body mulllte, quartz, A123, CeO2, SnO2, MgO, ZrO2, BN and mlxtures thereof are preferred. However, glassy materlals can be used provided their softening points are sufficiently high s~
that they do not undergo sinterlng when they are fired ln accordance with the lnventlon.
The constralnlng layer can be applied in the form of a flexible tape, a thick fllm paste, spray, dip, roll, etc. ~egardless of the form ln whlch the layer is applied, lt 18 essentlal that the layer be flexible in order to attain close conformance to the ceramic body surface to reduce and preferably minimize, the size of any gaps ~flaws) at the constraining layer/ceramic body lnterface and lncrease the crltlcal stress value at the interface. In general, the same binder polymers whlch are suitable for the unflred ceramlc tape will be suitable for the constrainlng layer when lt ls applled as a tape.
As used hereln, the terms ~thlck film~ and ~thick film paste~ refer to dlsperslons of flnely dlvlded solids in an organic medium, whlch dlsperslons are of paste conslstency and have a rheology whlch makes them capable of being applled by conventlonal screen printing. Other di~perslons having a consistency and rheology ~ultable for spray; dip or roll-coating may al80 be used. The organ~c media for such pastes are ordinarily comprlsed of liquld binder polymer and various rheological agents dissolved ln a solvent, all of which are completely pyrolyzable durlng the firlng process. Such pastes can be elther reslstive or conductive and, ln some ln-~tances, may even be dlelectrlc ln character. Such compo~ltlons may or may not contain an $norganic binder, depending upon whether or not the functlonal sollds are slntered durlng flring.
Conventional organic medla of the type used ln thick fllm pastes are also sultable for the constrainlng layer. A more detalled d$scu~sion of ~uitable organic media materials can be found ln U.S. 4,536,535 to Usala.
To ensure the formation of interconnected porosity in the constraining layer in order to provide an escape pathway for polymer decomposition products, the pore escape channels (void or pore structure) between the individual particles within the constraining layer must be sufficient ln slze and remaln open durlng heatup.
For the pore channels to remain open during heatup, the sinterlng rate of the constralning layer material must be less than the sintering rate of the ceramlc part belng fired as previously discussed. The pore structure in the constraining layer 18 determined by the characteristic particle arrangement or assembly within the layer. The arrangement or packing of particles ln the layer is influenced by several factors including:
the vol-ume fractlon of solids, the solid~ particle size, size distribution, and ~hape, the degree of di~persion of the particles ln the lnitial casting, the drylng characteristics of the castlng, whether the layer is applied by dip or spray slurrying, and how the layer ls applied. Furthermore, the pore or void structure ln a tape, spray, or dip layer that contains a polymer matrix wlll most llkely be different ln the layer after the polymer ls pyrolyzed. Keeping the foregoing condltlons 23 20~2210 ln mind, it ls posslble to pack particles to a bulk density of -90 vol % sollds. On the other hand, a lower limit on bulk denslty of - 10 vol % sollds should be practicable to provide sufficlently large pore channels B wlthout serious degradatlon of the X-Y compressive stress capabillty of the layer ~nd wlthout slqnlflc~nt Penetration of the glass lnto the layer.
p-oce~ Var~ es An essential characteristlc of the method of the lnvention ls that the constraining layer conform closely to the surface of the substrate. Where the constraining layer is applled as a flexible sheet, close conformance can be achleved by lamlnating the sheet to the unflred dielectric tape package.
The firlng cycle for the method of the invention is subject to the physical characteristics of the solids contained in both the ceramic body and the constraining layer and is further llmited by the heating rate capability of the oven or kiln in whlch the materials are fired. A typical batch furnace flrlng cycle whlch can be used for many applications ls to heat the assemblage at the rate of 3C per mlnute to 600C, then 5C per mlnute to a peak temperature of 850C, maintainlng the assemblage at peak temperature for 30 minutes, and then coollng the as-Qemblage by turnlng off the furnace. In a typlcal commercial lnstallation, the firlng characterlstlcs of the materlals are chosen so that they are sultable for the performance characteristlcs of the available furnace or kilnO
Firing can, of course, be conducted ln elther a batch, intermittent or contlnuous fashlon.
Upon completlon of flrlng, the constraining layer is in the form of a dry, porous layer ln whlch the partlcles are held together only weakly by van der Waals ~5 forces 3ince during flring the organic blnder 20~%210 volatilizes and the partlcles wlthln the layer have not sintered. Because the layer has llttle lntegral strength, it csn be easily removed by brushlnq. The removal of the flred constralnlng layer ls neverthele~
ch~racterlzed by the need for very llttlc mechanical energy, and certalnly grlndlng ls not requlred as lt ls for the prlor art processes ln whlch hot presslng ls used.
The lnventlon ls frequently used to make more complex multilayer parts ln which one or more of the dielectrlc layers has prlnted thereon a thlck fllm electrically functlonal pattern such as a reslstor or conductive llnes or both. When this is the case, the dielectrlc and electrlcally functional layers can be fired sequentlally or they can be co-fired. Moreover, multiple parts can be stacked vertically ln a single monolith and cofired. In such a monolith, a constrainlng layer lies between each part and on the top and bottom of the monollth, such that each part has a constralning layer ln clo-~e conformance to the top and bottom ceramlc surface as ~hown ln Flgure 3. Whether firing a s$ngle multllayer part or multiple multllayer parts assembled lnto a monollth, the f$rlng temperature proflle and/or the components of the dlelectrlc layers and electrically functlonal layers must be selected ln such manner that the organlc media of all the layers are completely volatillzed and the lnorganlc blnders of the respectlve layers are well slntered. In ~ome lnstances, lt may be necessary that the conductlve phase of the thlck fllm metalllzatlon be slntered as well. The ~electlon of components having these relative properties ls, of course, well wlthin the skill of the th$c~ fllm art.
The lnvention also permlts the firing of multllayer 3~ parts comprlslng multlple dielectrlc tape layers and '20~2210 thick fllm conductlve pastes on a rigld prefired ceramic substrate. The layers of the~e parts can be coflred ln one step or flred sequentlally, as dlscus~ed above, whlle maintainlng excellent X-Y dimensional ~tab$1ity in the dielectric layers.
The ability to cofire multiple layers of dielectrlc tape on a rigid substrate i8 attractive for several reasons. The rigid substrate lf made of a high strength material, such a~ alumina, provide-~ a mechanical upport. The rig$d ~ubstrate lf made of a high thermal conductivity material, such as AlN or beryllia, provides a method for removing heat from an electronic package.
R$gid substrates made of other materials, such as Si or other dielectric materials, are also potentially attractive. Being able to cofire mult~ple layers also reduces cost by reducing the number of firing steps.
The ability to cofire dielectr$c tape on to a rigid substrate has advantages over other tape on substrate methods (TOS) since the multllayer dielectric tape portion of the package can be formed by conventional methods. The dielectric layer~ are cut, printed with conductors or other dielectric materials, vias filled, layers stacked and laminated by conventional multllayer methodology. The constraining layer is then applied to the surface of the unfired dielectric tape. When using the constraining layer in a tape form, which i8 the preferred method, the constraining layer tape is laminated to the expoQed surface of the unfired dielectrlc tape such that intimate contact and close conformance is achieved between the dielectric tape and constraining layer. The dielectric tape, rigid substrate, and constraining layer tape can either be lamlnated together in one co-laminating step or laminated ~equentially. For ~equential lamlnation the dielectric tape layers are first laminated to the rig$d 205~210 substrate and then the constrainlng layer tape is laminated to the prevlously l~mlnated rlgld substrate/dlelectrlc tape l~mlnate. For colaminatlon, the rigid substrate, d$electrlc tape and constralning layer tape are laminated ln one step. If the constrainlng layer ls applled ln a paste or spray form, the dielectric tape and rigid substrate would first be laminated together ~nd then the constraining layer materlal applied in the proper form. Other stacklng and lamination methods are possible ~nd are obvious to those skilled in the ~rt.
After laminating, the entire rigid substrate, dielectric tape package, and constraining layer are fired ln one step ln accord wlth the process. Via filling is not an issue in tbls method.
For packages that are sequentially fired, the rigid substrate, dielectric tape, and constrainlng layer composlte would be constructed and flred as descrlbed above, however, additional layers of dielectric tape would then be added and laminated to the already fired package. In this case, the previously fired rlgid substrate/dielectric tape package acts as the rigid substrate onto wh~ch the dielectrlc tape and constrainlng layer material is applied to buildup additional layers of dielectric tape.
Thermally conductive rigid substrates and high strength rigid substrates are very attractive for hybrid appllcatlons. An attractlve conflguratlon for hlgh power IC chlp appllcatlons, 18 to put a cavlty ln the dielectric tape, cofire the cavlty conflguratlon onto a rlgld AlN substrate ln accord wlth the lnvention and then mount an lntegrated circuit chip ln the cavlty directly on AlN. A lld would then be attached over the cavlty to provlde hermeticity. The rigid AlN substrate provides a mechanical support and acts to remove heat 20~22~0 from the package. The concept of provldlng cavities or walls of dielectrlc materlal lnto whlch chlps are mounted ~s nttractlve because lt lncreaQes the level of package lntegration.
The ~blllty to coflre layers of dlelectric tape on a rigid substrate is limited by the thermal expanslon mlsmatch between the rlgld ~ubstrate, dielectric tape, and constralnlng layer materlal. If the thermal expanslon mi~match between the materlals of the laminated compos$te is large, defects at the lnterface between the materlals can occur durlng heat$ng whlch can lead to buckling. ~lso, for hybrid applications, the method requires that at least one side of the rigid substrate be flat (planar), so that the tape layers can be attached to the planar surface.
~e~l~ of tl~le ~ ;en~ly F~ rec Three embodiments of the lnventlon are shown in Figures 1-3. These embodiments are illustrative, not deflnltlve, of assemblies of the invention.
Figure l is a schematic representatlon of an arrangement of the components of the method of the inventlon in whlch a flexlble constralnlng layer is afflxed to both sldes of a ceramic tape part.
Both sldes of an unfired ceramlc tape part 5 (with or wlthout metalllzatlon) are lamlnated wlth flexible constralnlng layers 3 and 3a such that the constralnlng layers conform cloQely to the surface of the part. The thusly laminated ceramlc part can be fired in a conventlonal furnace by placlng the assemblage on the furnace belt l.
Flgure 2 is a schematlc representation of an arrangement of the components of the method of the inventlon in whlch a flexlble constralnlng layer i~
afflxed to only one s~de of a ceramlc tape part.

