Electromigration in Flip Chip Solder Joints J. W. Nah and K. N. Tu Dept. of Materials Science & Engineering UCLA, Los Angeles, CA 90095-1595 • Supported by NSF,SRC, IBM, Intel, Motorola, NSC Co-workers : Prof. Chih Chen, National Chiao Tung University, Hsinchu, Taiwan, ROC Prof. Chengyi Liu, National Centrl University, Chungli, Taiwan, ROC Prof. Taek Yeong Lee, Hanbat National University, Korea Dr. Woojin Choi, Intel, Santa Clara, CA Dr. Hua Gan, IBM T. J. Watson Research Center, NY Dr. Albert Wu, Advanced Light Source, LBNL
Electronic Thin Film Lab
Lead-free Technology Workshop TMS Annual Meeting, San Francisco, CA February 13, 2005
Materials Science & Engineering, UCLA
Outline 1. Introduction 2. Unique behavior of electromigration in solder joints 3. Electromigration in flip chip solder joints - SnPb vs. Pb-free (SnAgCu) vs. composite solder joints 4. Interaction among electrical, mechanical, & chemical forces 5. Future work using synchrotron radiation 6. Summary
Electronic Thin Film Lab
Materials Science & Engineering, UCLA
What is electromigration induced failure ? Electromigration example in 63Sn/Pb solder Si
e-
e-
Board
Failure was occurred at cathode in the downward electron-flown bump by the formation of void Peter Elenius, Flip Chip Technologies, (1999) Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Why electromigration is important in solder? Fast diffusion is not the key reason of EM failure in solder joints
Different diffusion mechanisms Melting point (oK)
Temperature ratio 373oK/Tm
Diffusivities at 100oC [373oK] (cm2/sec)
Cu
1356
0.275
Surface Ds=10-12
Al
933
0.4
Grain Boundary Dgb=6x10-11
Pb
600
0.62
Lattice Dl=6x10-13
Eutectic SnPb
456
0.82
Lattice Dl=2x10-9 to 2x10-10
Electromigration in Al is dominated by grain boundary diffusion Electromigration in Cu is dominated by surface diffusion Electromigration in solder is dominated by lattice diffusion Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Critical Product in Short Strip ∆x Cathode
Jσ Jem
Al
e-
J = Jσ + Jem Anode
TiN
D dσΩ D J = −C +C Z * eE kT dx kT
Jem: atomic flux driven by electromigration Jσ: atomic flux driven by back stress
If J =0, there is no net electromigration flux.
∆σ Ω = Z *e ρ j ∆x Critical product If j∆x < (j∆x )c Electronic Thin Film Lab
E= Electric Field (E = ρj )
(j ∆ x) critical
∆σΩ = * Z eρ
No electromigration damage Materials Science & Engineering, UCLA
Small Critical Product in Solders ( j∆ x ) c =
∆σΩ Z *e ρ
( j∆ x ) c =
Y∆ εΩ Z *eρ
Y(Gpa)
Z*
ρ(µΩ-cm)
Solder
~30
~30
~22
Al
~69
Cu
~110
2~4
~2
∆σ =Y∆ε, ∆ε=0.2% at elastic limit
0.5
( j ∆ x ) c_solder
Y∆ εΩ ≈ 5 x 10 = * Z eρ
10
−3
( j ∆ x ) c_Cu/Al
10
••At Atconstant constant∆x, ∆x,the thecurrent currentdensity densityneeded neededto tofail failsolder solderisis 2~3 2~3orders orderssmaller smallerthan thanthat thatneeded neededto tofail failAl Alor orCu Cu 5 6 2 3 4 2 ••IfIfAl Alor orCu Cufails failsat at10 105to10 to106A/cm A/cm2, ,solder solderwill willfail failat at10 103or or10 104A/cm A/cm2
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Unique Features of Electromigration in Solder Joints 1. Geometry (Line-to-bump) • Current crowding and local Joule heating
2. Eutectic Composition of solder alloys • No chemical potential gradient as a function of composition • It can lead to a large composition gradient or redistribution
3. Fast UBM dissolution • Fast diffusion and reactions of noble and near-noble elements in solder
4. Multiple driving forces • Thermo-mechanical, chemical, electrical
5. Sn and Sn-based Pb-free solders are anisotropic conductors • Grain rotation in electromigration Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Line-to-bump geometry z Current crowding in flip chip solder joints z Void formation induced Joule heating and melting of solder joints
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Materials Science & Engineering, UCLA
Current Density Distribution Current Crowding Position e-
Al line 2 µm
Solder
J = I/A AAl
3 1
* not failed, These MTTF (hrs) are averaged value of three samples
by W.J.Choi, UCLA, Now at: Intel, Chandler, AZ.
