Whisker & Hillock Formation on Sn, Sn-Cu & Sn-Pb Electrodeposits. William J. Boettinger, NIST ECTC, Orlando May 31, 2005

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1 Whisker & Hillock Formation on, -Cu & -Pb Electrodeposits 0 William J. Boettinger, NIST ECTC, Orlando May 31, 2005

2 Co-authors: C. E. Johnson, G. R. Staord, K.-W. Moon, M. E. Williams, L. A. Bendersky Metallurgy Division NIST Description o microstructure Measurement o stress and stress relaxation in, -Cu, -Pb electrodeposits and relation to whisker growth tendency Analysis o stress-ree strains in deposits (Initial Residual Stress) Analysis o creep response Ideas or nucleation & growth mechanism o whiskers Manuscript submitted to Acta Materialia: March,

3 Post-deposition stress measurement by cantilever beam Conirm that stress is compressive. Is stress more compressive in with Cu? Does Pb lower stress? How ast does the stress change with time? Correlation with whisker density? Phosphor bronze substrate Plating Conditions Commercial bright methanesulonate (MSA) prepared with 18.3 MΩ-cm high purity water Cu +2, Pb +2 methanesulonate added: 953 mg/l Cu 1906 mg/l Pb Fixed current density 60 ma/cm 2 Anode % sheet 100 rpm rotating cathode Plating at 25 ºC Alloy deposits: -3wt% Cu and -2wt%Pb 2

4 3 Microstructure o Electrodeposits & Whiskers

5 Hillock Formation on Pure Deposits FIB Cross Sections Courtesy: G, Gaylon and M. Palmer 4

6 Contorted Hillock and Whisker Formation on -Cu deposits Long whiskers 5

7 No Surace Disturbance on -Pb deposits FIB Cross Section Note equiaxed grains Back scatter image Pb grains 6

8 Metallographic cross-sections sections - 3 wt% Cu - 2 wt%pb 7

9 TEM micrographs o ine precipitates in Cu and -Pb deposits -Cu -Pb 8

10 Summary o microstructure o 16 µm m thick deposits -Cu -Pb Surace deects Conical hillocks Contorted hillocks and ilamentary whiskers None grain structure Columnar, mobile Columnar, immobile Equiaxed Initial alloy structure - Fine Cu 6 5 inside grains Fine Pb Aged alloy structure - Coarse Cu 6 5 on grain boundaries Sparse Pb grains Cu 6 5 orms slowly on the deposit / substrate boundary or all. 9

11 IMC Growth Rate dimc = Bt Onishi & Fujibuchi measured B = 6.23 x 10-6 exp(q/rt) cm 2 /s and Q = 57.7 kj/mol. At 298 K, the extrapolated value o B is 4.7 x cm 2 /s. dimc = B' t Tu and Thompson measured the growth rate o intermetallic at room temperature. They observed only Cu 6 5 and obtained with B = 4 x cm/s or IMC thicknesses up to 300 nm. Less than 20 nm o intermetallic can orm in 15 min Negligible eect on the stress in 3 to 16 µm thick deposits at 15 min On the other hand, 1.2 µm and 2.3 µm may orm in a year (3x10 7 s) and the stress o the deposit is inluenced at longer times. 10

12 11 Cantilever Beam Experiments, -Cu & -Pb

13 Measured delection curves or three materials and three thicknesses µm -Pb µm -Pb δ (µm) δ (µ m) Cu -40 -Cu E+003 1E+004 1E+005 1E+006 time (s) 40 3 µm -Pb -60 1E+003 1E+004 1E+005 1E+006 time (s) δ (µm) 20 0 Note: As shown below, a positive delection does not necessarily mean tensile stress in the deposit! -Cu -20 1E+003 1E+004 1E+005 1E+006 time (s) 12

14 Stoney s Stress Analysis d S Cu Substrate d In-plane stress (ater substrate release) Balance o Forces & Moments Substrate Tension L Compression d S d Distance 13

15 Initial Compressive Stress in Electrodeposits / Whisker Formation 0 -Pb Deposit Stress 900 s Cu Deposit Thickness (µm) 14

16 -Cu and -Pb Molar Volume vs. Composition Obtain V/V or various processes V M (cm 3 /mole o atoms) (Cu) two phase equilibrium mixture o and Cu 6 5 Cu 6 5 at raction (% Cu) rule o mixtures o and Cu Cu 3 Cu Ppt. o Cu 6 5 rom (,Cu) causes increase in molar volume γ β α V M (cm 3 /mole) (Pb) at raction (% Pb) Pb() two phase equilibrium mixture Ppt. o Pb rom (,Pb) causes drop in molar volume Pb Reaction + Cu Cu 6 5 causes 6% drop in molar volume 15

