Ing. Iva Králová

The aim of this work is to advance the development of an alternative eco-friendly method of manufacturing printed circuit boards (PCBs) using recycled and recyclable 3D printable polymers as the insulating substrate and a special silver paste for the conductive pattern. The components are embedded in the insulating substrate, and the connection to the PCB is made by overprinting the component with the silver paste. To improve mechanical properties and durability, in this work, the conductive pattern was overprinted on the 3D printer with another layer of recycled polymer, completely encapsulating the component. The results showed that the overprinting post-process led to a significant improvement in the mechanical properties of the interconnects with no negative impacts on the contact resistance of the components. The measurements of contact resistance indicated similarity to ECA joints. According to the pull-off test performed on the assembled two-terminal components, the pull-off force required to detach the component was similar to that of the soldered joints. Although the encapsulation did not apparently affect the change of the mechanical and electrical properties after accelerated aging tests, it showed an increase in reliability for rPLA samples. In this case, the encapsulation eliminated cracks in the conductive pattern. Despite the extensive development that still lies ahead, this alternative, environmentally friendly method could then be used in low-cost electronics or prototyping applications where sustainability is a priority.

The aim of this work was to evaluate the effect of 0.1 wt.% titanium dioxide nanoparticles (NPs) incorporated in low-temperature Bi58Sn42 solder paste on spreading behavior and mechanical and thermal properties. The spreading test was conducted on testing printed circuit boards with an Organic Solderability Preservative (OSP) surface protection of copper layer. Mechanical properties were evaluated by a shear strength test of 1206 resistors soldered on FR4 boards with OSP coating. Shear strength was also studied in dependence on accelerated thermal aging. The prepared test boards for the shear test were annealed in a climatic chamber at 85°C and 100°C for 1000 hours. Additionally, the solder alloys were subjected to Differential Scanning Calorimetry (DSC) to examine their thermal properties. The results showed a significant improvement in spreading when using solder paste with incorporated TiO2 NPs. Furthermore, TiO2 nanoparticles did not prominently deteriorate the thermal and mechanical properties.

Commercial powders made of two copper oxides were compacted with spark plasma sintering (SPS). Their dielectric properties were studied in a broad range of frequencies and temperatures. Various relaxation phenomena were documented. DC resistivity was measured as well. Microstructure and phase composition were studied, and phase purity was shown for CuO, whereas Cu2O was more sensitive to carbon contamination during the SPS processing. Influence of the sintering temperature on microstructure and electrical properties was described for both materials. Change of DC resistivity induced by visible light irradiation was monitored for 48 h. The overall difference between CuO and Cu2O from the electrical standpoint was finally not so dramatic as the stoichiometry indicated. Both materials exhibited giant permittivity, but extremely dependent on frequency and temperature. Application could find such systems mainly in the branch of sensors.

This research aimed to investigate the susceptibility of Sn-58Bi alloys to Electrochemical Migration (ECM) when combined with TiO2 nanoparticles tested in various solutions, including deionized water (DI), 1 mM Na2SO4, 500 mM Na2SO4, 1 mM NaCl, and 500 mM NaCl, using water drop (WD) test. The results revealed a heightened ECM susceptibility in Sn-58Bi alloys with the addition of TiO2 nanoparticles, indicating an adverse impact of TiO2 nanoparticle incorporation. Furthermore, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEMEDS) were utilized to analyze the surface morphology and elemental composition of dendrites formed after the WD tests. The outcomes showed the presence of dendrites and precipitates in both Sn-58Bi and Sn-58Bi-0.1% TiO2 cases. Sn was identified as the primary element in the dendrites, while Bi was not detected in the dendrites in any of the cases. Consequently, the reliability of electronics may be compromised when using Bi-Sn paste doped with TiO2 nanoparticles, particularly in terms of ECM. Nonetheless, these nanoparticles could enhance other properties associated with modified microstructure, such as mechanical or thermal properties, which warrant further investigation.

