Mathias Nowottnick

Characterisation of silver particles used for the Low Temperature Joining Technology

The Low Temperature Joining Technology (LTJT) is an important alternative to lead containing solders. Therefore the basic material, silver powders have been investigated. The particle morphology, the amount and composition of their organic coating and chemical impurities have been analysed. Scanning Electron Microscopy (SEM) has been used to determine the morphology of the silver particles. Differential Scanning Calorimetry (DSC) in combination with Thermogravimetry (TG) and Mass Spectroscopy (MS) reveals the occurrence of exothermic or endothermic reactions as well as information concerning mass loss and formed molecules. To determine the components of the organic coating Time of Flight — Secondary Ion Mass Spectroscopy (ToF-SIMS) has been applied. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) has been utilised to analyse chemical impurities. To remain a linkage between the characterised silver particles and the quality of a manufactured sintered joint, a shear test has been executed. One of the investigated powders reaches an averaged shear force of approximately 230 N. The mixture of small flake shaped and more spherical particles generates good adhesive strength.

Thermal peak management using organic phase change materials for latent heat storage in electronic applications

The inside of modern high power electronics consist of a large amount of integrated circuits for switching and supply applications. This technology has aside the benefits the problem of more and more increasing power density. Nowadays, heat sinks that are directly mounted on a device, are used as a thermal sink to reduce the on chip temperature and dissipate the thermal energy to the environment. This paper presents a concept of a composite coating for electronic devices on printed circuit boards or electronic assemblies that is able to buffer a certain amount of thermal energy, dissipated from a device. The aim of the idea is to suppress temperature peaks in electronic components during load peaks or electronic shorts, which could damage or destroy the device by using a phase change material to buffer the energy. The phase change material coating could be directly applied on the chip package or the PCB using different mechanical retaining jig approaches.

Energetic analysis of solder paste deposits as reference for soldering with selective heat

Strictly speaking standard reflow soldering processes are inefficient in terms of energy consumption. A large amount of energy is needed to heat up comparatively small solder joints. As a result the whole electronic assembly is stressed with heat, of which only a fraction is going into soldering. A reduction of process temperatures would improve this disproportion. To compensate for the resulting lack of energy, an exothermic reaction, releasing additional heat inside the solder paste deposits, could be applied. The potential of such a process has already been proven in earlier works [1], [5]. For the adjustment of such a sensitive process a better understanding of the energetic requirements for solder paste deposits in dependence of their size and temperature is required. In this work such results are generated by a practical measuring approach. Here, a chip resistor is used as a model to melt up particular solder joints through joule heating. The thermal energy is calculated by measuring electrical power over time.

Heat protection coatings for high temperature electronics

Highly-integrated electronic devices and printed circuit boards need to be protected against overheating. In this disquisition a concept is introduced to improve the thermal management of coated assemblies by means of functional additives based on earlier works [4]. It has already been proven that paraffin wax, zeolite molecular sieve and silica gel have the potential as thermal energy storage systems due to phase transition effects and sorption [1], [5], [6]. Here the cooling effects of these materials were analyzed by using a heating element (constantan on ceramic substrate) and chip resistors with different package sizes (2512, 1206, 0805, 0603). The covering of these testing elements with the investigated additives lead to a significant delayed heating and therewith to a short-period heat protection at pulse load condition. Nevertheless a re-cooling of the system could regenerate the cooling effects of the additives for multiple utilization; paraffin wax due to refreezing and zeolite/silica gel due to rehydration. The Application for coating technology indicates a significant improvement of thermal properties.

Behaviors of Printed Circuit Boards Due to Microwave Supported Curing Process of Coating Materials

Abstract The Application of a microwave supported curing process for coatings in the field of electronic industry poses a challenge. Here the implementation of this technology is represented. Within the scope of the investigation special PCB Test Layouts were designed and the polymer curing process examined by the method of dielectric analysis. Furthermore the coupling of microwave radiation with conductive PCB structures was analyzed experimentally by means of special test boards. The formation of standing waves and regular heating distribution along the conductive wires on the PCB could be observed. The experimental results were compared with numerical simulation. In this context the numerical analysis of microwave PCB interaction led to important findings concerning wave propagation on wired PCB. The final valuation demonstrated a substantial similarity between numerical simulations and experimental results.

