Search Results (6)

In this paper, the issue of an aging process’s influence on power LEDs’ properties is considered. Some measured DC characteristics of these devices and their thermal and optical parameters obtained are presented after considering different values of the aging process’ duration. Components soldered using different metal–ceramic composite pastes, e.g., with TiO₂, were tested. The tested devices and the used measurement setup are described. The measurement procedure is described in detail. The obtained measurement results are discussed. It is shown that after the aging process at elevated temperatures, worse properties were observed for the power LEDs soldered using classical SACX0307 alloy. Most of the samples soldered with reference alloy (not composite) were damaged during the test. The best properties were obtained for the samples soldered with solder paste with the addition of titanium oxide. Full article

Bi-doped Sn–Ag–Cu (SAC) microelectronic solder is gaining attention for its utility as a material for solder joints that connect substrates to printed circuit boards (PCB) in future advanced packages, as Bi-doped SAC is reported to have a lower melting temperature, higher strength, higher wettability on conducting pads, and lower intermetallic compound (IMC) formation at the solder-pad interface. As solder joints are subjected to aging during their service life, an investigation of aging-induced changes in the microstructure and mechanical properties of the solder alloy is needed before its wider acceptance in advanced packages. This study focuses on the effects of 1 to 3 wt.% Bi doping in an Sn–3.0Ag–0.5Cu (SAC305) solder alloy on aging-induced changes in hardness and creep resistance for samples prepared by high cooling rates (>5 °C/s). The specimens were aged at ambient and elevated temperatures for up to 90 days and subjected to quasistatic nanoindentation to determine hardness and nanoscale dynamic nanoindentation to determine creep behavior. The microstructural evolution was investigated with a scanning electron microscope in tandem with energy-dispersive spectroscopy to correlate with aging-induced property changes. The hardness and creep strength of the samples were found to increase as the Bi content increased. Moreover, the hardness and creep strength of the 0–1 wt.% Bi-doped SAC305 was significantly reduced with aging, while that of the 2–3 wt.% Bi-doped SAC305 increased with aging. The changes in these properties with aging were correlated to the interplay of multiple hardening and softening mechanisms. In particular, for 2–3 wt.% Bi, the enhanced performance was attributed to the potential formation of additional Ag₃Sn IMCs with aging due to non-equilibrium solidification and the more uniform distribution of Bi precipitates. The observations that 2–3 wt.% Bi enhances the hardness and creep strength of the SAC305 alloy with isothermal aging to mitigate reliability risks is relevant for solder samples prepared using high cooling rates. Full article

The contribution presents the verification of the methodology of accelerated creep tests from the point of view of obtaining more information about the stress–strain behaviour of the investigated materials using the Digital Image Correlation method. Creep tests are performed on SAC305 and SACX0807 lead-free solders and are supplemented by numerical modelling using the finite element method, considering the viscoplastic model based on the theory of Perzyna, Chaboche, and Norton. The stress–strain behaviour of both solders appears to be very similar at applied strain rates of 0.0002–0.0026%/s and applied creep stresses of 15–28 MPa. Initially, the viscoplastic model is calibrated using an analytical approach. Then, the finite element model updating approach is used to optimise the material parameters based on the simultaneous simulations of creep and tensile tests. As a result, the total objective function value is reduced almost five times due to optimisation. The proposed type of accelerated test with an hourglass specimen proves to be suitable for calibrating the considered class of viscoplastic models. The main benefit is that a single specimen is required to obtain creep curves on various stress levels. Full article

The effect of solder joint fabrication on the thermal properties of IGBTs soldered onto glass-epoxy substrate (FR4) was investigated. Glass-epoxy substrates with a thickness of 1.50 mm, covered with a 35 μm thick Cu layer, were used. A surface finish was prepared from a hot air leveling (HAL) Sn99Cu0.7Ag0.3 layer with a thickness of 1 ÷ 40 μm. IGBT transistors NGB8207BN were soldered with SACX0307 (Sn99Ag0.3Cu0.7) paste. The samples were soldered in different soldering ovens and at different temperature profiles. The thermal impedance Z_th(t) and thermal resistance R_th of the samples were measured. Microstructural and voids analyses were performed. It was found that the differences for different samples reached 15% and 20% for Z_th(t) and R_th, respectively. Although the ratio of the gas voids in the solder joints varied between 3% and 30%, no correlation between the void ratios and R_th increase was found. In the case of the different soldering technologies, the microstructure of the solder joint showed significant differences in the thickness of the intermetallic compounds (IMC) layer; these differences correlated well with the time above liquidus during the soldering process. The thermal parameters of IGBTs could be changed due to the increased thermal conductivity of the IMC layer as compared to the thermal conductivity of the solder bulk. Our research highlighted the importance of the soldering technology used and the thermal profile in the case of the assembly of IGBT components. Full article

The properties of Sn99Ag0.3Cu0.7 (SACX0307) solder alloy reinforced with ZnO nanoparticles were investigated. The primary ZnO particle sizes were 50, 100, and 200 nm. They were added to a solder paste at a ratio of 1.0 wt %. The wettability, the void formation, the mechanical strength, and the thermoelectric parameters of the composite solder alloys/joints were investigated. Furthermore, microstructural evaluations were performed using scanning electron and ion microscopy. ZnO nanoparticles decreased the composite solder alloys’ wettability, which yielded increased void formation. Nonetheless, the shear strength and the thermoelectric parameters of the composite solder alloy were the same as those of the SACX0307 reference. This could be explained by the refinement effects of ZnO ceramics both on the Sn grains and on the Ag₃Sn and Cu₆Sn₅ intermetallic grains. This could compensate for the adverse impact of lower wettability. After improving the wettability, using more active fluxes, ZnO composite solder alloys are promising for high-power applications. Full article

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. Full article