Bjoern Boehme

Investigations of carbon nanotubes epoxy composites for electronics packaging

The part of electronics packaging is steadily forced to adapt the requirements of the microelectronic industry. For future electronics application such needs will be: 1) steady miniaturisation of the electronic devices 2) high pin count up to 5000 i / o per device 3) pitches down to 20 mum 4) higher current density per devices 5) higher thermal dissipation loss This is only a small extract of the challenges facing the electronics packaging industry in the future. The aim and duty for electronics packaging is to realize a reliable package for future electronics. Commonplace materials for joining elements like solder are not able to solve these requirements. For example in [1] the authors describe that future ICs operating at high frequencies of 10-28 GHz, signal bandwidths of 20 Gbps and lower supply voltages require an estimated maximum of R (< 10 mOhm), L (<5-10pH) and C (<5-10 fF).[l] Current joining elements can not meet these requirements. To solve these problems the electronics packaging industry researches technologies and materials of the nanotechnology. Especially researches concerning new materials for electronics packaging rise up since the last three years. One of the most researched new materials are Carbon Nanotubes (CNT). Carbon Nanotubes have superior mechanical, electrical and thermal properties. Due to these properties CNT are considered as promising candidates in packaging technology. The most interesting field of application is the use of the Carbon Nanotubes as filler in electrical conductive adhesives. The aim is to improve the performance of conductive adhesives in comparison to common products. This study deals with characterization of carbon nanotube / epoxy adhesives in electronics packaging. For this study we optimize the CNT - adhesive system by modification of the CNT, use of different dispersion technologies and under variation of the epoxy matrix. The resulting adhesives are characterized by measuring their viscosity, mechanical strength and their thermal and electrical conductivity. For all studies Multi Wall Nanotubes were used which can be purchased at a reasonable price. For modification of the CNT they can be treated by low pressure plasma (cvd), UV / ozone treatment or modifiedchemically in solution to achieve a higher polarity resulting in a better dispersibility. Also bonding to the polymer matrix is improved. Success of the processes is studied by XPS and REM. For dispersion technology ultrasonic bath, speed mixing and/or treatment with a roll calander can be used. The polymer matrix is also varied in order to achieve an appropriate viscosity at the CNT-content of interest that enables good results in screen printing. Also CNT-polymer interaction can be adapted by varying polarity of the resin used. The distribution of CNT in the matrix is studied by TEM. The first investigations show that ultrasonic finger is the favourable dispersion technology to achieve well dispersed CNT. For modification of the CNT the plasma treatment came out to be efficient to give appropriate amounts of hydroxyl groups.

Comprehensive material characterization of organic packaging materials

In this study, two highly filled molding compounds were used as example to demonstrate the characterization scheme. In addition, two low filled packaging polymers are included for comparison. The characterization scheme consists of the steps sample preparation, measurement of the material data, and modeling the material behavior. The ‘sample preparation’ step included a DSC analysis to understand the cure reaction and to establish the cure kinetics model. In the ‘measurement’ step, two different sets of equipment were applied. The elongation modulus is determined by dynamic mechanical analysis (equipment: DMA ‘Q800’) in a wide range of temperatures and frequencies. The other parameters are measured by pressure-volume-temperature experiments (equipment: PVT ‘Gnomix’). Conducting these characterization tests, the bulk modulus (K), coefficient of thermal expansion (CTE), and the cure shrinkage was determined. The paper describes this comprehensive characterization with the measurement setups and parameter selection. E(T,t), K(T,t), CTE(T), Tg and cure shrinkage are determined to define a complete and consistent material model [JAN07]. Subsequently, the characterization results are presented, discussed and further work to implement the complete material model into FEM simulation tools like ANSYS™ is outlined.

