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 K. Ishida / Ni Solder Joints” by Y. Takaku, I. Ohnima, Y. Yamada, Y. Yagi, I. Nakagawa, T. Atsumi,

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

solid Transient Liquid Phase (TLP) Bonding solid interlayer(s) solid • 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

solid Transient Liquid Phase (TLP) Bonding solid T = const liquid solid solid solid T = const Diffusion, Reaction solid solid product solid Any system wherein a liquid phase disappears by diffusion, reaction (amalgamation), volatilization, or other processes is a candidate for TLP bonding !

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 after reaction at 215



C in vacuum for 225 hrs and Sn

In the



-Cu

6

Sn

5

:

J Sn J Cu



D Sn D Cu

 

V Cu V Sn



1.6

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



-Cu 3 Sn 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



-Cu 3 Sn was detected!

Diffusion zone morphology developed between Cu after reaction at 215



C in vacuum for 225 hrs and Sn

In the



-Cu

6

Sn

5

:

J Sn J Cu



D Sn D Cu

 

V Cu V Sn



1.6

215



C; 1600 hrs; vacuum

The binary Cu – Sn phase diagram

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

  

/ transition Simple Superlattice Long-Period Superlattice

215



C

 

phase ?

Cu5Ni Sn Cu5Ni Sn Ag Sn Cu5Ni 250



C 250



C Cu5Ni Cu5Ni (Cu,Ni) 6 Sn 5 Kirkendall plane (s) Cu5Ni (Cu,Ni) 6 Sn 5 (Cu,Ni) 6 Sn 5

Binary phase diagram Ni-Bi 250



C

250



C; 200 hrs; vacuum

250



C; 200 hrs; vacuum

Parabolic growth of the NiBi 3 intermetallic layers in the binary diffusion couples at 250



C k p = 5.2 x 10 -14 m 2 /s

50000 40000 30000 20000 10000 0 0 50 100 time (hr) 150 200 250

Knoop microhardness test on Ni-Bi and Cu-Sn systems Component Knoop hardness (kgf*mm -2 ) Ni NiBi 3 NiBi Cu Cu 3 Sn Cu 6 Sn 5 113.8

113.4

264.8

79.2

464.5

420.8

Ni Bi 280



C Ni Ni NiBi 3 Kirkendall plane (s)

250



C; 400 hrs; vacuum Kirkendall plane(s)

Ni Bi Ni Bi Ag Bi Ni 280



C 280



C Ni Ni NiBi 3 Kirkendall plane (s) NiBi 3 Ni NiBi 3

0.9

Liquidus surface 1 Ag 0.7

0.8

0.5

x( A g) 0.3

0.4

0.6

LIQUID 0.2

0.2

Ni x(Bi) x(Bi) FCC_A1 LIQUID BINI BI3NI Bi

0.9

250 C 1 Ag 0.7

0.8

0.5

x( A g) R 0.3

O M B 0.2

O _ A 7 + B I3 N I+ FC C _ A 1 0.2

0.4

0.6

Bi BI3NI+BINI+FCC_A1 x(Ni) x(Ni) FCC_A1+BINI+FCC_A1 Ni

0.9

268 C 0.7

0.5

x( A g) B I3 N I+ F C C _ A 1 + LI Q U ID 0.6

0.8

0.3

0.4

1 Ag 0.2

LIQUID+BI3NI 0.2

BI3NI+BINI+FCC_A1 Bi LIQUID+BI3NI+RHOMBO_A7 x(Ni) x(Ni) FCC_A1+FCC_A1+BINI Ni

268 C LIQUID+FCC_A1+BI3NI LIQUID LIQUID+BI3NI LIQUID +RHOMBO_A7 LIQUID+BI3NI+RHOMBO_A7 Bi x(Ni) x(Ni)

Concluding Remarks

• It is not possible to adjust and inexpensive elements (to increase above 260



C) liquidus temperature of any existing Sn-based solder alloys by simple alloying with environmentally friendly • Therefore, in the quest for (cost-effective) substitutes high-lead solders , attention has to be turned towards different base metals as well as the exploration of alternative joining techniques !

for • 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 !

• The TLP Bonding should be taken into further consideration as substitute for the high-lead soldering !