prof. Dr. Balázs György Illés

In this paper, the effect of different thermocouple constructions on the hot spot temperature are investigated with a modelling approach to achieve more precise soldering profile acquisition and failure analysis at different reflow soldering heat transfer methods. The paper focuses on convection-based reflow and condensation based vapour phase soldering processes and related heat transfer coefficients. Different thermocouple constructions were investigated. The parameters include the ASTM-type (K, T, J), the length of uninsulated wire length, the insulation material (PFA, PVC, PTFE, woven glass-fibre), the insulation thickness and the diameter of the hot-spot. The physical dimensions were obtained with optical microscopy for validation. Finite element analysis was performed. After validation, the results show that most parameter and geometry changes - most significantly, the uninsulated wire end - can result in non-negligible timely differences in 100 ms range between different sensors, pointing to the requirement of sorting and pairing in a symmetric measurement environment. The results can be applied in a modern manufacturing environment and in future process modelling and evaluation tasks.

Electrochemical Migration (ECM) is getting more attention in the microelectronics industry due to the continuing miniaturization, which increases the possibility of short circuit formation caused by the ECM induced dendrites. This work presents a 2D numerical model of the ECM based on the Nernst-Plank equation. The model contains the deterministic description of the metal dissolution, the changes of electrolyte properties, and the ion transport in the electrolyte. However, the reduction of the ions and the dendrite growth is described stochastically. The capability of the model was tested in the case of pure copper electrodes with a gap distance 200 mu m, 10 VDC bias, 20 degrees C temperature, and a contaminant-free electrolyte. The results of the model were validated by experimental water-drop tests. The results showed very good agreement between the experimental and the calculated mean time to failure values, dendrite morphologies, and the kinetics of the dendrite formation. The model showed that the developing dendrites consume most of the Cu2+ ions around them, which answers why only some dominant dendrites can develop in a given area. The model proved that not only the electric field but the diffusion of the ions is also dominant in given phases of the ECM process. Our model could be a useful tool for ECM failure prediction and for further ECM researches as the digital twin of the ECM process. Also the approach can be applied in various aspects of failure-prediction in modern reflow-soldering. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

The main joining process of the electronic industry is still soldering with different alloys of Sn. This work studied the relationship between Sn whisker growth and corrosion resis-tance of 99Sn0.3Ag0.7Cu-TiO2/ZnO (0.25wt%) composite solder alloys in a corrosive envi-ronment via scanning electron microscopy and focused ion beam techniques. The corrosive environment test showed that the application of TiO2 and ZnO nano-particles almost totally suppressed the Sn whisker growth and improved the corrosion resistance of the composite solder alloys compared to the reference 99Sn0.3Ag0.7Cu alloy. The microstructural analysis of the reference 99Sn0.3Ag0.7Cu solder alloy showed that the volumetric increase of the corroded b-Sn grains resulted in local mechanical stress in the solder joints, which was released by Sn whisker development. However, in the composite solder joints, the increased grain boundary fraction and segregation of the TiO2 and ZnO nano-particles to the grain boundaries resulted in a relatively uniform and compact pro-tective oxide layer at the b-Sn grain boundaries. This suppressed the formation of SnOx, and via this, the corrosion-induced whisker growth.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Flux contained in solder paste significantly affects the process of solder joint creation during reflow soldering, including the creation of an intermetallic layer (IML). This work investigates the dependence of intermetallic layer thickness on ROL0/ROL1 flux classification, glossy or matt solder mask, and OSP/HASL/ENIG soldering pad surface finish. Two original SAC305 solder pastes differing only in the used flux were chosen for the experiment. The influence of multiple reflows was also observed. The intermetallic layer thicknesses were obtained by the image analysis of micro-section images. The flux type proved to have a significant impact on the intermetallic layer thickness. The solder paste with ROL1 caused an increase in IML thickness by up to 40% in comparison to an identical paste with ROL0 flux. Furthermore, doubling the roughness of the solder mask has increased the resulting IML thickness by 37% at HASL surface finish and by an average of 22%.

