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On Pb-free (solder) Interconnections for High-Temperature
Applications
A.A. Kodentsov
Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, The Netherlands
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Cross-sectional view of flip-chip package
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• There is still no obvious (cost-effective) replacement for high-lead, high melting ( 260 - 320 C) solder alloys
• It is not possible to adjust (to increase above 260 C) liquidus temperature of any existing Sn-based solder alloys by simple alloying with environmentally friendly and inexpensive elements
• Therefore, in the quest for (cost-effective) replacements of the high-lead solders, attention has to be turned towards different base metals as well as the exploration of alternative joining techniques !
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Liquidus projection of the Zn-Al-Mg system
Ternary eutectic at ~ 343 C
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The binary Bi – Ag phase diagram
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TMS 2008 Annual Meeting, New Orleans March 9-13, 2008
“Interfacial behaviour between Bi-Ag Solders and the Ni -substrates” (Hsin-Yi Chuang and Jenn-Ming Song)
“Interfacial Reaction and Thermal Fatigue of Zn-4wt.%Al-1wt.% Cu/Ni Solder Joints” by Y. Takaku, I. Ohnima, Y. Yamada, Y. Yagi, I. Nakagawa, T. Atsumi, K. Ishida
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The binary Bi – Ag phase diagram
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The DSC heating curve of the eutectic Bi-Ag alloy
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Solidification microstructure of the Bi-Ag eutectic alloy (BEI)
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Solidification microstructure of the Bi-Ag hypo-eutectic alloy (BEI)
Ag
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Transient Liquid Phase (TLP) Bonding
solid
solid
solid interlayer(s)
• The interlayers are designed to form a thin or partial layer of a transient liquid phase (TLP) to facilitate bonding via a brazing-like process in which the liquid disappears isothermally
• In contrast to conventional brazing, the liquid disappears, and a higher melting point phase is formed at the bonding temperature
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Transient Liquid Phase (TLP) Bonding
Any system wherein a liquid phase disappears by diffusion, reaction (amalgamation), volatilization, or other processes is a candidate for TLP bonding !
solid
solid
solid
solid
solid
T=
const liquid
solid
solid product
T=
constDiffusion, Reaction
solid
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The effect of Ni additives in the Cu-substrate on the interfacial
reaction with Sn
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The binary Cu – Sn phase diagram
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The binary Cu – Sn phase diagram
215 C
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Diffusion zone morphology developed between Cu and Sn after reaction at 215 C in vacuum for 225 hrs
1.6VDVD
J
J
SnCu
CuSn
Cu
Sn
In the -Cu6Sn5:
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Reaction zone developed between Sn and Cu 1at.% Ni alloy after annealing at 215 C for 400 hrs
pores !!!
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Reaction zone developed between Sn and Cu 5at.% Ni alloy after annealing at 215 C for 400 hrs
No pores !!!
No -Cu3Sn was detected!
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Isothermal sections through the Sn-Cu-Ni phase diagram
P. Oberndorff, 2001 C.H. Lin, 2001
235 C 240 C
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Reaction zone developed between Sn and Cu 5at.% Ni alloy after annealing at 215 C for 400 hrs
No pores !!!
No -Cu3Sn was detected!
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Diffusion zone morphology developed between Cu and Sn after reaction at 215 C in vacuum for 225 hrs
1.6VDVD
J
J
SnCu
CuSn
Cu
Sn
In the -Cu6Sn5:
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215 C; 1600 hrs; vacuum
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The binary Cu – Sn phase diagram
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Part of the Cu-Sn phase diagram in the vicinity of the / transition
Long-Period Superlattice
Simple Superlattice
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215 C
- phase ?
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Cu5Ni
Cu5Ni
Sn
Cu5Ni
Cu5Ni
Cu5Ni
(Cu,Ni)6Sn5 250 C
Kirkendall plane (s)
Cu5Ni
Sn
Sn
Cu5Ni
Ag
Cu5Ni
Cu5Ni
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
250 C
Cu5Ni
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Binary phase diagram Ni-Bi
250 C
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250 C; 200 hrs; vacuum
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250 C; 200 hrs; vacuum
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0 50 100 150 200 2500
10000
20000
30000
40000
50000
squ
are
th
ickn
ess
(1
0^-
12
m^2
)
time (hr)
Parabolic growth of the NiBi3 intermetallic layers in the binary diffusion couples at 250 C
kp= 5.2 x 10-14 m2/s
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Component Knoop hardness (kgf*mm-2)
Ni 113.8
NiBi3 113.4
NiBi 264.8
Cu 79.2
Cu3Sn 464.5
Cu6Sn5 420.8
Knoop microhardness test on Ni-Bi and Cu-Sn systems
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Cu5Ni
Ni
Bi
Ni
Ni
Ni
NiBi3 280 C
Kirkendall plane (s)
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250 C; 400 hrs; vacuum
Kirkendall plane(s)
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Cu5Ni
Ni
Bi
Ni
Ni
Ni
NiBi3 280 C
Kirkendall plane (s)
Ni
Bi
Bi
Ni
Ag
Cu5Ni
Ni
NiBi3
NiBi3
280 C
Ni
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x(
Ag
)
x(Bi)
0.0
0.2
0.3
0.5
0.7
0.9
0.0 0.2 0.4 0.6 0.8 1.00 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
x(B i)
x(A
g)
Liquidus surface
N i
Ag
B i
L IQ U ID
L IQ U IDF C C _ A 1
BINIBI3N I
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x(A
g)
x(Ni)
0.0
0.2
0.3
0.5
0.7
0.9
0.0 0.2 0.4 0.6 0.8 1.00 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
x(N i)
x(A
g)
250 C
B i
Ag
Ni
FCC_A 1+BIN I+FCC_A 1
BI3N I+BIN I+FCC_A 1
RH
OM
BO
_A7+
BI3
NI+
FCC
_A1
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x(A
g)
x(Ni)
0.0
0.2
0.3
0.5
0.7
0.9
0.0 0.2 0.4 0.6 0.8 1.00 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
x(N i)
x(A
g)
268 C
B i
Ag
Ni
FCC_A 1+FCC_A 1+BIN I
BI3N I+BIN I+FCC_A 1
BI3
NI+
FCC
_A1+
LIQ
UID
LIQ U ID +BI3N I
LIQ U ID +BI3N I+RH O M BO _A 7
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x(A
g)
x(Ni)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.0 0.1 0.2 0.3
x(N i)
x(A
g)
268 C
B i
LIQ U ID +FCC_A 1+BI3N I
LIQ U ID +BI3N I
LIQ U ID +BI3N I+RH O M BO _A 7
FC
C_A
1+B
I3N
I+B
INI
LIQ U ID
LIQ U ID+RHOMBO_A7
0 0.1 0.2 0.30
0.01
0.02
0.03
0.04
0.05
0.06
0.07
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Concluding Remarks
• Through the judicious selection of Sn- or Bi-based interlayer between under bump metallization and substrate pad, (cost-effective) Transient Liquid Phase (TLP) Bonding can be achieved at ~ 250-280 C, and the resulting joints are capable of service at elevated temperatures !
• Therefore, in the quest for (cost-effective) substitutes for high-lead solders, attention has to be turned towards different base metals as well as the exploration of alternative joining techniques !
• It is not possible to adjust (to increase above 260 C) liquidus temperature of any existing Sn-based solder alloys by simple alloying with environmentally friendly and inexpensive elements
• The TLP Bonding should be taken into further consideration as substitute for the high-lead soldering !