260 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 31, NO. 3, JULY 2008

Processing of Silvar for MIC Packaging ApplicationsEluri Ravindranadh Tagore, Anish Upadhyaya, and Anand V. Pathak

Abstract—Materials with high thermal conductivity and thermalexpansion coefficient matching with that of GaAs are being usedfor packaging high-density microwave integrated circuits (MICs)due to their ability of faster heat dissipation. Powder metallurgy(P/M) has emerged as a promising technique for the processingof metal matrix composites (MMCs) in satellite applications suchas thermal management materials. Individual components inthese composites must retain their metallurgical identity. Thepresent study investigates the processing of silvar (Fe–Ni–Co–Ag)alloys for thermal management materials. For these silvar alloys,composition was varied by varying the silver content from 15 to35 wt. %. The compacts were pressed at 400 and 600 MPa andsintered/infiltrated at 1100 C, 1150 C, and 1225 C. Quantitativemetallographic measurements were performed on all sample andthe results were discussed. It was observed that all samples weresintered without any shape distortion. Microstructural evalua-tion reveals that higher compaction pressure resulted in highlycontiguous structure. In spite of density differences betweenconstituents, none of the alloys showed segregation. Coefficientof thermal expansion (CTE) values of the liquid phase sinteredcomposites is near to the values obtained by rule of mixtures.

Index Terms—Coefficient of thermal expansion (CTE) andthermal conductivity, electronic packaging, metal matrix com-posite (MMC), powder metallurgy (P/M), silvar.

I. INTRODUCTION

T HE continuing increase in the electronic packagingdensity has resulted in a need for materials with high

thermal conductivities. In addition, to minimizing thermalstresses that can cause component or solder failure, packagingmaterials must have coefficients of thermal expansion (CTEs)matching those of ceramic substrates (such as Al O , AlN),and semiconductors (Si and GaAs). Further, low densities arepreferred in aerospace and avionic applications. The use andpotential for composites in electronic packaging is very broad.By combining two or more constituents, it is possible to creatematerials with unique combinations of properties that cannotbe achieved in any another way.

In recent years, new composites have been developed thatprovide unique combinations of properties that make them

Manuscript received March 28, 2007; revised October 12, 2007. PublishedJuly 7, 2008 (projected). This work was supported in part by the Department ofSpace, Government of India, administered by the IIT-Kanpur Space Technologycell. This work was recommended for publication by Associate Editor I. Fidanupon evaluation of the reviewers comments.

E. R. Tagore was with the International Advanced Research Center forPowder Metallurgy and New Materials, Hyderabad 500005, India. He is nowwith the School of Industrial and Manufacturing Engineering, Oregon StateUniversity, Corvallis, OR 97331 USA.

A. Upadhyaya is with the Department of Materials and MetallurgicalEngineering, Indian Institute of Technology—Kanpur, Kanpur 208016, India(e-mail: anishu@iitk.ac.in).

A. V. Pathak is with the Space Application Center, Indian Space ResearchOrganization, Ahmedabad 380015, India.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TEPM.2008.926287

outstanding candidates for packaging applications. The fourclasses of materials are polymer matrix composites, metalmatrix composites, and carbon–carbon composites. Tradition-ally, invar (Fe–Ni), kovar (Fe–Ni–Co), Cu–W, Cu–Mo, Cu,and Al have been used as packaging, substrate carrier plates,or heat sinks [1], [2]. Kovar has been used as a microwavesubstrate carrier plate in order to provide mechanical supportto the alumina which has been used as a substrate. Althoughthe CTE of kovar nearly matches that of alumina, its thermalconductivity is poor. Cu–W and Cu–Mo have required CTE andthermal conductivity, but density and machining costs involvedare high. Although Cu and Al have high thermal conductivity,their CTE is much higher. Consequently, marginally thermalstress compensating materials, such as alumina are needed. Asa result, the ability of Cu or Al to dissipate heat faster can nolonger be realized due to the additional thermal resistance ofalumina. The inability of traditional materials to dissipate theheat generated due to ever increasing power densities led to thedevelopment of alternate packaging materials such as Al–SiCcomposites. The CTE of Al–SiC can be tailored anywherebetween 5–18 ppm/K while maintaining a thermal conductivityof 170–180 Wm K by changing the Al-to-SiC ratio on thecomposite. However, Al–SiC composites are very difficult tomachine [3].

