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Fluxless Microjoining by Au-In-Ni Isothermal Solidification

Published online by Cambridge University Press:  10 February 2011

M. Waelti,
N. Schneeberger,
O. Brand and
H. Baltes
M. Waelti
Affiliation:
Physical Electronics Laboratory, ETH Zurich, Hoenggerberg HTP-H6, CH-8093 Zurich, Switzerland
N. Schneeberger
Affiliation:
Physical Electronics Laboratory, ETH Zurich, Hoenggerberg HTP-H6, CH-8093 Zurich, Switzerland
O. Brand
Affiliation:
Physical Electronics Laboratory, ETH Zurich, Hoenggerberg HTP-H6, CH-8093 Zurich, Switzerland
H. Baltes
Affiliation:
Physical Electronics Laboratory, ETH Zurich, Hoenggerberg HTP-H6, CH-8093 Zurich, Switzerland
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Abstract

A reliable, fluxless microjoining technique based on isothermal solidification is reported. The Au-In-Ni system has been chosen for bonding, because gold is often applied for wafer bumping, and Ni is commonly used for substrate plating. To demonstrate the potential of this microjoining scheme, optical filter have been directly attached to a smart CMOS thermoelectric IR system. Since further packaging of the microsystem includes SMT assembly, the bond has to withstand subsequent SnPb reflow-soldering.

The influence of bonding time and temperature on the bond was investigated. Phase formation and transformation were analyzed by light and scanning electron microscopy (SEM). Bonding performed at temperatures below 200°C remained stable even after multiple soldering cycles with peak temperatures of 235°C. The shear strength of the bonds was found to be more than 70 MPa. Long term stability was confirmed by extended anneal at 160°C. The method is generally well suited for processes involving sequences of joining steps, and for bonds demanding high strength and high thermal stability at low bonding temperature.

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