An Investigation on Nanoscale Tensile Mechanical Properties for Pd-coated Copper Wire

Transcription

1 An Investgaton on Nanoscale Tensle Mechancal Propertes for Pd-coated Copper Wre H.C. Hsu 1, J.H. Chen 1, L.M. Chu 1, S.P. Ju 2, Y.T. Feng 2 and S.L. Fu 3 1 Department of Mechanc and Automaton ngneerng, I-Shou Unversty 2 Department of Mechanc ngneerng, Natonal Sun Yat-Sen Unversty 3 Department of lectronc ngneerng, I-Shou Unversty 1,3 No.1, Sec.1, Syuecheng d., Dashu Dstrct, Kaohsung, Tawan 84001,.O.C. 2 No. 70, Lenha d., Kaohsung, Tawan 80424,.O.C. Abstract The mechansm of ntermetallc compound (IMC) between Copper (Cu) free ar ball (FAB) and Alumnum (Al) bond pad has been carefully nvestgated n ths paper. The test vehcle s Pd-coated Cu wrebonds on Al pad n plastc ball grd array (PBGA) package. Palladum (Pd), the ant-oxde materal coated on Cu wre wll be blended n the Cu FAB when the ball s formed by an electrcal flame-off (FO). Prelmnary results demonstrated that Cu- Al IMC cracks orgnally begns from the edge of bondng nterface and spreads nto the center area. Ths s the cause of open fal. Nanoscale bondabltes on IMC of Cu/Cu 9 Al 4 and CuAl 2 /Al are also cautously nvestgated by usng molecular dynamcs (MD) smulatons. Nanoscale tensle force and tensle strength are predcted to examne the bondng strength of two IMCs along the bondng nterface. Interfacal fracture s dfferent n dfferent tensle speed as well as the workng temperature. A seres of expermental works and MD smulatons are conducted n ths research. Introducton Wrebondng process usng ultra-thn copper wre has been mplemented n semconductor packagng ndustry for the past few years for ts easy applcaton and low cost. However, hgher rgdty and strength of Cu wre would result n Al pad squeezng beneath the mashed FAB. Cu oxdzaton ssue can be mproved by coatng Palladum (Pd) over the copper wre surface. However, falures caused by hgher rgdty of Cu materal and ntermetallc compound (IMC) layer along Cu-Al nterface have been caught attenton by many researchers [1-3]. It s reported that the hardness of CuAl IMC s also stffer than ether copper or alumnum materal. The nfluence of Cu-Al IMC s fully examned by OM, SM, TM and DS. It s observed that the dffuson rate of Cu nto Al s faster than that of Al nto Cu. Prelmnary results shows the thckness of IMC affect the bondblty of copper wre and the vod s propagated along copper FAB boundary. As soon as FAB s formed by FO, the Pd coatngs would spread nto Cu ball based on TM- DX analyss. Only a small porton of Pd concentraton can be occasonally examned at the bondng nterface. It s observed that IMC layers are spontaneously formed beneath the crushed FAB surface. A thn layer of IMC s observed n the center area. The lower IMC layer s consdered as CuAl 2 protruded from Al pad surface. IMC layer n the upper sde s determned as Cu 9 Al 4. The atomc rato of Cu- Al n the mddle IMC layer would be vared or ntermxed of the two phases of IMCs. Al splashes are easly found at the outsde of Cu ball bonds. Thus, crack begns from the edge and gradually propagates nsde along the lne of IMC bondng nterface. Vods are observed at all nterfaces between Cu and Cu-Al alloy phase. Therefore, the bondng strengths of Cu/Cu 9 Al 4 and CuAl 2 /Al have essental nterests to defne the bondng nterface. Because many materal propertes of Cu wre and Al pad are scarcely realzed, wrebondng process s essentally dffcult to smulate by numercal approach. As the characterstc sze of the thn flm reduces to nanoscale, fnte element method s not capable to nvestgate the stran-stress behavor on the nanostructure. Molecular dynamcs (MD) has been wdely appled to smulate the materal behavors n a varety of materals n nanoscale movement [4]. Temperature and tensle speed are two maor factors on determnng the bondng strength. Materal n hgh temperature has greater knetc energy and has better formablty. Hgher pullng speed n tensle test would results n materal much easer fracture. Based on densty functonal theory n quantum mechancs, potental functon s derved to descrbe metallc propertes and behavors.

