recrystallization and grain growth of cold-drawn gold bonding...

Recrystallization and Grain Growth of Cold-DrawnGold Bonding Wire

J.-H. CHO, J.-S. CHO, J.-T. MOON, J. LEE, Y.H. CHO, Y.W. KIM, A.D. ROLLETT, and K.H. OH

Recrystallization and grain growth of gold bonding wire have been investigated with electron back-scatter diffraction (EBSD). The bonding wires were wire-drawn to an equivalent strain greater than11.4 with final diameter between 25 and 30 mm. Annealing treatments were carried out in a salt bathat 300 °C, and 400 °C for 1, 10, 60 minutes, and 1 day. The textures of the drawn gold wirescontain major ^111&, minor ^100&, and small fractions of complex fiber components. The ^100& ori-ented regions are located in the center and surface of the wire, and the complex fiber components arelocated near the surface. The ^111& oriented regions occur throughout the wire. Maps of the localTaylor factor can be used to distinguish the ^111& and 100& regions. The ^111&oriented grains havelarge Taylor factors and might be expected to have higher stored energy as a result of plastic defor-mation compared to the ^100&regions. Both 111& and 100& grains grow during annealing. In par-ticular, 100& grains in the surface and the center part grow into the ^111& regions at 300 °C and400 °C. Large misorientations (angles .40 deg) are present between the ^111& and 100& regions,which means that the boundaries between them are likely to have high mobility. Grain average mis-orientation (GAM) is greater in the ^111&than in the ^100&regions. It appears that the stored energy,as indicated by geometrically necessary dislocation content in the subgrain structure, is larger in the^111& than in the ^100&regions.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY 2003—1113

I. INTRODUCTION

FINE wires of pure Au, Cu, or Al, are used for inter-connection in semiconductor packaging. As packaging tech-nology continues to advance, improved properties in bond-ing wire are needed. In particular, the ball shape, breakingload, elongation and the homogeneity of microtexture andmicrostructure are important. These characteristics are relatedto the purity of the original materials, the drawing processand the annealing process. The homogeneity of the micro-texture and the microstructure can affect the swing or loop-ing of bonded wires and theses are major factors for failureof packaging. Most bonding wires also undergo a finalannealing before the packaging process. During final an-nealing, breaking load decreases and elongation increases.Therefore it is necessary to optimize annealing processesin order to obtain optimum bonding wire.

High purity gold (99.9999%wt Au) is too soft and unsta-ble for obtaining good properties for bonding when it is drawnand annealed. Generally, the annealing and recrystallizationtemperature for pure gold is in the range of 150-200 °C andit has been reported that highly deformed pure gold willshow recovery and recrystallization at room temperature [1].Therefore bonding wire commonly has various dopants at

parts per million (ppm) level in order to control annealingresponse and to obtain better thermal and mechanical prop-erties. Impurities, even at these low levels, are important forcontrolling the final mechanical properties and microstruc-tures of gold wire by raising the recrystallization tempera-ture and preventing grain growth.[2,3] Such small concen-trations of impurities are known to strongly affect themigration rate of grain boundaries in many materials.[4,5] Re-crystallization, recovery and grain growth all occur duringannealing, and they affect the microstructures, microtexturesand mechanical properties of gold wires.[6-9]

The evolution of textures during wire drawing has beeninvestigated by many researchers. The wire drawing texturesof fcc metals typically have 111& and 100& fiber texturecomponents. The textures of aluminum, copper, and brasswires have been investigated for cyclic symmetry.[10] Thetexture of drawn silver wires have radial symmetry, whichis related to twinning.[11] Recrystallized grains are formed nearthe surface due to frictional heating in the die. Heizmannet al. have reported that the strength of the cyclic textureincreases as the die angle increases.[12]

Taylor’s theoretical analysis shows that all grains rotateso that either a ^111& or ^100& crystal direction is alignedwith the extension axis, depending on which direction theextension axis is closest to at zero strain. The crystal axesof grains near ^101&will tend to rotate toward the ^111& orthe 100& axis.[13] English et al. has shown that the ratio ofthe 111&and 100&fiber texture components varies dependingon the stacking fault energy.[14] Low stacking fault energymetals, i.e., silver, have stronger ^100&components than ^111&.High and intermediate stacking fault energy metals, however,such as aluminum or copper, exhibit a stronger ^111& than^100&. Work on wire drawing textures in fcc metals, i.e.,silver, gold, copper, aluminum, and brass, has also shownthat the final texture components are ^111& and 100&.[15,16]

J.-H. CHO, Researcher, Y.W. KIM, Research Associate Professor, andK.H. OH, Professor, are with the School of Materials Science and Engi-neering, Seoul National University, Seoul 151-744, Korea. Contact e-mail:[email protected]. J.-S. CHO, Senior Researcher, J.-T. MOON, Princi-pal Researcher, and J. LEE, R&D Head, are with MKE Electronics,Kyunggi-do 449-810, Korea. Y.W. CHO, Principal Researcher, is withthe Nano-materials Research Center, Materials Science and TechnologyDivision, KIST, Seoul 130-650, Korea. A.D. ROLLETT, Professor, is withthe Materials Science and Engineering Department, Carnegie Mellon Uni-versity, Pittsburgh, PA 15213-3890.