In this paper, the effect of different thermocouple constructions are investigated from the aspect of reflow soldering, to achieve more precise soldering profile acquisition and failure analysis at different soldering heat transfer methods. The paper focuses on convection-based reflow and vapour phase soldering processes and related heat transfer coefficients. Different thermocouple constructions were investigated. The parameters involve ASTM-type (K, T, J), length of uninsulated wire length, insulation material (PFA, PVC, PTFE, woven glass-fibre), insulation thickness and dimensions of the hot-spot. The physical dimensions were obtained with optical microscopy for validation. Finite element analysis with multiphysics modelling (convection- and condensation-based heat transfer, and heat conduction in solids) was applied. After validation, the results show that most parameter and geometry changes – most significantly the uninsulated wire end – can result in non-negligible timely differences in 100 ms range between different sensors, pointing to the requirement of sorting and pairing in symmetric measurement environment. The results can be applied in modern manufacturing environment and in future process modelling and evaluation tasks.

In the past years, the use of low silver content SAC0307 (99Sn0.3Ag0.7Cu wt%) solder alloy grew considerably due to its good soldering quality. In the latest version of this alloy, manganese, and bismuth are used to improve further its soldering parameters. In this study, a reliability issue, the whisker growth from SAC0307-Mn07 and SAC0307-Bi1-Mn07 solder alloys was investigated. Ultra-thin films (in 100-150 nm thickness) were vacuum evaporated from the investigated alloys onto Cu substrates. The samples were stored at laboratory conditions for 28 days. Whisker growth was followed by a scanning electron microscope, and the ultra-thin film layers structures were investigated in focused ion beam cuts. In the case of the SAC0307-Mn07, the mechanical stress due to the intermetallic layer growth resulted in whisker formation right after the layer deposition. However, Bi addition could increase the stress relaxation ability of the ultra-thin film layer; the whisker formation started only three days after the evaporation in the case of the SAC0307-Bi1-Mn07 layer. Besides, the SAC0307-Bi1-Mn07 alloy could entirely suppress the formation of filament-type whiskers that might cause reliability issues in microelectronics. Furthermore, a unique phenomenon was observed that the SAC0307-Bi1-Mn07 layer produced mostly Bi-Sn whisker couples.

The effect of recrystallization of 99.3Sn-0.7Cu wt. % solder alloy on the allotropic transition of beta to alpha-Sn (so-called tin pest phenomenon) was investigated. Bulk samples were prepared, and an InSb inoculator was mechanically applied to their surfaces to enhance the transition. Half of the samples were used as the reference material and the other half were annealed at 180 degrees C for 72 h, which caused the recrystallization of the alloy. The samples were stored at -10 and -20 degrees C. The beta-Sn to alpha-Sn transition was monitored using electrical resistance measurements. The expansion and separation of the tin grains during the beta-Sn to alpha-Sn transition process were studied using scanning electron microscopy. The recrystallization of the alloy suppressed the tin pest phenomenon considerably since it decreased the number of defects in the crystal structure where heterogeneous nucleation of beta-Sn to alpha-Sn transition could occur. In the case of InSb inoculation, the spreading of the transition towards the bulk was as fast as the spreading parallel to the surface of the sample.

The kinetics of Sn whisker growth was investigated on vacuum-evaporated Sn thin-films. Sn film layers were deposited on a Cu substrate with 0.5 and 1 mu m thicknesses. The samples were stored in room conditions (22 +/- 1 degrees C/50 +/- 5RH%) for 60 days. The Sn whiskers and the Cu-Sn layer structure underneath them were investigated with both scanning electron and ion microscopy. Fast Cu-Sn intermetallic formation resulted in considerable mechanical stress in the Sn layer, which initiated intensive whisker growth right after the layer deposition. The thinner Sn layer produced twice many whiskers compared to the thicker one. The lengths of the filament-type whiskers were similar, but the growth characteristics differed. The thinner Sn layer performed the highest whisker growth rates during the first 7 days, while the thicker Sn layer increased the growth rate only after 7 days. This phenomenon was explained by the cross-correlation of the stress relaxation ability of Sn layers and the amount of Sn atoms for whisker growth. The very high filament whisker growth rates might be caused by the interface flow mechanism, which could be initiated by the intermetallic layer growth itself. Furthermore, a correlation was found between the type of the whiskers and the morphology of the intermetallic layer underneath.

