Anna Stoynova

Thermal behavior study of corroded magnesium alloys used in electronic devices

The aim of the study is to develop a suitable method for thermographic control of corrosion in parts of magnesium alloys used in electronic packaging. Specimens with different thickness, representative of magnesium alloys of the three typical groups are studied: AZ91, AM50 and AE42. Thermographic investigation is conducted for the both specimens: without corrosion and corroded specimens in an environment of 3.5% NaCl for 120 h through 24 h. The corrosion rate is estimated by the method of weight loss and the corroded surface morphology is estimated using metallographic microscope. The experimental results after eddy current thermography (ECT) and lock-in thermography (LIT) are analyzed and compared with results from weight loss method. Two different thermographic approaches for corrosion evaluation of magnesium alloy are proposed. Different error sources decreasing precision of results are noted. The received results can be used in detection of the corrosion level from temperature measurements on the surface.

Application of CFD modeling to solve problems in thermal design of LED applications in the initial project phase

The article examines the use of compact models and the thermal design of LED applications. The study shows that despite the simplified representation of the thermal behavior, compact models predict the transition temperature with a sufficiently good accuracy in the initial project phase. An evaluation of methods for further improvement in predictive accuracy is made.

Multi sensor control based on fuzzy logic

Recently fuzzy logic has increasing its popularity. One of the main reasons is that it gives abstraction, so a system could be controlled, even if it is not fully describable. On the other hand, this approach simplifies integration of data from multiple sources and its proper usage. The paper is focused on a mature area of temperature control, so the data is homogenous and simple. This applicability demonstration is not limitation and the approach could be used for any type and quantity of data. The paper shows how usage of multiple sensors improves controlled parameters behavior, giving example with the automotive industry. In this type of products that consist of a network of sensors, it is beneficial to use all available data. The result of this research is analyzed, demonstrating better accuracy as result of a fusion of sensors.

Infrared survey in electrical preventive maintenance

The article discusses the basic elements of procedure applicable to programs for maintenance of electrical equipment with infrared inspection. Based on accumulated experience of experimental thermographic measurement of electrical equipment in different companies was proposed approach to creating a typical procedure, taking into account the specificities of the studied electrical equipment and targets set.

Corrosion evaluation with stepped induction thermography

The aim of the paper is to studying the possibilities of stepped induction thermography for corrosion evaluation. It is proposed non-destructive and contactless corrosion evaluation method. It is presented accuracy decreasing factors and used solutions in proposed method. Test specimens made from different magnesium alloys is prepared. The results are presented in graphical form and discussed.

Influence of the heat generated in flexible OLEDs on their electrical behavior

In this work, variation of the main electrical parameters of flexible organic light-emitting diodes (FOLEDs) is studied as a function of the thermal conditions. Tested structures are consisted of polyethylene terephthalate (PET) as a flexible substrate, indium-tin oxide transparent conductive anodes, polyphenylene vinylene derivative based emissive films and aluminum cathodes. Thermal degradation analysis of the FOLEDs characteristics is performed by IR thermography in broad range of voltages and driving current densities at different temperatures. The study of thermal distribution with the time provides important information for the thermal events in the operating FOLED, as well as for the local temperature change, due to the presence of hidden defects. CFD thermal simulations of two types of test FOLED samples were conducted. Experimental measurements of the distribution of surface temperature of the samples were carried out with an infrared camera. The obtained difference between the simulated and measured values is not greater than 5 °C at the beginning of degradation processes.

Hidden solder joints inspection used in surface-mount technology

In this work, the capabilities of three (standard convection reflow, vacuum reflow and vapor phase reflow) soldering methods of surface mounting technology (SMT) are studied for high quality and reliability hidden solder joint formation. Comparative evaluations of the voids percent, which are formed in the hidden solder joints, were carried using X-ray inspection and testing the mechanical strength of the joints. To determine the contents of the hidden voids in the solder joints, X-ray inspection is used as the calculation of the area percentage occupied by the available voids is carried out by specialized software. Thermography measurements were also performed and a correlation has been established between the temperature distribution in the tested boards and the presence of voids within the hidden solder joints. A method for digital processing of infrared images to determine the percentage of voids in soldered joints is proposed. Three groups of 20 boards were studied and the percentage of voids within the hidden solder joints of power MOS transistors are evaluated. The use of thermography approach allows automate the control process of the voids percentage in hidden solder joints with a large area.

Degradation detection as a result of ageing organic solar cells by thermography

In this work, two types organic solar cell (OSC) modules (with diferent organic semiconductor layer - zinc phtalocyanine (ZnPc) and DPP(TBFu)2-C60) are studied for determination of the effects of degradation in different zones of the OSC. Therefore is used thermographic approach for the monitoring and characterization of this effects. The dynamic infrared thermography is imposed as effective approach for diagnosis of solar cells and for detect invisible sub-surfaced defects. A method for digital processing with LoG filter is presented.

Back-bias voltage influence on Hall-effect microsensors

In this paper we have examined the back-bias effect and its influence on Hall microsensors behavior, which is manufactured on 0,18 μm CMOS technology. Then the sensors parametric reliability was tested at the temperature range of -40°C to +150°C. The purpose of the constructed Hall cell is to improve sensors characteristics, achieving high sensitivity with minimal parasitic offset effects.