As described in the previous sections, thermal managementmaterials are generally processed through powder metallurgicaltechniques, either by liquid phase sintering or infiltration.W- and Mo-based materials are often made by P/M methodsincluding pressing and sintering followed by infiltration withcopper [4]. While these materials have the desirable range ofthermal expansion and conductivity, their poor machinabilityand platability have been a matter of concern. Elsewhere, ce-ramic-based materials were processed by using AlN as startingmaterial. Due to the significant amount of dissolved oxygenin the starting material, it forms a fine layer of Al O on thesurface and resists densification [5]. Further work aimed atdensification of AlN through liquid phase sintering by additionof small amounts of CaO Y O . This combines with Al Ofrom AlN and forms liquid phase at 1780 C resulting in99% sintered density [6]. Cuvar is another suitable materialprocessed by extrusion [7]. It is a composite of invar andcopper and stable in the range of 70 C to 400 C. However,exposure to higher temperatures has shown to cause diffusionalinteraction between the phases. As higher density is not desiredfor avionic applications, several researchers processed a newtype material Al–SiC [8]. Generally, these composites wereprocessed by infiltration of liquid metal into ceramic preform[9]. Capillary-induced infiltration has distinct advantages overother methods in that it does not require expensive toolingand equipment. The first such kind of infiltration of Al–Mgalloys were made by Aghajanian et al. [10]. They showed thata variety of ceramics, including Al O , SiC, AlN, and TiB ,could be infiltrated in the range of 750 C to 900 C. Rao et al.

1521-334X/$25.00 © 2008 IEEE

TAGORE et al.: PROCESSING OF SILVAR FOR MIC PACKAGING APPLICATIONS 261

[3] have demonstrated that these materials can be processedby pressureless infiltration of Al alloys instead of Al–Mg alloyand without any protective atmosphere. In addition to the infil-tration technique, these Al–SiC composites can be processedby plasma-sprayed coating. Manchang et al. [11] showed thatAl with SiC (40 to 60 vol. %) can be obtained by mixing thepowders of desired composition and spraying on to a targetmaterial by using plasma energy. However, Al–SiC is stillrestricted in use because of its poor machinability. Recently,it has been proved that thermal management materials can beprocessed through conventional casting techniques. James et al.[12] produced Al/Graphite composite having lower densityand higher thermal conductivity than Al–SiC materials. Mostrecently, invar and kovar have been used as one of the con-stituents in thermal management materials like cuvar, kuvar,and silvar. Systematic studies of these excellent systems arenot reported in literature. Our objective is to study the effectof sintering parameters on the densification and properties ofsilvar (composite of kovar and silver) alloys.

II. EXPERIMENTAL DETAILS

For the present investigation, kovar and silver powders weresupplied by Carpenter Powder Products (USA) and Modison,Ltd., (india), respectively. The composition and characteristicsof the as-received powders are presented in Table I. The silvarcomposites (with Ag varying from 15 to 35 wt. %) were preparedby mixing in a Turbula mixer (model: T2C, supplier: Bachoffen,Basel, Switzerland) for 2 h. The mixed powders were uniaxiallycompacted at 400 and 600 MPa using a semi-automatic hydraulicpress (model: CTM-50, supplier: FIE, Ichalkaranji, India) with afloating die. All the powder mixes were pressed in a cylindricalpellet form with diameters of 8 and 16 mm and height of 5 mm.and transverse rupture strength (TRS) bars mm .Zinc stearate was used as die wall lubricant during compaction tominimize friction. Sintering and infiltration were carried out in aMoSi -heated horizontal tubular furnace (model: OKAY 70T-7,supplier: Bysakh, Kolkata, India). The samples were heated ata constant rate of 5 C/min in pure hydrogen atmosphere (dewpoint: 35 C). The compacts were liquid phase sintered for1 h in H atmosphere at 1100 C, 1150 C, and 1250 C andinfiltrated at 1150 C, respectively. The sintered density wasmeasured through dimensional measurements and by usingArchimedes principle. The densification response was expressednumerically in terms of the densification parameter, which isexpressed as

