Ing. Markéta Klimtová

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.