This study aimed to analyze the reliability of solder joints on different types of printed circuit board (PCB) substrates. Conventional glass-epoxy (FR-4) laminate and aluminum substrate with glass-epoxy dielectric layer were included in the evaluation as the rigid types. Tested flexible PCBs were made from polyimide. Moreover, FR-4 substrates varied in used surface finishes. Soldering pads were protected by hot air solder leveling (HASL) or organic solder preservative (OSP). Components mounting was accomplished using low- temperature solder alloy Sn42Bi57Ag1 and consequent convection reflow soldering. Assembled PCBs were subjected to 500, 1000, 1500, and 2000 cycles in the thermal shock chamber. The first evaluation approach was the measurement of electrical resistance during and after the thermal cycling. Shear strength of solder joints was also assessed. A thickness of intermetallic layers (IML) was observed in relation to the mechanical properties of solder joints. Thermomechanical analysis (TMA) was employed to determine the glass transition temperature (T g ) of organic substrates and to obtain the coefficient of thermal expansion (CTE) of all tested substrates. Solder joints on the substrate with an aluminum base layer exhibited the greatest degradation due to thermal cycling. The electrical resistance of the joints on this substrate type increased significantly, and the occurrence of extensive cracks was very frequent. A substantial increase in the width of the IML reduced the shear strength.

So far, only a few studies have been focused on the electrochemical migration (ECM) of gold or gold surface finishes of printed circuit boards. Therefore, this work focused on the evaluation and comparison of electroless nickel immersion gold (ENIG) and galvanic gold in terms of ECM. The copper surface was also included in the experiment as a reference value. The water drop test (WDT) was conducted with distilled water on the comb pattern electrodes with three different applied voltages, and time to failure (TTF) was measured. Energy Dispersive X-ray (EDX) analysis of the dendrites was subsequently performed regarding their composition. The results showed an apparent difference in ECM behavior between the gold surface finishes. ENIG surface finish is more resistant to ECM than galvanic gold. Bare copper is the most susceptible to ECM among tested surface finishes. The difference in susceptibility to ECM among gold surface finishes is caused by their structure and how they cover the copper electrodes.

It is well known that the reflow temperature profile has an influence on the thickness of intermetallic layers. The higher the thermal treatment, especially above the melting temperature of solder alloy, the higher the thickness of intermetallic layers. The heating factor (time-temperature integral of temperature profile above the melting temperature) is commonly used for comparison of the heat treatment. This paper deals with a comparison of three different reflow temperature profiles (with three different heating factors: 242, 1061 and 1652 s.K), three surface finishes (Organic Solderability Preservative - OSP, Hot Air Solder Leveling - HASL and Electroless Nickel Immersion Gold - ENIG) of copper soldering pads and two solder pastes with the same composition of solder alloy (Sn96,5/Ag3,0/Cu0,5) but with different flux type (less aggressive - ROL0 and more aggressive - ROL1). The cross-sections of the samples were prepared after the reflow soldering process, followed by analysis by scanning electron microscope and measurement of intermetallic layers thicknesses. Results confirmed that the heating factor significantly influences the intermetallic layers. The highest thickness of intermetallic layers was achieved with the highest value of the heating factor. The surface finish HASL had the highest values of intermetallic layer thicknesses from all used surface finishes, whereas ENIG had the lowest. The higher values of intermetallic layer thickness were observed for the solder paste with a more aggressive flux, ROL1, than for ROL0.

Electrochemical migration (ECM) on the surface of printed circuit boards (PCBs) continues to pose a significant reliability risk in electronics. Nevertheless, the existing literature lacks studies that address the solder mask and solder pad design aspects in the context of ECM. Therefore, the objective of this study was to assess the impact of solder mask type with varying roughness and solder pad design on the susceptibility to ECM using a water drop test and thermal humidity bias test. Hot air solder leveling-coated PCBs were tested. Furthermore, the ECM tests were conducted on PCBs with applied no-clean solder paste to evaluate the influence of flux residues on the resulting ECM behavior. The results indicated that the higher roughness of the solder mask significantly contributes to ECM inhibition through the creation of a mechanical barrier for the dendrites. Furthermore, lower ECM susceptibility was also observed for copper-defined pads, where a similar effect is presumed. However, the influence of the no-clean flux residues can prevail over the effects of the solder mask. Therefore, the use of a rough solder mask and a copper-defined pad design is recommended if the PCB is to be washed from flux residues after the soldering process.

