Daniel May

Characterization of Thermal Interface Materials

In this paper new characterization equipment for thermal interface materials is presented. Thermal management of electronic products relies on the effective dissipation of heat. This can be achieved by the optimization of the system design with the help of simulation methods. The precision of these models relies also on the used material data. For the determination of this data an experimental set-up for a static measurement is presented, which evaluate thermal conductivity of thermal interface materials (e.g. adhesive, solder, pads, or pastes). The paper gives an overview over the set-up and the measurement technique and discusses experimental and simulation results

Limitations and accuracy of steady state technique for thermal characterization of thermal interface materials and substrates

The steady state method is a commonly used and in principle simple way to measure thermal resistance and conductivity of thermal interface materials (TIMs). The sample must be positioned between a hot and a cold plate with constant temperatures, whereby a heat flow through the sample and temperature gradient across the sample are generated. To determine the thermal resistance of the sample the heat flow and the temperature gradient have to be measured. This is also defined by the ASTM standard ASTM D5470 [4]. However, for the new generation of highly conductive and thin TIMs, die attach materials and substrate the resolution of the common steady state technique often reaches its limit. To increase the resolution of the steady state equipment beyond the state-of-the-art the test systems must be analyzed and parasitic effects be studied. Some options for increasing the resolution of the steady state method will be studied analytically and by FE simulation within this paper. Accuracy and resolution depend not only on the precision of the setup, but decisively on the selection and execution of the measuring method conformed to the specific measurement task. We will also present our test stand TIMA Tester for thermal characterization of TIMs, die attach materials and substrates based on the mentioned steady state method. It has been developed as a platform which allows the integration of various modules for characterization of different materials under different conditions, e.g. mated surface, finish, operation temperature, pressure, aging etc. Finally, selected studies of different materials will be presented in order to demonstrate the functionality and the accuracy of the test stand.

Processing-structure-property correlations of sintered silver

This paper presents a systematic study of sintered silver. In order to investigate the correlation between processing conditions, microstructures, thermal and electrical properties, sintered silver samples have been prepared in 27 variations of sintering temperature between 200°C and 270°C and sintering pressure between 5 MPa and 25 MPa. For the thermal and electrical characterization, the innovative test stand LaTIMA has been used. The microstructures of the samples have been analyzed by focused ion beam (FIB) and scanning electron microscope (SEM). The results of the thermal and electrical characterizations as well as the structure analysis showed clear correlation to the process conditions of sintered silver.

Microfabricated sensor platform with through-glass vias for bidirectional 3-omega thermal characterization of solid and liquid samples

Abstract A novel microfabricated, all-electrical measurement platform is presented for a direct, accurate and rapid determination of the thermal conductivity and diffusivity of liquid and solid materials. The measurement approach is based on the bidirectional 3-omega method. The platform is composed of glass substrates on which sensor structures and a very thin dielectric nanolaminate passivation layer are fabricated. Using through-glass vias for contacting the sensors from the chip back side leaves the top side of the platform free for deposition, manipulation and optical inspection of the sample during 3-omega measurements. The thin passivation layer, which is deposited by atomic layer deposition on the platform surface, provides superior chemical resistance and allows for the measurement of electrically conductive samples, while maintaining the conditions for a simple thermal analysis. We demonstrate the measurement of thermal conductivities of borosilicate glass, pure water, glycerol, 2-propanol, PDMS, cured epoxy, and heat-sink compounds. The results compare well with both literature values and values obtained with the steady-state divided bar method. Small sample volumes (∼0.02 mm³) suffice for accurate measurements using the platform, allowing rapid temperature-dependent measurements of thermal properties, which can be useful for the development, optimization and quality testing of many materials, such as liquids, gels, pastes and solids.

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.

Novel test stand for thermal diffusivity measurement of bulk and thin films

This paper deals with the development of a new test stand for thermal diffusivity measurement based on Ångströms method (called TIMAwave™). The concept of the test stand has been proved by FE simulation and experiments on standard samples. The test stand has been realized and integrated into the hardware of the already existing test stand LaTIMA™, which was developed for the measurement of thermal conductivity of highly conductive materials. The combination of these two test stands in one device is a great advantage, since the diffusivity and the conductivity of a sample can be measured at one specimen in one device. The results allow the calculation of the specific heat capacity or the density of the sample. Several materials have been characterized by using the new test stand. Some selected results will be discussed in this paper.

Precision determination of thermoreflectance coefficients for localised thermometry

The continual trend of microelectronics to miniaturization, accompanied by the increase of the power density has given modern science a huge challenge. The thermal transport in an electronic device has to be addressed. Therefore, the fast and precise measurement of thermal properties like temperature, thermal conductivity and diffusivity is needed. Especially the spatial resolution needs to be enhanced since the thermal structure size lies within several nanometres nowadays. A contactless measurement device for the surface temperature is presented in this work for later use in a full frame measurement. It utilizes the thermoreflectance effect, which connects the reflectance of a surface with its temperature. A measurement setup has been built and the thermoreflectance coefficient of different metals has been measured. They are in accordance with the values for metals with e.g. 4,9E-5 for pure Si. Additionally the surface temperature of a sample was measured after a proper calibration with a temperature resolution of 5K.

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.

Combined method for thermal characterization of high power semiconductors

Commonly computational methods are used to determine and enhance the lifetime of electronics and electronic systems. The validity of such methods highly depends on the used material data and therefore on the quality of the accompanying experiments. For this reason different methods were combined to better determine the thermal state of a desired device of variable size, while providing this data within in a reasonable short time. The method allows the in-situ measurement of: · the surface temperature of the top and bottom side by IR thermography and a steady state technique · the junction temperature using transient method · the generated heat flow by the tested device The correlating temperatures of the heating and cooling phase can be monitored at different geometries and setups which allows to build up static and transient simulation models and therefore make the reliability assessment of the used setup or device for many application cases possible.