20~2210 A pre-fired ceramic substrate 7 ~with or without metallization) and an unfired ceramic tape part (wlth or without metallizat~on) S are aligned and colaminated. A
flexible constralning layer 3 may be separately laminated to the exposed surface of ceramlc tape part 5, or all three components, l.e., the constraining layer 3, the tape part 5 and the pre-fired ~ubstrate 7 may be colaminated. The assemblage ls then flred ln a conventional furnace by placing the ~3semblage on furnace belt 1.
Figure 3 is a schematic representation of an arrangement of the various components of the lnvention in which multiple (n) ceramic parts are alternated with n + 1 constraining layers to form a monolith. In this figure, n is three. But, n can be any positive integer.
Unfired ceramic tape parts (wlth or without metallization) 5a, 5b and 5c are aligned alternatively with flexible constraining layers 3a, 3b, 3c and 3d.
~he entire assemblage can be colaminated or subassemblies can be laminated to form the entire assembly. For example, ceramlc tape part 5a and constraining layer 3a can be laminated. Constraining layer 3b and the other layers of the assemblage can then be laminated in turn to the subassembly. Alternatively, a subassembly such as ceramic tape part 5a and constraining layer 3a and 3b can be aligned and laminated. A second ~ubassembly such as ceramic tape part 5b and 5c and constraining layer 3c and 3d can be aligned and laminated. Then, the first subassembly and the second ~uba~sembly can be aligned and laminated.
After lamination, the assemblage or monolith is fired ln a conventional furnace by placing the monolith on furnace belt 1.

