Ing. Denis Froš

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.

Biodegradables are a promising path for the future of electronics in a greener mindset. The review study focuses on their applications and past and current research results. The paper also investigates the application of nanomaterials as fillers to control or increase the physical (electrical, mechanical, thermal) properties of biodegradable biopolymers. These biodegradables and nanocomposites are already effectively used in prototypes and advanced application areas with demanding requirements, such as flexible and wearable electronics, implantable or biomedical applications, and traditional commercial electronics. The nano-enhanced biopolymer substrates (e.g., with improved gas and water barrier functionalities) sometimes also with integrated, nano-enabled functionalities (such as electromagnetic shielding or plasmonic activity) can be beneficial in many electronics packaging and nanopackaging applications as well.

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 work evaluates the impact of latent heat (LH) absorbed or released by a solder alloy during melting or solidification, respectively, on changes of dimensions of materials surrounding of the solder alloy. Our sample comprises a small printed circuit board (PCB) with a blind via filled with lead-free alloy SAC305. Differential scanning calorimetry (DSC) was employed to obtain the amount of LH per mass and a thermomechanical analyzer was used to measure the thermally induced deformation. A plateau during melting and a peak during solidification were detected during the course of dimension change. The peak height reached 1.6 mu m in the place of the heat source and 0.3 mu m in the distance of 3 mm from the source. The data measured during solidification was compared to a numerical model based on the finite element method. An excellent quantitative agreement was observed which confirms that the transient expansion of PCB during cooling can be explained by the release of LH from the solder alloy during solidification. Our results have important implications for the design of PCB assemblies where the contribution of recalescence to thermal stress can lead to solder joint failure.

New types of filaments were formed by adding various fillers into polymers during the fabrication. We designed two new composites in cooperation with a filament producer. Biodegradable thermoplastic polyester polylactic acid (PLA) was filled with carbon black. Carbon black (CB) formed 30 wt.% of in the final mixture. The second composite was made by compounding the polyethylene terephthalate glycol-modified (PET-G) with titanium dioxide. Two different contents of TiO2, namely 10 and 20 wt.%, were selected for filament preparation. The goal of the testing was to evaluate the change of mechanical and thermomechanical properties in contrast to pure filaments. The testing specimens were prepared by Fused Deposition Modeling (FDM), which belongs to 3D printings techniques. The printed samples were subjected to mechanical tensile testing and to microhardness measurement by the Vickers method. Thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA) were performed too. In particular, glass transition temperature, coefficient of thermal expansion (CTE), and storage modulus were analyzed. All composites showed a lower ultimate tensile strength than the neat polymer. Notable changes in thermomechanical behavior were also detected. The most significant difference was the drop in CTE above the glass transition in the case of filled PLA samples.

This comprehensive study dealt with the reliability issue of no-clean flux spattering from the solder paste during the reflow soldering process. Several factors entering the process were investigated: the type of flux in the solder paste (ROL0/ROL1 according to the IPC J-STD-004B standard), the type of solder mask (liquid photo imageable/directly printed), and the type of solder pad design (copper-defined/solder mask-defined). A unique design of the test board was developed for a proper evaluation of flux spatter spots spattered from the solder paste. The testing board included real-sized soldering pads and a boron-silicate glass slide as a target area, ensuring the same evaluation process for all samples. The results showed a strong influence of flux chemistry on spattering. Changing the flux from ROL1 to ROL0 reduced the flux spot occurrence by 62 % on average. The type of solder pad design also slightly affected the flux spattering. No significant difference was found between the used solder masks; therefore, it is possible to use the novel, directly printed one without increasing the risk of flux spattering.