Densification parametersintered density – green density

theoretical density – green density

The sintered compacts were prepared for metallographicstudy without using any etching reagent. From microstructuralobservations, quantitative analysis were carried out throughan image analyzer (model: Q5001W, supplier: Leica ImagingSystem, Ltd., Cambridge, U.K.) attached to a computer. Afterthe metallographic studies, hardness of the composites wasmeasured by using a Vickers hardness tester (Supplier: BlueStar, Mumbai, India) at a load of 5 Kg using a diamond indenter.Composites were measured for electrical resistivity by usinga digital micro ohmmeter (Model: 5893, Supplier: Tinsley,Delhi, India) with the advantage of the Weidmann–Frenz law,

TABLE IANALYSIS OF THE AS-RECEIVED POWDERS

Fig. 1. SEM micrograph of the as- received powders. (a) Kovar. (b) Silver.

and thermal conductivity of composites was obtained from theelectrical conductivity. CTE was measured by using a verticalpushrod dilatometer, (Model: Unitherm 1161, supplier: AnterLaboratories).

III. POWDER CHARACTERIZATION

The results of particle size and size distribution analysis ofas-received powder are summarized in Table I. It is evident thatsilver powder is relatively finer as compared to kovar. This kindof mixed powder sized gives the better green density. Fig. 1shows the SEM micrographs of the as received powders. It isclear from rounded powder morphology that both powders wereprocessed by gas atomization process. Such morphology givesbetter powder flowability. Furthermore, silver powder appearsbimodal. Bimodal distribution of silver powder is effective togive better packing. EDS analysis was carried out to find theelemental composition of the powders. The composition of boththe powders is given in Table I. While silver is highly pure, kovarconsists of Fe, Ni, and Co. Fig. 2 show the SEM micrograph ofcomilled (kovar and silver) powders. It shows the typical milledflaky morphology. Kovar powder got coated by the ductile silverpowder. With the effect of milling, the kovar particle size wasreduced by five times. This was revealed after sintering.

IV. RESULTS AND DISCUSSIONS

A. Densification Response

1) Liquid Phase Sintering: Fig. 3(a) shows the density varia-tion of silvar alloys sintered at different temperatures. All com-pacts were pressed at 600 MPa. It is observed that final sintered

262 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 31, NO. 3, JULY 2008

Fig. 2. SEM micrograph of comilled kovar and silver powders.

Fig. 3. Effect of sintering temperature and Ag content on the (a) sintered den-sity and (b) densification parameter.

density does not improve significantly with sintering temper-ature. Likewise, higher silver content too does not contributetowards densification. Even though, these alloys did undergoliquid phase sintering. The constituent elements (Fe, Ni, andCo) have insignificant solubility in Ag, and furthermore thissolubility limit is not affected by the temperature. Hence, insilvar alloys, the solution-reprecipitation mechanism does notcontribute towards densification. The only possibility of densi-fication is through solid-state sintering of kovar. Since the di-hedral angle is higher the for Ag-kovar system, kovar formsstable necks at the incipient melting of silver. This subsequentlyhinders densification through capillary-induced rearrangementpore filling and solid-state sintering of kovar. Fig. 3(b) shows thedensification parameter variation with sintering temperature andAg content. The density parameter is relatively high at 1100 C;however, the final density is the same. Alloys sintered at 1150 Chave a lower densification parameter indicating less shrinkage.As the temperature does not have an influence on densification,1150 C was decided as an optimum temperature for subsequentinvestigation.

