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Introduction Joining two dissimilar materials,such as ceramics and metals, is criticalfor various applications, such as elec-tronic devices and thermoelectric gener-ators (Refs. 1–6). The most popularjoining process in the electronics indus-try remains soldering because of theease of application in individual orbatch operations, low cost, and good ex-perience (Refs. 7, 8). For higher temper-ature applications, similar furnace braz-ing techniques are used for higherstrength metal-ceramic joints (Refs. 6,7, 9, 10). However, conventional soldering and brazing processes have severaldrawbacks. They often require lengthyand undesirable preheating of the ma-

terials to be joined, due to the fact thatconventional processes heat the wholejoint, including the base materials(Ref. 14). Long processing times arenecessary because rapid temperaturechange can cause thermal shock andcracking of the ceramic (Ref. 15). If thetemperature-induced stresses are toohigh because of lack of ductility andtoughness, cracking occurs (Ref. 15).Another difficulty with joining ceram-ics to metals is their mismatch in coef-ficient of thermal expansion (CTE)that can produce shear stresses at thejoint interface, which can cause the ce-ramic to crack (Ref. 16). Diffusion is acommon problem that plagues ceram-ic-metal braze and solder joints (Ref.17), as brittle intermetallics can formand joint decohesion occurs in service.

Microwave-assisted joining could bean attractive alternative to conven-tional soldering and brazing processes.Microwaves are electromagnetic waveswithin the frequency range of 1–300GHz (Ref. 18), generated typicallyfrom a magnetron. Microwaves havebeen used for joining plastics using di-electric heating, which is produced bythe friction in a dielectric materialduring the rearrangement of polarmolecule groups following the changeof electric field direction. Dielectricheating is directly proportional to theimaginary part of the dielectric per-mittivity of a material (Ref. 11). Themore dielectric loss, the more heat isgenerated, which can be effectively uti-lized for joining two materials withhigh dielectric loss. In the case of join-ing low dielectric loss materials, an ad-hesive with high dielectric loss hasbeen used as an interfacial filler be-tween them (Ref. 12). Advantages of microwave joiningrelative to furnace soldering or brazingtechniques were considered to be 1)lower risk of brittle interfacial com-pound formation (Ref. 13) and atomicdiffusion of the filler into the base ma-terial because of faster processingtimes (Refs. 7, 14, 17); 2) a more ener-gy-efficient process than conventionalbrazing techniques (Ref. 19); 3) signif-icantly shorter cycle times (Ref. 14);and 4) less impact on the base materi-als being joined. In this paper, we present a novelclosed-loop controlled microwave sol-dering system for joining of two dis-similar materials. Closed-loop control

Microwave Joining — Part 1: Closed­LoopControlled Microwave Soldering of

Lead Telluride to CopperA new plasmaless, closed­loop microwave heating device was

designed, built, and tested for ceramic to metal joining

BY D. HOYT, Y. ADONYI, AND S. KIM

ABSTRACTCeramic­to­metal joints require faster, more energy efficient techniques to enhance

the manufacturing productivity and reduce costs in electronics manufacturing. Mi­crowave heating is such a technique, especially when using a closed­loop controller in aplasmaless process to improve process reproducibility. This paper shows that mi­crowaves are able to heat and melt an interface filled with tin powder in glycerol suspen­sion, where dielectric heating of the glycerol was used to melt the metal powder, whilethe dielectric material boiled off. The ceramic­metal solder joint was made significantlyfaster, with minimal diffusion at the interface, and without damage to the ceramic usingthis microwave process, as opposed to typical furnace soldering processes. This new uni­versal microwave system (Microwave Welder, patent pending) could potentially be usedfor a multitude of ceramic­to­metal joints, as well as other special applications wherecurrent joining methods are prohibitively expensive and time consuming or unavailable.

D. HOYT, Y. ADONYI (yoniadonyi@letu.edu), and S. KIM are with LeTourneau University, Longview,  Tex.

KEYWORDS • Microwave Soldering • Thermoelectric Elements • Lead Telluride • Plasma Suppression • Metal­Ceramic Interfaces • Dielectric Heating • Eddy Current Heating

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and arc suppression were to be used toensure repeatable joining of differentcoatings, semiconductors, and metals,two specific features of our system forwhich a patent is pending. We at-tempted to validate the concept of di-electric heating by microwaves and theuse of this heating mechanism at theinterface of a ceramic-metal joint tomelt the metal filler powder used tobond the joint.