FEM assisted development of a SHM-piezo-package for damage evaluation in airplane components

Structural Health Monitoring (SHM) is a wide spread field for material condition observation of differential structure components. At the IZFP the guided wave (Lamb wave) technology is under higher investigation. Actual investigations are in progress to apply SHM-systems at structures in airplanes to perform condition monitoring. New materials like Carbon Fibre Reinforced Plastics (CFRP) will be placed in airplanes partially, because they provide very high stiffness, high rupture strength and reduced total mass. These SHM-systems are using different damage indicators, which are based on differences in amplitude or phase relation between two measured signals at two different times points (condition). Additionally, these signals are affected by environmental loads, sensor setup and changes in material properties of the adhesive layer. A successful material application can only be achieved by using an integrated reliable SHM-system. The validation of reliability comes along with high probability of detection and high robustness regarding environmental loads. This study tests and analyses the robustness of a novel piezo-sensor-package. The sensor package is very slim and consists of LTCC ceramic, which encloses a PZT piezo ceramic sheet and carries electronic components on its surface. Using the piezoelectric effect the package generates lamb waves and transmits them into a base substrate. The package is assembled on a 2 mm (thick) aluminium plate for study purposes, because aluminium possesses an isotropic material behaviour. Frequency ranges from 25 kHz up to 400 kHz produce excited symmetrical S0 and asymmetrical A0 lamb waves that are guided into the aluminium plate. Subsequently, a FEM-model of the package is calibrated to ensure correct physical behaviour of the simulation using analytical solutions of lamb wave propagation and experimental data. The calibration of the FEM-model provides the base for further investigations. The principle of wave propagation based on the new package configuration is studied and effects resulting from the package shape and construction are defined. Also, influences of the adhesive layer between the ceramic package and the aluminium plate are determined as a function of thickness and temperature depended stiffness and for the case of a delaminating progress.

CNTs - a comparable study of CNT-filled adhesives with common materials

Electronics packaging must be designed to meet the increasing requirements of the microelectronics industry. Future packages will have an even higher number of I/Os and pitches down to 20 microns resulting in high dissipation losses and extreme current densities. When using conventional materials, design engineers will face physical barriers and limitations in performance and new material solutions have to be found.

Reliability of embedding concepts for discrete passive components in organic circuit boards

This work presents the research towards two promising embedding approaches for electronic components with a possible application to integrate a sensor node for structural health monitoring into the structure itself. The production process for both integration methods has been assessed through experimental and simulation effort. Samples with passive components for both presented approaches have been successfully built. To evaluate the quality of the integration processes, the embedding samples along with regular SMT assemblies were subsequently tested towards reliability using tensile and temperature cycling tests with in-situ measurement. Both testing methods have also been supported with advanced finite element modeling to understand the structural behavior for the integration. The results have shown that both approaches can successfully be applied to integrate electronics into organic material. It was also found that the damage mechanism during the thermo mechanical loading for embedded components differs from conventional SMT assembly because the solder is completely fixated. With a correct manufacturing process this will yield an increase of life time for the electronics.

Study of nanosilver filled conductive adhesives and pastes for electronics packaging

This research deals with the investigation of novel nanosilver filled conductive adhesives and pastes for application in electronics packaging. Compared to conventional soldering-based interconnection technology, electrical conductive adhesives are believed to have the following advantages: finer pitch capability, lower processing temperature requirements, more environmentally friendly than lead-containing solders. The intention of this research is to investigate the application of silver nanoparticles in conductive adhesives and pastes with an objective to improve the desired properties such as electrical conductivity and mechanical stability. Silver has the highest room temperature electrical and thermal conductivity among all metals. It is expected that adhesives with nanosilver will show higher conductivity due to better sintering and that is why the influence of curing temperature on electrical properties of the adhesives and pastes was also investigated. The dependence of resistivity and curing temperatures was shown. Resistivity was measured and compared for different adhesives and pastes: 1) conventional samples without nanoparticles, 2) mixes which contain both micro- and nanoparticles and 3) only with silver nanoparticles.

