Klaus-Juergen Wolter

Primary and tertiary creep properties of eutectic SnAg3.8Cu0.7 in bulk specimens

The paper presents extended creep properties of eutectic SnAg3.8Cu0.7 solder using dog bone shaped tensile specimen to extend the data for the Alpha-Omega Model. The tensile test machine was carefully modified with respect of solder material requirements in terms of stress free specimen mounting and assurances of high stable stress conditions. Furthermore the tensile tester is able to measure the deformation of the specimen directly on the specimen, which allows more precise tests and more independence from temperature and stress dependent tester parts like the clamping and the rods. The tests were performed in three temperatures: 25 °C, 75 °C and 125 °C and three different stresses at each temperature.

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.

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.

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.

Biostability issues of flash gold surfaces

Being chemically inert and non-toxic and having excellent electrical properties gold is known as one of the most suitable materials for fabrication of long-term implantable electronic devices. In spite of this fact, stability of structures with flash gold finish layers (electroless Ni @ immersion Au or electroless Ni @ electroless Pd @ immersion Au), widely used in electronics packaging, is questionable. Such layer configurations are often characterized as not sufficiently stable by applications in humid or corrosive environments and are poorly investigated under the influence of physiological factors as living tissues, microorganisms and body fluids. In this work biostability of FR4 @ 50 µm Cu @ 4 µm Ni @ 0.1 µm Au in simulated blood plasma was studied. The samples were stressed dynamically in a special circulation system, keeping the values of temperature, pressure and flow velocity similar to the natural parameters of a human body. The alterations in topography and structure integrity, chemical composition and wetting properties of the samples surfaces have been investigated by optical microscopy, atomic force microscopy (AFM), laser profilometry, contact angle measurements, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Cracks formation and delamination of a gold layer, significant mass losses and precipitation of the foreign substances have been observed.

Modeling, simulation and calibration of the chip encapsulation molding process

The transient process of filling the mold cavity in microelectronics packaging has been simulated applying the finite element method (FEM). The results have been compared to those of so-called ‘short shot’ experiments, in which the molding process is interrupted at predefined points in time. After determining the quantitative discrepancies between the simulation and the experimental results, conventional model calibration was performed covering the full dimensional, material, and load parameter space, in which the actual test conditions could have deviated from the target values considered in the first simulation run. This way, parameter deviations could be ruled out as main reason as most of the simulation inaccuracy persisted. Instead, a subsequent investigation revealed the following additional effects having caused the discrepancies: the effect of cavity vacuum, the systematic concentration of filler particles on top of the dies, the gas bubbles formed at the mold front by the chemical reactions, and the coasting flow of mold material after process interruption. Accounting for these effects as well, the simulation accuracy was improved substantially. It now allows the virtual design optimization of the microelectronics packages and the molding tool as well as that of process conditions and material selection prior to experimental tests boosting both, manufacturing efficiency and product reliability.

Influences of sterilisation procedures on polymer surfaces

This paper describes the influences of different sterilisation procedures regarding to the behaviour of biocompatible packaging materials. The effects of these procedures on coated substances are researched, i.e. adhesive strength towards the substrate, surface energy and roughness. Non-sterilised, coated samples are used as references. Additionally studies of former samples are analysed to detect the influences of long exposure times, especially in relation to adhesion. The coatings are the polymers named the Poly-Para-Xylylen “Parylene C” and the silicone “Med 6-6606”. Glass is used as carrier. The applied sterilisation procedures are electron beam sterilisation, gamma radiation sterilisation and the influence of ethylene oxide.

Parylene C and silicone as biocompatible protection encapsulants for PCBs

During the last years a lot of active implantable medical devices like neural prostheses [1] and sensors have been produced. For such systems a long term reliability under the influence of aggressive physiological conditions is very important. To protect them a stiff housing is normally utilised. In some cases, if the requirements for a low weight, flexibility and perhaps a need to transmit optical signals have to be fullfiled, they arent the best choice. Another possibility is the use of flexible protection materials like polymers, which have the advantageous properties. Finally their structure isnt completely impermeable for different components contained in physiological fluids. Some promising polymers are Poly-Para-Xylylen (Parylene C) and silicone rubber (MED6-6606). This paper shows the results of our tests to their ability to function as encapsulants for the PCB configurations FR4+Cu+Ni+Au, FR4+Cu+chem. Sn and FR4+Cu+HAL Sn.