Ing. Markéta Klimtová

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.

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.

This work aimed to evaluate the susceptibility of copper and tin to the electrochemical migration (ECM) in electrolytes that are well-known from real-life experience. Additionally, the aim was to increase awareness of the ECM phenomenon through familiar examples. The experiment involved testing high-purity water, tap water and selected commercial beverages, such as cola and an energy drink. The study determined the mean time to failure (MTTF) of the ECM behavior through the water drop test (WDT). A bias voltage of 10 VDC was applied to the test comb pattern after depositing a selected liquid. The results indicated a strong correlation between MTTF and the liquid type. Surprisingly, tin-coated samples showed an increase in MTTF when commercial beverages were applied, compared to tap or high-purity water. The MTTF of cola was 270 seconds, while tap water only had 6 seconds. On the other hand, dendrites grew at the same rate in cola as they did in both high-purity and tap water when placed between 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.

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.

This work aimed to investigate the susceptibility of novel nanocomposite solder paste to electrochemical migration (ECM). Bismuth-tin solder paste was filled with 0.1 wt.% of TiO2 nanoparticles. The electrochemical migration behavior was evaluated using a water drop (WD) test. High-purity water and 10 VDC bias voltage were applied. The experiments were performed on two types of samples differing in the surface finish of conductive pattern- bare copper without surface protection and copper with applied galvanic gold finish. The results revealed a trend when the mean time to failure decreased for samples with deposited nanocomposite solder paste, regardless of the surface finish. Thus, the electronics’ reliability may be lowered when using Bi-Sn paste doped with TiO2 nanoparticles, at least in terms of ECM. This deterioration of the solder paste should be considered a significant drawback for further development. However, the nanoparticles could improve other properties related to modified microstructure, such as mechanical or thermal. These aspects will be the subject of further research.

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.

This work aimed to analyze a reliability issue that occurred on printed circuit boards (PCB) primarily designed to study electrochemical migration (ECM) phenomena. The test boards were supplied directly from the PCB producer, and the copper traces were covered with a hot air solder leveling (HASL) surface finish. However, the solder layer was clearly inconsistent and poor, caused by contamination from improper solder mask application, as was confirmed by analysis using the scanning electron microscope. On these boards, a water drop test with distilled water (bias voltage of 10 V) and thermal humidity bias test (85 °C/90% R.H./25 V/168 h) was conducted to evaluate predisposition for electrochemical migration of boards with a such poorly fabricated solder mask. PCB without solder mask and with correctly applied solder mask was also included in this study for comparison. The results clearly showed that the test boards with the poorly fabricated solder mask were significantly more inclinable to electrochemical migration – in the case of the water drop test, the forming dendritic structures shorted the electrodes up to 6 times faster than on PCB without a solder mask, while the samples with correct solder mask exhibited the best resistance against ECM. During the thermal humidity bias test, the electrical short appeared after only 2 hours on PCB with the bad solder mask compared to PCB without the mask, where the dendrites grew after more than 27 hours. Energy dispersive spectroscopy confirmed that the migrating element was tin from the HASL cover layer.

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.

This work focuses on evaluating the anisotropy of the structures manufactured by two 3D printing technologies, Fused Filament Fabrication (FFF) and Stereolithography (SLA). According to the manufacturing process, printed structures are expected to have some level of anisotropy. It is essential to consider it while designing a model for 3D printing. Printed specimens were subjected to tensile testing, thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA). Measurements were based on the ASTM D638, E1545 and D4065 standards. The specimens were printed in three different orientations for tensile tests and DMA, respectively in four orientations for TMA. The FFF specimens were prepared from poly-lactic acid (PLA) with different extruder temperatures, 210°C and 240°C. In the case of SLA specimens, photopolymer based on epoxy resin was used, the process differed in curing time (8 s and 16 s). The results clearly showed a higher level of anisotropy of the FFF specimens compared to SLA specimens. The FFF specimens show in the orientation XZ 1,5 times lower tensile strength than in the other two orientations, compared to SLA, where the tensile strength was similar. The anisotropy of the thermomechanical properties was high for FFF specimens; in contrast, the SLA specimens showed no difference in thermal expansion for different printing orientations.