Material characterization of organic packaging materials to increase the accuracy of FEM based stress analysis

Organic packaging materials gain a steady increasing importance for electronics packaging assemblies. They are used in various ways in substrate materials, adhesives, encapsulations, underfills and many more. This paper outlines the importance of thermo-mechanical characterization of these polymeric packaging materials to improve the accuracy of Finite Element Modeling for advanced reliability analysis of electronics packaging solutions. Therefore the effects of including temperature-and time dependent mechanical material properties of a PPS molding compound were investigated. This molding compound should be used as a coupling element to decrease the occurring stresses in an array of solder connections between substrate and package. The setup was analyzed by FEM (Finite Element Modeling). For the material characterization a DMA 2980 equipment was utilized to determine the time- and temperature dependent elongation modulus of the molding compound material. A description of the measurement setup and parameter selection is given. Subsequently the measurement results are presented. To use this measurement results in a material model for time dependent elongation modulus the results needed to be fitted to a Prony series model which allows implementing this complex material behavior in the FEM simulation software Ansysreg. Additional the WLF (Williams-Landel-Ferry) shift function was determined and implemented to add the temperature effect to the viscoelastic material data used for simulation. For the stress analysis the package setup was implemented as geometric model of the real structure and the loading conditions were defined. The simulations showed that there are significant differences in the occurring stress levels in the setup. For higher temperatures the stress levels were decreased due to stress relaxation in the polymer.

Resistance of conductive adhesive joints on non-noble surface finishes

Electrically conductive adhesives (ECAs) have number of advantages comparing to solder joints. They are environmentally friendly and have lower processing temperature that makes them suitable for manufacturing of LCDs or connecting flexible solar cell arrays in modules. However, in flexible solar cells the metal contact is usually made of non-noble metal, and this joint can suffer from decrease of conductivity at the operation environment. In this work the joint behavior between different metal surfaces (Au, Sn, Mo) and ECAs during different ageing conditions is investigated. The aim of the work is to investigate the degradation of non-noble metal-ECA joints in order to predict the reliability of such an electrical contact. The results of the literature review and ageing experiments are presented. The ageing conditions were 120 °C thermal ageing and damp heat test (85 °C/85RH). The test samples were manufactured on FR-4-boards with metal stripes and applied ECAs between them. The experiments are focused on the electrical conductivity. The experiments have shown significant increase in joint resistance after 85 °C/85RH, while under 120 °C ageing a slight decrease was observed.

Measurement of viscoelastic material properties of adhesives for SHM sensors under harsh environmental conditions

This paper addresses the influence of the viscoelastic material properties of adhesives on the functionality of SHM (Structural Health Monitoring) sensor applications. Adhesives behave viscoelastically and show a strong temperature and time dependency of their mechanical properties. Creep processes increase the deformation under mechanical load and relaxation can decrease the stress in the adhesives (polymer) with time. These processes are most effective in the temperature range of glass transition (Tg), at which the material behavior switches between the glassy and rubbery state and all material parameters change drastically. Additional the viscoelastic material properties are influenced by environmental loading like moisture as it behaves like a plasticizer in the epoxy matrix.

Influence of accelerated aging on the acoustic properties of a ceramic microsystem for structural health monitoring

Carbon fibre reinforced polymers (CFRP) are widely used wherever weight has to be reduced without compromising the mechanical properties of parts. One popular application is in aerospace. To reduce failure analysis (maintenance) time and eventually detect impacts even during flight it is necessary to durably mount an array of miniaturized sensors on CFRP elements. Especially the acoustic properties of the interface (adhesive) between ultrasound transducer and CFRP need to be constant over time (or at least show an predictable behaviour). LTCC-based transducers allow the integration of reliable electronics into the ultrasound sensor. For electrical circuits on ceramic substrates, lifetime predictions are basically known. In contrast, no investigations about the “acoustic lifetime” of an ultrasound sensor interface are known. The paper describes the fabrication and assembly of realistic test specimens as well as the accelerated aging by temperature cycling and temperature / humidity test. To evaluate the adhesive, lifelike ultrasound signals are sent and received with different types of solid mount LTCC-based sensors. Furthermore the electrical impedance of the piezoelectric transducers is analyzed.