Objectives For the purpose of demonstratingthe dielectric heating mechanism andjoining two dissimilar materials withthe proposed system, we chose ceram-ic thermoelectric elements (lead tel-luride and bismuth telluride) and cop-per (Cu). Joining these thermoelectricmaterials to Cu substrate is crucial forthermoelectric generators. A mi-crowave joining system has been de-signed, built, and tested, as no com-mercial systems were available. Safeand reproducible operation was to beensured by using a plasma-discharge-free environment, although metals

were involved inthe joining process.

Experimental Procedures A conceptual schematic of the sys-tem is shown in Fig. 1, while Fig. 2shows the third prototype of the sys-tem with key components labeled. The microwave generator producesa 2.45-GHz microwave guided througha WR-340 waveguide that has a crosssection of 3.4 × 1.7 in. and supportsonly the TE10 mode. The waveguide isconnected first to the isolator (whichdiverts the reflected wave), then to adual-power coupler that measures for-ward and reverse power, third to thesample chamber where heating occurs,and finally to the movable backwallwhere reflection happens. A FLIRT300 thermal camera is used to ob-serve the temperature of the sample,which is fed to a computer running aLabVIEW program that contains theclosed-loop control platform. This con-troller uses temperature and heatingrate to control magnetron power andbackwall position, respectively. A touchscreen computer interfaceallows the operator to input parame-ters, such as sample mass and material

properties, and observe runtime re-sults, such as temperature of the sam-ple and magnetron power. For the theoretical modeling ofthis microwave system in terms ofelectromagnetic field distribution inthe waveguide, we employed theCOMSOL multiphysics software. Atypical calculated electric field norm(V/m) along the waveguide is shownin Fig. 3. Due to the reflected wavefrom the backwall, constructive anddestructive interferences between theforward and the reflected waves oc-curs, and a standing wave pattern isformed inside the waveguide. At thepeak locations, the magnitudesquared of electric field is doubledfrom that of the forward wave onlycase while, at valleys, it becomes zero.Therefore, the backwall is used toproduce a variation of intensity levelsat a desired location in the waveguidedepending upon the backwall’s loca-tion (Refs. 21, 22). The dielectric heat is directly pro-portional to the magnitude squared ofelectric field (|E0|2), as shown in Eq. 1.Note the dielectric heat is also propor-tional to the dielectric loss term (”),which is a unique material characteris-tic of the target.

Fig. 1 — A diagram of the Microwave Welder showing key components.

Fig. 2 — The Microwave Welder with key components labeled.Note, the generator control panel and most of the touchscreenpanel are hidden from view in this image.

Fig. 4 — A schematic of the actual closed­loop control system usedby the Microwave Welder.

Fig. 3 — A COMSOL simulation of the electric field distribution inthe microwave waveguide with a reflecting backwall.

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dialectric = 2 0”|0|2 (1)

where is the 2.45 GHz frequency ofthe wave, and 0 is the permittivity offree space. Control of the electric fieldintensity distributions and modula-tion of magnetron power providesclosed-loop control of the heat deliv-ered to the sample using temperatureof the sample as feedback into the con-trol system. This dual parameter con-trol (backwall position and power)coupled with continuous temperaturemonitoring using a thermal cameraprovides the means for the actualclosed-loop control for stable heatingat a joint interface — Fig. 4. Sample heating is accomplished bycontrol of magnetron power and back-wall position and set by a target heat-ing rate. The thermal camera measuresthe temperature of the sample, whichis fed back into the control algorithm(implemented on a LabVIEW plat-form) along with instantaneous heat-ing rate in two feedback loops. The first feedback loop controls mag-netron power by feeding the desiredtemperature as a set point and the actu-al temperature as the process variableinto a proportional-integral controller,the output of which is scaled to themagnetron’s power output range.