The purpose of the paper is to improve the control of vapour phase soldering (VPS). To enable better productivity and assembling quality, the industry needs to provide precise control and measurements during assembling. In the paper, a special monitoring method is presented for VPS to enable improved process control and oven state identification.

The effect of copper substrate roughness and tin layer thickness were investigated on whisker development in the case of Sn thin-films. Sn was vacuum-evaporated onto both unpolished and mechanically polished Cu substrates with 1 mu m and 2 mu m average layer thicknesses. The samples were stored in room conditions for 60 days. The considerable stress-developed by the rapid intermetallic layer formation-resulted in intensive whisker formation, even in some days after the layer deposition. The developed whiskers and the layer structure underneath them were investigated with both scanning electron microscopy and ion microscopy. The Sn thin-film deposited onto unpolished Cu substrate produced less but longer whiskers than that deposited onto polished Cu substrate. This phenomenon might be explained by the dependence of IML formation on the surface roughness of substrates. The formation of IML wedges is more likely on rougher Cu substrates than on polished ones. Furthermore, it was found that with the decrease of layer thickness, the development of nodule type whiskers increases due to the easier diffusion of other atoms into the whisker bodies.

In this paper, the relationship between the crystallographic structure of Cu-Sn intermetallic grains and Sn whisker growth was investigated. In order to prevent the influence of the elements in the alloy composition and the effect of the soldering process on the formation of the intermetallic layer, 99.99% pure Sn was vacuum evaporated onto Cu substrates. The Sn layer thickness was sub-micron region (~400 nm in average) to reach considerable and rapid compressive stress on the tin layer originated by the intermetallic formation. The samples were stored at room temperature for 1 month. Different types of whiskers (nodule and filament) and the layer structure underneath were studied with a scanning ion microscopy and transmission electron microscopy. It was found that not only the thickness of the intermetallic layer and shape of the intermetallic grains affects the whisker growth but the crystallo- graphic structure of the intermetallic grains as well. The susceptibility of the Sn layer to whisker development is higher in those regions where the intermetallic layer is composed of monocrystalline grains instead of those regions, where it is composed of polycrystalline grains. This effect can be explained by the higher compressive stress generated by the monocrystalline intermetallics compared to the polycrystalline ones.

Vapour phase soldering brings besides many advantages for reliability in electronic production an increased incidence of tombstoning errors. The imbalance of wetting forces has a decisive effect on the tombstoning phenomena, and the changes in wetting forces during the vapour phase soldering are therefore analyzed and evaluated. Two types of solders (leaded Sn63Pb and lead-free SAC387 alloy) and two types of heat transfer fluids (Galden LS 230 and Galden HS 240) were used. The use of soldering fluids changes the surface tension equilibrium vectors for both used solders and changes the measured wetting force by up to 20%. The measurement was performed by wetting balance method using non-wetting sample.

The reflow soldering process of large size components was always problematic in microelectronics manufacturing due to the possibility of component displacement failures after soldering; like tombstone formation or skewing, which can be traced back to the different heating of the opposite component sides. During vapour phase soldering, the efficiency of heat transfer highly depends on the thickness of the condensate layer. In this paper, the inhomogeneity of condensate layer formation and its effects were investigated at large size components during vapour phase soldering by numerical simulations. For this purpose, a 3D computational fluid dynamic model was established. According to the condensate layer formation in different cases, the onset differences in the melting of the solder alloy at the opposite leads of the component were calculated. By the results, the risk of the component displacement during reflow soldering was analysed. It was found, that the congestion of the condensate layer around the large size components can cause considerable differences in the onset of the solder alloy melting, which can yield in component displacement failures after soldering. The extent of difference in the onset of melting depends on the location of the component on the board and on the applied soak temperature. Keep-out zones on the board were suggested to reduce the possibility of the component displacement failures during the vapour phase soldering process.