205~210 = 1 ~2 The following set of experlments was conducted to show that the method of the lnventlon ellmlnates radial ~hrinkage (l.e. X-Y shrlnkage) during flring and provldes a means for fabrlcating multllayer packages with tlght dimensional tolerances. The examples show the precise llnear dimenslonal control provided by the process. Samples measured ln the study were prepared from Du Pont Green Tape~ (dielectrlc constant -6). The technlque used to measure llnear dimension changes during firing $.Q also reviewed.
Samples were prepared by standard multllayer Du Pont Green Tape~ processing techniques which included cutting blank layers of dielectric tape and laminating the layers under low temperature (e.g. 70C) and pressure (e.g. 3000 psi) to produce a monollthic unfired multilayer body. In some lnstances, as indicated below, metal conductor pastes were screen printed onto the tape layers prior to lamination. In some instances, layers of constraining tape were added to the top and bottom of the multilayer stack prior to laminatlon. In other instances, the dielectric layers were first laminated without constraining layers. In these instances, the constraining layers were simply added to the top and bottom of the laminated dielectric layers, and the entlre stack was laminated an additional time to adhere the constrainlng layers.
The 2~ x 2~ samples of Examples 1 through 5 ln Table 1, were made from elght 3" x 3~ planar blanks.
For thoQe samples where me~alllzation ls indicated, elther two or ~lx of the elght layers were screen prlnted with Du Pont 6142 Ag conductor metallizatlon, in a croQshatched test pattern. The test pattern was designed to replicate a high density conductor pattern.

