Andrej Novikov

Cooling of electronic assemblies through PCM containing coatings

A novel concept of using phase change materials (PCM) to improve the thermal management of electronic components was investigated. The main idea is the smoothing of temperature peaks produced by the component itself or by high ambient temperature. This was realized by heat absorption during melting process of the PCM. Such materials can be used in powder form as additive to the standard coating material like resin or will be applied directly on the electronic component and encapsulated by a polymer material.

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.

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.

Low-temperature assembling process with nanoscaled solder layers

Further development of electronics with higher requirements to integration density and reliability needs new technological solutions for the production of extremely small interconnections with less material and power consumption. This problem can be solved with the usage of nanoscaled solder materials, which have in comparison with bulk materials a reduced melting temperature due to another surface-to-volume ratio. A new concept based on the usage of ultra thin films as solder material and its combination with carrier foil was investigated. For this purpose the main solder material tin was sputtered with the thickness between 5 and 100 nm and then characterized by high resolution methods of scanning and transmission electron microscopy (SEM/TEM) and atomic force microscopy (AFM). The oxidation of nanoscaled layers was studied by X-ray and electron diffraction methods. The melting behavior of such nanoscaled foils was investigated by the method of fast chip DSC. In the first soldering experiments different passivation coatings and separation interlayers were studied on their convenience to produce a stable solder joint. For the improvement of the stability of a solder joint thin carrier metal foils can be used. During soldering process nanoscaled solder layers are melting at lower temperature and going into the intermetallic reaction. Whereas the carrier foil dominates the mechanical, electrical and thermal properties of the final solder joint.

Synthesis and characterization of nanoscaled solder material

A new concept based on the usage of ultra thin films as solder material will be presented here. For this purpose nanoscaled solder films of pure tin with the maximal thickness 100 nm were synthesized through physical vapor deposition method and then characterized with high resolution methods of scanning electron microscopy and atomic force microscopy. For the protection from oxidation during phase change analysis and also during storage and soldering process silicon nitride and carbon were sputtered in the same process chamber. The function of these coatings was tested through x-ray diffraction. The crystalline film under protective layer after cooling down is a sign for impermeability of oxygen. The thermodynamic properties like melting point and undercooling were researched with the sophisticated method of chip calorimetry that allows the measurements at very fast heating and cooling rates and therefore very small amounts of material can be studied. In the first soldering experiments passivation coatings were also tested on their convenience to produce a stable solder joint. Very promising solution for the production of stable solder joints was seen on the system consisting of alternating nanoscaled metal layers, which react during soldering process by building of an alloy. One of the components of such reactive solder systems has to serve for passivation at the same time. Carrier foil with the sputtered solder structures on its both sides can noticeably improve the stability of solder joint. After assembling at low temperature the solder structures transform into diffusion zone and the main physical properties like electrical and thermal conductivity and mechanical strength of the final solder joint are determined by the properties of the carrier foil. In this work such system consisting of silver carrier foil with nanoscaled solder layers of tin and gold was successfully tested.

Nanoscaled solder for low-temperature assembling processes

The change to lead-free soldering due to RoHS directive demands the search for alternative solder materials. At the moment the favorite lead-free solder for reflow processes is SnAgCu that in comparison to eutectic SnPb has higher melting and soldering temperature. The higher soldering temperature leads to more power consumption for the assembly process and can also lead to damage of some sensitive electronic components. By scaling of soldering material size down to nanometer scale the melting temperature can be reduced. For practical usage multilayer structure of alternating nanoscaled tin and temporary interlayer will be interesting. After soldering process at reduced temperature tin layers are melting together and final solder joint has the properties of bulk material that means the higher unsoldering temperature and accordingly higher stability. The properties of nanoscaled layers of pure tin have been studied. The layers were produced by sputtering in different thicknesses between 5 and 100 nm. The surface topography and the influence of process parameters on the sputtering rate were investigated using REM and AFM methods. For the protection from oxidation carbon and silicon nitride layers were RF-sputtered in the same process chamber. The layer thickness was varied between 2 and 10 nm. The function of this coating was tested by x-ray diffraction. The crystallographic structure of tin layer under silicon nitride was measured as sputtered, after melting and after cooling processes. The crystalline tin film after cooling down is a sign for impermeability of oxygen. For investigation of phase change the method of fast differential calorimetry was used. The prepared samples with tin layers scaled down to 15 nm were measured with this method. Measured values have certain deviation from theoretical model values that can be explained with another surface tension due to final preservation by silicon nitride layer.