The authors' institutions are Partners in the VINMES network (Visegrad Network for Microelectronics Engineering Scientists), and they are conducting a Visegrad Fund project about “V4 Seminars for young scientists on publishing techniques in the field of engineering science”. The aims of the project are the enhancement and support the scientific publication of young scientists from V4 and the surrounding countries, with a knowledge transfer about presentation techniques at conferences, journal article writing, and publication practice. Furthermore, facilitate the publication possibilities on youth conferences and in special issues of scientific journals. Twelve hours long teaching system was developed about the basics of publication; best practices of the different types of publication (journal and conference paper, posters, oral presentation etc.); related topics like image preparation and statistical evaluation. Up to now, three pilot courses were organized at three Central European Universities. The feedback of the participating students was evaluated and the positive result of the pilot tests encourages us to consider developing our courses to e-Iearning materials. This may help to disseminate the information about our courses and attract more students to take the courses on-line. In this paper the conclusions of our work are summarized

The thixotropic behaviour of different type lead-free solder pastes (Type 3: particle size: 20–45 μm; Type 4: 20–38 μm; Type 5: 10–25 μm) was investigated and compared. The viscosity decrease during the first stencil printing cycles is also described. The measurements were carried out with a rheometer using serrated parallel plates by sweeping the shear rate between 0.0015–100 s−1 with ten recordings per decade. Thixotropic behaviour of the solder pastes within the cycles of stencil printing was evaluated by including a “rest-period” between every rheological measurement to address the idle time between the printings. The “rest-period” was set to 15 s, 30 s and 60 s in this research The viscosity curve parameters η0 and η∞ were determined and were used to describe the viscosity change of solder pastes (from initial to stabilised state). Oscillatory stress sweep tests for different idle time periods were conducted in the stabilised state of the solder pastes. Significant difference was shown in the viscosity between the initial and the stabilised state of the solder paste; the η0 parameter decreased even to one thirds, and the η∞ decreased approximately to two thirds during the measurement cycles.

In this paper, an automatic method was developed to characterise whisker growth quantitatively in SEM images. The key step of the automatic methods in this case is the determination of the optimal threshold value for image segmentation, i.e. separation of the objects (whiskers) from the background (substrate). A thresholding method was developed for this purpose and was compared to manual and to general purpose automatic methods as references. As it was proven in previous studies the vacuum deposited tin layers on copper substrates can produce numerous tin whiskers in various shapes and lengths in a short time. This layer deposition technology was therefore chosen for the comparison of the thresholding methods. Images of the produced whiskers were captured by a FEI Inspect S50 Scanning Electron Microscope. By executing the automatic methods in the captured images, the area density of the whiskers, and the maximum and the mean length of the whiskers were measured. Based on the results of area density, the automatic methods were compared to manual counting and the Mean Absolute Percentage Error (MAPE) was determined. Finally, the reference automatic methods were compared to the self-developed method from the maximum and mean length of whiskers point of view

Purpose - The aim of this paper is to present a review of the tin whisker growth phenomena. The study focuses mainly on whisker growth in a corrosive climate when the main inducing factor of the whisker growth is oxidation. The tin whisker phenomenon is still a big challenge in lead-free reflow soldering technology. Modern lead-free alloys and surface finishes with high tin content are considered to be possible sources of whisker development, also the evolution of electronic devices towards further complexity and miniaturization points to an escalation of the reliability risks. Design/methodology/approach - The present work was based on a worldwide literature review of the substantial previous works in the past decade, as well as on the results and experience of the authors in this field. Findings - The effect of corrosion on tin whisker growth has been under-represented in reports of mainstream research; however, in the past five years, significant results were obtained in the field which raised the corrosion phenomena from being a side effect category into one of the main inducing factors. This paper summarizes the most important findings of this field. Practical implications-This literature review provides engineers and researchers with a better understanding of the role of corrosion in tin whisker growth and the current challenges in tin whisker mitigation Originality/value - The unique challenges and future research directions about the tin whisker phenomenon were shown to highlight rarely discussed risks and problems in lead-free soldering reliability.