The measured MTTF was much smaller than the calculated values at the higher current density Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Model of Bump Heating & Melting Al
void
SiO2
Contact opening
solder
3-D simulation model of joule heating in a flip chip solder bump with a pancake-like void below the contact to the Al line By Prof. A.M. Gusak, Cherkasy State University Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Simulation Results Cross-sectional view
Electric filed distribution
Joule heating distribution
Resistance distribution
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Materials Science & Engineering, UCLA
Melting of Solder Joints • High Pb- eutectic SnPb composite solder joints • 5 µm thick Cu UBM
e-
e-
0.6 hour
Room temperature, 5.00 ×104 A/cm2
e-
100 µm
J.W. Nah, J. O. Suh, and K. N. Tu, submitted to JAP, 2004
A large Joule heating inside solder bumps can cause melting of the solder bump and the failure occurred quickly. Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Electromigration in different flip chip solder joints • Electromigration in eutectic SnPb • Electromigration in eutectic SnAgCu • Electromigration in composite solder joint of high-Pb (97Pb3Sn) and eutectic SnPb
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Failure Mode in Pb-free (SnAgCu) Solder Joint Initial state
140 oC, 3.00×104 A/cm2 Al/Ni(V)/Cu UBM
Formation and propagation of void caused the failure
After 14 hours
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2002 Electronic Components and Technology Conference
UBM (Under Bump Metallization) Study for Pb-free Electroplating Bumping : Interface Reaction and Electromigration Se-yeong Jang, Juregen Wolf, Woon-Seong Kwon, Kyung-Wook Paik Dept. of Materials Science and Engineering KAIST (Korea Advanced Institute of Science and Technology)
Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Test Sample of Composite Solder Joints
Si chip
Al
TiW/Cu/electroplated Cu
electron flow, 97Pb/Sn
e-
97Pb/Sn underfill
37Pb/Sn
eCu
Solder resistor
Ni/Au
e-
Organic substrate
Samples were prepared at KAIST Electronic Thin Film Lab
Materials Science & Engineering, UCLA
SEM Images & Elemental WDS Map of Sn 155 Before current stressing
, 2.55X104 A/cm2
Upward electron flow for 20 hrs
e-
50 µm
Upward electron flow; No void. Downward electron flow for 3 hrs
Downward electron flow for 20 hrs
e-
e-
Void formed J. W. Nah, K. W. Paik, J. O Suh, K. N. Tu, JAP, 94, 7560, (2003) Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Sequential Change of Catastrophic Failure 155
Cathode side in downward e- flown bump
, 2.55X104 A/cm2
e-
Cu
Cu
Chip
e-
Cu3Sn
Cu6Sn5
10 µm
0hr
3hrs
Cu3Sn
Sn
Substrate
e-
Cu
e-
12hrs
Cu
Cu3Sn
Cu3Sn Cu6Sn5
e-
Cu
Sn 18hrs
Cu6Sn5
Cu3Sn Sn
Cu6Sn5 20hrs
Sn
V oid was for m e d at the left corner of contact window an d propagated to the right. Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Solder reaction with UBM during electromigration z Enhance the diffusion of Sn to the cathode z Dissolution of thick UBM z Formation of a very large quantity of IMC in the solder joint
Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Electromigration-driven Phase Separation in eutectic SnPb Solder Before EM
After EM
Pb
eFrom ASE, Taiwan
Sn
Current density = 5.0×103 A/cm2 Temp. = 160oC, time = 82 hrs
Electromigration in flip chip solder joints at high temperature enhanced the diffusion of Pb to the anode & Sn to the cathode Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Failure of SnAgCu Solder Bump on Thick Film UBM Before EM
Electroless Ni
Before failure
261 hrs at 125 & 2.0 A
e-
50 µm
After failure
25hrs at 150 & 2.5 A
e-
In the same solder on thin film UBM sample, MTTF is 14.2 hrs at 140 oC & 2.4 A
Thick film UBM shows more MTTF than thin film UBM Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Effect of current crowding on electromigration in SnPb solder joint Current = 1.27A, Temp. = 100 Si Chip
Open
e-
Cu dissolution
e0 min
e-
Cu conducting trace Cu6Sn5 IMC
15 mins
e-
90 mins
ePCB
100 µm
From Professor C. R. Kao in National Central University in Taiwan 30 mins
e-
Time of current stressing ↑ Left-hand side of Cu UBM disappeared gradually and was replaced by solder
45 mins
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Materials Science & Engineering, UCLA
Future studies z Effect of combined forces on solder joints 1. Electrical and mechanical forces → Electromigration & tension, shear, or creep 2. Electrical and chemical forces → Polarity effect on IMC formation
z Synchrotron radiation 1. Stress distribution on solder joint cross-section 2. Grain rotation induced by electromigration 3. Chip-packaging interaction : Thermal stress of solder joints on Cu/ultra low k multi-layer
Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Sample for Combine Test Easy to apply tensile stress and current on one solder joint serially or simultaneously Outward appearance
Cross-section
~440 µm
Cu
Solder
Cu 300 µm Electronic Thin Film Lab
By Fei Ren, UCLA Materials Science & Engineering, UCLA
Tensile Test before & after Electromigration No electromigration
Stress MPa
50
48hours, 145ºC,
40
1.68×103 A/cm2 30
48hours, 145ºC 5.03×103 A/cm2
20
10
0 0.0
0.1
0.2
0.3
0.4
0.5
Strain
Electromigration in solder joint affects on the results of mechanical test Max. stress ↓ Current density ↑ Fracture mode changed Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Fracture Image after Tensile Test No electromigration
96 hours electromigration e-
300µm
Current stressing time ↑ Electronic Thin Film Lab
144 hours electromigration e-
5×103 A/cm2, 145ºC
Fracture mode changed Materials Science & Engineering, UCLA
Future Study on Combine Forces
σ I
σ
A weight
Simultaneous electromigration and creep test Both are time-dependence processes Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Sample for Electrical and Chemical Interaction Cross-section view of a v-groove with metallization layers 100 µm
Multi-segment v-groove sample Si (001)
Cu or Ni
Solder 69.4 µm µm 69.4
SiO2(0.1µm)
Cu or Ni wire
Cu or Ni wire
Si V-groove
Ti(0.05µm) Cu(1µm) or Cu (1 µ Ni (0.31µm) Au(0.05µm)
Top view of a multi-segment v-groove Si piece
Easy to change the length of solder joints and compare them at once
By Shengquan Ou, UCLA Electronic Thin Film Lab
Cu or Ni wire
Solder line
Materials Science & Engineering, UCLA
Length Dependence – Ni/SnAg Solder Segment
L1
L2
L3
Length (µm)
110
290
710
Ni L1
Ni
Ni
L2
SnAg
Ni
L3 500µm
Ni
Solder Segment
L1
L2
L3
Length (µm)
290
440
860
L1
Ni
L2
Ni
Ni
L3 500µm
SnAg
Ni
The length of the solder segments can be varied from 100 µm to 900 µm Microstructure of the solder/electrode interface is similar to the real flip chip sample.
Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Length Dependence – Cu/SnAgCu Solder Segment
L1
L2
L3
Length (µm)
20
780
530
At 150°C, EM at 2×104 A/cm2 for 72 hrs
Depletion Cu L1
Cu
Cu
L2
e-
Cu
L1
100 µm
No depletion After Polish L1
10µm
Back stress exists in SnAgCu solder lines Electronic Thin Film Lab
Huge amount of IMC formation in the shortest solder lines Materials Science & Engineering, UCLA
Grain rotation in Sn during EM z Sn is an anisotropic conductor z Grain rotation occurs in electromigration z A torque may exist across a Sn grain in electromigration, and it leads to grain rotation z Grain rotation may occur in Pb-free solders
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Grain Rotation in Sn to be Studied by Synchrotron Radiation Sn stripe
2x104 A/cm2 at 100oC
The morphology of the stripe shows that grains rotate under electromigration. (Courtesy of Prof. C. R. Kao at National Central University, Taiwan) Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Chip-Packaging Interaction of a Flip Chip Sample Cutting line
Eutectic SnPb
Si chip
Si chip 1 cm
UBM
Center
1 cm
Solder ball and UBM
Organic substrate
Organic substrate
∆x = 10 µm
Right
∆x
Gap ~ 20 µm, Shear ≈ 10/20
Left Electronic Thin Film Lab
∆x
100 µm
For 50 µm solder bump, the gap will be smaller!! Materials Science & Engineering, UCLA
Cross-sectioned pure Sn Bump in Diagonal Direction
Si chip
Center
Sample from KAIST
Pure Sn Organic substrate
10 µm
We can scan the entire bump by micro beam in synchrotron radiation with a spatial resolution of 1 µm. Electronic Thin Film Lab
Materials Science & Engineering, UCLA
Sample Preparation Procedure for in-situ EM Investigation substrate
-
-
+
+
chip
Underfill
5mm
Top-view of flip chip sample
-
Soldering of Cu wire
+
Mounted sample in epoxy
Bottom view after polishing
In-situ observation by
synchrotron radiation is now possible Polishing direction
-
+
Polished sample until solder bumps were shown
Electronic Thin Film Lab
Chip
Substrate 20 µm
100µm
Materials Science & Engineering, UCLA
Summary of Future Studies 1. Micro beam diffraction in synchrotron radiation has opened a new field of study of reliability of flip chip solder joints with 1 µm spatial resolution 2. Chip – packaging interaction Stress distribution in solder bump and UBM Stress distribution in Cu/ultra-low k multi-layers 3. In-situ study of interaction among electromigration, mechanical stress, thermal stress, and chemical reactions in flip chip solder joints 4. Grain rotation in Sn during electromigration Electronic Thin Film Lab
Materials Science & Engineering, UCLA
