Ing. Martin Plaček, Ph.D.

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.

Sn–Bi alloys are desirable candidates for soldering components on printed circuit boards (PCBs) because of their low melting point and reduced cost. While certain tin–bismuth solders are well characterized many new alloys in this family have been developed which need proper characterization. The following study looks at the behavior of four different Sn–Bi alloys—traditional 42Sn58Bi and 42Sn57Bi1Ag and two new tin–bismuth alloys—in solder paste during the reflow soldering process. Each alloy was processed using different reflow profiles that had varying times above liquidus (TALs) and peak temperatures. The PCBs were then analyzed to see how the processing variables influenced wetting, voiding, microstructure, intermetallic layer composition, and thickness. After analysis, the PCBs were then subjected to thermal cycling experiments to see how reflow profile impacted microstructure evolution. The results demonstrated that reflow profile affects properties such as metal wetting and voiding. It does not however, greatly impact key metallurgical properties such as intermetallic layer thickness.

Electrical components can fail during their function in an electrical circuit, especially in a harsh environment. One of the possible failure reasons is the electrochemical migration, which leads to a short circuit or change of electrical parameters of components. This paper focuses on electrochemical effects and explains mechanisms leading to the formation of a conductive path within a component molded in a plastic package. Appropriate diagnostic methods SEM/EDS and penetration tests have been chosen in order to find the root cause for short circuit creation. It was found that the conductive path appeared due to electrochemical migration of silver between the Electrically Conductive Adhesive (ECA) joints connecting the capacitor package. A tiny gap that was found between the molding compound and the leadframe of the package, showed to be the necessary condition for the electrochemical migration to appear. The main aim of this work was not just to identify the cause of an inadvertent conductive path creation, but also to identify the part of the manufacturing process, where similar problems can be prevented.

The thermal behavior of solder alloys during melting and solidification is important from point of many aspects. It has influence on formation of joint structure together with intermetallic compounds, reliability issues like tombstoning effect or warpage effect, dimensional changes of components and printed circuit board etc. This article deals with influence of released latent heat from solder joints on the surrounding temperature. Printed circuit boards with soldering pads designed side by side in the matrix and solder paste based on SAC387 solder alloy particles was used in experimental study. The influence of increased number of solder joints in a defined area was examined. The temperature profiles were measured below soldering pads during reflow soldering to find the thermal influences. The results show that released latent heat from solder joint has influence on surrounding temperature and it can delay the solidification of neighbor joint.

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.

Purpose – The purpose of this paper is to increase the reliability of manufactured electronics and to reveal reliability significant factors. The experiments were focused especially on the influence of the reflow oven parameters presented by a heating factor. Design/methodology/approach – The shear strength of the surface mount device (SMD) resistors and their joint resistance were analyzed. The resistors were assembled with two Sn/Ag/Cu-based and one Bi-based solder pastes, and the analysis was done for several values of the heating factor and before and after isothermal aging. The measurement of thickness of intermetallic compounds was conducted on the micro-sections of the solder joints. Findings – The shear strength of solder joints based on the Sn/Ag/Cu-based solder alloy started to decline after the heating factor reached the value of 500 s · K, whereas the shear strength of the solder alloy based on the Bi alloy (in the measured range) always increased with an increase in the heating factor. Also, the Bi-based solder joints showed shear strength increase after isothermal aging in contrast to Sn/Ag/Cu-based solder joints, which showed shear strength decrease. Originality/value – The interpretation of the results of such a comprehensive measurement leads to a better understanding of the mutual relation between reliability and other technological parameters such as solder alloy type, surface finish and parameters of the soldering process.

