David Busek

Flux effect on void quantity and size in soldered joints

Abstract 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.

Thermomechanical analysis of electrically conductive adhesives

Mechanical properties of isotropic electrically conductive adhesives (ICAs) are mainly influenced by their formulation and thermal processing. Thermomechanical analysis of ICAs was used with the goal to find influence of addition of two types of nanoparticles, silver nanoballs and carbon nanotubes, and two types of curing processes, on air and in vacuum, on the glass transition temperature and storage modulus of bisphenol epoxy adhesives filled with silver flakes (70 to 80 % by weight). Specimens were formed as blocks. Three groups of specimens were fabricated: specimens of adhesives used as received and cured according to the recommendation of a supplier in air; specimens of adhesives used as received and cured according to the recommendation of a supplier in vacuum, specimens of the same adhesives added with low fraction (0,3 to 10 % by weight) of silver nanoballs, colloidal dispersion of silver, or carbon nanotubes. There were found differences in the glass transition temperatures and storage and loss moduli in dependence on the type of particles added into adhesive. It was also found that the adhesive cured in vacuum had lower thermal expansion coefficient and higher the storage and loss modulus in comparison with adhesive cured in air.

Investigating the effect of solder paste viscosity change on the pressure during stencil printing

The effect of viscosity change of solder pastes during stencil printing on the pressure distribution along the stencil line and on the results of numerical modelling is investigated. Rheological properties of Type 4 SAC305 (Sn96.5Ag3Cu0.5) solder paste were measured for several cycles in our previous work, to investigate the viscosity change of pastes during stencil printing. Parameters of the Cross model were fitted to the results then. In this work, the parameters of the Carreau-Yasuda model were calculated from Cross model and they are included in a finite volume model of stencil printing. The model includes the stencil, the squeegee with 55° attack angle and the solder paste as the domain of interest. The pressure distribution and the shear rates are calculated by utilising the rheological properties of the solder paste in different states (fresh, and stabilised after the 9th printing cycle). The error of numerical results is analysed for the case if viscosity parameters of the fresh paste are applied instead of that of stabilised one.

Correlations between mechanical and electrical parameters of modified electrically conductive adhesives

In our previous research, only electrical properties in dependence on different substrates and after various straining were measured or only mechanical parameters of a bulk adhesive were observed. The aim of this work is to put in contrast electrical resistance and mechanical properties (shear strength) of joints realized using electrically conductive adhesives. Two types of 2-component adhesives and seven modifications of two single-component adhesives were tested.

Correlation among Mechanical and Electrical Properties of Conductive Adhesive Joints

The paper is focused on investigation of correlation among changes of the resistance and nonlinearity of adhesive joints fabricated of 6 types of isotropic electrically conductive adhesives with epoxy resin binder and silver filler. Long-time static mechanical load, climatic load caused by ageing of adhesive joints at the temperature of 125degC, climatic load at the relative humidity of 100%, and combined load temperature-humidity (80degC, 80% RH) have been used for study of changes of loaded adhesive joints. Correlation among changes caused by different types of load, applied on one type of adhesive joints, has been examined. Correlation among changes caused by the same type of load, applied on different types of adhesive joints, has also been investigated. Good correlation has been found rarely. Therefore it is possible to conclude that correlation among changes caused by different types of climatic and mechanical load have not been found.

Studying heat transfer on inclined printed circuit boards during vapour phase soldering

Purpose The paper aims to present an investigation of heating during vapour phase soldering (VPS) on inclined printed circuit board (PCB) substrates. The PCB is a horizontal rectangular plate from the aspect of filmwise condensation with a given inclination setting. Design/methodology/approach The paper focuses on the measurement of temperature distribution on the PCBs with a novel setup immersed in the saturated vapour space. The measuring instrumentation is optimized to avoid and minimize vapour perturbing effects. Findings The inhomogeneity of the heating is presented according to the lateral dimensions of the PCB. The inclination improves temperature uniformity, improves heat transfer efficiency; however, a minor misalignment may affect the flow and result in uneven heating. Practical implications The results can be implemented for practical improvements in industrial ovens with the use of intended inclination. The improvements may consequently point to more efficient production and better joint quality. Originality/value The novel method can be used for deeper investigation of inclination during and can be complemented with numerical calculations. The results highlight the importance of precise PCB holding instrumentation in VPS ovens.

Problem with no-clean flux spattering on in-circuit testing pads diagnosed by EDS analysis

Abstract Surface Mount Technology (SMT) assembly often faces the issue of residues on In Circuit Testing (ICT) pads. These residues may have non-conductive character and therefore in-circuit test may mark tested product as failed though they would have worked normally. Since it is not possible to export such marked products, the total production quality decreases. In this work, we analyze the real manufacturing problem using Energy Dispersive Spectroscopy (EDS) analysis of the stains with the aim to find the possible source of residues that appear on the testing pads during the mass electronic assembly or during Printed Circuit Board (PCB) production. Analysis of potential source of residues together with its diagnostic and confirmation of its source is presented in this work.

Assembly method and its influence on electrically conductive adhesives and solder pastes joints durability

Reliability of electronic devices is heavily dependent on joint quality and durability under various ambient conditions. This article is focused on comparing of different assembly methods, particularly stencil printing and dispensing paste deposition and compares lead and lead-free solder pastes with one-component and two-component epoxy based electrically conductive adhesives. The evaluation method is based on resistance measurement after accelerated stress tests. The aging was conducted using automatic mechanical cyclic stress machine tester and joint resistance to environment was tested using long term temperature straining and combined humidity and temperature straining. Shear strength measurement was done in order to determine mechanical stability of the connection.

Comparison of shear strength of soldered SMD resistors for various solder alloys

Mechanical stability of solder joints has influence on total reliability of electronic products. Mechanical stress affect on properties of solder joints during function, transport and storage of the electronic products. A method which is commonly used in electronic industry to classify the mechanical properties and reliability of solder joints is shear test. This article deals with the comparison of mechanical stability of soldered joints from the point of shear strength of soldered SMD resistors. Three types of solder pastes, one lead Sn62Pb36Ag2 and two lead free Sn42Bi58 and Sn96.5Ag3Cu0.5, were used for solder joint preparation. In addition, influence of solder paste amount on shear strength of soldered SMD component, were compared. The type of solder paste and its applied amount together with the temperature profile are interesting parameters from point of economic aspects, especially in case of mass production, where it is possible to achieve considerable savings. The results shows, that the amount of solder paste has influence on the share strength of solder joins. The best results, highest share strength, have been achieved for Sn96.5Ag3Cu0.5 solder paste.

Study of thermomechanical properties of one- and two-component conductive adhesives

The goal of this work was to find if there is some difference in thermomechanical properties of one- and two-component electrically conductive adhesives. All adhesive used in this study were of an epoxy type filled with silver flakes and all had isotropic electrical conductivity. The storage modulus, loss modulus and tan delta were studied using thermodynamic mechanical analysis carried out by a penetration probe. The samples were formed on glass substrates, the thermodynamic analysis was carried out by the force 0.3 ±0.2 N at 1Hz, the maximum temperature was 140 °C. It was found that there are no significant differences between the thermodynamic parameters of adhesives tested. It was also found that all adhesives were not optimally cured though curing was carried out in accordance with recommendation of a producer of adhesives.