17 Metallurgical Explanation or Dierent Initial Stress in Alloy Deposits Intrinsic plating stress or pure plus ppt. eect rom alloying Alloy Measured initial stress (MPa) Corresponding stress-ree strain Deviation o stressree strain rom that or pure 1/3 o volume change due to ppt. o Cu 6 5 or Pb rom Cu or Pb supersaturated x %Pb x x x %Cu x x x 10-3 Correct trend compared to pure deposit: Addition o Pb reduces compressive stress in deposit Addition o Cu increases compressive stress in deposit Predicted Cu 6 5 eect too large! Probably due to zero partial molar volume assumption or interstitial Cu 16

18 17 Stress Relaxation / Creep

19 Modiication o Stress in deposit by IMC ormation d S d IMC d Balance o Forces & Moments Cu Substrate In-plane stress (ater substrate release) Cu 6 5 Tension Substrate Cu 6 5 Compression d IMC d S d Distance 18

20 Model or Beam Delection δ (t); or d IMC & d << d S 2 δ ( t) 1 Let = Kt ( ) = = beam curvature 2 L r() t T T 2 I dimc and d << ds ; ε = ε = ds K ; (zero strain = lat) 3 IMC plating substrate δ < 0 K < 0 ε T, IMC > 0 T E 0 εimc = εimc + εimc IMC: elastic strain + stress-ree strain T E P 0 ε = ε + ε + ε : elastic strain + plastic strain + stress ree strain Value determined rom initial delection ' ES 2 σimcdimc + σ d = ( ds ) K Try dierent values 6 ' E ' 2 0 σimc = EIMCεIMC = EIMC ds K ε with IMC 3 = 0 1/2 ' E 2 d 0 plastic dimc ( t) = Bt "IMC Growth Rate" & σ = E & ε = ( dsk) & ε & ε 3 dt 0 1 ( ) ( ) "Consumption o " plastic d t = d dimc t & ε = ( σ ) extracted rom measured delections ds( t) = ds dimc( t) "Consumption o Cu" 2 See i creep response makes sense 19

21 Using delecti on data rom pure sample 7 µm Deposit Strain total 1E+002 1E+004 1E+006 Time (s) elastic δ (µm) ε IMC 0 =0 7 µm stress ree + plastic Stress and strains or dierent ε 0 IMC values -Pb -Cu -60 1E+003 1E+004 1E+005 1E+006 time (s) Deposit Stress (MPa) Deposit Strain E E+004 1E+006 Time (s) -5-1e-3 0-2e e ε IMC =-2e ε 0 IMC total Time (s) elastic stress ree + plastic 20

22 Creep curves derived rom delection measurements 1E-007 Probable value Blue, ε 0 IMC =0 Red, ε 0 IMC = (IMC stress = 125 MPa tensile). Green, ε 0 IMC = (IMC stress = 250 MPa tensile). - Plastic Strain Rate (s -1 ) 1E-008 1E-009 1E-010 -Pb -Cu Layer IMC has no eect at early times The prediction o [Hutchinson et al., Mat. Sci. For 2004; : 465] or whisker growth rate (converted to average strain rate in the in-plane direction) 21 1E Stress (MPa)

23 Creep as a mechanism o whisker ormation Consideration o 3-D important Surace grain necessary in mostly columnar structure. Localized Nabarro-Herring-Coble creep has higher chemical potential on grain aces normal to stress has lower chemical potential on grain aces parallel to stress Flux along grain aces rom high to low chemical potential. Accretion o on aces most parallel to stress Push up o whisker grain 22

24 Eect o grain shape and mobility on whisker & hillock ormation -Cu -Pb Columnar grain boundaries are mobile hillocks Columnar grain boundaries are pinned whiskers Equiaxed grain boundaries general Coble creep 23

25 Conclusions -Cu: Whiskers and contorted hillocks Initial stress MPa : Conical hillocks Initial stress MPa -Pb: no surace disturbances Initial stress -3.8 MPa Initial Stress (15 min ater plating) Not due to IMC ormation on /substrate interace Increase/decrease in -Cu/-Pb due to volume changes due to ast ppt. o Pb or Cu 6 5 within grains The analysis o the changes in beam delection indicates that the additional compressive stress in the deposit due to intermetallic ormation on the deposit / beam interace over time is o the same order o magnitude as the plating stress. & -Cu: columnar grain structure; -Pb: equi-axed grain structure. To relieve compressive stress: Columnar structures, lux toward oblique grain aces near but under the ree surace that orces the grain upward. (transport along ree surace inhibited by oxide) Equiaxed structure, lux to ubiquitous internal grain aces parallel to stress Lack o lateral grain boundary mobility o the columnar structure in the -Cu deposits caused by precipitate pinning promotes whisker ormation by maintaining a laterally small grain where lux accumulates. Broad hillock ormation occurs in pure due to extensive lateral grain boundary migration. 24