Climatically Accelerated Material Processes Determining the Long-Term Reliability of Light-Emitting Diodes.

LEDs (Light-Emitting Diodes) are widely applied not only in decorative illumination but also in everyday lighting in buildings, flats, public areas, and automotive fields. These application areas often mean harsh environments, for example, regarding the humidity content of the surrounding air: besides outdoor and automotive illumination, even the household use cases (kitchen, bathroom, cellar) may represent extreme temperature and humidity variations (often reaching relative humidity levels close to 100%) for these devices; thus, their reliability behaviour in such circumstances should be better understood. Thermally activated processes were studied in several previous publications, but less information is available regarding high-humidity environmental tests. Moisture and temperature ageing tests with appropriate environmental parameter settings were performed as accelerated lifetime tests to investigate not only the effect of temperature but also that of humidity on the ageing and reliability of LED packages containing RGB (red green blue) chips and phosphor-converted white (pcW) LEDs. The ageing was followed not only through monitoring optical/electrical/spectral parameters but also with material analysis. Moisture-material interaction models were proposed and set up. It was found that humidity-accelerated ageing processes are more severe than expected from previous assumptions. RGB and pcW LEDs showed strongly different behaviour.

On the Electrochemical Migration Mechanism of Gold in Electronics-Less Reliable Than Expected?

Electrochemical migration (ECM) forming dendritic short circuits is a major reliability limiting factor in microcircuits. Gold, which is a noble metal, has been regarded as a metallization that can withstand corrosion and also ECM, therefore its application in high-reliability metallization and surface finishing systems became widespread although it has a relatively high and fluctuating price. Gold electrochemical short circuits have been found only in the case of halogen (e.g., chloride containing) contaminants that can initiate the anodic dissolution of gold via complex ion formation. The experimental results of the study demonstrate that gold can form dendritic shorts even without the presence of halogen contaminants, therefore the direct anodic dissolution of gold must also be supposed. This could also be a serious reliability influencing factor even when applying gold metallization systems and must be taken into consideration. The theoretical background of the classical (contaminant-free) model of gold is also discussed in the paper.

Investigation of the Mechanical Properties of Mn-Alloyed Tin-Silver-Copper Solder Solidified with Different Cooling Rates.

Manganese can be an optimal alloying addition in lead-free SAC (SnAgCu) solder alloys because of its low price and harmless nature. In this research, the mechanical properties of the novel SAC0307 (Sn/Ag0.3/Cu0.7) alloyed with 0.7 wt.% Mn (designated as SAC0307-Mn07) and those of the traditionally used SAC305 (Sn96.5/Ag3/Cu0.5) solder alloys were investigated by analyzing the shear force and Vickers hardness of reflowed solder balls. During the preparation of the reflowed solder balls, different cooling rates were used in the range from 2.7 K/s to 14.7 K/s. After measuring the shear force and the Vickers hardness, the structures of the fracture surfaces and the intermetallic layer were investigated by SEM (Scanning Electron Microscopy). The mechanical property measurements showed lower shear force for the SAC0307-Mn07 alloy (20-25 N) compared with the SAC305 alloy (27-35 N), independent of the cooling rate. However, the SAC0307-Mn07 alloy was softer; its Vickers hardness was between 12 and 13 HV, whereas the Vickers hardness of the SAC305 alloy was between 19 and 20 HV. In addition, structural analyses revealed rougher intermetallic compound layers in the case of the SAC0307-Mn07 alloy, which can inhibit the propagation of cracks at the solder-substrate interface. These two properties of SAC0307-Mn07 alloy, the softer nature and the rougher intermetallic layer, might result in better thermomechanical behavior of the solder joints during the lifetime of electronic devices.

Localization factor: a new parameter for the quantitative characterization of surface structure with atomic force microscopy (AFM).

In this work we present the possible application of a new parameter called localization factor for the quantitative characterization of surface structures with atomic force microscopy (AFM). For this purpose contact mode AFM images were taken from technologically different polycrystalline gold thin films and were evaluated according to the following parameters: surface roughness (R(a), R(RMS)), roughness factor (f(r)) and localization factor. The localization factor was compared with the other surface parameters. We demonstrate that this new parameter can be used to identically characterize these gold thin film surfaces with contact mode AFM in the 1-1000 µm(2) scan range. The mathematical background and possible application fields of the localization factor are also discussed in our paper.

[Microelectrodes and their application in diagnostic medicine]. / Mikroelektródok az orvosi diagnosztikában.

Microelectrodes are in common use in medical detection systems. The binding of two complementary nucleic acid sequences is called hybridization. Today the major obstacle of large-scale hybridization approaches is the large time-dependency of a single reaction, which is up to 16 hours. As the DNA molecules can be electronically charged, the binding could be facilitated and confirmed using an electronic control system. The authors' team aimed to develop a microelectrode system capable for the detection and control of hybridization. A microelectrode head is immersed in small liquid drop. Here, the platinum counterelectrode is surrounded by a non-conducting quartz capillary. The reference electrode is chloridized silver immersed in saturated Ag/Cl dilution. The Ag/AgCl/1 M KCl +AgCl microelectrode in stabilized against the calomel electrode in the first hours, and remains stable between 7th and 30th hours. This can be verified by the minimal drop in the potential difference. Thus the AgCl saturated KCl electrode is usable for several days for actual measurements. The detector is controlled by an attached computer. The system can be used to detect hybridization in a micro-cell located on a gold-plate. The electrode can be dismounted and reused after repeated chloridization of the Ag wire. The microelectrode is simple, cheap; thus is best suited for application in future automated diagnostic detection systems.