None of the comixed silvar alloys result in full density; thelater being a primary requirement for all electronic packagingmaterials. To achieve full density, the sintered parts were rolledto 50% reduction in thickness. The density of rolled materialswas measured subsequently using Archimedis principle. Theobtained density was equal to the theoretical density of the alloy.The rolled alloys were annealed at 700 C to relieve the inducedstresses. Sintering followed by rolling and annealing are neces-sary for getting the final desired material.

2) Infiltration: An alternative processes to consolidate silvaralloys to full density was through infiltration. Kovar powder is

Fig. 4. Sintered density variation of infiltrated silvar alloys.

highly magnetic. During the compaction stage, die filling wasvery difficult because it sticks to the die wall which leads toblockage of the punch. To avoid this, silver was partially pre-mixed with kovar powder, and the remaining Ag was infiltratedat 1150 C. Fig. 4 shows the sintered density variation of silvaralloys processed through this route. It is evident that all the al-loys attained above 95% density. In all the cases, invariably,some silver was leftover at the top of the compact even after in-filtration. This could be because of higher green density of thecompacts used for infiltration. So the amount of silver infiltratedis just sufficient enough to fill the interconnected pores, and theremaining portion of the silver was left on the surface.

To overcome the problems faced during compaction of purekovar, the die was demagnetized by heating prior to the com-paction. Compacts were pressed at different pressures (200, 400,and 600 MPa) to get different volume fractions of porosity.However, compacts pressed at 200 MPa did not have enoughstrength to handle it. Initially, an equal amount of silver wasmade in the form of pellet and placed at the top of the compactfor infiltration. Due to this, some silver was invariably left onthe top of the compact which was pressed at 600 MPa. In thesecond stage, the amount of silver was exactly calculated basedon the volume fraction of porosity in the compact. This resultedin complete infiltration of silver into kovar. Fig. 5 shows thepictures taken after infiltration. Finally, near full density was at-tained with the infiltration in both the cases (400 and 600 MPa)without any shape distortion. This was also confirmed by mi-crostructural studies of the top and bottom surfaces and axialcross section.

3) Comilling and Liquid Phase Sintering: In addition toliquid phase sintering, infiltration silvar alloys were also pro-cessed by sintering comilled silver and kovar powders. Silverand kovar powders were milled for 24 h in acetone and driedat 1250 C for 6 h. Fig. 6 shows the effect of milling andcompaction pressure on both green as well as sintered density.Milling resulted in flaky morphology (Fig. 2) and silver coatingon kovar. This kind of morphology is difficult to compact andresults in lower green densities [13]. However it is evidentform Fig. 6 that all alloys, despite lower green densities, re-sulted in higher densities. There was a significant amount of

TAGORE et al.: PROCESSING OF SILVAR FOR MIC PACKAGING APPLICATIONS 263

Fig. 5. Photographs of silvar alloys infiltrated at 1150 C.

Fig. 6. Effect of compaction pressure and Ag content on the (a) green and sin-tered density and (b) densification parameter of comilled kovar–Ag alloys.

shrinkage observed after sintering of these alloys at 1150 C.This could be attributed to communition of particle size duringmilling and more homogeneous Ag distribution, both of whichenhance sintereability. Ag coating on kovar makes the system0 dihedral angle unless otherwise the kovar and Ag system isassumed to be high dihedral angle. A similar kind of behaviorwas observed in a W–Cu system.

B. Microstructures

Fig. 7 shows the optical microstructures of silvar alloys sin-tered at various temperatures. It is evident that all microstruc-tures are liquid phase sintered. It can also be seen that despite in-creasing the sintering temperature, no change in the microstruc-tures is discerned. This is because of negligible intersolubilitybetween the phases. In all micrographs, continuous network ofkovar is interspersed in the silver matrix. Kovar–35Ag alloysshow higher porosity as compared to lower Ag content. How-ever, in all the cases, the pores are completely isolated from thesilver phase, which is due to the formation of highly contiguouskovar network well before the liquid formation. The presumedreason for high contiguity is the high dihedral angle betweenkovar and silver systems.

