Electromigration in Solder Joints and Lines - Engineering · Electromigration in Solder Joints and...

Electronic Thin Film Lab Materials Science & Engineering, UCLA

Electromigration in Solder Joints and Lines

K. N. Tu, Xu Gu, Hua Gan, and W. J. ChoiDept. of Materials Science &Engineering, UCLA

Los Angeles, CA 90095-1595

1. Introduction2. Unique behavior or electromigration in solder joints 3. Electromigration of flip chip solder joints

- SnPb vs. Pb-free (SnAgCu)4. Electromigration of solder lines in V-groove

- Temperature, composition, and polarity5. Summary

• Supported by NSF,SRC, IBM, Intel, MotorolaCo-workers : Dr. Everett C. Yeh, Intel, Santa Clara, CA

Prof. C. Y. Liu, National Central University, TaiwanProf. C. Chen, National Chiao Tung University, TaiwanDr. Taek Yeong Lee, Bell Lab. Lucent TechnologiesDrs. P. Elenius & H. Balkan, Flip Chip Technologies


Peter Elenius, Flip Chip Technologies, (1999)


1. Geometry (Line-to-bump)• Current crowding and local Joule heating

2. Eutectic Composition• No chemical potential gradient as a function of composition• It can lead to a large composition gradient or redistribution

3. UBM dissolution• Fast diffusion of noble and near-noble elements in solder

4. Multiple driving forces• Thermo-mechanical,chemical, electrical

Unique Features of Electromigration in Solder Joints


Current Density Distribution

2 µm UBM

0.20

0.58

1.66

4.80

13.9

40.0x103 A/cm2

Solder J = I/AAAl<<Asolder

Al line2 µm

e-Current Crowding Position


0.20

0.58

1.66

4.80

13.9

40.0

x103 A/cm2

(a)

(b)

X

Y

Current D

ensity (x104A

/cm2)

Z

3.0 -

2.5 -

2.0 -

1.5 -

1.0 -

0.5 -

Simulation of current crowding in the solder bump

• Current carried by solder bump : 0.2 Amps• Thickness of Al interconnection : 2 µm• Contact window (opening) : 100 × 100 µm2


Fig. Sequence of the void propagation at 125 oC, and 2.25×104 A/cm2

(a) 37 hrs (b) 38 hrs (c) 40 hrs (d) 43 hrs

Void Propagation (e-SnPb solder)

(a) (b)

(c) (d)

contact window

e-

e-

e-


e-

Si

Cu

e-

e-

Void propagation

(c) (b) in fig. 2

Time - Potential Curve

Fig. Potential change of the solder bump due to the electromigration

(b) (c) (d) in fig.2

0 10 20 30 40 50

1

2

3

4

5

6

Pote

ntia

l (V)

Time (hrs)

Resistance does not change much


* not failed, These MTTF are averaged value of three samples

(hrs)

Mean Time To Failure

= −

ktQexpAjMTTF n n = 1.8, Q = 0.8 eV

(By Flip Chip Technologies)

1.5 A (1.9x104 A/cm2)

1.8 A (2.25 x104 A/cm2)

2.2 A (2.75 x104A/cm2)

Expected Actual Expected Actual Expected Actual

100 oC 380 97 265 63

125 oC 108 573* 79.6 43 55.5 3

140 oC 46 121 34 32 24 1

24 samples tested (three samples for each test condition)8 samples cross-sectioned by W.J.Choi


Failure Mode in Pb-free (SnAgCu) Solder Bump

Fig. Void propagation and failure at 140 oC and 2.4 Amps (3.0E4 A/cm2)(a) Before current stressing (b) After 14 hours : not failed (c) Magnified picture of (b)

(a)

(b)

(c)

This sample failed after 14.2 hrs


Failure in SnAgCu Solder bump on Thin Cu/Ni(V)/Al UBM

140 oC , 2.4 Amps

2 hrs21 hrs99 hrs160 oC

14.2 hrs94.5 hrs

132 hrs140 oC

83 hrs112 hrs580 hrs125 oC

2.4 A3.0E4 A/cm2

2.2 A2.75E4 A/cm2

1.8 A2.25E4 A/cm2

0 2 4 6 8 10 12 14 16

1.8

2.0

2.2

2.4

2.6

2.8

3.0

Pote

ntia

l (V)

Time (hrs)

MTTF on SnAgCu solder bump

(b) In last Fig.


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 PaikDept. of Materials science and Engineering

KAIST (Korea Advanced Institute of Science and Technology)


No chemical potential gradient as a function of composition below the eutectic temperature

Pb-Sn

Ni-Sn


Failure of SnAgCu Solder Bump on Thick Film UBM

Fig. Failure mode of solder bump on Thick UBM(a) No UBM dissolution(b) Before failure after 261 hrs at 125oC

and 2.0 Amps(c) Failure after at 150oC 25 hrs

at 150 oC and 2.5 Amps• In the same solder on thin film UBM sample,

MTTF is 14.2 hrs at 140 oC and 2.4 Amps

(b)(a)

(c)

50 µm


Experiment

Experimental Conditions:• Temperature of environment: 100℃• Time of passing electric current:

15min, 30min, 45min, 60min, 75min, 90min, 95min (short) •Current 1.27 A per joint, Current density 2× 104 A/cm2

Chip

PCB

e-

e-

Chip

PCB

e-

e-

Au/Ni/Cu UBM

Cu conducting trace Cu padCu pad

100μm 63Sn/Pb

Flip Chip Solder Ball

100μm 63Sn/Pb

Flip Chip Solder Ball

Current direction, chip side metallurgy (pure Cu), and substrate side metallurgy (Au/Ni/Cu) used in this study.