Mechanical stress act on solder joints during transport, function and storage of the electronic equipment. Therefore, the knowledge of solder joints resistance against it is very useful. This article deals with comparison of mechanical resistance of SnCu0,7 and Sn42Bi58 solder alloys. The experiments were based on two PCBs (smaller and larger one, with soldering pads designed according to commonly used chip package) which were soldered together. Surface finishes of soldering pads were OSP, ENIG and HASL. Mechanical test were based on sharp tip, which were driving between two PCBs. From overall point of view a slightly better results were achieved for Sn42Bi58 solder alloy. Additionally the thicknesses of intermetallic layers were compared and voids frequency inside solder joints were inspected.

Mechanical properties of solder joints have influence on reliability of electronic products. Therefore the knowledge of the mechanical behavior of solder joints is very important. This article deals with comparison of mechanical properties of solder joints prepared with conventional soldering methods and ultrasonic soldering methods. Material for samples preparation was based on two PCB (material FR4) with soldering pads which were soldered together. As solder alloy was used Sn965Ag3Cu0.5 for convention soldering method and active soldering alloy for ultrasound soldering. The mechanical resistance, changes according to used surface finishes the highest share strength, have been achieved for solder joints which ware prepared by ultrasonic soldering on pads with copper surface finish, worst results were achieved for pads with ENIG and HASL surface finishes.

The article deals with wetting balance method comparison of solderability of the following materials combinations: two lead free solder alloys (Sn99Cu1, Sn95.5Ag3.8Cu0.7), one leaded solder alloy (Sn63Pb37), one soldering substrate type - copper wire (0.7 mm diameter) and four type of fluxes. The surface of tested samples was additionally evaluated by subjective optical analysis. The results confirm that the best results were achieved for Sn63Pb37 solder alloy - it has the fastest wetting speed (lowest zero cross time). Out of the lead free solder alloys, better results were achieved with Sn95.5Ag3.8Cu0.7 solder.

One of the most vulnerable components of electronic circuits is the solder joints. The most common technique of soldering used in electronics is the reflow soldering. During the soldering process, different issues may occur leading to the inception of some defects (tombstone effect, flux spattering, etc.). To avoid these defects, it is mandatory to follow the parameter values of the reflow process, especially temperature values. In this paper, the temperature distribution for three types of samples (two interconnected soldering pads–without solder, with solder on one pad, and with solder on both pads) during the reflow soldering process, calculated with Comsol Multiphysics software (heat transfer 3-D module) is presented. The calculated values are then compared to the ones obtained by experiments in order to verify the accuracy of the numerical model. To carry out the computations, the latent heat of melting/solidification and the variation with the temperature of the solder parameters were considered. The results show that there is a good correlation between the measured and calculated values, the differences (for all types of samples) being lower than ±3 °C.

This article is focused on both macro and microvoids in soldered joints and the use of additional flux to reduce their frequency and minimize their negative effect on the soldered joint reliability. In total five fluxes were used, three were gel based (NC559, MTV-125R, TSF-6516) and two of them were liquid based (Topnik G-5, JBC FL-15). They were used within two solder pastes, both lead and lead-free. The reflow process was identical for all of the combinations and was within the range of manufacturer recommended profiles. The amount of voids was evaluated using X-ray analysis. It was found that the use of increased amount of proper flux, specifically flux with higher activity, in the solder paste may significantly lower the void occurrence.

Temperature profile is a very important parameter which has a high impact on reliability issues in electronic assembly manufacturing. The wrong shape of temperature profile (temperature/cooling gradients, maximum temperature, preheat together with flux activation) can lead to issues like flux/solder spattering, pad cratering, unwetting, excessive interrmetallic growth, tombstone effect shrinkage etc. This article deals with influence of latent heat on the shape of temperature profile, which starts to be more attractive due to higher integrity of electronics equipment. The experiments were based of two lead-free solder alloys (Sn96.5Ag3.5, Sn96.5Ag3Cu0.5), which were remelted on the copper pads in continual convection furnace. The temperature profile, were measured directly under the copper pads. Results shows, that the measured temperature profiles under soldering pads differs against the temperature profile which were measured on the PCB. Additionally the temperature profiles which were measured under soldering pads differ according to used solder alloys as well.