Manuscript submitted January 31, 2002.

Montesin and Heizmann have reported an X-ray diffractionprocedure for fine wires that included a diffraction volumecorrection.[17] Rajan and Petkie measured wire textures withelectron backscatter diffraction (EBSD) and displayed theresults with Rodrigues–Frank maps in addition to inversepole figures and standard pole figures.[18] The presence ofan inhomogeneous distribution of twins and twinning reac-tions in copper wire was characterized, and it was suggestedthat the variations in the mesotexture could contribute tomechanical anisotropy.

Recrystallization textures in drawn wires are also ^111&and 100& fibers, which are similar to the deformation tex-tures. The ratio of the ^111&to the ^100&varies with the an-nealing time and temperature. This suggests that the growthrates of 111&and 100&are different and that they are grow-ing and competing against each other. It has been reportedthat copper wire develops ^100&or 112& texture componentsin low-temperature annealing, but at high temperature, themajor texture components are a mixture of ^111&and 112&fiber textures.[19]

During annealing, grain growth occurs as a result of grain-boundary migration. Grain boundaries adopt curvatures basedon local equilibrium at triple junctions, which are describedby Herring’s equations.[20,21,22]These curvatures lead to grain-boundary migration. The grain-boundary migration ratesdepend on the grain-boundary energy and mobility, whichin turn are a sensitive function of the grain-boundary struc-ture. Second-phase particles result in resistance to migra-tion with eventual pinning of the grain structure. Measure-ment of the misorientation distribution function (MDF)provides some information on grain-boundary types, grain-boundary energy, and, thus, annealing characteristics. Aknowledge of the thermodynamic and kinetic properties ofgrain boundaries according to crystallographic type will beuseful for materials optimizations in the context of annealing.

Up to now, the research on gold bonding wires has fo-cused on their mechanical properties and recrystallizationbehavior in the heat-affected zone (HAZ) during bonding.For good bonding wire, it is necessary to understand thealloy design, optimized drawing process, and annealingprocess together. In this research, the microtextures andmicrostructures of gold bonding wires during drawing andannealing are investigated with high-resolution–electron back-scatter diffraction (HR-EBSD). In order to understand thegrain-boundary characteristics, the MDF and the frequencyof coincident site lattice (CSL) boundaries are also calcu-lated from EBSD data.

II. EXPERIMENTAL

A. Materials and Sample Preparation

The purity of gold wire used in this research is more than99.99 pct and it has some (intentional) dopants, such as Caand Be that total less than 50 ppm by weight. Even at PartsPer Million levels, these dopants strongly affect (decrease)grain-boundary mobility and, hence, increase the recrystal-lization temperature. A typical recrystallization temperatureof this gold is 320 °C, based on isothermal annealing test afterrod rolling a cast gold bar to an area reduction of 85 pct.

The original cast gold bar was drawn through a seriesof diamond dies to a von Mises equivalent strain of 11.4.

Each die has less than 10 pct reduction in area in order toachieve homogeneous deformation. The EBSD measure-ments were performed on gold wires with diameters of25 and 30 mm. For statistical reliability of the EBSD data,at least three wires were measured for the cold drawn andeach of the annealed states. The number of grains measuredand analyzed was approximately 5000 for the cold drawnwires and 500 for the annealed wires. Given the smalldiameter of the bonding wires, EBSD was more convenientand reliable than X-ray diffraction, as noted by Montesinand Heizmann.[17] Isothermal annealing for wires was car-ried out for 1 minute, 10 minutes, 60 minutes, and 24 hoursat 300 °C and 400 °C.

B. EBSD Measurement

The bonding wire was mounted in epoxy and then sectionedand polished. The polished specimens were cleaned withion milling. HR-EBSD (JEOL* 6500F with INCA/OXFORD

*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

EBSD system) was used for measurement and the data analy-sis was made by Reprocessing of EBSD Data in SNU(REDS).[23] The operating voltage was 20 kV and the probecurrent was 4 nA. A rectangular grid was used and the pixelspacing was 0.239 mm. The EBSD maps were measured fortransverse and longitudinal sections. The orientations mapswere used for texture representations, and the misorienta-tion distribution function (MDF) was used to characterizethe grain-boundary characteristics. Taylor factor maps arealso shown because they distinguish the ^111& and 100&fiber texture components. The Taylor factor is calculatedbased on the standard slip systems for fcc metals and ve-locity gradient appropriate to uniaxial extension.[24,25] The{111}^110& slip systems are assumed and the velocity gra-dient, «ij, is as follows:

[1]

“Image quality” or “pattern quality” is the term given todescribe the quality of an electron backscatter diffractionpattern (EBSP). Many factors control the quality of theEBSP, which can be assigned a numerical value. This pat-tern quality value is derived from the Hough transform ofeach diffraction pattern.[26]

In order to calculate the grain size, the number of data pointsor pixels in the grain is calculated. Using the known pixel stepsize and numbers, the grain area is calculated. The most con-venient measure of grain size from grain area is the equiva-lent circle diameter (ECD) or equivalent grain size, which isthe diameter of a circle having the same area.[27] Other mea-sures of grain size are available but not used in this work.