The secondfeedback loopfinds the heating rate error and multi-plies it by a set point using the dielec-tric character of the target, whichspecifies the volume fraction of thetarget that is composed of a dielectricsubstance versus a metal. The productis used to calculate the target wall po-sition, which is fed into a proportion-al-integral-derivative (PID) controllerwith appropriate constants optimizedfor speed and accuracy. This design al-lows the control system to select be-tween heating the dielectric portion ofa target with use of a local electric fieldmaximum or heating of metallic parti-cles by the use of a local magnetic fieldmaximum. The position of these fieldmaxima are adjusted by the movementof the backwall. The advantage of this closed-loopcontrolled device lies in the combina-tion of a feed-forward component —the dielectric character — with a dou-ble-nested loop design feedback com-ponent utilizing the temperature andheating rate for control of the mag-netron power and the backwall posi-tion (Ref. 22). The dielectric characterdefines the composition of the targetbeing heated — what portion is a di-electric material versus a metal. The

magnetron input power and the back-wall positions are constantly adjustedto maintain the desired heating rate ofthe sample.

Results and DiscussionDielectric Heating Validation

To demonstrate the microwaveheating of a dielectric material, a sim-ulation was performed using a Bis-muth Telluride Bi2Te3 block by placingit in a waveguide terminated by a mov-able backwall. As shown in Fig. 5, theBi2Te3 block is heated with microwaveenergy mostly by the dielectric mecha-nism. Figure 5A shows the tempera-ture of the Bi2Te3 for a backwall posi-tion giving particularly low electricfield intensity at the sample location.Figure 5B shows the temperature de-veloped in the Bi2Te3 block for a back-wall position extended by 0.38 cm. Itis obvious that the Bi2Te3 temperatureis different and depends upon the rela-tive location of the movable backwalldue to the standing wave pattern pro-duced in the waveguide, which in turnshows the controllability of the sampleheating using the backwall.

Fig. 5 — A COMSOL simulation plot of the heating of bismuth tel­luride in the Microwave Welder, demonstrating the difference infinal temperature for two different backwall locations.

Fig. 6 — Experimental results from a microwave heating experi­ment that validated the COMSOL simulation of Bi2Te3 heating.

Fig. 7 — PbTe­Cu solder joint configuration in the Microwave Welder.Fig. 8 — Magnetron power, the temperature set point, and the ac­tual temperature of the interface as a function of time for the Cu­PbTe soldering experiment.

A

B

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An experiment with the actual Mi-crowave Welder machine prototype wasperformed to validate the simulatedheating at the backwall location produc-ing the maximum temperature in theBi2Te3 block. The block of Bi2Te3 wasplaced on a stage within the waveguideto center it in the waveguide. Figure 6shows the heat generated in the samplefor a 120-s run. The final temperatureon the block at the backwall positionproducing maximum heating (the elec-tric field hotspot) was 69.9°C, while thefinal temperature from the simulationfor heating Bi2Te3 in the electric fieldhotspot was 74°C. This represents anerror of 4.1°C, or only about 6%. The experiment shown in Fig. 6demonstrates the ability of the systemto heat a dielectric material using amovable backwall. It also validates thesimulation results used to predict thewave pattern generated in the Mi-crowave Welder’s single mode wave-guide. Indeed, finite element analysis(FEA) simulation was used to find thebackwall location for which the great-est heat would be generated in the di-electric target — the backwall positionfor which the hotspot would exist atthe target’s location. The temperaturedeveloped in this simulation was close-ly correlated to an experimental resultusing the same operating parametersas existed in the simulation. Thus, theMicrowave Welder was shown to workas designed, with more or less degreesof accuracy, and more work will be re-quired to perfect the system.

Microwave Soldering ValidationExperiment

To validate the ceramic-metal join-

ing process, a mi-crowave solder jointwas made betweenlead telluride (PbTe)and Cu, a commonconductor material

in the electronics industry. Both ofthese materials have a low susceptibili-ty to microwave heating as defined inEq. 1, which means that selective heat-ing will exist at the interface. Minimalheating of the base materials is key forceramic-metal joining as it reduces thestresses induced by disparities in theCTEs of the materials and thermalshock in the ceramic. A dielectric liq-uid, glycerol, was identified as having amuch greater dielectric loss factorthan bismuth telluride. In addition,antimonious tin powder (Sn) was se-lected as the metal filler, having a lowmelting point and being commonlyused in the soldering industry. A 90%mass tin/10% mass glycerol interlayerpaste composition was selected for itsviscosity. The joint configuration wasplaced on a metal stage within the Mi-crowave Welder, as shown in Fig. 7. The magnetron power and temper-ature data for the experiment is shownin Fig. 8. In this example of microwavesoldering, the closed-loop control sys-tem favored modulation of the mag-netron power over backwall adjust-ment. Thus, there was no significantmovement of the backwall during theexperiment. The inset image in Fig. 8shows the sample used in this experi-ment. As shown, the actual heatingrate of 5.41°C per second (red solidline) outpaced the expected heatingrate of 3°C per second (orange dashedline) while the magnetron power isgradually increased from 0 to 1400 W(green dotted-dashed line). While the mismatch between theactual heating rate and the expectedheating rate shows that certain miscal-ibration still exists in the system, theactual heating rate was linear, demon-