In Example S, the metal was applied to only h~lf the surface of each prlnted layer. Four layers of 3 mils thlck constraining tape were added to both the top and the bottom of each stack, for an overall tot~l of 16 layers of tape. All 16 layers were lamlnated together at 3000 psl and 70 C for 10 minute~. The ~amples were then cut to the 2~ x 2~ slze. The unflred constralnlng layer tape/clrcult parts were placed on ~mooth, non-porous alumlna setters and burned out at 275C for 1 hour. Wlthout removlng them from the ~etters, the parts were then passed through a belt furnace and sintered at 850C. After coollng, the constralnlng layers were removed by dusting.
The S~ x S~ samples of Examples 6 through 9 in Table 1, were made from elght S~ x S~ planar dielectric blanks. In Examples 6 and 7, three layers of constraining tape were added to both the top and bottom of the stack prlor to laminatlon. The 14 layers of tape were then laminated at the indicated pre~sures, at 70C
for 10 minutes. For Examples 8 and 9, the eight dielectrlc layers were first laminated alone, ~t 3000 psi and 1000 psi respectively, at 70C for S minutes.
Three layers of constraining tape were tben added to both the top and bottom of each, and the 14 layered tape parts were laminated a second time, for an additlonal S
minutes, at 70C and 3000 psl.
In order to preci~ely and accurately measure linear dimenslonal changes durlng flrlng, which are ln accord wlth the tolerances requlred ln multilayer packages, a photollthographlc proce~q was used to place a relatively hlgh resolutlon pattern of 25 to 2B Au cro~-~e-Q wlth 1 mil llne wldths on the surface of the blanked diclectrlc tape layer~ ln a ~lmple matrlx. The dielectrlc layers so marked became the top dielectric layers of each of the test parts. The matrlx of cros-qes 20~2210 was examined by an automated travellng optlcal microscope before and after flring. The locatlons of the $ndividual crosses wlthin the natrlx were dlgltlzed and recorded ln computer memory. Uslng the computer to S drlve a preclslon X-Y table, the matrlx was lmaged and the llnear dlstances between indlvldual crosses anywhere on the surface of a sample were calculated to an accuracy of ~-0.2 mll. A total of 300 to 378 linear dlmenslon changes were measured for each of the nine sample conflguratlons 113ted ~n Table 1.
Table 1 shows mean llnear dimenslon changes, ~l/lo, where ~1 ls the change ln llnear distance between two selected crosshatches aQ a result of firlng and lo ls the lnltlal llnear distance between them. "Alternated"
refers to the orlentatlon of the lndlvldual tape layers within ~he sample. During doctor blade casting, particles have a tendency to allgn themselves ln the machlne castlng dlrectlon which has been shown to affect shrinkage durlng flrlng. Thus lt ls often deslrable to alternate the castlng dlrectlon of the lndlvldual tape layers to mlnlmize castlng effects.

~a~
Ex. Shrinkage No. SamPle Confi~uration (~ o) ,Std. Dev.