Energy dispersive spectroscopy (EDS) analysis was carriedout on the sintered microstructure, grain (dark spot), and liquid(colored phase) to determine the elemental distribution in eachphase. The composition of grain is the same as that of as-re-ceived kovar powder, whereas the liquid phase composition cor-responds to pure silver.

Fig. 7. Optical micrographs showing effect of sintering temperature on the mi-crostructure of silvar alloys containing (a) 25%, and (b) 30% silver. All thesealloys were compacted at 600 MPa.

Fig. 8. Elemental mapping of kovar25Ag alloy. The alloy was prepared bycomixing and subsequently pressed at 600 MPa and sintered at 1150 C.

Fig. 9. Optical microstructures showing the effect of rolling on the microstruc-ture of silvar alloys containing 25% silver. The alloy was rolled to 50% reductionand annealed at 700 C.

Fig. 8 show the elemental distribution maps determined usingEDAX. As the kovar powder is in pre-alloyed condition, Fe, Ni,and Co appears distributed homogeneously in kovar grains. Thepresence of sliver is seen only in liquid phase.

As described earlier, sintered silvar alloys were rolled andannealed to achieve full density. Fig. 9 shows the optical mi-crostructures of kovar25Ag in sintered, rolled, and annealedcondition. It is clearly visible that kovar grains were elongated.Furthermore, due to the rolling, the pores got closed, and italso induces some residual stresses. After complete annealing,stresses will relieve and grains regains their shape. However, inthe present study, the annealing temperature was insufficient tocause recrystallizing.

264 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 31, NO. 3, JULY 2008

Fig. 10. SEM micrograph of comilled kovar20Ag pressed at 400 MPa and sin-tered at 1150 C.

Fig. 11. Optical microstructures of (a) top and (b) regions of kovar25Agpressed at 400 MPa (left) and kovar15Ag pressed at 600 MPa (right).

Fig. 10 shows the SEM micrograph of the comilled andsintered kovar-20%Ag alloy. It is found that milling not onlyresulted in densification enhancement but also significant (fivetimes) reduction in kovar grain size. As the milled powderparticle size was finer, the number of interparticle contacts in-creases. In addition to this, the dihedral angle is also high. Thisleads to the highly contiguous and connective network of kovar.The extent of connectivity and contiguity is such that silverphase is completely isolated from one another. This would af-fect the conductivity of the alloy which relies on the continuityof the conducting phase. Elemental mapping of the above alloywas carried out to verify whether any alloying occurred duringmilling. It is clearly observed that the elemental distribution issimilar to the liquid phase sintered alloys. Milling does not leadto any compositional change in the phases.

Fig. 11 represents the optical microstructures of the top andbottom surfaces of silvar alloys processed through the infiltra-tion technique. In both (400 and 600 MPa) the cases, the mi-crostructures look self similar. This attributes to the perfect in-filtration. To reinforce the above statement, segregation studieswere carried throughout the axial cross-section. Except a fewareas, which would have formed in the earlier stage of com-paction, there was no such kind of segregation. The microstruc-tures were homogeneous all over the cross section.

Fig. 12. Variation of bulk hardness for various silvar alloys consolidated by(a) pressing and sintering (600 MPa; 1150 C); rolling (50% reduction) andsubsequent annealing (700 C); 1 h) and (b) milling and infiltration.

Fig. 13. SEM microstructures of kovar30Ag (a) as sintered and (b) cold rolledto 98% reduction.

C. Material Characterization

1) Bulk Hardness: Fig. 12(a) shows the hardness variationof liquid phase sintered, cold worked and heat-treated silvar al-loys. It is clear from the figure that rolling up to 50% reduc-tion in thickness resulted in increasing hardness. Usually, coldrolling results in work hardening of the material, and it alsoinduces stresses that contribute to hardness enhancement. An-nealing at 700 C results in lowering of hardness, but it is stillhigher as compared to the sintered alloy. Furthermore, 700 Cdoes not result in recrystallization of grains. This observed phe-nomenon could be because of insufficient annealing tempera-ture. Increasing the silver content resulted in lowering of bulkhardness which is due to the soft nature of silver.