An intermetallic layer (IML) between the solder alloy and the soldered surface affects the mechanical and electrical performance of the resulting joints. Numerous studies have explored the possibilities of influencing the IML to achieve more reliable interconnections. However, the type and composition of the used flux, crucial for the proper creation of solder joints, is rarely included as a possible influencing factor. In this article, a comprehensive study on the interfacial microstructure evolution of lead-free SnAgCu solder joints, accounting for the flux type and the temperature of the preheating phase of reflow soldering, where the flux contained in the solder paste becomes active, is presented. In the results, it is shown that the IML of as-reflowed and thermally aged solder joints depends significantly on the flux. The IML activation energy is 57% higher for rosin-based low-activity (ROL)0 flux compared to ROL1 flux. The ROL0 flux, containing fewer active components, also outperforms the ROL1 flux in both the mechanical and electrical properties of the joints. Furthermore, the temperature profiles also show slight differences in measured properties, with the fluxes responding differently to changes in preheating temperature. In the presented results, importance of the used flux on solder joint microstructure is demonstrated.

To enhance the solderability, highly active fluxes are commonly employed in lead-free soldering. However, there are industry-wide efforts to use less active fluxes to avoid possible issues associated with corrosion processes and minimize subsequent cleaning processes, thereby reducing potential environmentally harmful waste. Therefore, in this study, the effect of the plasma treatment (N2/H2 97:3) on the wettability of the soldered surface (copper connectors) was investigated. Wettability measurements were conducted using SAC305 solder alloy and six different fluxes. The wetting balance test revealed a significant improvement in wetting for all tested fluxes, regardless of their composition. On the other hand, non-wetting occurred when no flux was applied to the plasma-treated surface, attributed to a thin residual oxide layer detected by X-ray photoelectron spectroscopy. Thus, the plasma treatment of the surface supports the flux effect, which cannot be entirely omitted from the soldering process. However, incorporating plasma treatment in the soldering process allows for the use of much less active or even expired fluxes.

A conductive filament proposed for Fused Deposition Modeling was produced and evaluated in terms of electrical resistivity. Thermoplastic polyester polylactic acid (PLA) was selected as a pristine polymer. Carbon black (CB) was added to PLA to improve the electrical conductivity. The incorporation ratio of the CB in PLA was set to 30 wt.%. Neat polymer and another commercially available filament with CB filling were included in the measurements and comparison. Volume and surface resistivity was measured perpendicularly to the printing direction. Further, the resistivity evaluation of the materials was performed in the same direction as the filament deposition. This resistivity was determined using a four-wire measuring method and purposefully designed specimens. A decrease of twelve orders of volume resistivity was observed for filled PLA. Mechanical properties were determined via tensile testing. The behavior of the material under thermomechanical loading was observed by dynamic mechanical analysis (DMA). Glass transition temperature was determined from DMA diagrams. A touch sensor in the form of circular electrodes was prepared for the purpose of practical use assessment. The functionality of the sensor was verified by switching the transistor, which controlled the LED. Magnitudes of electric current flowing through the sensor at various supplied voltage levels were monitored. The second practical utilization was demonstrated by the strain gauge. The sensor for bending detection was designed to exhibit resistivity in hundreds of kiloohms. The strain sensing behavior of the strain gauge was determined by conducting tensile loading.