The reflow process of SAC305 solder paste was investigated by differential scanning calorimetry (DSC) and measurement of the temperature profiles in a real continual convection reflow furnace. Melting and solidification processes of two solder joints were investigated by both methods to determine the influence of latent heat on the surrounding temperature. While one peak was present during melting in DSC, two peaks were present during solidification of two joints connected by a resistor. The released latent heat from one joint increased the temperature of the surroundings and delayed the solidification process of the second joint. Analysis of the temperature profiles obtained during the reflow processes in a real furnace shows that latent heat influences the surrounding temperature not only during the phase transformation, but also during the whole following passage of the sample in the furnace. The maximum temperature increase of the joint is substantial: 25 mg of the solder paste increases the joint temperature by about 15 °C. When two joints are placed on the sample the temperature increase is close to 20 °C.

This article is focused on measurement of temperature gradients in different locations on test-boards during Vapor Phase Soldering (VPS) with different materials as PCB substrates. VPS is usually applied in electronics assembly technologies due to its ability to heat the board fast and uniformly during reflow soldering. The heating rate differs significantly even on a moderate size PCB. The measurements were performed on different substrates, particularly on FR4, FR4 with aluminum core, ceramic, and polyimide flexible board samples. The temperature gradients were uniform with low heat capacity boards, contrary, the high heat capacity aluminum based PCB showed significant non-uniformity in heating, where observed temperature differences reached up to 20 °C.

Solder is usually an alloy of two and more metals. Very often is used tin, lead, copper, silver, bismuth, nickel, etc. for its production. If the alloy is exposed to a sufficiently high temperature, it is induced phase change from solid to liquid phase for it. Phase is changed again from liquid to solid during cooling substance. At this point, the substance generates a latent heat that is manifests differently for each alloy. Each component contributes its thermal and conductivity parameters into the overall properties of the alloy. The result determines quantity of the generated latent heat. And by final amount of the temperature acts on their surroundings. There have been several experimental measurements for determining of decomposition of temperature in a volume of electrically conductive solder joints during a phase change (from liquid to solidstate). Measurements were carried out on the reference alloy Sn-Pb (eutectic) and three lead-free solder alloys.

Many aspects such as different materials used during the electronic assembly, increasingly finer dimensions of components, different processes and their settings have an influence on reliability of electronics products. This article deals with BGA assembly, specifically the determination of BGA solder joint detachment cause - with a warpage effect. Samples for diagnostic were obtained from manufacturer, who already pointed to the probable problematic location (BGA component) by electric in-circuit test. Cross-section of BGA assembly was made and an open joint - thin longitudinal gap between BGA component and solder joints connection was observed. The task was to identify a cause that produced axial detachment of BGA solder ball from component pads. The use of Scanning Electron Microscopy (SEM) determined that detachment was caused by a warpage effect.

This research focuses on the possible emergence of tin crystalline formations on thick layers of lead-free solders. For this test were selected two lead-free solders, which are commonly used in the electronics industry, 99Sn1Cu and 96Sn3.8Ag0.2Cu. The created samples were exposed to any mechanical static stress and suitable climatic conditions under the defined climatic conditions. The aim was to verify whether under the conditions that occur in many electrical devices, there is the growth of whiskers.

The research is focused on whiskers growth on thick layers of two commonly used lead-free solders that were plated on two different substrates. There were two objectives. The first was to examine the influence of the dynamic stress applied in the standard climatic conditions on solders and subsequent whiskers growth and confirm that an additional mechanical stress increase internal stress as well as whiskers growth. This can occur in many applications in an electronic industry. The second was to check the presence of whiskers in this material combination with the thick layer of solder. The experiment confirmed the occurrence of beginning whiskers grow long several micrometres on samples covered by SnCu as well as SnAgCu solders.

