Transient Liquid Phase Bonding as a Potential Substitute for Soldering with High-Lead Alloys A.A. Kodentsov Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, The Netherlands

There is still no obvious (cost-effective) replacement for high-lead, high melting (  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

Transient Liquid Phase (TLP) Bonding 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 T = const liquid solid solid product T = const Diffusion, Reaction solid

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

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 In the  -Cu 6 Sn 5 :

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, 2001C.H. Lin,  C240  C

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

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

Binary phase diagram Ni-Bi 250  C

250  C; 200 hrs; vacuum

Cu5Ni Ni Bi Ni NiBi  C Kirkendall plane (s) Ni Bi Ni Ag Cu5Ni Ni NiBi  C Ni

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 ~  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 !

Diffusion zone morphology developed between Cu and Sn after reaction at 215  C in vacuum for 225 hrs

Parabolic growth of the Cu-Sn intermetallic layers in the binary diffusion couples at 215  C 1.58 x m 2 /s 7.55 x m 2 /s

Diffusion zone morphology developed between Cu and Sn after reaction at 215  C in vacuum for 225 hrs

Determination of the ratio of intrinsic diffusivities of species in line-compounds

Diffusion zone morphology developed between Cu and Sn after reaction at 215  C in vacuum for 225 hrs In the  -Cu 6 Sn 5 :

The Cu 3 Ti – type lattice (oP 8 ), the basic structure of the long-period superstucture of the  -Cu 3 Sn View down [010] TiCu TiCu The hexagonal analog of L1 2 -structure of Cu 3 Au !

In the Long-Period Superstructure of  -Cu 3 Sn (oC80 ) antiphase shifts occur at every fifth period along the b 0 -axis The basic structure (oP 8 ) c0c0 b0b0 a0a0 (a=2a 0 ; b=10b 0 ; c=c 0 ) Projection onto (001) plane Sn has 12 Cu NN Cu has 4 Sn and 8 Cu NN There are no Sn - Sn NN The hexagonal analog of the Cu 3 Au!

“The ordered Cu 3 Au rule” L1 2 - type structure (A 3 B) The nearest neighbour (NN) arrangement of A -atoms The nearest neighbour arrangement of B -atoms

Reaction zone developed between Sn and Cu after annealing at 215  C for 400 hrs markers !!!

Reaction zone developed between Sn and Cu after annealing at 215  C for 400 hrs

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  -Cu 3 Sn was detected!

Isothermal sections through the Sn-Cu-Ni phase diagram P. Oberndorff, 2001C.H. Lin,  C240  C

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

Isothermal section through the Sn-Cu-Ni phase diagram at 235  C. (P. Oberndorff, Ph. D. Thesis, Eindhoven University of Technology, The Netherlands, 2001) Cu 1at.% Ni

Isothermal section through the Sn-Cu-Ni phase diagram at 240  C. (C.H. Lin, Master Thesis, National Tsing-Hua University, Republic of China, 2001) Cu 1at.% Ni

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

Isothermal section through the Sn-Cu-Ni phase diagram at 235  C. (P. Oberndorff, Ph. D. Thesis, Eindhoven University of Technology, The Netherlands, 2001) Cu 5at.% Ni

Isothermal section through the Sn-Cu-Ni phase diagram at 240  C. (C.H. Lin, Master Thesis, National Tsing-Hua University, Republic of China, 2001) Cu 5at.% Ni

Diffusion zone morphology developed between Cu and Sn after reaction at 215  C in vacuum for 225 hrs In the  -Cu 6 Sn 5 :

215  C; 1600 hrs; vacuum

The binary Cu – Sn phase diagram

The NiAs- type lattice the basic structure of the  -Cu 6 Sn 5 hP 4 tetrahedral hole trigonal hole octahedral hole

Pictorial view of the NiAs (hP4 ) structure As Ni

Pictorial view of the NiAs (hP4 ) structure A B A C A B A C A Ni has 6 As NN As is surrounded by 6 Ni The As octahedra share faces normal to the c-axis The Ni-atoms are direct neighbours along [001] direction

The composition “Cu 6 Sn 5 ” is achieved by adding additional Cu-atoms in one tenth of the tetrahedral interstices in the hexagonal Sn-sublattice Cu Sn

1. Ordering of the excess Cu -atoms in the tetrahedral interstices results in the  / - Long-Period Superlattice Cu Sn

An arrangement of the unit cells along the three principle axes in the sequence ABABAABABA … results in the supercell of the  / -phase Type A  c/ 2 Type B c a2a2 a1a1 Cu Sn 2. The excess Cu-atoms occupy the tetrahedral interstices at random

215  C  / - Cu 6 Sn 5

Binary phase diagram Ni-Bi 250  C

250  C; 400 hrs; vacuum

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

The reaction zone developed in the incremental couple based on Cu and  - Cu 6 Sn 5 (215  C; 225 hrs) “Unstable” Kirkendall plane ?

The Cu 3 Ti – type lattice (oP 8 ), the basic structure of the long-period superstucture of the  -Cu 3 Sn View down [010] TiCu TiCu The hexagonal analog of L1 2 -structure of Cu 3 Au !

In the Long-Period Superstructure of  -Cu 3 Sn (oC80 ) antiphase shifts occur at every fifth period along the b 0 -axis The basic structure (oP 8 ) c0c0 b0b0 a0a0 (a=2a 0 ; b=10b 0 ; c=c 0 ) Projection onto (001) plane Sn has 12 Cu NN Cu has 4 Sn and 8 Cu NN There are no Sn - Sn NN The hexagonal analog of the Cu 3 Au!