Electrochemical migration (ECM) is an ongoing reliability issue in the microelectronics industry. ECM is characterized by the presence of dendrites, which form between two electrodes with a nonzero electric potential difference and can lead to short circuits. This work is focused on the evaluation of the influence of electrode shapes and voltage polarity on dendrite growth. Several combinations of straight, oval, and angular shapes were tested, along with the changing polarity of the applied electric field. Water drop test (WDT) with 1.5 wt.% and 0.01 wt.% NaCl solution was used to examine ECM. The results show a difference in the dendrite growth depending on the electrode’s shapes and the voltage polarity. The longest time to failure (TTF) and the most remarkable differences between the different voltage polarities occurred when the combinations with the angular electrodes were tested.

The goal of this work was to design a new methodology for the solderability measurement of solder alloys in vias (plated through holes) on printed circuit boards (PCB). As a key measurement device, a wetting balance tester was chosen. The sample holder was modified to be able to fix a copper tube, which simulated the plated through hole (PTH). The copper tubes were covered by a non-wetting coating on the outside; therefore, only the inside of the tube was wetted during immersion. This methodology was used for experiments with SAC305 solder in combination with colophony-based flux in order to verify its suitability. Three solder bath temperatures (255 °C, 270 °C, and 285 °C) were chosen for the measurement. The performed experiment showed the effect of a solder bath temperature and a diameter of PTH on the evaluated parameters, such as zero-cross time, non-wetting time, maximum wetting force, and height of capillary rise of the solder. The higher the temperature, the shorter the zero-cross time and non-wetting time. The bigger the diameter, the higher the maximum wetting force and the longer the non-wetting time. With the increasing vias’ diameter, the decreasing trend of the zero-cross time can be observed. The obtained results prove that the proposed methodology is appropriate for evaluating the alloys’ solderability in vias, providing a complex view of their wetting behavior during soldering.

This work aimed to evaluate a new approach to electronics manufacturing using recycled and recyclable 3D-printed polymers as an insulation substrate and printing of conductive silver ink as a conductive pattern. Furthermore, connection to the board is realized via embedding the components into the substrate and the ink's overprints. The first results from the measurements showed that this type of connection is comparable to conductive adhesive joints regarding contact resistance. Also, the behavior of the joints during accelerated aging by thermal shocks is relatively acceptable from a reliability point of view. A significant advantage of this manufacturing method is the absence of the high-temperature heating processes and, thus, energy savings compared to conventional production processes. Also, the final product is easily recyclable after its functional life. On the other hand, the overall quality is lower than that of standard printed circuit boards consisting of FR4 substrate and the copper conductive layer. The range of available and suitable components is also much smaller. The proposed solution could find applications primarily in low-cost electronics or prototyping.

The wetting balance method is used for the precise classification of solderability of chosen substrates by solder alloys. This work deals with a weakness of the wetting balance method during the wettability measurement of connectors with the wetting issue. The wetting issues at examined pins connector appeared during the serial manufacturing production, and therefore, the connector pins were analysed using the wetting balance method. The wetting balance method showed a good wetting of the connector pins. The wetted pins were examined by scanning electron microscopy (SEM) to find the reason for the wetting issue. This analysis showed a non-wetted area at pins edges. Following investigation of pins microsections using confocal/optical microscopy showed the reason for the wetting issue, when the surface finish was much thinner or was missing on the edges of the pin. This was the reason for the wetting issue of the connector pins in serial manufacturing, even though the wetting balance test showed good wettability results because most parts of the pin surface had good wetting.

Due to the restrictions on lead solder alloy, there is an ongoing search for alternative lead-free alloys with the best possible solder properties similar to Pb-based solder alloys. Therefore, phosphorus and gallium are, for example, added to solder alloys, which could lead to an improvement in their solder properties (wettability, mechanical, lower melting point). These additives are usually used for the improvement of SAC alloys. This work focused on lead-free low-temperature solders based on tin and bismuth. The effect of added 1 wt.% gallium and traces of phosphorus on wettability and spreading was studied. These properties were observed on FR4 boards with three different surface materials: copper, copper with hot air solder leveling surface finish (HASL) and copper with electroless nickel-immersion gold surface finish (ENIG). Examined alloys were Bi58Sn42, Bi58Sn42P, Bi59Sn40Ga1 and Bi59Sn40Ga1P. The results showed that although the addition of Ga and P exhibited no significant improvement or even decrease in the wetting and spreading ability of the solder alloy on copper and ENIG surfaces, the wetting behavior of the doped alloys was better on HASL surface compared to the eutectic solder alloy.