C. Statistical Analysis of Microstructures

The fluctuations or variations of material microstructurescan be described by so-called second-order characteristicssuch as the variance of the volume of a microstructural com-ponent or phase. When the mean or the first moment, E, ofthe volume, V, of a component Ξ restricted to a spatial win-

«ij 5 £20.5 0 00 20.5 00 0 1

≥

1114—VOLUME 34A, MAY 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A


dow W is given by EV(Ξ > W), then the variance, var, ofthe volume of Ξ> W is

[2]

where EV2 (Ξ > W) is the second moment of volume of theΞ phase. If f(x) and f(y) are the probabilities of random vari-ables, x and y, the covariance of the pair (f(x), f(y)) is given:

[3]

The normalized covariance function is called the correlationfunction, rf(x)f(y), and takes values between 21 and 11.

[4]

where sf (x) and sf (y) are the standard deviations of f(x) andf(y), respectively. A value of 21.0 or 11.0 indicates per-fect linear correlation between f(x) and f(y), whereas a valueof zero indicates absence of correlation.[28] Here, f(x) andf (y) are the Taylor factor and pattern quality at positions xand y, respectively. This correlation function was used tocheck if the ^100&grains of the cold drawn wire exhibiteda higher image quality than ^111& grains.

D. Average Lattice Orientation, Grain OrientationSpread, and Grain Average Misorientation

The average orientation can be calculated for each grain,and it is useful for characterization of grain substructure.Recently, Barton and Dawson defined an average orienta-tion based on misorientation angles, which leads to a non-linear least-squares problem that can be solved numerically(Appendix).[29,30,31] In addition to average orientation, grainorientation spread (GOS) and grain average misorientation(GAM) can be calculated.[24] Considering Pi as an orienta-tion at a point (xi), the GOS in a grain can be calculated withmisorientation angles, which are given for two arbitrarilychosen orientations.

[5]

where S is the symmetry operator belonging to the appro-priate crystal class concerned[32,33] and the subscripts inrotation P refer only to position. The term 2Cn?m is the com-bination that selects a subset containing two orientationsamong a given set with n? m orientations. The GAM isdetermined in the same way as the GOS, but it includes onlythe first nearest neighbor orientations within a grain in theaverage in contrast to GOS. Therefore, it is a more localmeasure of orientation spread. Both GAM and GOS are sizedependent and they increase as the grain sizes increase.

III. RESULTS

A. Transverse Section of as-Drawn Wire (30 mm)

In order to analyze the fiber texture of the drawn wire,both transverse and longitudinal transverse sections of wireswere investigated with EBSD. The main fiber componentsobserved during drawing are ^111&//ND and 100&//ND. These

an21

i51 a

n

j5 i11

min c acos e trace ((Pi ? P21j ) ? S) 21

2f d

2Cn?m

rf(x)f(y) 5

cov( f(x), f(y))sf (x)sf(y)

cov ( f(x), f(y)) 5 E[( f(x) 2 Ef(x))( f(y) 2 Ef(y))

var V (Ξ > W) 5 EV 2 (Ξ > W) 2 [EV (Ξ > W)]2

two texture components are known as the typical fibers offcc wires. A set of EBSD maps of the as-drawn wire is shownin Figure 1. The color index for the orientation maps is shownin Figure 1f. Each grain in the wire can be partitioned intotwo types by their Taylor factors (Figure 1(b)) or by theircrystallographic orientation, i.e., ^111&or ^100&//ND (Fig-ure 1(a)). The partitioning of orientations is based on the mis-orientation angle, 15 deg, between grains. The average Tay-lor factor of cold drawn wire was calculated using the standard12 slip systems for fcc metals, and its value was found to beabout 2.87. Regions with a Taylor factor lower than 2.87 arepredominantly 100&oriented, whereas regions with a Taylorfactor greater than 2.87 are predominantly ^111&. As expected,the images partitioned by orientation or by Taylor factor arevery similar to each other. Most of the ^100& or low Taylorfactor regions are located in the center and some on the sur-face regions of the wire, while ^111&fibers or high Taylor fac-tor regions are located throughout the wire. The ^100&orientedgrains in the surface regions are not axisymmetric. The regionbetween the center and surface regions contains complex ori-entations, which deviate from ^111& and 100& by more than15 deg. Figure 1(c) shows this deviation by coloring only thosegrains that lie more than 15 deg from either ^100&or ^111&.