+-Power Supply

Si Chip

PCBe- e-

e-

Schematic diagram of experimental setup.No current


From Professor R.Kao

in National Central Univ. in Taiwan

openCu

conducting trace

Cu dissolution

Cu6Sn5IMC

Si Chip

PCB

e-

e-

90mins

Electromigration in SnPb solder bump

I = 1.27A, T = 100℃,

e-15mins

30mins

e-

45mins

0min


The Top View of the Cu Conducting Trace at Chip Side (solder joint had been polished away carefully)

• The dissolved Cu region was back-filled with solder.

• The failure took place between back-filled solder and Cu conducting trace

Optical micrograph of the top down cross-section at Cu conducting trace


Proc. 6th Internl. Vacuum Congr. 1974Japan. J. Appl. Phys. Suppl. 2, Pt. 1, 1974

Room Temperature Interaction in Bimetallic Thin Film Couples

King-Ning Tu and Robert RosenbergIBM thomas J. Watson Research Center

Yorktown Hights, New York 10598

PtSn4PtPb4PtPdSn4PdPb2PdNi3Sn4-NiAuSn4AuPb2AuAg3Sn-AgCu6Sn5-Cu

SnPb

Cu, Ag, Au, Ni, Pd, Pt

Si, Ge, Sn, PbInterstitial Diffusion

Table. Intermetallic compounds formed at room temperature


0 day

4 days

8 days

8 days

SiO2(0.12µm)

Cr or Ti (0.05µm)

Cu(1µm)

100µm

69.4µm

Si V-groove

Cu wireCu wire

Si (001)(a)

(b)

Polished Surface

IMC

Electromigration inV-Groove Line

SEM Images

J = 2.8 x 104 amp/cm2

T = 150 oC


~0.5(Eutectic)


Room Temperature Electromigration in Eutectic SnPb solder on V-groove

67.42 %66.76 %68.11 %-69.63 %23.6

68.42 %79.95 %-86.02 %86.49 %13.8

69.34 %86.00 %86.04 %89.33 %91.66 %0

18013590450

Polishing from anode to cathode sidePolishing down from the surface

(µm)

Fig. Electromigration phenomenon in Eutectic SnPb solder line stressed by a current density 5.7x104A/cm2 at RTfor 4 days, 8 days, and 12 days


The Polarity Effect of Electromigration The Polarity Effect of Electromigration on Intermetallic Compound (IMC) Formationon Intermetallic Compound (IMC) Formation

in Solder Vin Solder V--groove Samplesgroove Samples

• Experiment and Results–Morphology change of IMC: polarity effect–Thickness change of IMC: polarity effect

• Analysis and Discussion


Polarity Effect of EM on IMC Thickness

Cu Cu

100µm

PCB

Si chip

Soldere-

IMC

Solder 10hr

Cu

IMC

Solder

Cu

10hr e-

Anode

Cathode

3.2x104 A/cm2, 180oC


Sample Preparation

<110>

Solder line

Cu wireSi Wafer • Advantage of V-groove sample:

- Comparable dimension and current density as real flip chip solder joints - Easy to make and observe- Higher current density comparing with traditional samples

• Advantage of V-groove sample:- Comparable dimension and current density as real flip chip solder joints - Easy to make and observe- Higher current density comparing with traditional samples

SiO2 (0.1 µm)

Cr or Ti (0.05 µm)

Cu (1 µ

100 µm

Cu (1µm)

69.4 µm69.4 µm

Au (0.05µm)

Solder


• Width of solder line: ~ 100µm

• Length of solder line: ~ 600-800µm

• Width of solder line: ~ 100µm

• Length of solder line: ~ 600-800µm

Typical Sample View

Solder V-groove sample after reflow Polished sample ready for EM test

Si wafer

Cu Cu

CuCu

Solder line Solder line

Si wafer

e-Cu Cu

Cu

Cathode Anodee- e-

Cu

IMC

Solder


Comparing of Morphology Change

Cathode(4x104 A/cm2, 150oC)

Anode(4x104 A/cm2, 150oC)

No Current(150oC)

157hr0min

0min

0min

35min

35min


EM effect on Thickness Change of IMC

0 hr

Anode (3.2x104 A/cm2, 180oC) Cathode (3.2x104 A/cm2, 180oC)

10hr

e-

Anode Cathode

e-0 hr

10hr

Cu6Sn5

Cu6Sn5

Anode

Anode(a)

(b)

Cu

Cu

Solder

Solder

0 hr

10hr

Cu6Sn5

Cu6Sn5

Cathode

Cathode(d)

(c)

Cu

Cu

Solder

Solder


EM effect on Thickness Change of IMC (cont.)

e-

Anode Cathode

e-

Anode (3.2x104 A/cm2, 180oC) Cathode (3.2x104 A/cm2, 180oC)

21 hr

87hr

Cu6Sn5

Cu3Sn

Cu6Sn5

Anode

Anode

Cu

Cu

(b)

(a) Solder

Solder

21 hr

87hr

87hrCu3Sn

Cathode

Cathode

Cu

Cu

(d)

(c)Solder

SolderVoid

Cu6Sn5

Electromigration in Solder Joints and Lines - Engineering · Electromigration in Solder Joints and...

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