Electromigration damage characterization

Electromigration Damage Characterization in Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce Solder Joints by Three-Dimensional X-ray Tomography and Scanning Electron Microscopy

H.X.XIE,1D.FRIEDMAN,2K.MIRPURI,2and N.CHAWLA 1,3

1.—Materials Science and Engineering,Arizona State University,Tempe,AZ 85287-6106,USA.

2.—Assembly Test and Technology Development,Intel Corporation,Chandler,AZ 85226,USA.

3.—e-mail:nchawla@https://www.360docs.net/doc/226757954.html,

Cerium (Ce)-containing Sn-3.9Ag-0.7Cu alloy exhibits desirable attributes of microstructural re?nement,increased ductility,and mechanical shock per-formance,while possessing better oxidation resistance than other rare-earth-containing solders.In addition to the bene?cial mechanical properties,it is imperative to study the reliability performance of novel solder alloys in the form of electromigration experiments,in comparison with Sn-3.9Ag-0.7Cu.In this study,electromigration tests were conducted on solder joints at elevated temperature with a constant current using a V-groove testing methodology.The microstructural change of solder joints during electromigration was investigated by scanning electron microscopy,and the void growth was mon-itored utilizing the three-dimensional (3D)x-ray microtomography imaging technique.The current density inside the solder matrix was determined by 3D microstructure-based ?nite-element modeling.Finally,the product of diffu-sivity and effective charge number of solder joints during electromigration was calculated from both marker displacement and 3D void growth.It was found that electromigration-induced Cu diffusion in Sn-3.9Ag-0.7Cu-0.5Ce alloy was greatly accelerated,and void formation at the cathode side was retarded as a result of ?ner microstructure and existence of CeSn 3intermetallic particles.Key words:Rare earth,cerium,lead-free solder,electromigration,x-ray

tomography

INTRODUCTION

The shift in semiconductor manufacturing from Pb-based to Pb-free solder has caused reliability concerns for future electronic packaging platforms,due to the low ductility and poor mechanical shock resistance of Sn-Ag-Cu solder alloys.1–3Recently,a series of rare-earth-containing Sn-3.9Ag-0.7Cu Pb-free solders have been proposed as high-ductility alternatives.4–6It has been shown that the addition of a trace amount of rare-earth elements can re?ne the microstructure,reduce the interfacial interme-tallic layer thickness,and,most importantly,increase the ductility and shock performance of

solder joints.4–8Oxidation behavior studies on rare-earth-containing solder indicate that Ce-based sol-der alloy possesses the best oxidation resistance 7and thermal stability 8among several rare-earth-containing solders.

Previous studies have established the feasibility of a Ce-based solder alloy as a replacement for conventional SAC alloys.Nevertheless,the electro-migration behavior of Ce-containing solder still needs to be investigated.9The electromigration behavior of Sn-Ag-Cu lead-free solder has been studied and compared with conventional Sn-Pb solder.10–13Generally,Sn-Ag-Cu solder has much better electromigration performance,owing to the lower homologous temperature and higher Young’s modulus.However,very few studies have been carried out on the electromigration behavior of rare

(Received April 10,2013;accepted June 17,2013;published online July 11,2013)

Journal of ELECTRONIC MATERIALS,Vol.43,No.1,2014

DOI:10.1007/s11664-013-2667-z ó2013TMS

33

earth(RE)-containing lead-free solder,and these show a large discrepancy in the literature.Lin et al.14reported that the electromigration resis-tance of Sn-Ag-Cu and Sn-Zn solders15decreased with addition of0.5wt.%Ce.This was attributed to the?ner grain size in Ce-containing solder,serving as the main diffusion path for diffusion of atoms.In contrast,He et al.16observed enhanced electromi-gration resistance in RE-containing Sn-Bi solder due to the aggregation of RE at grain boundaries, which results in a barrier to the diffusion of atoms. However,the mechanisms for electromigration in these materials are still unclear,and much work remains to be done in order to understand the electromigration behavior of Ce-based Pb-free solders.