Figure 1(d) shows the overall structure of the gold wireafter the drawing process. Most of the grains show orienta-tions parallel to 111&, whereas the center and some parts ofthe surface are parallel to ^100&. The complex regions are lo-cated between the center and the surface. These multilayerstructures of wire are related to shear deformation and orig-inal microstructures. The fcc metals such as gold typicallyexhibit a majority of 111&and a minority of 100&fiber. The^100& fiber in the center is related to the microstructure andtexture of the initial gold bar, which has mainly large ^100&grains. The image quality map of the transverse section ofthe cold drawn wire is shown in Figure 1(e) and suggeststhat 100& regions are associated with high image quality.

The 100& region on the surface in Figure 1(e) shows ahigher pattern quality than other regions. In order to quan-tify this observation, a correlation function was calculatedwith the two variables, orientation (or Taylor factor) andimage quality. This approach is based on the fact that highTaylor factor regions have high stored energy and exhibit alow pattern quality, whereas the low Taylor factor regionshave low stored energy and higher pattern quality.[24] Usingthe orientations, the Taylor factor was calculated, and thenthe image quality of the orientations was combined, as shownin Eq.[4]. The resulting value was 20.22, which is a mildnegative correlation. This suggests that high Taylor factorregions have low pattern quality and low Taylor factorregions have high pattern quality in keeping with the qual-itative observation made previously. The ^100& orientedmaterial on the surface is likely to be a consequence of sheardeformation or dynamic recrystallization.[11] The 100&grainsin the center part are related to original cast bar and theyare also highly deformed regions as are the ^111&grains. Dis-crete inverse pole figure maps for the pixels partitioned byeither their orientation or Taylor factor in Figure 1 are shownin Figures 2(a) and (b). The complex regions of cold-drawnwire are shown in Figure 2(c). The drawn wire apparentlyhas major ^111&and minor ^100&texture components. TheTaylor factor can separate the ^111&and 100& regions suc-cessfully, as shown in Figure 2(b).


Fig. 1—Orientation maps for a cold-drawn gold bonding wire (30mm). TheEBSD data are separated into ^111&1 ^100&and high& 1 ^low& Taylor fac-tor regions. Their differences are complex regions mainly: (a) orientation imagemaps for 111& 1 ^100& regions, (b) orientation image maps for ^high& 1^low& Taylor factor regions, (c) complex regions, (d) schematic plot for struc-ture of gold wire, (e) image quality, and (f ) orientation color key.

Fig. 6—Orientation maps of transverse sections of 25-mm gold wires. Mis-orientation angle, 5 deg is used for identifying the grains: (a) 1min,(b) 10 min, (c) 60 min and (d) 1 day at 300 °C; and (e) 1 min, (f ) 10 min,(g) 60 min, and (h) 1 day at 400 °C.

Fig. 8—Orientation maps of 25-mm gold wires after isothermal annealing. Misorientation angle, 5 deg is used for identifying the grains. Circles show islandswith S3 boundaries: (a) 1 min, (b) 10 min, (c) 60 min and (d) 1 day at 300 °C; and (e) 1 min, (f ) 10 min, (g) 60 min and (h) 1 day at 400 °C.


Fig. 2—Inverse pole figure (ND) of drawn wire in Fig. 1: (a) crystallographic 111& and 100& regions, (b) Taylor factor high& and low& regions, and(c) complex regions.

(a)

(b) (c)

Fig. 3—Misorientation angle/axis distribution of the drawn wire. These data come from crystallographic ^111& 1 ^100& regions in Fig. 1(a): (a) ^111& re-gions, (b) 100& regions, and (c) all part.

(a) (b) (c)

B. Misorientations in the as-Drawn Wire

Misorientation distributions in Rodrigues–Frank spacebased on the crystallographic partitioning of the as-drawnwire are given in Figure 3. These maps show that the mis-orientation distributions of 111&fibers, as expected, are con-centrated on ^111&misorientation axes and their Rodriguesvector components (R1, R2, R3) take values from (0, 0, 0)

to . The latter Rodrigues vector is equivalent to a

60 deg ^111&misorientation angle/axis pair. The length of

a1

3,

1

3,

1

3b

the Rodrigues vector is equal to the tangent of half the mis-orientation angle; therefore, ^111&regions have a large rangeof misorientation angles, i.e., from 0 to 60 deg, around the^111& misorientation axis.[32,33,34] By contrast, the 100&regions are concentrated on ^100& misorientation axesand their Rodrigues vectors are located between (0, 0, 0) and( ). The latter Rodrigues vector is equivalent toa 45 deg 100&misorientation. Grain boundaries in the ^100&regions have smaller misorientation angles than in the ^111&regions. Some boundaries in the ^111&and complex regionshave 110& misorientation axes (Figure 3(c)).

