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Improved sinterability of particles to substrates by surface modifications on substrate metallizations
Keywords: particle sintering, surface modifications, microelectronics packaging
The rapid pace of electrification in the automobile industry through electric vehicles and hybrid electric vehicles requires high power density devices such as electric vehicle batteries and motors. These are the driving forces for the development of high-power semiconductors apart from aerospace applications in the form of unmanned aerial vehicles (UAVs) and space exploration requirements. Die-attach bonding is a key process to realize high-temperature operation of power semiconductor devices. Solder alloys have low electrical conductivities compared to sintering solutions with Ag or Cu and are susceptible to fatigue failure under cyclic loading. With the increase in the usage of wide bandgap (WBG) seminconductor devices, it is imperetive to find sustainable and reliable alternatives both on the economical and the technical fronts, able to fulfill the challenging requirements of the WBG semiconductor devices and to realize mass production. Sintering has been in the forefront of the research and development over the past decade, moreso Ag sintering as an alternative to high temperature soldering and die-attach bonding for high temperature electronics[1][2][3]. However, the use of high bonding pressure during the sintering process has been a deterrent to mass-scale industrialization[4]. A high bonding pressure is applied during sintering to achieve a closed packing structure in the joint and the surface contact between the substrate metallizations and the sinter particle paste. One way suggested to limit the use of a high bonding pressure during sintering is by inducing surface modifications which enhance sinterability of the particles[5]. However, substrate metallizations are observed to play an important role while sintering. Topological modifications to the substrate has been reported to increase the shear strength in case of Ag sintering [6]. A double stencil printing process has also been suggested[7]. We propose nanostructured surface modifications on the substrates. These nanostructures surface modifications serve two purposes namely; to increase surface contact between the substrate and the particles and to increase surface energy due to the increased curvatures. Both these factors are well known to enhance sinterability which is mainly driven by diffusion bonding and particle consolidation. Three different types of surfaces have been investigated as part of our sintering with the in-house developed hybrid Cu and commercially available Ag sinter pastes, namely; Cu, standard electroless nickel immersion gold (ENIG) and surface modified brass substrates. The initial experiments have been conducted on 0,8mm thick brass substrates (max. 37% Zn). These substrates are etched in HCl and scanning electron microscopy analysis show the development of nano-strcutured surface modifications on the substrates. The substrates are therafter cleaned to remove any traces of the etchants which could affect bondability and cause corrosive effects on the joint. Preliminary shear tests on samples with surface modified brass substrates are promising with results of ~21 MPa, which is greater by a factor 1,75 compared to sintering with Cu-Cu bonding surfaces and factor 1,5 compared to sintering with Au-Au bonding surfaces with in-house developed hybrid Cu paste under sintering conditions of low bonding pressure (1MPa) at 300C for 60min under N2 atmosphere. In our research, we propose surface modifications on substrate metallizations thorugh a wet chemical etching process and sintering with hybrid particles pastes, which are capable of sintering at 250-300C. Initial tests show a marked improvement compared to traditional surface metallizations. Temperature cycle tests to validate the quality of the interconnect under thermo-mechanical stress are ongoing and shall be presented during the submission of the full paper.
Sri Krishna Bhogaraju, Research Associate
Institute of Innovative Mobility - Technische Hochschule Ingolstadt
Ingolstadt, Bavaria

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