Selective cooling of sensitive PCB components by means of silica gel based cover coatings

The safe operation of electronic assemblies and their components requires smart thermal management. Here a concept based on sorption processes is introduced to avoid local overheating. It is shown, that water desorption effects of silica gel can significantly delay the heating of sensitive electronic components. Furthermore the influence of ambient conditions (temperature, humidity) on cooling performance was tested by means of a climatic exposure test cabinet. After complete dehydration, silica gel is able to regenerate the water storage under ambient conditions for re-utilisation. Finally a polymer binder (acrylic resin) was used in order to produce an open porous silica gel based cover coating. The covering of sensitive components of a test PCB demonstrated the exceeding cooling effects of the new designed coating.

Reliability investigation of large area solder joints in power electronics modules and its simulative representation

Abstract Power electronics (PE) modules for inverter units in hybrid/electric vehicles (H/EV) generate a large amount of heat which needs to be dissipated. This is often done via a liquid cooled metallic baseplate which acts as a heat sink. The interconnect between the PE module and the baseplate is realized using a large area lead-free solder joint which under passive temperature cycling (pTC) tests, develops adhesive cracks (delamination) at the solder-intermetallic compound (IMC) interface. Such cracks reduce the capability of the solder joint to effectively transfer heat to the baseplate and potentially lead to device failure due to overheating. Considering the large number of potential designs for various application of such PE modules, an understanding of the influence of the mechanical behaviour of individual assembly components such as the power substrate, baseplate or the solder joint itself on reliability is of great interest and utility. This study therefore provides an in-depth reliability assessment of multiple physical variants aged under three different pTC profiles. The investigation reveals certain clear trends with respect to warpage, stiffness and size of the joining partners. A detailed Finite Element Method (FEM) simulation methodology was also developed that represents the delamination behaviour for lifetime assessment.

Study on the interdependence of soldering profile, ageing conditions and intermetallic layer thickness in large area solder joints and its influence on reliability

This study was aimed at quantifying critically relevant topics about the influence of intermetallic compounds (IMC) within power electronics reliability such as the influence of soldering profile on the growth rate of IMC in large area solder joints, estimation of IMC thickness during non-isothermal ageing and the impact of IMC thickness on reliability. To this end, test samples were soldered using four different soldering profiles by varying hold time at peak temperature and cooling rate. Isothermal ageing was performed at 125°C, 150°C and 175°C. IMC thickness measurements were taken at ageing durations of 200h, 500h and 1000h respectively. Using linear regression on experimental data and the finite differences method, a differential equation was derived and applied to predict IMC thickness for any arbitrary thermal ageing profile, for example, active or passive temperature cycling. Additionally, finite element method (FEM) simulations were carried out to evaluate the influence of IMC thickness under three potential failure modes. These failure modes result from either substrate war page or in-plane shear deformation. The study has been able to show the effects of soldering profiles on the IMC growth rate and also the influence of IMC thickness on stress state near crack locations using FEM simulations.

Material study for full-faced heating layers, integrated in printed circuit boards

Different materials which are suitable for heating the complete area of an electronic assembly when embedded within a printed circuit board (PCB) are studied in this work. The focus being, two foil type materials with a conductive layer and two paste materials with carbon filler. The suitability to integrate these materials in the process technology for PCB manufacturing is described and used to construct test samples for heating and perform preliminary tests.

Highly variable Sn-Cu diffusion soldering process for high performance power electronics

Abstract A Sn-Cu-based diffusion soldering process is presented that is capable of producing high temperature resistant joints for small area dies, like MOSFETs as well as for larger IGBTs and even large area baseplate or cooler-contacts. It is shown how the lateral infiltration process, needed to produce these joints, is influenced by the choice of the respective Cu-paste system. Furthermore, materials analysis on bulk material samples, including tensile testing, metallography and fracture analysis, show how the choice of paste system also affects the microstructure and hence the mechanical behavior of the bulk material. First results from power-cycling investigations on one of the selected paste-systems show a more than tenfold increase of lifetime compared to a SnAgCu305 solder reference.