Fig. 12(b) shows the effect of milling and infiltration onbulk hardness of silvar alloys. This is clear from the figurethat hardness of the milled and infiltrated alloys is higher thantheir liquid phase sintered counterparts. Due to the highersintered density, infiltrated alloys exhibit higher hardness ascompared to the sintered alloys. In the case of infiltrated alloys,due to the less silver content, compacts pressed at 600 MPa(kovar15Ag) showed marginally higher hardness than thosepressed at 400 MPa (kovar25Ag). In spite of higher sinterdensity, hardness of infiltrated alloys is inferior to comilledalloys which have finer grain size.

2) Machinablility: Machinability is the important propertyrequired for electronic packaging materials. Fig. 13 shows themicrostructures of silvar alloy. Fig. 13(a) and (b) represents the

TAGORE et al.: PROCESSING OF SILVAR FOR MIC PACKAGING APPLICATIONS 265

TABLE IICTE AND THERMAL CONDUCTIVITY OF DIFFERENT SILVAR ALLOYS

kovar30Ag alloy in as-sintered and 98% cold rolled conditions,respectively. Even after severe rolling, the slivar alloy shows noevidence of crack or any defect, which confirms good ductility.This can attribute to the good machinability. The final compo-nent was machined without any difficulty.

3) CTE Measurement: The CTEs of these alloys were mea-sured using a vertical pushrod dilatometer. Table II summarizesthe CTE values obtained by experiments and rule of mixtures. Itis evident from the table that CTE values of experimental resultsare near to the values obtained by rule of mixtures. Elsewhere,Rao et al. [3] proved that rule of mixtures is reliable for CTE ofAl-SiC composites. The observed variation is due to the pres-ence of porosity in the sintered material.

4) Thermal Conductivity: Thermal conductivity of the silvaralloys were not measured experimentally. However, using elec-trical resistivity measurements, thermal conductivity was mea-sured from Wiedemann Franz’s law [14]. The results obtainedare summarized in Table II. This table also contains the conduc-tivity values predicted using rule of mixtures. Note that thereis significant deviation between the experimental and modelvalues. The observed deviation in the results could be due tothe presence of pores in the sintered parts which possibly con-tributed to errors in measuring the resistance of these alloys.

V. CONCLUSION

For the first time, silvar alloys were consolidated throughP/M by premixing the kovar (Fe–Ni–Co) and Ag powder vialiquid phase sintering, infiltration, and liquid phase sintering ofcomilled powders. The CTE values of these alloys were close tothe values predicted by rule of mixtures. In addition to the above,none of the alloys resulted in macro-segregation of phases.

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Eluri Ravindranadh Tagore received the B.Tech.degree from the National Institute of Technology,Tiruchirappalli, India, and the M.Tech. degree fromthe Indian Institute of Technology, Kanpur. He iscurrently pursuing the Ph.D. degree from OregonState University, Corvallis.

His areas of interest are powder metallurgy, corro-sion, and microfluidic devices.

Anish Upadhyaya received the M.S. and Ph.D. de-grees from The Pennsylvania State University, StateCollege.

He is currently an Associate Professor in the De-partment of Materials and Metallurgical Engineeringat the Indian Institute of Technology, Kanpur. Hisarea of specialization is in powder metallurgicalprocessing of ferrous, nonferrous, and refractoryalloys with special emphasis on sintering.

Anand V. Pathak received the B.Eng. and M.Eng.degrees in mechanical engineering from Gujarat Uni-versity, Gujarat, India, and the Indian Institute of Sci-ence, Bangalore, respectively.

He is currently a Project Manager with the SpaceApplications Center, Indian Space Research Organ-ization (ISRO), Ahmedabad, India, and is workingon INSAT and GSAT spacecraft payloads. He is re-sponsible for design, development, and fabrication ofcommunication payloads. In the past, he had deliv-ered various payloads for the Indian National Satel-

lite System (INSAT). Currently, he is interested in the development of new ma-terials for MMIC technology.