X-rays have long been utilized as a powerful imaging tool for visualizing intrinsic microstruc-tures of solder joints.17X-ray microtomography,in particular,provides a better understanding of the pore size,distribution,and evolution during mechanical or electromigration testing,especially the characteristics of pores.18–21The3D synchro-tron tomography imaging technique has been con-ducted on Sn-Pb solder bumps subjected to electromigration,to fully understand the shape and size evolution of current-induced voids.21

In this study,the electromigration behavior of Ce-containing Sn-3.9Ag-0.7Cu was compared with that of Sn-3.9Ag-0.7Cu by examining the microstructure evolution using scanning electron microscopy (SEM)and laboratory-scale x-ray tomography.The current-induced vacancy?ux in Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce solder joints was calcu-lated accurately based on the3D void volume change from x-ray tomography.Finally,the current density distribution in the actual solder joint was studied using microstructure-based?nite-element modeling(FEM),where the microstructure from x-ray tomography was directly incorporated into the FEM model.Here,the effect of preexisting voids on the current distribution was studied and is discussed.

EXPERIMENTAL PROCEDURES Vacuum-melted ingots of Sn-3.9Ag-0.7Cu with a trace amount(0.5wt.%)of Ce were prepared.High-purity Sn-3.9Ag-0.7Cu ingots(Indium,Clinton,NY) were cut into small rectangular pieces (6.5mm96.5mm913mm)and mixed with Ce shot(ESPI,Ashland,OR).Due to the reactive nature of pure Ce with oxygen,materials were mixed in a quartz ampoule(12mm diameter)in a sealed glove-box with helium atmosphere.The quartz ampoule was then evacuated to10à5torr and sealed.The sealed ampoules were heated to1000°C for4h,and periodically mixed by rotation of the ampoule in order to homogenize the liquid metal.The ampoules were water-quenched from1000°C to room temperature removed from the ampoule,and sectioned.

To study the effects of electromigration and the microstructural change of Ce-containing Sn-3.9Ag-0.7Cu solder,micron-sized Cu/solder/Cu butt joints(500l m diameter)were fabricated,using a V-groove testing methodology.22,23Silicon V-grooves were made on a silicon wafer using photolithogra-phy and wet KOH etching techniques,as shown in Fig.1.The as-processed solder ingots were ma-chined to500-l m-thick solder discs,then punched to small solder discs500l m in diameter.Copper wires (500l m diameter)were polished to a0.05-l m?nish with a colloidal silica solution.A rosin mildly acti-vated(RMA)?ux was applied to the Cu end bars to improve the wetting between the Cu wires and sol-der.The butt joints were assembled by aligning the two Cu wires and the solder disc on the Si V-groove. The joints were re?owed on a hot plate using a re?ow pro?le that consisted of heating the?xture to 170°C for2min to allow the excess?ux to vaporize, followed by heating to20°C above the melting point and holding for50s,then cooling in air on Al blocks.

A reproducible cooling rate of1.4°C/s was obtained in these experiments.

Before electromigration tests,the joints were reinforced by a high-temperature epoxy resin to avoid brittle failure while transferring the sample to the x-ray tomography system and SEM.The as-re-?owed sample was polished down to about half of the initial solder joint thickness in order to obtain a higher current density,as well as for microstruc-tural characterization in the SEM.The butt joint and test?xture were preheated inside a furnace at 100°C for1h to stabilize the sample and minimize thermal shock damage to the joints.Electromigra-tion tests were performed at elevated temperature (100°C)with a constant current to obtain an aver-age current density of59103A/cm2in all samples. The actual temperature of the specimen during testing was monitored by attaching a thermocouple on the cathode side of the solder joint.

The microstructural evolution of the Ce-contain-ing solder joint during electromigration was exam-ined by optical and SEM during current stressing, and compared with that of Sn-3.9Ag-0.7Cu joints. The electromigration-induced void growth was examined by a laboratory-scale x-ray tomography technique.Two-dimensional(2D)projection images of solder joints were obtained using a commercial laboratory-scale x-ray tomography system(Xradia, Concord,CA).These images were reconstructed using Xradia Reconstructor software into a3D model of the microstructure,which was subsequently sliced into a stack of parallel cross-sections.

After tomography,the reconstructed data were segmented and3D models were obtained by com-mercially available software(MIMICS;Materialise, Ann Arbor,MI,USA).After obtaining the3D microstructure,FEM was utilized to predict the current density distribution based on the actual geometry and distribution of voids in the solder joint.To conduct?nite-element analysis on the

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sample,a volume mesh of the reconstructed model was generated by commercial meshing software (Hypermesh;Altair Engineering Inc.,Troy,MI)using linear tetrahedral elements.The meshed model was then exported to Abaqus (Dassault Sys-tems Simulia Corp.,Providence,RI)for current density analysis.In the model,all the boundary conditions followed the experimental setup,with electrical current passing through cathode to anode.The electrical conductivity of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce solder was taken to be 8.669106/X m and 8.399106/X m,respectively.24