22 2 1, 0, 0

Figure 4 shows the drawing deformation and misorienta-tion fundamental zone in Rodrigues space. The wires undergoan axisymmetric uniaxial deformation during drawing suchthat they have a C∞ symmetry axis aligned with the wire axis(Figure 4(a)). As the grains are elongated during deformation,most of the boundary area has a normal perpendicular tothe axis. Thus, most boundaries are either ^111&or 100& tiltboundaries. Figure 4(b) shows a projection of the fundamentalzone in Rodrigues space and the location of most of the low-sigma coincident site lattice (CSL) boundary types. In thisfigure, the 111&axis falls on top of the 110&axis along thehypotenuse of the triangle. The ^100& axis projects alongthe lower, horizontal edge. The CSLs along the ^111& axis(Figure 4(b)), with S3, 7, 13b, 21a, and 31a, are found fre-quently in the 111& fiber regions. The most frequent CSLsin the 100& component are S5, 13a, 17a, 25a, and 29a onthe 100&axis. Almost all of the boundaries between the ^111&and 100& components have misorientation angles above40 deg so that the CSLs are mainly S3, 9, 11, 17b, 25b, 31b,and 33c.

Before showing the misorientation angle distribution of thedrawn wire, the well-known Mackenzie plot for randomly dis-tributed cubic crystals is shown in Figure 5(a).[35,36] The peakin frequency (fraction number) occurs at a misorientation anglenear 45 deg and the maximum angle is 62.8 deg; this maxi-mum misorientation for two cubic crystals is found for com-binations such as the rotated cube, {100}^011&, and Goss,{110}^100& orientations. The misorientation angle distribu-tions of 800 combinations of randomly distributed single crys-tals are shown with symbols on the same plot. The twodistributions are very similar. The misorientation angle dis-tribution for cold-drawn gold wire is shown in Figure 5(b).Three different distributions are plotted separately based oneach of the fiber regions, i.e., ^111&, ^100&, and intermediateorientations. All three exhibit nonrandom distributions. Mostmisorientation angles in the ^100&regions are less than 40 deg,whereas the 111& regions exhibit angles up to 60 deg. Highmisorientations predominate for boundaries between the ^111&and 100&regions. The peak in the misorientation distributionfor boundaries between the ^111&and 100&regions is located


Fig. 5—Misorientation angle distribution of initial wire: (a) random case from Mackenzie–Handscomb and (b) as-drawn gold wire.

(a) (b)

(a) (b)

Fig. 4—Tilted grains in a drawing wire and projected CSL grain boundaries in the Rodrigues space in the ^111& and 100& regions and between them:(a) ^100& type tilted grains and (b) Rodrigues vectors of CSLs.


Fig. 7—Aspect ratio, grain size, and volume fraction of gold wire along cross section during isothermal annealing at 300 °C and 400 °C in Fig. 6: (a) aspectratio, (b) equivalent grain size, and (c) volume fraction.

(a)

(b) (c)

between 45 and 60 deg. Note that this peak increased to 55 degfrom about 45 deg in the random distribution shown in Fig-ure 5(a). The large misorientation angles of boundaries betweenthe 111& and 100& regions mean that these boundaries willtend to have higher energy and, possibly, higher mobility thanthe average boundary in this system.

C. Recrystallization and Grain Growth during Annealing(Transverse and Longitudinal Sections; 25-mmDiameter Wires)

The gold wires were characterized by EBSD after isother-mal annealing at 300 °C and 400 °C for 1 minute, 10 minutes,60 minutes, and 24 hours, and orientation maps are shown inFigures 6 and 8. As in the as-drawn wire in Figure 1, theorientation maps during annealing show that most of the ma-terial is aligned with either ^111&or ^100&. During annealingat 300 °C (Figures 6(a) through (d)), grain growth occurs inboth the 111& and 100& regions. During this grain growth,some 111&grains consume other ^111&grains and some ^100&grains of the center and surface regions grow into ^111&regions. The wire surface is not uniformly covered by ^100&grains and so growth of the ^100&fiber in the surface is cor-respondingly nonuniform. After 24 hours, the ^100& regionshave obviously coarsened. Consequently, the ^111& volumefraction decreases and the ^100& volume fraction increasesduring annealing.

Isothermal annealing at 400 °C (Figures 6(e) through (h))causes faster growth of ^111&and 100&grains than at 300 °C,as expected for a thermally activated process. Coarseningoccurs in all regions in the wire during annealing. At bothannealing temperatures, coarsening of the ^100& regions isclear also and the ^100& volume fraction increases withannealing time.

