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Development of more efficient sintering processes to attach power devices using commercially available dry film and pastes of Ag nanoparticles
Keywords: sintering of Ag nanoparticles, die attachment, efficient sintering process
Sintering of Ag particles and nanoparticles has been demonstrated as one of the most promising lead-free solder alternatives for power die attachment. This is because the sintering process can be carried out at temperatures similar to those used in soldering processes, but the sintered Ag joints have higher creep resistance and higher electrical and thermal conductivities than the conventional solder joints. There have been a wealthy of work in the Ag sintering technology covering the formulation of Ag nanoparticle pastes; effects of manner and pattern of printed paste, sintering behaviour of Ag paste, metallization and surface topology of substrate on the adhesion strength and mechanical, thermo- mechanical properties and thermal cycling and power cycling reliability of the sintered Ag joints. The Ag sintering processes for power die attachment can briefly be classified into three types depending on whether or how much pressure is applied during the sintering process. The first one is the high pressure- assisted sintering of microscale Ag particle paste which was invented in 1990. The second one is the pressure- less sintering of Ag nanoparticle pastes pioneered in early 2000 and the mixed micro/nanoparticle Ag pastes developed in a few years later. The third one is the lower pressure- assisted sintering of Ag nanoparticle pastes or dry films which was almost appeared and proliferated almost at the same periods as the second type of pressure-less sintering process. For all these three types of sintering processes, emphasis of optimizing sintering parameters has been placed on the “best” die attachment in terms of either high shear strength or high reliability, and the suppliers of the pastes or dry films in general recommend a relatively narrow window of the sintering parameters for each of them to achieve the “optimized” die attachment. As far as the recommended sintering times are concerned, they increase in the order of the high pressure-assisted, lower pressure- assisted and pressure-less sintering, from several seconds to several hours. Both low pressure sintering and pressureless sintering are highly desirable because they are relatively easy to be carried out and can reduce the risk of damaging power devices. The present work is concerned with the development of more efficient sintering processes to attach power devices using the low pressure sintering and pressureless sintering of commercially available dry film and pastes of Ag nanoparticles. In particular, efforts have been placed on significant reduction in the recommended sintering time, while the sintered Ag joint power attachments should have sufficient shear strengths and high thermos-mechanical reliability better than or at least similar to the conventional solder joints. In the present work, the low pressure- assisted sintering parameters cover the sintering temperatures of, sintering pressures of and sintering times of, and the pressure-less sintering parameters cover the sintering temperatures of and sintering times of . The effects of the sintering parameters on the shear strength and microstructure of the sintered joints have been investigated. The objectives of this paper are: (i) to find the quickest sintering processes with the selected dry film and pastes of Ag nanoparticles; (ii) to compare the microstructure and shear strength of the presently different samples; (iii) to identify the trade-off between processing time and microstructure and shear strength of the sintered Ag joint die attachments; and (iv) to compare the present results with the typical data reported in the existing literature.
Jingru Dai, PhD student
University of nottingham
Nottingham, Nottinghamshire
United Kingdom

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