Factors influencing microstructural evolution in nanoparticle sintered Ag die attach

Abstract

The behaviour of sintered silver die attach at high temperature has been investigated. Assemblies were made by sintering a commercially available paste composed of Ag nanoparticles with zero applied pressure on the die. The morphology of the cross sectioned surface of assemblies remains stable even at temperatures of up to 400 °C. This behaviour remained consistent even inside vacuum or after acid cleaning of the free surface. In contrast, the same sintered Ag material in the interior of a joint or sintered under a glass cover slip showed rapid microstructural changes even at 300 °C. These samples were investigated using an optical microscope to analyse the changes in the microstructure after storage at 200 to 500 °C. The observations showed a 20% increase in silver grain size after only 5 h storage at 300 °C. However, in the case of a free surface, no changes were observed after 60h storage at 400 °C. These observations were combined with DSC experiments in order to suggest the cause of the difference in behaviour. The results suggest ways of stabilizing sintered silver materials so that they can be used in applications up to 400 °C without significant structural changes occurring in the material.

Microstructural evolution of sintered silver at elevated temperatures

“…They do however show that a low temperature passivating layer exists with the same characteristics as those found in the thermal experiments, preventing atomic surface diffusion only at the exterior of the porous material. 20 Considering the results from sample sets 1-5 as a whole, the passivating layer cannot be due purely to organic compounds from the original paste, as these exist also in the interior of the material, and would have been removed from the exterior by the perchloric acid solution. The presence of oxygen on the free surface of sintered silver has been detected by EDX at room temperature in a similar study [24].…”

“…The high surface energy available and noble metal surface of silver also allows sintering to take place without 3 pressure being placed on the die; an added benefit for manufacturers, but one which leads to the final sintered structure containing 20-30% of pores. Although many studies have been performed on the sintering behaviour of nanoparticles [10,11], and also the mechanical properties of their sintered structures [8,9,12,13], there exists few detailed studies on the high temperature behaviour and stability of sintered silver at temperatures above 300 °C as a high temperature die attach (excepting [14][15]), while previous studies below 300 °C concentrate on bulk mechanical properties and statistical averages of microstructural properties [8,9,[16][17][18][19][20]. In the present work, we determine the nature and speed of microstructural changes in sintered silver throughout the 200-400 °C temperature range and investigate the atomic migration mechanisms that lead to these changes.…”

Arresting high-temperature microstructural evolution inside sintered silver

“…The grain sizes were analysed by taking optical images of the samples and analysing the average grain sizes of the samples using ImageJ 1.46 and the Matlab® image processing toolbox. Detailed description of grain size calculations can be found in references [10], [13]. The measured average grain sizes were then investigated to estimate the limitations of the sintered silver stabilisation technique at each storage temperature.…”

“…COMSOL Multiphysics® software has been utilised in an attempt to further understand the effects of an oxide layer by modelling the diffusion behaviour and resulting microstructural evolution of individual grains in the absence of an oxide. While there have been empirical studies to investigate the microstructural evolution inside sintered silver previously [13][6], these have not probed the underlying mechanisms. A few studies have been carried out on the modelling of its long-term reliability using Finite Element (FE) methods, using visco-plastic continuum mechanics models [14], again without detailed understanding of the underlying mechanisms.…”

Ultra-Stable Sintered Silver Die Attach for Demanding High-Power/Temperature Applications

“…The optical images from these samples were then analysed using ImageJ 1.46 and the Matlab® image processing toolbox to investigate the microstructural changes of sintered silver grains. Refer to references [9,14] for further information on the image processing techniques utilised and algorithms for extracting grain size, which references also include further information regarding the benefits and implications of observations of sintered silver microstructure through a cover-slip.…”