2 xpermental works For comparson, ultra thn (ψ=15um) 99.99% (4N) Pd-coated Cu wre and 4N no-coatng Cu wre were selected for the test materals. lectrc flame off (FO) processes for Cu wres were conducted by K&S 1488 plus wre bonder. Mcro-tensle mechancal property Buld up layers are lamnated on core consstng of several conductve layers wth a vacuum hot press. Based on ASTM standard F219-96, mcroscale tensle pull tests were conducted for Cu wres. Samples were prepared n 10mm length, wre pull speed s controlled n 1mm/mn and the yeld strength s measured at 1% of total elongaton. All data were repeatedly measured at least three tmes to receve consstent results. The Instron-3365 unversal test system wth 5N±0.5% load cell s used. Thermal effects (25, 150, 175 and 200 O C) on materal propertes are taken nto account. The chamber needs to be Ntrogened to avod oxdzaton. Fgure 1 shows the true stress-true stran curve of mcro-tensle pull tests for Pd-coated/no-coatng Cu wre at 25 O C. The correspondng mcro tensle mechancal propertes for all cases studes are lsted n Table 1. Fgure 1 True stress vs. true stran curve from mcrotensle test (25 O C). Table 1 Mcro-tensle mechancal property for Pdcoated and no coatng Cu wre. Cu Wre nocoatng Pdcoated T( O C) (GPa) Yeld Strength (MPa) 0.2% Stress (MPa) Theoretcal dervaton Among many potental models n MD smulatons, Tght-Bndng (TB) model derved by osato [5] s used n ths research to nvestgate the dynamcal propertes, surface energes and reacton forces behavor of face-centered cubc (FCC) atoms by manybody potental energy functons. For metal materal, TB model s capable to explore the varaton of mechancal propertes durng tensle test. The potental energy, attractve energy, repulsve energy and atomscale stress are expressed as follows Potental energy c ( B ) (1) U r and r p( 1) r U 0 r Ae Attractve energy r p( 1) r0 Ae and B 1 2 r 2 0 r e r 2q r 1 epulsve energy 1 r 2 2q r B e (3) Substtute equaton (2) and (3) nto (1) receves full tght-bndng potental n equaton (4). (2)

3 r 2q r 2 e c r p( 1) r Ae (4) A, p, q, ξ, r 0 are expermental constants, or obtaned from fttng usng tght-bndng potentals. Molecular dynamcs MD has been wdely appled to evaluate the materal behavors n a varety of materals n atomc-level moton. Temperature and tensle speed are two maor factors on determnng the bondng strength between two materals. Materal n hgh temperature possesses a greater knetc energy and holds a better formablty. Hgher pullng speed n tensle test would results n materal easer fracture. Based on densty functonal theory n quantum mechancs, potental functon s derved to descrbe metallc propertes and behavors. Fgure 2 llustrates the predcted tensle model usng MD smulaton for pure Al, Cu-Al (IMC) and pure Cu materals. The thckness of IMC Cu-Al layer s less than 8Å. Fgure 3 demonstrates the boundary condtons and number of atoms used n the predcted tensle model. Both Cu and Al atomc scale used n ths study s real sze. xpermental constants (bondng propertes) A, p, q, ξ, r 0 n equaton (4) are lsted n. Table 2 for Cu. Al and Cu-Al [6]. Fgure 3 Boundary condtons and number of atoms used n the predcted tensle MD model. Table 2 Bondng propertes n tght-bndng potental functon. Cu Al CuAl A(eV) P ξ(ev) Q r 0 (Å) The presented bondng property of Cu-Al IMC s fttng by usng the exstng potental [7]. Atomc-level stran/stress Based on theory of Contnuum Mechancs, the entre materal s assumed to be contnuous wthout any defect (vod) n macro vew. However, the strength of materal s not capable to descrbe the stran/stress behavor n nanoscale of molecular/atomc motons. Fgure 4 llustrates the predcted atomc-level enlarged length (ncludng length and elongaton) from n step to n+1 step. Atomc-level stran calculaton can be then easly obtaned from equaton (5). Atomc-level stress calculaton n ths research s BDT stress model derved by Myazak [8]. The dea of BDT stress model s a sngle atomc stress can be obtaned to accumulate the average stress n the localzed area. Fgure 5 demonstrates the accumulated average stress n localze area S for a sngle atom. The atomc level stress n the localzed area can be obtaned from equaton (6). Fgure 2 eal sze of the predcted tensle MD model.

