Paper 5.2

Phenomenon of Back-diffusion in Formation of Intermetallic Structure Within Ni/Al/Ni Interconnection

Janczak-Rusch J.¹, Kloch , J.², Guzik E.³, Wołczyński W.⁴

¹ Laboratory of Joining and Interface Technology, Swiss Federal Laboratories for Materials Testing and Research (EMPA), 129 Ueberland St., 8600 Duebendorf, Switzerland,

² Institute of Mathematics, Polish Academy of Sciences,

31 027 Kraków, Św.Tomasza 30, Poland,

³ Faculty of Foundry Engineering, University of Science and Technology,

30 059 Kraków, Reymonta 23, Poland,

⁴ Institute of Metallurgy and Materials Science, Polish Academy of Sciences,

30 059 Kraków, Reymonta 25, Poland,

The Ni-Al phase diagram has been used to place on its liquidus line the nominal Al - solute concentration measured previously within the Ni/Al/Ni joint. Moreover, a real temperature imposed for isothermal solidification has also been put on the Ni-Al phase diagram. A difference between equilibrium temperature and real temperature is treated as a motive force for solidification which occurred during formation of the Ni/Al/Ni interconnection. The calculation of a nominal concentration of the Ni-Al solidifying alloy is based on the EDS measurement of the Al - solute profile within the real Ni/Al/Ni joint frozen just at the disappearance of the liquid Al – filler metal. Simulation of solidification is performed starting from planimetered nominal Al - solute concentration and completing before a cross-section of equilibrium and real temperature on liquidus line of the Ni-Al phase diagram. The formation of each sub-layer within the joint formed during diffusion soldering depends on a value of back-diffusion parameter calculated separately for partitioning and redistribution, next for peritectic reaction and finally for solid/solid transformation. The computer simulation of the width and segregation within different peritectic phases appearing in sequence is in accordance with the Ni-Al phase diagram. The sub-layers of a proper peritectic phases are created during simulation starting from the Ni – substrate and completing at the axis of symmetry of the joint contrary to the technology according which the sequence of sub-layers is created just in inverse direction that is from the axis of symmetry to the Ni -   substrate. At first, the Al- reach phase is appearing at the axis and Ni – reach phase is appearing at the substrate due to the dissolution/solidification phenomena governing the technology. The constant partition ratio is used to simulate solidification of the Al₃Ni₂ peritectic phase but the use of variable partition ratio is required to simulate the formation of the Al₃Ni – peritectic phase. The variable partition ratio has a hyperbolic dependence on the solute concentration in the liquid.

The greater is the value of back-diffusion parameter the bigger is the width of a given sub-layer. The analysed segregation and width of each sub-layer can be reproduced within a joint for which solidification was stopped and morphology frozen. It was possible because the applied theory contains a parameter responsible for a freezing of solidifying alloys.