1 2~2~, 8 layora, ltern-tod, 0 001304 0 000291 no ~otal 2 2~x2~, 8 layerJ, not ltornatod, 0 001404 0 000245 no retal 3 2~x2~, 8 layor~, lternatod, 0 000285 0 000401 t~o layera of motal 4 2~x2~, 8 layer~, ltornatod~ -0 00017 0 000581 ~x layer~ of ~etal 20~210 2~2~, 8 layer~, ~lt~rnated, -0 00015 0 000647 ~lx layer~ ~alf et~ ed 6 5~5~, B l-y~r~, not lt~rn-t~d, 0 002000 0 000265 no rotal, 3000 p~ m~n-tlon 7 5~5~, 8 l-y~r~, not ~lt~rn~t-d, 0 0025~6 0 000360 no ~et-l, 2000 p~ n~n-t~on 8 S~s5~, 8 lay ro, not lt-rnat~d, 0 000~65 0 000337 no m~t-l, 2 ~t-ge l-~lnatlon, dl~l~ctrlc l-y ro t 3000 pol, ~th con~tralnlng layero t 3000 p~
9 5~5~, ~ l-y~ro, not alternated, 0 00~067 0 000413 no metal, 2 otage lamlnatlon, d~electric l-y~ro t 1000 p~, ~ith con~tr~ning layero at 3000 p~i The slight dimensional changes measured for these parts is largely due to a material thermal expansion effect and a constralning layer compactlon effect and is not attributed to sintering. The results show that shrinkage during firing for a number of sample configurations ls virtually eliminated and that linear dimenqions can be controlled to a degree of accuracy previously unattainable. The results also show that sample geometry and metallization density do not affect shrinkage behavior. For comparison, typical free ~intered (i.e. not constrained) multilayer Du Pont Green Tape~ part~ have a (~l/lo) of 0.12 and a standard deviatlon of ~ 0.002 where shrlnkage is hlghly lnfluenced by part geometry and conductor metal density.
Since the process offers such tight dimension tolerance during processlng, dimenslonal control ls not an important ls~ue when fabricatlng multilayer parts by thi~ technlque.

Claims

1. A method for reducing X-Y shrinkage during firing of a ceramic body comprising the sequential steps of a. Providing an unfired ceramic body comprising an admixture of finely divided particles of ceramic solids and sinterable inorganic binder dispersed in a volatilizable solid polymeric binder;
b. Applying to a surface of the unfired ceramic body a flexible constraining layer such that the constraining layer conforms closely to the unfired ceramic body, the constraining layer comprising finely divided particles of non-metallic inorganic solids dispersed in a volatilizable polymeric binder, the Penetration of the sinterable inorganic binder of the ceramic body into the constraining layer being no more than 50 µm;
c. Firing the assemblage at a temperature and for a time sufficient to effect volatilization of the polymeric binders from both the ceramic body and the constraining layer, forming interconnected porosity in the constraining layer and sintering of the inorganic binder in the ceramic body;
d. Cooling the fired assemblage; and e. Removing the porous constraining layer from the surface of the sintered ceramic body.

2. The method of claim 1 in which the inorganic binder is an amorphous crystallizable glass.

3. The method of claim 1 in which the inorganic binder is an amorphous vitreous glass.

4. The method of claim 1 in which the contact angle of the inorganic binder on the non-metallic solids of the constraining layer is greater than 60 degrees.

5. The method of claim 1 in which the viscosity of the sinterable inorganic binder is at least 1 x 105 poise.

6. The method of claim 1 in which the interconnected pore volume of the fired constraining layer is at least 10% of the total volume of the fired constraining layer.

7. The method of claim 1 in which the sintering temperature of the non-metallic inorganic solids in the constraining layer is at least 50°C higher than the sintering temperature of the inorganic binder in the ceramic body.

8. The method of claim 7 in which the sintering temperature of the inorganic binder in the ceramic body is 600-900°C.

9. The method of claim 1 in which the non-metallic inorganic solids in the constraining layer are ceramic solids.

10. The method of claim 9 in which the ceramic solids in the constraining layer are selected from mullite, quartz, Al2O3, CeO2, SnO2, MgO, ZrO2,BN and mixtures thereof.

11. The method of claim 9 in which the ceramic solids in both the ceramic body and the constraining layer are the same material.

12. The method of claim 1 in which the constraining layer is laminated to the unfired ceramic body.

13. The method of claim 12 in which the ceramic body comprises one or more layers of unfired ceramic tape.

14. The method of claim 13 in which the ceramic solids in the unfired ceramic tape are selected from Al2O3, SiO2, and mixtures and precursors thereof.

15. The method of claim 13 in which the ceramic solids and inorganic binder contents of the unfired ceramic tape constitute 30-70% by volume of the unfired ceramic tape and the non-metallic inorganic solids content of the constraining layer constitutes 10-90% by volume of the fired constraining layer.