Figure 7 shows the aspect ratio, equivalent grain size, andvolume fraction of 111&and 100&grains in transverse section.The aspect ratio of grain shape in the transverse section (Fig-ure 7(a)), is in the range 1.5 to 2 (grains are elongated alongthe drawing direction) and annealing time and temperatureshave little effect on the aspect ratio. Grain growth occurs inall areas of the wire and is more rapid at 400 °C than at 300 °C,as expected for thermally activated motion of grain bound-aries (Figure 7(b)). The average grain size in the ^111& and^100& regions bracket the average grain size. The equivalentgrain size increases gradually from 0.7 to 5 mm. The 100&region grows at the expense of the ^111&region such that thevolume fraction of 100& increases (Figure 7(c)), whereasthe 111&volume fraction decreases. The volume fraction ofcomplex orientations, i.e., the balance of the material frompartitioning the orientations into ^111&, ^100&, and complexfibers, increases at first and then decreases at longer times.

Figure 8 shows EBSD orientation maps for longitudinalsections of isothermally annealed gold wire after 1 minute,10 minutes, 60 minutes, and 24 hours at 300 °C and 400 °C.

Statistically, a longitudinal section contains many more grainsthan a transverse section. In contrast to the equiaxed shapesobserved in the transverse sections, most grains have elon-gated shapes in the longitudinal sections during annealing.These grain shapes suggest that growth occurs in both thetransverse and longitudinal directions. The as-drawn struc-ture in Figure 8 shows ^100& oriented regions both in thecenter and at the periphery, with ^111& oriented materialoccupying most of the volume of the wire. As annealingtime accumulates, both ^111& and 100& grains grow and^100&grains grow into ^111&grains. After 24 hours anneal-ing (Figures 8(d) and (h)), there are islands of small grainswithin large 100&or 111&grains that are surrounded by S3boundaries and appear to be stable. Annealing twins withimmobile S3 boundaries are often observed in metals withlow to moderate stacking fault energies such as gold. Theisland regions may be stable to further coarsening becausetheir perimeter is a low mobility boundary.

Figure 9 shows the aspect ratio, equivalent grain size, andvolume fraction of 111&and 100&grains in longitudinal sec-tions. The initial aspect ratio of grains is about 4.5 and it de-creases gradually during annealing at 400 °C. Consideringthat grain growth occurs in both the transverse and the lon-gitudinal directions, the decrease in the aspect ratio duringgrain growth shows that coarsening is fastest in the transversedirection. The wires of 300 °C annealing show more inter-esting results.

The aspect ratio for 1 minute at 300 °C decreases slightlyand it reflects the effects of newly recrystallized grains or sub-grain growth from dislocation tangles during recovery or firststage of recrystallization. There is also a slight decrease inthe equivalent grain size. As annealing time increases, the as-pect ratio for 10 and 60 minutes at 300 °C increases againand it shows that most of the ^111& grains merge with other^111&grains along the longitudinal direction, i.e., coalescenceoccurs. After 24 hours, grain growth occurs between grainswith large misorientation angles and the aspect ratio drops toabout 2. Boundaries between grains of the same fiber com-ponent tend to be tilt boundaries when the grain centers areconnected by a radius (Figure 4(a)) and, by contrast, twistboundaries when they are lying along the wire axis. Pure tiltboundaries are known to exhibit higher mobility than twistor mixed boundaries.[4,37] Gold wire during annealing at 300 °Cshows that the mobilities of twist boundaries seem to be higherthan tilt boundaries and they move rapidly at the beginningof recrystallization. Initial grains have an elongated shape withaspect ratio 1.5 along transverse and 5.5 along longitudinal.As annealing time increases, it converges to about 2 in bothdirections. The aspect ratio after 24 hours along the longitu-dinal direction is similar to that of the transverse section atboth 300 °C and 400 °C.

Equivalent grain size increases gradually according withannealing time, and the ^100&grains are slightly larger thanthe 111& grains at 300 °C and 400 °C. As observed in the


Fig. 9—Aspect ratio, grain size, and volume fraction of gold wire along longitudinal section during isothermal annealing at 300 °C and 400 °C in Fig. 8:(a) aspect ratio, (b) equivalent grain size, and (c), volume fraction.

(a)

(b) (c)

transverse sections, longitudinal sections show that ^111&and 100& grains at 400 °C grow faster than at 300 °C and^100&grains seem to grow faster than ^111&. The larger grainsizes in longitudinal sections than transverse comes fromthe elongated grain shapes.

Volume fraction changes measured in longitudinal sec-tions are similar to the transverse sections. The ^100&grainsincrease and ^111&grains decrease as annealing time accu-mulates. At 300 °C, 100&grains grow continuously and the^100&volume fraction is larger than that of ^111&. At 400 °C,individual grains grow to sizes comparable to the wire di-ameter after 24 hours annealing and the ratio of ^111& and^100& approaches 1:1, which means that ^111& grains growagain through grain boundary movement.