RESULTS AND DISCUSSION

Microstructural Evolution of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce During Electromigration

Joule heating generated during current stressing causes higher local temperature in solder joints than ambient temperature,which may accelerate the effect of electromigration and alter the domi-nant diffusion species in the solder joint.Kao et al.25reported that test temperature plays a cru-cial role in the degradation mechanisms of electro-migration (e.g.,pancake-type voids versus metallization consumption).The actual tempera-ture at the solder joint was monitored during the electromigration test by a thermocouple attached at the cathode side.To minimize thermal shock on the joints,the test ?xture was stabilized inside the

furnace at elevated temperature (100°C)before testing and the electromigration test was performed inside the furnace.It was found that,for a current density of around 104A/cm 2,the temperature increased 34°C and 35°C for Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce,respectively.This is reason-able because the resistance coef?cient of Sn-3.9Ag-0.7Cu is similar to that of Sn-3.9Ag-0.7Cu-0.5Ce alloy.19Based on the following equation,25where

Z ?Cu and Z ?

Sn are the effective charge numbers of Cu in Sn and Sn in Sn,respectively,k is the Boltzmann constant,and T is the absolute temperature,one can obtain the electromigration ?ux ratio of Cu to Sn as a function of temperature.Thus,current-in-duced Sn self-diffusion can be expected to be the dominant failure mechanism with elevated tem-perature.It was found that this statement is only true for Sn-3.9Ag-0.7Cu based on the test condition used in this study,which will be further discussed in the following sections.

J Cu J Sn ?1:0?10à4Z ?

Cu

Z ?Sn exp 37:9ekJ =mol TkT :

To characterize the electromigration-induced

microstructural evolution of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce solder joints,backscatter SEM was utilized due to its atomic number (Z)contrast.Figures 2and 3show the microstructures of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce

at

Fig.1.Procedure for making silicon V-groove using photolithography and KOH etching technique.

Electromigration Damage Characterization in Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce Solder Joints by Three-Dimensional X-ray Tomography and Scanning Electron Microscopy

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different electromigration times.The observed region was the Cu/solder interface because deple-tion and extrusion are most likely to occur there,and current crowding occurs in this region,as indicated by our FEM results (presented in the next section).For the as-re?owed joints,the scalloped Cu 6Sn 5layer formed at the Sn-3.9Ag-0.7Cu-0.5Ce/Cu interface was thinner than for the Sn-3.9Ag-0.7Cu solder joints,as reported previously.4–8After 96h of current stressing,a noticeable amount of Cu was consumed and the intermetallic layer was becoming thinner at the cathode side,indicating Cu dissolution into the solder matrix as a result of current-induced Cu diffusion from the Cu wire and intermetallic layer at the cathode side.Cu then dissolved into the solder matrix,and excessive intermetallic compounds formed at the anode side due to Cu diffusion from the cathode to anode.11,14It was also noted that pancake-type voids started to form between the Cu 6Sn 5layer and solder matrix at the cathode side in the Sn-3.9Ag-0.7Cu joint after 96h of stressing,as a result of current-accelerated Sn self-diffusion.Such a phenomenon is a clear indication that both Cu consumption and pancake-type void formation in Sn-3.9Ag-0.7Cu joints are the dominant failure mechanism in Sn-3.9Ag-0.7Cu.This is consistent with observations in the literature that the Sn and Cu electromigration ?uxes turn out to be comparable when the solder temperature is above 80°C.14As a result of Sn self-diffusion,com-pressive stresses built up inside the solder joint.The cross-sectioned surface of Sn-3.9Ag-0.7Cu solder remained rather ?at due to the mechanically harder matrix of Sn-3.9Ag-0.7Cu solder,9as indicated in Fig.4.The compressive stress was relaxed by squeezing out hillocks of intermetallic at the anode side.10

For comparison with Sn-3.9Ag-0.7Cu solder joints,the microstructural evolution of Sn-3.9Ag-0.7Cu-0.5Ce solder joints is shown in Fig.4.

Similar

Fig.2.Backscattered electron microscopy images of cross-section of Sn-3.9Ag-0.7Cu/Cu solder joint after current stressing for:(a)0h,(b)48h,and (c)96

h.

Fig.3.Backscattered electron microscopy images of cross-section of Cu/Sn-3.9Ag-0.7Cu-0.5Ce/Cu butt joints after current stressing at current of 10A and 100°C for 0h and 200h.