16. The method of claim 13 in which the average particle size of the solids in the unfired ceramic tape and constraining layer is 1-20 microns with less than 30% by volume of such particles having a particle size less than 1 micron.

17. The method of claim 13 in which unfired ceramic tape is laminated to a pre-fired planar ceramic substrate prior to firing.

18. The method of claim 17 in which unfired ceramic tape is laminated to both sides of the planar ceramic substrate.

19. The method of claim 17 or 18 in which at least one surface of the substrate contains a conductive pattern.

20. The method of claim 19 in which the prefired planar ceramic substrate comprises a material selected from the group consisting of Al2O3, AlN, and Si.

21. The method of claim 13 in which a thick film conductive pattern is applied to the fired tape after removal of the constraining layer and the pattern is fired to effect volatilization of the organic medium therefrom and sintering of the conductive solids therein.

22. The method of claim 21 in which the conductive material in the pattern is a noble metal or mixture or alloy thereof.

23. The method of claim 22 in which the noble metal is gold, silver, palladium or alloys thereof.

24. The method of claim 21 in which the conductive material in the pattern is copper or a precursor thereof.

25. The method of claim 13 in which at least one layer of unfired ceramic tape has printed thereon an unfired pattern of thick film electrically functional paste and the assemblage is co-fired.

26. The method of claim 25 in which the thick film electrically functional paste is a conductor.

27. The method of claim 25 in which the thick film electrically functional paste is a resistor.

28. A method for reducing X-Y shrinkage during firing of a ceramic body comprising the sequential steps of:
a. Providing a monolith comprising n unfired ceramic bodies alternated with n + 1 flexible constraining layers wherein n is a positive integer, each unfired ceramic body comprises one or more layers of ceramic tape comprising an admixture of finely divided particles of ceramic solids and sinterable inorganic binder dispersed in a volatilizable solid polymeric binder, each constraining layer comprises finely divided particles of non-metallic inorganic solids dispersed in a volatilizable polymeric binder, and each constraining layer conforms closely to the surface of each adjacent ceramic body, but the Penetration of the sinterable inorganic binder of the ceramic body into the constraining layer is no more than 50 µm;
b. Firing the monolith at a temperature and for a time sufficient to effect volatilization of the polymeric binders from the ceramic body(ies) and the constraining layers, form interconnected porosity in the constraining layers and sinter the inorganic binder in the ceramic body(ies);
c. Cooling the fired monolith, and d. Removing the porous constraining layers from the surfaces of the sintered ceramic body(ies).

29. A composite unfired ceramic body comprising an admixture of finely divided particles of ceramic solids and sinterable inorganic binder dispersed in a volatilizable solid polymeric binder having affixed and closely conformed to a surface thereof a flexible constraining layer comprising finely divided particles of non-metallic inorganic solids dispersed in a volatilizable solid polymeric binder, the Penetration of the sinterable inorganic binder of the ceramic body into the constraining layer being no more than 50 µm.

30. A method for making the composite unfired ceramic body of claim 29 comprising the sequential steps of a. applying to at least one surface of the unfired ceramic body a constraining layer such that the constraining layer conforms closely to the surface(s) of the ceramic body, the unfired ceramic body comprising finely divided particles of non-metallic inorganic solids dispersed in a volatilizable organic medium comprising solid polymeric binder dissolved in volatile organic solvent, the constraining layer comprising finely divided particles of non-metallic inorganic solids dispersed in volatilizable polymeric binder, and the Penetration of the sinterable inorganic binder of the ceramic body into the constraining layer being no more than 50µm; and b. removing the organic solvent by evaporation.

31. The method of claim 30 in which the constraining layer is laminated to the surface(s) of the ceramic body.

32. The composite unfired ceramic body of claim 29 in which at least one surface thereof has printed thereon an unfired pattern of thick film electrically functional paste.

33. The composite unfired ceramic body of claim 32 in which the thick film pattern is printed on the constraining layer side(s) of the ceramic body.

34. The composite unfired ceramic body of claim 33 in which the thick film pattern is conductive.

35. The composite unfired ceramic body of claim 33 in which the thick film pattern is a resistor.

36. The composite unfired ceramic body of claim 32 or claim 33 having both resistor and conductor patterns printed thereon.