IV. DISCUSSION

The fcc metals typically exhibit a mixture of ^111& and^100&fiber components, as expected from Taylor’s originalanalysis of the effects of crystallographic dislocation glideon the reorientation of crystals during plastic deformation.The gold wire studied here exhibits this classical combi-nation of texture components and, to first order, annealingonly changes the relative volume fractions. In displayingthe microstructures, the Taylor factor provides a convenientmeans of partitioning the material because the ^100& ori-ented grains have the minimum Taylor factor, whereas the^111&oriented grains have the maximum Taylor factor undertensile deformation[38] (Figure 1(b)). The Taylor factor is ameasure of the ratio of microscopic shear or glide to macro-scopic strain. Large values mean that more slip must takeplace in order to accommodate the imposed strain and,equivalently, that those grains will bear a higher (macro-scopic) flow stress for the same critical resolved shear stress.This suggests that the ^111& grains would be expected tohave higher stored energies than the ^100& grains. Thisshould provide a driving force for ^100&grains to grow intothe 111& regions.

In the cold-drawn wire, the ^111& grains exist throughoutthe wire and they arise from the drawing deformation. The^100& grains are located in the center and the surface of thewire. The presence of the ^100& component at the center ismost likely inherited from the texture of casting bar, whichprobably had a columnar grain structure. The presence of ^100&at or near the surface is related to friction between the wireand the dies during drawing. Figures 1, 6 and 8 show that^100& grains are distributed over the surface and that theycoarsen independently of the ^100&grains in the center regionduring annealing.

Figure 10 shows the transmission electron microscopy(TEM) and EBSD image for as-drawn gold wire. The grainsare elongated along drawing directions. The TEM imageshows that grains have less than 1 mm along the transversedirection and the elongated grains have subgrain boundaries.Comparing the TEM image to the EBSD image maps, a tol-erance angle of 5 deg for grain identification resulted in morereasonable grain shapes than a choice of 15 deg.

The aspect ratio and equivalent grain size of the as-drawngold wires were investigated as a function of tolerance angleused for grain identification (Figure 11). Both aspect ratioand equivalent grain size increase sharply as the tolerance

angle increases to 5 deg. After 5 deg, the equivalent grainsize increases slowly and continuously, but the aspect ratiodecreases slightly. The variation in equivalent grain sizeshows that there are many subgrains with less than 5 degbetween them. The aspect ratio changes show that most ofthe subgrains with low misorientation angles are alignedalong the longitudinal direction and they appear to be twistboundaries. Most of the tilt boundaries along the transversedirection have a misorientation angle greater than 5 deg.The increase of aspect ratio of the wires under 300 °C an-nealing during 10 and 60 minutes seems to be related tothe motion of twist boundaries leading to coalescence ofsubgrains, as shown in Figure 9(a). After 24 hours, thegrains grow in the transverse direction also. It seems thattwist boundaries with low misorientation angles have highermobility than others. After subgrain growth based on mo-tion of twist boundaries, grain growth occurs by motion oftilt boundaries and high-angle grain boundaries between^111& and 100&.

Figure 12 shows the variations of aspect ratio during an-nealing as a function of tolerance angle. Grain aspect ratiosfor ^111&and 100&grains decrease after 1 minute at 300 °C.The 111&grains exhibit a maximum aspect ratio at 60 min-utes and ^100&grains have a maximum at 10 minutes. Thissuggests that coalescence in the longitudinal direction pro-ceeds at different rates in the two fiber components. Thesetrends are independent of tolerance angles used for grainidentification.

During primary recrystallization, boundaries of nucleatednew grains sweep through a deformed structure and removethe dislocations that were stored during the prior plastic


Fig. 10—Grain shapes for cold-drawn gold wire. The elongated grains areshown in (a) TEM image, (b) EBSD image (tol 515 deg), and (c) EBSDimage (tol 55 deg). Tol: Tolerance angle for grain identification.


deformation. Higher dislocation densities therefore representa higher driving force for recrystallization. It is reasonableto suppose that higher Taylor factor orientations such as ^111&should contain higher stored energy. There is indeed evi-dence that the ^100& grains grow into the 111& regions at300 °C and 400 °C. To set against this view, however, thereis little evidence for new grains growing in a deformed struc-ture with the accompanying contrasts in image quality, forexample. Instead, it appears that a general coarsening occursthroughout the material, i.e., subgrain growth. Figure 13shows the orientation image and pattern quality map alongthe longitudinal direction. High pattern quality regions areassumed to be newly recrystallized or growing grains. Thecircles marked in Figure 13 show that most of them haveelongated shapes and are growing by subgrain coarseningor grain growth.