strating the efficacy of the closed-loopcontrol. The temperature reached theboiling point of glycerol at about 40 s,at which point the Sn solder meltedand formed a bond while the glycerolevaporated. The interface temperaturestarted to drop after the Sn melted be-cause the dielectric liquid was nolonger present, removing the primarysource of microwave heating. Figure 9 shows a scanning electronmicroscope image of the Cu-PbTe joint.Complete bonding to the copper basematerial was achieved, while partialbonding to the ceramic occurred. Thisis due to the absence of pressure on thejoint during the soldering process. Withaddition of joint pressure, completewetting to the ceramic could beachieved, which will improve jointstrength. Most importantly, no crack-ing is observed in the PbTe, indicatingthat the copper and lead telluride basematerials were not unevenly overheat-ed to produce stresses induced by theirdisparity in thermal expansion. In addition, this figure shows somePb diffusion into the Sn interlayer.However, although the Sn interlayer isonly about 100 microns thick, no Pb-Cu intermetallics formed because thespeed of the joining process preventedthe base materials from mixing chemi-cally. Note the entire microwave sol-dering process was completed in 75 s, as opposed to the typical 3600 sused during furnace soldering of thisspecific joint (the reason for the longsoldering cycle is that lead telluridehas very low thermal conductivity andany rapid heating or cooling via con-vection will crack the ceramic). An EDS (electron-dispersive x-rayspectroscopy) scan at a point in theinterlayer close to the Cu base mate-rial is shown in Fig. 10, revealing noappreciable amount of lead. This in-dicates that atomic diffusion was in-sufficiently rapid to produce the de-fects that are common to this kind of

Fig. 9 — SEM images of the Cu­PbTe interface showing eachcomponent of the joint and an absence of cracking, although afew voids exist between the tin interlayer and the ceramic.

Fig. 10 — Electron­dispersive x­ray spectrum of the solder interfacematerial.

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joint in longer duration processes.Again, when compared to a furnacesoldering process on the order of anhour, this nearly minute-long mi-crowave soldering approach appar-ently did not allow time for brittle in-termetallics to form. Finally, some adjustments to the sys-tem could improve this joint, such aseliminating the voids at the ceramic sideof the interface. With the addition ofpressure upon the sample during themicrowave soldering process, completewetting to the ceramic could beachieved, which will improve bondstrength. In addition, if the dielectricliquid could be replaced completely by ametal soldering composition, then high-er temperature soldering and brazingcould be accomplished with this samemicrowave process. Eddy currents andmagnetic hysteresis are both mecha-nisms by which microwaves interactwith metals to produce heat, paramag-netic metals only utilizing the first andferromagnetic metals utilizing both. Ap-plication of pressure to the sample iscurrently part of this ongoing researcheffort at LeTourneau University and isexpected to generate further notewor-thy results. This improvement will bepresented in Part 2 of this paper serieson microwave joining.

Conclusions

A novel plasmaless, closed-loop mi-crowave heating device was designed,built, and tested for ceramic to metaljoining. It was shown that furnace sol-dering could successfully be replacedby microwave soldering in a particularsemiconductor/metal interface forthermoelectric generator application.The novelty consisted in delivering di-electric heating to melt a metal fillerpowder at the joint, which succeededin avoiding thermal shock to the low-thermal conductivity lead telluridethermoelectric element and producedless brittle intermetallics at the jointinterface. Accordingly, more efficient

and longer lasting thermoelectric gen-erators could be built in the future, asmore work remains to be performed inimplementing the technology from el-ements to full-scale modules.

This work was supported in part bythe II-VI Foundation and by the Amer-ican Welding Society. We are gratefulto Allen Worcester who completed hisMSc thesis work on this topic and themany LeTourneau University under-graduate students, such as IthamarGlumac and others, who brought im-portant contributions to this project.

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Acknowledgments

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