The Behaviour of Au-Au Wire Bonds in Extreme Environments

Abstract

Microelectronics are being required to show high reliability within aero-engines, oil-gas wells and other extreme applications where devices can experience over 250°C, 1000bar, corrosive environments and significant vibration. Currently operation may be only possible for a small number of hours, which necessitates expensive shut-down and replacement. Wire bond interconnects continue to be critical to overall reliability though previous studies have shown that traditional Au-Al bonds fail in such environments due to microstructural instability. Au-Au bonds offer a potentially stable solution that is increasingly desirable as microelectronics are required to endure ever harsher conditions. Au ball bonds on two representative substrates with Au surfaces and ceramic bases have been exposed to 250 and 300°C in N2 for up to 2000 hours. Substrate 1 was a high temperature co-fired ceramic (HTCC) with a Au coating above Ni-Co and W layers, and Substrate 2 was a Au thick film on alumina. Wire pull and ball shear tests for both sample types significantly exceeded industry specifications, though a decrease in yield strength was observed. Key samples were cross-sectioned and grain structure was revealed by ion channelling contrast within a focused ion beam (FIB) system. The yield strength decrease was attributed to the Hall-Petch effect through elimination of twin grains formed during bonding deformation. Electron probe microanalysis (EPMA) of underlying Ni showed little interdiffusion into the Au. Hence the mechanical robustness and microstructural stability of Au-Au wire bonds in extreme environments has been demonstrated and understanding of thermal ageing mechanisms has been improved.

Electronics design, assembly and reliability for high temperature applications

“…For wire bonding, Au bumping of aluminium bond pads with a suitable barrier material to create a monometallic joint between the Au wire and Au bond pad surface is attractive and has been shown to produce stable bonds up to at least 300 o C for 2000 hours [5].…”

Effectiveness of Barrier Layer Metallizations in Long Term High Temperature Endurance Tests on Wire Bond Interconnections

“…2 For wire bonding, Au bumping of aluminum bond pads with a barrier material to create a monometallic joint between the Au wire and Au bond pad surface is attractive. Au wire bonding to Au thick film on alumina has been shown to produce stable bonds up to at least 300 C for 2,000 h [6]. There are several options available commercially for the bumping of devices with Au surface metallization, but little work has been undertaken to examine the long-term aging characteristics of Au wire bonds with bumped devices.…”

“…The devices were then Au wire bonded using 25 mm diameter wire using conditions established from previous work [6]. The devices were hermetically sealed using Au-Sn solder between the package seal ring and an Au/Ni plated Kovar lid.…”

Mechanical and electrical characterisation of Au wire interconnects in electronic packages under the combined vibration and thermal testing conditions

“…Shepherd et al [5] reported small deterioration of the ball shear and wire pull strength after long-term thermal ageing at 250˚C with the ball shear and wire pull strength levels remaining high even after 2000 hours of testing. The low mechanical strength levels resulted from the combined tests in our work indicate the contribution-to-failure of the vibration effects to the temperature related effects.…”

“…Furthermore, the bond is not susceptible to interface corrosion or other bonddegrading conditions. Shepherd et al [5] examined the changes in bond mechanical behaviour and the underlying microstructural stability of Au-Au metallurgical systems under 3 thermal ageing at 250˚C and 300˚C. They reported that these systems are mechanically robust in extreme environments with a stable microstructure.…”