4 r 1 a 2 2 r Materal propertes of dfferent contents of Cu-Al IMC were lsted n Table 3 [6]. As can be seen, the hardness of IMC s larger than that of pure materals, for nstance, mcrohardness of CuAl s 628 HV5. The hardness of Cu s HV5 and the hardness of Al s HV5. The IMC s much harder than pure Cu and pure Al materals. In MD smulaton, the crystal structure of CuAl s monoklnc whch s dfferent from BCC of pure Cu and Al. Fgure 4 Atomc-level length from n step to n+1 step. ( l n 1 ln ) ( n 1) (5) ln S Fgure 5 Accumulated average stress n localze area S for a sngle atom. From a sngle atom, the average atomc-level stress n the localzed area s Table 3 Materal propertes of Cu-Al IMC. Concentrato Crystal Hardness n Structure HV5 (at.% Cu) Al Cubc CuAl Tetragonal 324 CuAl Monoklnc 628 Cu 9 Al Cubc 549 Cu Cubc esults and dscusson The surface between CuAl IMC s assumed to be perfectly flat wthout any roughness. elaxaton movement was performed to assure the stablty of MD tensle test smulaton. After the entre structure was reachng stable equlbrum, the atomc weght of Cu=63.546, Al= , v= å/fs and ntal temperature T=300K were appled to the model. M m n 1 V mn (6) N m n s 1 ( r ) x x 2V r r N s s the number of atoms n regon S, M s the atomc weght of atom, m s the velocty n m drecton for atom, r s the relatve dstance between atoms and, ( r ) s the potental and x m s the dsplacement n s m drecton for r. V s the total atom volume n the stressed regon and a s the average radus of atom n stressed regon. 4 V 3 a 3

5 demonstrated. Acknowledgments The authors would lke to extent ther apprecaton to Natonal Scence Councl, Tawan, OC, proect No. NSC and NSC for fnancal supports. eferences Fgure 6 Tensle test smulatons usng MD. (a) relaxaton (b) pull (c) neckng (d) before rupture (e) after breakage (f) cease. Fgure 6 llustrates the predcted tensle test usng MD smulaton for pure Al, Cu-Al IMC and pure Cu. MD predcted results are also shown that pressng depth between two bondng materals wll affect the bondng strength. Ths s very mportant for thermoultrasonc ball bond process. Fgure 7 shows the predcted nanoscale elastc property. Fgure 7 Predcted nanoscale elastc property. [1] Bschoff, A. et al, elablty Crtera of New Low- Cost Materals for Bondng Wres and Substrates, Proc. 34 th lectronc Component Conerence., New Orleans, p 411, [2] Wulff, F. W. et al, Charactersaton of ntermetallc growth n copper and gold ball bonds on alumnum metallzaton, Proc. of 6 th lectroncs Packagng Technology Conference, pp , [3] Alder, B. J. et al, Phase Transton for a Hard Sphere System, J. Chem. Phys, Vol. 27, No. 5, pp , [4] Hsu, Q. C. et al, Study on Bondng Propertes of Copper and Alumnum n Nano Scale Usng Molecular Dynamcs Smulaton, Appled Mechancs and Materals, Vols (2012) pp [5] osatoa, V. et al, Thermodynamcal and structural propertes of f.c.c. transton metals usng a smple tght-bndng model, Phlosophcal Magazne, seres A, Vol.59, No.2, pp , [6] Karolewsk, M. A., Tght-bndng potentals for sputterng smulatons wth fcc and bcc metals, adaton effects and defects n solds, Vol.153, No.3, pp , [7] roless, F. et al, Interatomc Potentals from Frst- Prncples Calculatons: The Force-Matchng Method, urophyscs Letters, Vol.583, No8, pp , [8] Myazak N. et al, Calculaton of Mechancal Propertes of Solds Usng Molecular Dynamcs Method, JSM, seres A, Vol. 39, No. 4, pp , Summary As the CuAl IMC emerged from bondng process, mechancal propertes of Cu-Al were found to be much better than Cu and Al materals. From Fgure 6 (a) to (f), neckng s observed n the Al atoms and rupturng s also found n the regon of Al atoms. Bondng strength of CuAl IMC s better than pure Al, but s weaker than pure Cu. MD smulatons to predct the nanoscale IMC bondablty n ths research are