Purpose – The main advantages of vapour phase soldering are a non-oxygen environment, the elimination of overheating and the possibility of the vacuum application, which can guarantee undeniably higher quality of solder joints, especially as regards void formation. These features are less affected by the alloy composition. The paper aims to discuss these issues. Design/methodology/approach – The quality of solder joints made in two VPS options (with and without vacuum) was investigated in terms of voids formation. Solder alloys of 37%Pb63%Sn (PbSn) and 96%Sn3.5%Ag0.5%Cu (SAC 305) were applied to an etched Cu layer on a glass-epoxy substrate using the screen-printing method. 1206 SMD resistors were placed on the solder pads with a Quadra pick-and place machine. For the inspection of joint structure and void identification, 3D X-ray images of samples were taken using a computed tomography system with a 180 kV/15 W nanofocus. For comparison, traditional cross-sections of the samples were performed using a metallographic polisher. The cross-section analysis was done in a scanning electron microscope (SEM). To confirm the relevance of these data, a statistical analysis was carried out. Findings – The paper shows that alloy composition has less impact on the quality of joints as regards void formation. The tendency for a different arrangement of voids in a junction depending on the distance SMD element and the thickness of the solder layer was investigated using X-ray computed tomography. Originality/value – The use of 3D computed tomography for void investigation gives full information about the internal structure of the joint and allows for precise void identification. Vacuum application during the soldering allows significant voids elimination.

The paper reports on the influence of processing on microstructure and electrical properties of multilayer varistors based on zinc oxide doped with Bi 2 O 3 , Sb 2 O 3 , Co 2 O 3 , MnO, Cr 2 O 3 , B 2 O 3 , and SiO 2. 0.5-1 wt% of AlF 3-CaB 4 O 7 was used as a new effective sintering aid. The behavior of green laminates during heating was characterized using differential thermal analysis and a heating microscope. As revealed by XRD, SEM, and EDS methods, the varistor layers are composed of ZnO grains of 1-5 μm size, submicrometer spinel and pyrochlore grains situated at the ZnO grain boundaries, and nanometric Bi 2 O 3-rich films surrounding ZnO grains. Complex impedance studies carried out in the frequency range of 0.01 Hz-2 MHz at temperatures changing from-30 to 150 ℃ imply the formation of semiconducting grains and insulating grain boundaries. Frequency dependence of dielectric permittivity shows a high plateau at lower frequencies, typical for barrier layer capacitance effect. The fabricated multilayer varistors show nonlinear current-voltage characteristics with a high nonlinear coefficient of 26-38. The breakdown voltage was found to decrease within the range of 66-130 V with sintering temperature increasing from 1000 to 1100 ℃. Good surge current capability of the varistors was confirmed by the tests using 8/20 μs pulses.

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The effect of the microstructure of solder joints on the thermal properties of power LEDs is investigated. Solder joints were prepared with different solder pastes, namely 99Sn0.3Ag0.7Cu (as reference solder) and reinforced 99Sn0.3Ag0.7Cu–TiO2 (composite solder). TiO2 ceramic was used at 1 wt.% and with two different primary particle sizes, which were 20 nm (nano) and 200 nm (submicron). The thermal resistance, the electric thermal resistance, and the luminous efficiency of the power LED assemblies were measured. Furthermore, the microstructure of the different solder joints was analyzed on the basis of cross-sections using scanning electron and optical microscopy. It was found that the addition of submicron TiO2 decreased the thermal and electric thermal resistances of the light sources by 20% and 16%, respectively, and it slightly increased the luminous efficiency. Microstructural evaluations showed that the TiO2 particles were incorporated at the Sn grain boundaries and at the interface of the intermetallic layer and the solder bulk. This caused considerable refinement of the Sn grain structure. The precipitated TiO2 particles at the bottom of the solder joint changed the thermodynamics of Cu6Sn5 formation and enhanced the spalling of intermetallic grain to solder bulk, which resulted in a general decrease in the thickness of the intermetallic layer. These phenomena improved the heat paths in the composite solder joints, and resulted in better thermal and electrical properties of power LED assemblies. However, the TiO2 nanoparticles could also cause considerable local IMC (Intermetallic Compounds) growth, which could inhibit thermal and electrical improvements.

Purpose – The purpose of this paper is to assess the reliability of thermoelectric generators after ageing at elevated temperature and to determine the influence of the technology used (i.e. type of thermoelectric material, type of substrate and soldering technology) for thermogenerator (TGE) assembly. Design/methodology/approach – In this paper, the Seebeck coefficient and the current voltage were measured for lead telluride doped with either manganese (PMT), germanium (PGT) or sulfur (PST) TGEs. The Seebeck coefficient measurements were taken at temperatures between 230 and 630 K. Findings – The Seebeck coefficient determined for PMT, PGT and PST TGEs increases approximately linearly with increasing temperature and is greater by about 40 per cent for PST and about 30 per cent for PMT than in commercially available PbTe TGEs. The best outcome in terms of stability after long-term ageing was that of PMT material. Originality/value – The choice of proper technology (i.e. thermoelectric materials, type of substrate and soldering technology) for the TGE assembly is essential for their functioning overtime and reliability.