As noted previously, there is also some competition be-tween the two main texture components, ^111&and 100&. Allthis suggests that more attention should be paid to the prop-erties of the boundaries involved. In order to make a moreaccurate estimate of the stored energy of the grains as a func-tion of orientation, GAMs are shown in Figure 14. The pres-ence of dislocations is associated with variations in orienta-tion within each grain. The GAM is the average misorientation(angle) between all neighboring pairs of points in a grain.

The slight increase in GAM observed during grain growthis the result of the accumulation of low-angle boundarieswithin grains. In general, the frequency of high energyand high mobility boundaries decreases during annealing,whereas the frequency of low energy and low mobilityboundaries such as low-angle boundaries and twin bound-aries increases.

Fig. 12—Aspect ratio variations during annealing at 300 °C according to grain identification angle or tolerance angle: (a) ^100&grains and (b) ^111&grains.

(a) (b)

Fig. 11—Aspect ratio and equivalent grain size variations of as-drawn gold wires according to grain identification angle or tolerance angle: (a) aspect ratioand (b) equivalent grain size.

(a) (b)


Fig. 13—Orientation image and pattern quality along longitudinal sections. High pattern quality regions are marked with circles at (d), (e), and (f ):(a) ^111& regions, (b) ^100& regions, (c) pattern quality for as-drawn wire, (d) ^111& regions, (e) ^100& regions, (f) pattern quality for 1 min at 300 °C,(g) ^111& regions, (h) 100& regions, and (i) pattern quality for 1 min at 400 °C.

Fig. 14 —The GAM of the gold wire along longitudinal section during annealing: (a) 300 °C and (b) 400 °C.

(a) (b)

The GAM has a slightly lower value in ^100& than otherregions during annealing in 300 °C and 400 °C up to60 minutes, which means that a lower density of geomet-rically necessary dislocations exists in ^100& and it has alower stored energy. The ^100& can grow over other re-gions during recrystallization. In addition, the competi-tion in coarsening depends on the characteristics of the var-ious grain-boundary types such as energy and mobility.These important characteristics have yet to be measured.It should also be noted that the fact that grains in eachmajor component are aggregated together means that theinterfacial area between the two components is minimized.This in turn limits the rate at which one component cangrowth into the other.

V. CONCLUSIONS

In this study, recrystallization and grain growth of goldbonding wire have been investigated during isothermalannealing at 300 °C and 400 °C.

1. The cold-drawn bonding wire has a major ^111&fiber com-ponent and a minor ^100&component. The 100&orientedgrains are located in the center and the surface regions.

2. There is a weak correlation between image quality andorientation in the cold-drawn wire, which suggests a lowerstored dislocation density in the ^100& component thanin the ^111&.

3. During annealing, both the ^100& and 111& orientedregions coarsen. The ^100&grains grow into 111&grainsat 300 °C and 400 °C and increase the ^100&volume frac-tion. At 400 °C for 24 hours, ^111& fraction approaches^100&.

4. The misorientation angle distributions show that grainboundaries within the ^111& fiber have larger misorien-tation angles than in the ^100& component.

5. The GAM for individual grains shows that ^100& grainshave lower orientation spreads than ^111&-oriented grainsduring annealing. This suggests that strain energy basedon geometrically necessary dislocation content in ^100&is smaller than ^111&.

6. Other than low-angle boundaries, CSL boundaries inthe 111& regions are predominantly of ^111&axis type.Similarly the CSL boundaries in the ^100& regions areof the 100&misorientation axis type. The CSLs between^111&and 100&have larger misorientation angles, greaterthan 40 deg.

7. Most of the low-angle boundaries under 5 deg in as-drawn wires consist of twist boundaries, and they arethe source of subgrain growth from dislocation tanglesat the beginning of recovery or recrystallization duringannealing.

ACKNOWLEDGMENTS

This research is supported by the BK21 project of the Min-istry of Education & Human Resources Development (Seoul,Korea) and MKE Electronics. Partial support of theMesoscale Interface Mapping Project, Carnegie Mellon Uni-versity, under NSF Grant No. 0079996 is acknowledged.

APPENDIX

Average lattice orientation by nonlinear approach

Lattice orientation data are determined for a large num-ber of pixels within each grain in an EBSD map, and theaverage of these pixel orientations is useful in data analy-sis. In order to define the average orientation based on mis-orientation angles, the nonlinear least-squares approach canbe used.[29,30,31]The average orientation (Ca) correspondingto a given set of lattice orientations, C(i), with i in the rangefrom l to n, where nis the number of orientations, pro-duces a misorientation Cm between Ca and C(i):

[A1]

[A2]

where u(i)m is the misorientation angle between Ca and C(i),

and Sj describes one of the psymmetry operations belong-ing to the appropriate crystal class concerned.[28,29] Becausethe misorientation angle is the metric by which distance ismeasured in orientation space, an average of a set of orien-tations can be found by minimizing the following functionof the misorientation angle, u(i):

[A3]

where f(i) is the weight of each of the ith misorientation andthe sum of the weights is unity.

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