Abstract: The study attends to the evaluation of solder joints from a mechanical point of view. Two lead-free alloys Sn99.7Cu0.3 and Sn96.5Ag3Cu0.5 (SAC305) were selected for samples preparation. To compare the results of current alloys with the most utilized solder in the past, the study comprises eutectic lead solder paste Sn63Pb37, too. Solder joints intended for observation were created by assembling chip resistors. Assembled boards were reflowed employing a hot air convection oven. Mechanical properties were evaluated by performing shear tests and microhardness measurements. Microhardness of bulk solder and intermetallic compounds (IMC) was measured using the micro-indentation Vickers test. For this purpose, a metallographic cross-section of some samples was accomplished. Solder pads on testing boards were coated by Organic Solder Preservative (OSP) and Hot Air Solder Levelling (HASL). Depending on solder paste type, the results show more or less impact of surface finish on the solder hardness. Some difference in microhardness of IMC layers and shear strength was also detected but, in these tests, results are not in favour of one surface treatment.

This work aimed to evaluate the change of properties of joints based on bismuth-tin eutectic alloy soldered under various reflow conditions during long-term accelerated aging tests. Four temperature profiles were used for the reflow soldering of testing boards. The profiles differed in the reflow phase and the pre-heat phase when the soldering flux is activated. Subsequently, the first half of the testing boards was aged for 1000 hours in a climatic chamber at 85 °C and relative humidity 85 %. The second half was closed up in a temperature-shock chamber for 1000 cycles with the following settings: a hot temperature of 125 °C, a cold temperature of -40 °C, and a cycle time of 30 minutes. The solder joint properties - electrical resistance, shear strength, and thickness of intermetallic layers - were measured four times during the aging to observe trends. It turned out from the measurement results that the most stable during temperature-humidity aging were the joints soldered with higher temperature in the pre-heat phase and lower temperature in the reflow phase. On the contrary, the joints soldered with a lower temperature in the pre-heat phase and a higher temperature in the reflow phase were more resistant against the thermal shocks. The shear strength was even higher than before the start of the test. Overall, both pre-heat and reflow settings play an important role in solder joint reliability.

The purpose of this study was to compare the strength of the bond between resin and glass cloth for various composites (laminates) and its dependence on utilized soldering pad surface finishes. Moreover, the impact of surface finish application on the thermomechanical properties of the composites was evaluated. Three different laminates with various thermal endurances were included in the study. Soldering pads were covered with OSP and HASL surface finishes. The strength of the cohesion of the resin upper layer was examined utilizing a newly established method designed for pulling tests. Experiments studying the bond strength were performed at a selection of laminate temperatures. Changes in thermomechanical behavior were observed by thermomechanical and dynamic mechanical analyses. The results confirmed the influence of the type of laminate and used surface finish on bond strength. In particular, permanent polymer degradation caused by thermal shock during HASL application was observed in the least thermally resistant laminate. A response to thermal shock was detected in thermomechanical properties of other laminates as well, but it does not seem to be permanent.

Electrochemical migration is one of the issues that appears due to specific conditions when the electrolytic solution is present between two neighbouring conductors with different electrical potentials. This article deals with a student’s workplace, which demonstrates the electrochemical migration phenomena. The workplace can be included as a part of practical education in labs. The students can directly observe the electrochemical migration process, respectively, the growth of dendrites at prepared, printed circuit boards (PCB). The workplace is based on Peltier modules which enable to cool the sample (PCB) below dew point temperature. This ensures conditions for moisture condensation on the PCB with the following movement of metal ions in the presence of electric potential. The students obtain knowledge and experience from the field of electrochemical migration processes, moisture and its condensation. One of the advantages is that the workplace can be captured with a camera, and it can be used as a part of on-line education.

The goal of this study is to evaluate the bond between the soldering pad and the substrate in dependence on the solder pad finish as well as on ambient temperature. Stated differently, measuring of pulling force necessary for detaching the pad from the substrate is a subject of submitted study. A conventional substrate (laminate) made of woven glass cloth and epoxy resin with glass transition temperature value of 135 °C was available. OSP and HASL were chosen as surface finishes for evaluating the impact on the studied concern. Bond strength test was performed in room temperature and at an elevated temperature around 60 °C. An influence of IMC layers on test performing and results is included. Achieved results suggest a difference in the rate of adhesion of soldering pad to the substrate considering surface finish. However, other testing variables don't affect the pad adhesion significantly.