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to Sn-3.9Ag-0.7Cu,both void formation and metal-lization consumption were observed at the cathode side.The pancake-type void formed at the cathode side is signi?cantly smaller than that of Sn-3.9Ag-0.7Cu,and a substantial amount of Cu 6Sn 5inter-metallic compound accumulated near the anode and central regions.The accelerated Cu atom diffusion in Ce-containing Sn-3.9Ag-0.7Cu solder can be explained by the ?ner microstructure 4and the fas-ter diffusion path provided by grain boundaries of Sn.As indicated in Fig.5,Cu 6Sn 5intermetallic compound accumulated along the grain boundary and numerous newly formed Cu 6Sn 5intermetallic dispersed in the middle of the solder alloy.Mean-while,Sn grain sliding occurs at grain boundaries to minimize the total resistance along the current direction due to the anisotropic resistance proper-ties of Sn.26,27The Cu 6Sn 5intermetallic layer at the anode side also showed an inhomogeneous growth rate with respect to the Sn grain boundary.In Fig.5,one can observe that Cu 6Sn 5intermetallic at region B (right side of grain boundary)grew faster than at region A.These phenomena clearly indicate that Sn grain boundaries can act as an effective diffusion barrier for migration of Cu atoms driven

by the current,as indicated in the image.28In Sn-3.9Ag-0.7Cu-0.5Ce solder,one may expect more high-angle grain boundaries present inside the ?ner solder matrix than in Sn-3.9Ag-0.7Cu,due to the existence of heterogeneous nucleation centers (CeSn 3intermetallic particles).4,7Therefore,Cu diffusion might be blocked by the grain boundaries,as it is dif?cult for Cu atoms to diffuse through grain boundaries (as shown in Fig.5),and an excessive amount of Cu 6Sn 5intermetallic compound formed in the central region of the solder joint.Such accelerated Cu atom diffusion and substantial Cu 6Sn 5intermetallic compound formation inside the solder joint may retard the Sn atom diffusion,since the critical product of diffusivity and effective charge number for Cu is four to ?ve times larger than that of Sn.29It is interesting to note that,un-like for Sn-3.9Ag-0.7Cu,Sn extrusion and surface bulging are observed in the central solder region,where CeSn 3particles are densely distributed.Sn extrusion always formed at the top side relative to nearby CeSn 3particles.Considering that the cur-rent-induced Sn diffusion direction is from top to bottom,it is hypothesized that the formation of Sn extrusion is due to the blocking effect of

immobile

Fig.4.Backscattered electron microscopy images of central region of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce joints after current stressing for 200h (arrow indicates current direction).

Electromigration Damage Characterization in Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce Solder Joints by Three-Dimensional X-ray Tomography and Scanning Electron Microscopy

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CeSn 3precipitates,30as well as solder matrix soft-ening by CeSn 3particles.31Therefore,one may expect that current-induced Sn atom migration will be impeded by CeSn 3particles,and the compressive stress localization at the anode side,which may initiate cracks,10can be de?ected.However,the blocking effect of CeSn 3particles does not contrib-ute to the lesser void formation at the cathode side of the Ce-containing solder in this case due to the existence of a free surface,as the back stress is released by forming Sn extrusions at the solder surface.Thus,the suppressed pancake-type void formation in the Ce-containing solder is mainly be-cause of the quick formation of Cu 6Sn 5intermetallic compound.Although consumption of Cu metalliza-tion could cause catastrophic failure in thin-?lm Cu under bump metallization (UBM),11rapid formation of Cu 6Sn 5intermetallic compound is favorable for electromigration resistance in thick Cu UBM.32X-ray Tomography and Finite-Element Modeling Results

It is well known that ?ux vaporized during re?ow can produce pores near the solder/Cu interface.17Void formation inside the solder joint is a compli-cated process with several different sources,and it is extremely dif?cult to control during the re?ow process.It has been reported that the presence of voids can increase solder electrical resistivity.33Therefore,it is important to determine the effect of void characteristics on the electromigration behav-ior of solder joints.Instead of characterizing the void size and distribution using metallographic techniques,x-ray microtomography was utilized to understand the effect of pore size,distribution,and location on the current density distribution as a simple and nondestructive technique.

