On Pb-free (solder) Interconnections for High-Temperature

Applications

A.A. Kodentsov

Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, The Netherlands

Cross-sectional view of flip-chip package

• 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 !

Liquidus projection of the Zn-Al-Mg system

Ternary eutectic at ~ 343 C

The binary Bi – Ag phase diagram

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

The binary Bi – Ag phase diagram

The DSC heating curve of the eutectic Bi-Ag alloy

Solidification microstructure of the Bi-Ag eutectic alloy (BEI)

Solidification microstructure of the Bi-Ag hypo-eutectic alloy (BEI)

Ag

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

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

The effect of Ni additives in the Cu-substrate on the interfacial

reaction with Sn

The binary Cu – Sn phase diagram

The binary Cu – Sn phase diagram

215 C

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:

Reaction zone developed between Sn and Cu 1at.% Ni alloy after annealing at 215 C for 400 hrs

pores !!!

Reaction zone developed between Sn and Cu 5at.% Ni alloy after annealing at 215 C for 400 hrs

No pores !!!

No -Cu3Sn was detected!

Isothermal sections through the Sn-Cu-Ni phase diagram

P. Oberndorff, 2001 C.H. Lin, 2001

235 C 240 C

Reaction zone developed between Sn and Cu 5at.% Ni alloy after annealing at 215 C for 400 hrs

No pores !!!

No -Cu3Sn was detected!

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:

215 C; 1600 hrs; vacuum

The binary Cu – Sn phase diagram

Part of the Cu-Sn phase diagram in the vicinity of the / transition

Long-Period Superlattice

Simple Superlattice

215 C

- phase ?

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

Binary phase diagram Ni-Bi

250 C

250 C; 200 hrs; vacuum

250 C; 200 hrs; vacuum

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

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

Cu5Ni

Ni

Bi

Ni

Ni

Ni

NiBi3 280 C

Kirkendall plane (s)

250 C; 400 hrs; vacuum

Kirkendall plane(s)

Cu5Ni

Ni

Bi

Ni

Ni

Ni

NiBi3 280 C

Kirkendall plane (s)

Ni

Bi

Bi

Ni

Ag

Cu5Ni

Ni

NiBi3

NiBi3

280 C

Ni

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

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

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

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

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 !