In this paper, the allotropic transition of ß to α-Sn (so called “tin pest”) was identified with electrical resistance measurements. Samples were prepared from SnCu1 alloy using mechanical treatment of two different sample sizes, artificially inoculated with InSb and stored at −18°C for 9 weeks. The electrical resistance measurement showed that in the case of SnCu1 solder alloy the α-Sn transition has three stages: nucleation, growth and the saturation phase, when the transition is almost stopped. It was proven by focused ion beam cross-sectioning and Mösbauer spectroscopy that the developed α-Sn phase can enclose the metastable non-transitioned ß-Sn preventing its further transition and resulting the saturation phase of the phenomenon. In addition, the results point out that the rate of vertical expansion of the α-Sn into the sample body can be equivalent or higher than the horizontal expansion.

Purpose - A measurement method has been developed to reveal the viscosity change of solder pastes during stencil printing. This paper aimed to investigate thixotropic behaviour, the viscosity change of a lead-free solder paste (Type 4). Design/methodology/approach - The viscosity change of the solder paste during stencil printing cycles was characterised in such a way that the time-gap between the printing cycles was modelled with a rest period between every rheological measurement. This period was set as 15, 30 and 60 s during the research. The Cross model was fitted to the measurement results, and the eta(o) parameter was used to characterise the viscosity change. The number of printing cycles necessary for reaching a stationary state in viscosity was determined for various rest periods. Findings - It was found that the decrease in zero-shear viscosity is significant (25 per cent) in the first cycles, and it starts to become stationary at the sixth-seventh cycles. This means a printing process can provide the appropriate deposits only after the 7th cycle with the investigated Type 4 solder paste. Originality/value - Time-dependent rheological behaviour of solder pastes was studied in the literature, but only the viscosity change over continuous time at constant shear rates was examined. The time-gap between stencil printing cycles was not considered, and thixotropic behaviour of solder pastes was also neglected. Therefore, the authors developed a measurement set which is able to model the effect of time-gap between printing cycles on the viscosity change of solder pastes.

The heat and mass transport processes were investigated with numerical simulations in a vacuum vapour phase soldering system during the vapour suctioning process. Low vapour pressure/concentration is applied during vacuum soldering to decrease the number of gas voids in the solder joints. Three-dimensional numerical flow model was developed which based on the Reynolds averaged Navier-Stokes equations with the standard k-ε turbulence method. The decrease of the vapour concentration and its effects on the solder joints were studied in the case of different oven settings. It was found that vapour suctioning has considerable effects on the heat transfer processes in the soldering chamber which might lead to early solidification of the solder joints and reduces the efficiency of the void removal. Different oven settings were simulated in order to decrease the heat loss of the soldering chamber during the vapour suctioning. It was shown that with appropriate setting of the vacuum vapour phase soldering technology, the efficiency of the void removal can be increased.

Leider was the first to summarize the knowledge on VPS in recent years (Leider, 2002), covering the basic formulation of heat transfer during the process, which was based on the Nusselt theory of filmwise condensation. Leider also investigated different condensation media types, and the dependency of the PCB alignment, from the aspect of vertical and horizontal positioning in the work zone. It was clearly indicated that, with a vertical alignment, the heat transfer coefficient may rise by 25 to 50% depending on the condensation medium. More recently, it was found by Illés that the inclination of the board had a serious effect on the heating, and the heat distribution (Illés, 2016.); the results were obtained from verified modelling software. No actual practical investigation was published about the exact effect of inclination during VP soldering, so a deeper investigation was needed with novel measurement approaches.