The goal of this work was to evaluate changes in mechanical properties and microstructure of the lead-free bismuth-tin alloys when adding gallium and phosphorus. These elements were added originally on account of melting temperature decrease (Ga) and improvement of wetting (P). Six different solders were examined: the eutectic one Bi58Sn42 as a reference, then Bi59Sn40Ga1 and Bi57Sn40Ga3. In addition, the investigation was also performed on all the mentioned alloys with an added trace amount of phosphorus. Mechanical properties were measured by the shear test of a solder ball on a copper substrate with Organic Solderability Preservatives (OSP) surface finish. Furthermore, the small balls of solder alloys were reflowed on boards with three different surface finishes: copper-plated, Hot Air Solder Levelling (HASL), and Electroless Nickel Immersion Gold (ENIG). These coupons were left to aging in the climatic chamber for 500 and 1 000 hours at a temperature of 100 °C. The metallographic cross-sections were made, and the microstructure of the intermetallic layer (IML) was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The shear test showed a significant decrease in the shear force by adding gallium. It was also found that the addition of phosphorus has only a minor (but still statistically significant) impact on the shear force. The addition of gallium affected the IML thickness and caused a considerable decrease compared to alloys without Ga. The reason was the composition of the IML. IML of Bi-Sn solder joints consisted of Cu-Sn, whereas the gallium-containing alloys formed IML consisted of Cu-Ga.

The aim of this work is to evaluate a wettability improvement and microstructure changes by addition of gallium and trace elements of phosphorus to novel low-temperature lead-free Bi-Sn solder alloys. Four different alloys Bi59Sn40Ga1, Bi57Sn40Ga3, Bi60Sn40 and the eutectic alloy Bi58Sn42 were chosen. Furthermore, all these solders were investigated with an added small amount of phosphorus as well. For the wettability comparison, the wetting balance test in combination with three different fluxes was used. Moreover, these alloys were soldered to a copper plated test board and aged in a climatic chamber at the temperature of 80 °C for 24 days. Subsequently, metallographic cross-sections were made and analyzed by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). The results of the wettability analysis showed the dominance of the chosen flux while soldering. However, it is still possible to draw the conclusion that phosphorus as an additive in Bi-Sn-Ga alloys supports the wetting, which is a crucial property of the solders. On the other hand, by the addition of gallium to the Bi60Sn40, the wetting force decreased. Regarding the microstructure, two different intermetallic compounds were identified. Namely, Cu6Sn5 at the interface between Cu board and alloys Bi60Sn40P and the eutectic one Bi58Sn42. The second detected IMC was CuGa2 between Cu and solder alloys with one and three weight percent of added gallium.

The objective of this work is to examine the mechanical and thermal properties of novel lead-free bismuth-tin solder alloys with low melting temperature, each with a different percentage of added gallium and with phosphorus as a trace element. The motivation was to determine solder alloy with decreased melting point and maintain mechanical properties of eutectic bismuth-tin solder alloy at the same time. BiSn40, BiSn40P, BiSn40Ga1, BiSn40Ga1P, BiSn40Ga3, and BiSn40Ga3P were among the analyzed solder alloys. In addition, the same properties of BiSn42 were analyzed for comparison as a reference. The thermal properties were examined by differential thermal analysis, where the melting and solidification points were determined. The Vickers micro-hardness test was conducted for the observation of mechanical attributes. Moreover, the microstructure of the alloys was observed in a scanning electron microscope. The result of this study showed that the temperature of the melting point significantly decreased in the alloy BiSn40Ga3. Also, this alloy seems to have as sufficient mechanical properties as BiSn42. Therefore, it could possibly be used as a suitable substitution for the eutectic solder alloy.