Ivan Nikitin

Mechanical properties of porous silver materials depending on sintering parameters

We investigated the mechanical properties of a silver sinter paste dependent on sintering and drying conditions. For these experiments we fabricated dedicated samples without silicon and leadframe to investigate the pure material properties. Our investigation on the influence of drying and sintering conditions demonstrated that the longer the sintering time and the higher the temperature the higher is the stiffness of the sintered layer. We found that a sintering pressure in the range of 2, 5-10 MPa has no influence on the sintered layer for the investigated paste. An optimum drying temperature of 150°C was identified. Microstructure evaluation showed that the concentration of the pores was about 19% ±3%. We observe that the sintering conditions only influence the pores size but not the pores area/volume. These material studies allow us to achieve material parameter inputs for further simulations to describe the die attach process. In addition we achieve the optimum material parameter for the sintering process.

Accelerated reliability testing and modeling of subsystems based on sintered silver thermal interface materials

This paper proposes a guideline for the mechanical acceleration of end-of-lifetime prognostics of metal based die attach materials. As a such, we used first an advanced nano-effect sintered silver layer as interface between die and substrate which has very god electrical and thermal conductivities. Two pairs of experiment/simulation are scheduled. An isothermal mechanical 3-pt bending experiment to induce fatigue the specimens rapidly as well as a thermal induced strain fatigue by thermal chamber for validation. The manufactured specimens are designed to be used for both. With a FEM of this subsystem to simulate the failure parameter which is the accumulated von Mises strain, lifetime modelling is possible.

Thermo — Mechanical characterization and reliability modelling of sintered silver based thermal interface materials

Within this paper, we present a guideline for the mechanical acceleration of reliability experiments for end-of-lifetime prognostics of metal based die attach materials. First, we used an advanced hybrid nano-effect sintered silver layer as interface between die and substrate which has very good electrical and thermal conductivities. Two pairs ofexperiment/simulation are scheduled. An isothermal mechanical 3-pt bending experiment to induce fatigue the specimens rapidly as well as a thermal induced strain fatigue by thermal chamber for validation. The manufactured specimens are designed to be used for both. With a FEM of this subsystem to simulate the failure parameter which is the accumulated von Mises strain, lifetime modelling can be performed.

Reliability experiments of sintered silver based interconnections by accelerated isothermal bending tests

Abstract Integration of more functionality and smaller chips into decreasing package volume leads to increasing heat generation. In addition, the use of new compound semiconductors like SiC and GaN require a high thermal conductivity of the interconnect materials. One of the promising solutions is a layer of sintered silver between semiconductor and substrate. The advantages compared to conventional solders are significant. A higher thermal and electrical conductivity in combination with a higher duty temperature due to a higher melting point should enhance the reliability of the package. However, even as the large scale commercial usage of the material has been started by the industry recently, many important details of the mechanical properties and the reliability behavior are still unknown. While the thermal properties could be characterized relatively easy and are quite repeatable and stable, the mechanical properties - important for the reliability - are extremely process-dependent and wide-spreading. The hunt for lowest feasible sintering process parameters - such as temperature, time and especially pressure - even amplify that behavior and led to an impasse in some cases. Also their failure mechanisms, to be identified in lifetime investigations, are yet unknown as well as their stability and predictability. In order to enable prolonged function of these interfaces, thermo-mechanical reliability has to be assured. Within this paper, we show the status of silver sintering and the problems regarding mechanical material characterization found in literature. Additionally, we present a guideline for the mechanical acceleration of reliability experiments by isothermal bending tests. Finally a proof of concept by failure analysis will be presented.

Modeling of the effective thermal conductivity of sintered porous pastes

The thermal conductivity of sintered porous pastes of metals is modelled, based on an analytical and a numerical approach. The first method arises from the differential effective medium theory and considers the air voids as ellipsoidal pores of different sizes, while second one is based on the finite element method to analyse the scanning-electron-microscope images of the paste cross sections. It is shown that the predictions of both approaches are consistent to each other for a sample of porous silver paste. Pancake-shaped pores are the main sources of reduction of the thermal conductivity of porous pastes, and they block the thermal conducting pathways more efficiently than spherical and cigar-shaped pores. The decrease of the thermal conductivity of the matrix due to the presence of pores can be minimized with spherical pores. The predictions of the proposed methodology could provide useful insights on the thermal behaviour of porous pastes.

Reliability of advanced thermal interface technologies based on sintered die-attach materials

This paper proposes a guideline for the mechanical acceleration of end-of-lifetime prognostics of metal based thermal interfaces. As die attach material, we used an advanced nano-effect sintered silver layer as interface between die and steel substrate which has very good electrical and thermal conductivities. Two types of experiments/simulations are scheduled. A mechanical 4-pt bending experiment to cause the specimens to undergo fatigue failure rapidly as well as a thermal strain induced fatigue by thermal cycling for comparison. The manufactured specimens are designed to be used for both. With a Finite Element (FE)-model it is possible to simulate the accumulated von Mises strain as failure parameter to generate a lifetime model. Most of the work is currently in progress and results will be delivered soon as full paper.