During the surface mount technological process the assembled PCB is exposed to heat when molten solder alloy wets the surfaces and creates future solder joints. The PCB with all components should ideally be exposed to the same temperature profile during the soldering process. The temperatures differ throughout the PCB due to different thermal capacities of used materials and components even though the temperature profile is stable. The aim of the study was to assess the solderability of the copper material with different thermal capacity and to consider the effect of different types of fluxes. The measurement was done by wetting balance method, where the wetting force is measured as a function of time during which the test specimen is immersed into, remains immersed and then it is pulled out from the solder bath. In our experiments we used copper wire of four different diameters (0.7 mm, 1 mm, 1.5 mm), three types of solders - one lead (Sn63Pb37) and two lead free (Sn99Cu1, Sn97Cu3) solders and three types of fluxes.

The transition from a highly widespread lead solders to lead-free solder alloy types meant many changes in the electrical industry. This transition has brought many technological phenomena that deserve our attention and they have to be still addressed in order to improve the quality and reliability of manufacturing printed circuit boards. One of them is the presence of small crystalline formations on the surfaces of tin and some tin alloys which are called whiskers. This article focuses on the influence of the used electrical materials and their surface treatments on the growth of tin whiskers. For our experiment were selected two lead-free solders with a high percentage representation of tin, by which are Sn95,5Ag3,8Cu0,7 (SAC 387) and Sn99Cu1. Solders were plated in a thick layer on the three types of metal substrates with various surface finishes. For this experiment was chosen copper, brass and phosphor bronze. The created samples were exposed to static mechanical stresses of compressive force under the action of permanent constant temperature at 50 degrees of Celsius.

Electrical properties of electrically conductive adhesives are the DC resistance, the impedance, noise and nonlinearity od the current vs. voltage characteristic. The methods of measurement of these parameters are described - four termination and three termination measurement of the resistance, the impedance measurement, the current noise measurement and the odd and even nonlinearity measurement. Results of some basic types of the measurements are presented graphically.

Many studies show that if an organization is using ana-lytical tools to solve fundamental problems, their solutions are more efficient and less demanding. Therefore, these types of instruments have been spreading, particularly in recent years. Expert systems can be used to design solutions to the situation on the basis of observations and hypotheses, where there are no solution using traditional algorithms. Entering the correct terms by human expert is still the most important condition for correct functioning of an expert system. Tin whiskers are one of the issues of today lead-free technology. These crystals complicate design of new high-reliable products. Tin whiskers are electrically conductive thin single crystal structure of spontaneously growing out of metal surfaces - often tin, cadmium and zinc. Whiskers pose a serious risk to the reliability of electrical equipment because of their conditions are not yet fully specified. That is why there is another use of expert systems. In this paper, the possibility of using expert systems in managing risks associated with the formation of whiskers in industrial systems have been studied in detail. An example of basic expert system is discussed in this paper.

Impedance of nine types of electrically conductive adhesives of epoxy type with silver filler was measured at the frequency range of 20 Hz to 1 MHz with the goal to find if the filler concentration can influence the the frequency dependence of the joint impedance, due to the formation of the conductive net in the joint of the filler particles. Concentration of filler particles was in the range of 60 to 75 % (b.w.). It was found that the filler concentration influences the frequency dependence of the joint impedance, but the influence is not too high.

After elimination of lead from soft solders used for the electrical industry had to find new alternatives for creating electrically conductive connections. For some time no, one of the widespread options is using lead-free solders. Also another alternative comes to the fore. This alternative is using electrically conductive adhesives. The quality of their electrical and mechanical properties far does not reach the quality of lead-free solders. And therefore they are used only on places where is necessary.

Six capacitors of metalized polymer film were measured - nonlinearity of impedance was investigated. Then the capacitors were thermally aged at the temperature of 90 oC for the time of 300 hours and the measurement of nonlinearty was repeated. It was found that the temperature ageing of capacitors causes change of the nonlinearity. The cause is change of the structure of the thin film electrode caused by ageing. This approach could be used to inspection of level of ageing of the capacitors as well as to forecast of the rest life-time of the capacitors.