Figure 6shows a typical ‘‘virtual’’cross-section through the 3D tomography reconstruction of a solder joint.The solder matrix and pores can be separated based on the different gray scale,which is determined by the attenuation of x-rays through the joint.34To accurately reconstruct the size and shape of pores,a novel and semiautomatic segmentation algorithm known as Livewire òwas used to create 2D and 3D masks for all the pores (Mimics;Mate-rialise,Ann Arbor,MI).35A typical 3D recon-structed model of Sn-3.9Ag-0.7Cu is shown in Fig.6.Most of the pores are distributed close to the inter-faces between the Cu and solder.Some fairly large pores appear to accumulate at both surfaces.Note that the pore size and distribution are extremely inhomogeneous,therefore the results from two-dimensional images always misrepresent pore https://www.360docs.net/doc/226757954.html,ing the Mimics software,the pore size and the volume of porosity can be quantita-tively measured.

After obtaining the 3D reconstructed microstruc-ture,FEM was utilized to predict the current dis-tribution based on the actual geometry and distribution of voids in the solder joint.Figure 7shows the simulation results for the current density distribution in a joint with a total applied current of 10A at the cathode side,compared with a joint with the same geometry but excluding all preexisting voids.It is well known that electromigration-in-duced failure in conventional ?ip-chip structures is dominated by the current-crowding effect in the solder region induced by the geometry of the solder joint,where the maximum current density at the contact window can be several orders higher than in the solder matrix.36,37This current-crowding effect increases the complexity of comparison between solders and accelerates joint failure.In contrast,the V-groove testing methodology provides a more uni-form current density distribution without this crowding effect.38It can be observed in Fig.7that the current distribution in the solder region is very uniform,and no current-crowding area can be ob-served in the solder joint without preexisting voids.However,in the solder joint with preexisting voids,local geometry distortion by re?ow porosity leads to current crowding at the interface between solder and porosity.It is important to note that the current density in the crowding region is only $4times larger than that in the solder matrix,which is sig-ni?cantly lower than the current-crowding effect in a ?ip-chip structure.The current-crowding region is also relatively small and highly localized at

the

Fig.5.Backscattered electron microscopy images of anode side of Cu/Sn-3.9Ag-0.7Cu-0.5Ce/Cu butt joints after current stressing for 0h,48h,and 96h.

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solder/Cu interface,as the current distribution in the remaining solder region is uniform.Therefore,the crowding effect induced by the preexisting voids does not appear to play as crucial a role in the V-groove methodology as it does in ?ip-chip struc-tures,although failure in the current-crowding region can be accelerated due to larger electromi-gration driving forces.Moreover,the 3D structure-based FEM results were also used to determine the current to be applied in a Sn-3.9Ag-0.7Cu-0.5Ce joint to obtain the same current density as in a Sn-3.9Ag-0.7Cu joint.

Electromigration Behavior of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce Solders

The drift velocity of a Sn atom and the product of the diffusivity and effective charge are normally measured for quantitative comparison of the elec-tromigration resistance of different solder alloys.One way to quantitatively measure the drift velocity in butt solder joints is to measure the marker dis-placement on the solder surface.Markers can be polishing compound introduced during polishing or visible intermetallic compound,such as CeSn 3.

Only

Fig.6.Image of butt solder/Cu joint showing direction of virtual cross-sectioning.Two-dimensional virtual cross-section data are also shown,along with segmented gray scale image and 3D reconstructed structure of solder joint;the 3D structure of the solder joint is then meshed for

FEM.

Fig.7.Finite-element modeling results of current density inside Sn-3.9Ag-0.7Cu solder:(a)with porosity,and (b)without porosity.

Electromigration Damage Characterization in Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce Solder Joints by Three-Dimensional X-ray Tomography and Scanning Electron Microscopy

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markers that are 20l m away from the cathode side are selected for measurement,to eliminate the effect of a back stress gradient.39The average dif-fusivity can be calculated by assuming that current-induced Sn migration is uniform across the solder cross-section:

J ?

D X át

;where D is the displacement measured from marker movement,X is the average atomic volume of Sn-rich solder alloy,which can be approximately calculated from the lattice parameter of Sn (0.0027nm 3),40and t is the duration of the electromigration test.Thus,the electromigration-induced Sn atomic ?ux can be obtained as J EM =6.59105atoms/cm 2s.As pro-posed by Huntington and Grone,the driving force for electromigration can be calculated by Ref.41as

F EM ?Z ?eE ;

where Z *is the effective charge number of the migrating ion in electromigration (Sn in this case),e is the charge of an electron,and E is the electric ?eld.Assuming that the back stress is completely released at the free surface,the atomic ?ux in units of atoms/cm 2s can then be taken as

J EM ?C

D kT

Z ?

eE ;where C is the concentration of atoms per unit vol-ume and D /kT is the atomic mobility of a Sn atom.On the basis of the above equations,we can obtain the product of diffusivity and effective charge number for Sn-3.9Ag-0.7Cu as 0.774910à10cm 2/s.The result obtained from a Blech-strip test struc-ture is slightly larger than was obtained in this study,because of the smaller current density and higher silver content in this study.42The DZ *value

of Sn-3.9Ag-0.7Cu is one order smaller than that of Sn-Pb solder,which is due to the smaller modulus and the lack of diffusion barriers in Sn-Pb solder.21It should be noted that the DZ *value of Ce in Sn-3.9Ag-0.7Cu solder could not be measured from marker displacement on the solder surface as the pancake-type void growth in Sn-3.9Ag-0.7Cu-0.5Ce was extremely limited.

The 3D void growth of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce solder joints was also studied using the high-resolution x-ray tomography tech-nique.Three-dimensional rendering was performed at 0h,100h,200h,300h,and 550h.The voids at the cathode and anode sides were reconstructed separately for side-by-side comparison of the sam-ples,as shown in Figs.8and 9.It is clear that the Sn-3.9Ag-0.7Cu solder showed massive void growth and movement after only 100h of stressing,which correlates very well with the surface observations.After 300h of stressing,the cathode side of the Sn-3.9Ag-0.7Cu solder joint is saturated with voids.By examining the size and distribution of individual voids during electromigration,it was found that the growth of preexisting voids is less pronounced than that of newly nucleated voids,which may be due to the fact that the current-crowding zone exists at the intermetallic/solder interface.Voids also prefer to nucleate in a void-dense region,where the current density is higher due to the current-crowding effect.In comparison,little void growth or void nucleation took place in the Sn-3.9Ag-0.7Cu-0.5Ce solder joint,even after 550h of current stressing.The volume change of the pancake-type void growth during electromigration can be accurately measured from the 3D rendered structure.The electromigration-induced atomic ?ux calculated based on the 3D structure is J EM =5.89105atoms/cm 2s,which is slightly smaller than that obtained from surface marker movement.This is reasonable because of

the

Fig.8.Sn-3.9Ag-0.7Cu void growth over time,showing 3D rendering (top)and cathode void top view (bottom).

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overestimated cross-sectional area in the previous result.The Sn ?ux in Sn-3.9Ag-0.7Cu-0.5Ce,how-ever,could not be measured due to the smaller void growth in the Ce-containing solder.As formation of Cu 6Sn 5is the dominant phenomenon in Ce-con-taining solder,it would be interesting if one could resolve Cu 6Sn 5phases from x-ray tomography images,which is possible due to the difference of x-ray attenuation coef?cient between Cu 6Sn 5and Sn.Unfortunately,the x-ray ?ux of a laboratory-scale x-ray tomography tool is too low to visualize the small contrast difference between Cu 6Sn 5and Sn.This problem could be solved by utilizing syn-chrotron tomography.17

CONCLUSIONS

The electromigration behavior of Sn-3.9Ag-0.7Cu-0.5Ce solder joints was studied by the silicon V-groove methodology and compared with that of conventional Sn-3.9Ag-0.7Cu solder.Based on the experimental results,the following conclusions can be drawn:

1.Preexisting voids can alter the current distribu-tion inside the solder by introducing current-crowding regions,regardless of the solder com-position.

2.Microstructure characterization indicates that a potential degradation mechanism of Sn-

3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce in this study is due to both pancake-type void formation and metallization consumption.

3.As a result of a ?ner solder microstructure due to Ce addition,Cu atomic diffusion is accelerated in Sn-3.9Ag-0.7Cu-0.5Ce solder,and rapid Cu 6Sn 5formation retards pancake-type void formation at the cathode side.

4.CeSn 3intermetallic particles can block the Sn diffusion,which may also bene?t the electromi-gration performance of Ce-containing solder.

5.The product of effective charge number and diffusivity was calculated based on surface marker movement,and it was found that the electromigration resistances of Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.5Ce are signi?cantly high-er than that of Sn-Pb solder alloy.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge ?nancial support for this research from the Semiconductor Research Corporation (SRC)(with Drs.M.Renavi-kar,A.Aleksov,K.Mirpuri,P.Brofman,K.Zeng,Z.Zhang,and M.Varughese as industrial liaisons).The authors also thank David Wright of the Center for Solid State Science,Arizona State University for his assistance with evacuating the quartz tubes.We also thank Dr.Mario Pacheco (Intel)for providing access to the microtomography tool and E.Padilla (Intel)for his assistance with scanning the samples.

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