Influence of Carbon Nanotubes Added to a Commercial Adhesive

This paper describes the influence of carbon nanotubes onto electrical properties of electrically conductive adhesives. This work is related to our previous research that described electrical connection network within electrically conductive adhesive and possible advantages of non-spherical particles distributed within the adhesive matrix. Two different commercial conductive adhesives were used in this experiment and different amounts of carbon nanotubes were mixed using different methods (ultrasound/rotation/spatula mixing) into the matrix. The positive effect in terms of mechanical properties can be expected according to previous research in the material field. The goal of this work is to describe the influence of this additional filler onto electrical parameters and generally to improve the adhesive properties

Measurement of Nonlinearity as a Diagnostic Tool for Quality Determination of Lead-Free Solder Joints

At present there are directions RoHS-WEEE in European Union which restrict the use of certain hazardous substances in new electric and electronic products. Therefore mass usage of lead-free soldering in electronic production started. A typical alloy for lead-free soldering is based on Sn, Ag and Cu (SAC). For the most of types of lead-free solders is necessary to use technological process of soldering in reduced oxygen concentration atmosphere for improvement of wettability of soldered surfaces. Quality of these soldered joints is evaluated according of their electrical and mechanical properties. This paper is focused on measurement of nonlinearity, electrical resistance and mechanical strength. Especially evaluation of nonlinearity as a diagnostic tool for the prediction of solder joints reliability has been tested. Also effect of different surface finish of PCBs and influence of reduced oxygen concentration on quality of joints has been examined

Electrochemical migration of lead-free solders and Ag-based electrically conductive adhesives in NaCl solution

In this article, the electrochemical migration of lead-free solders and electrically conductive adhesives with an Ag content of more than 70% was studied by using a water drop test in NaCl solution. The results of solder pastes were compared with the results of electrically conductive adhesives. It was found that some solder pastes with a low Ag content have a lower susceptibility to electrochemical migration than, for example, solder paste Sn42Bi58. The highest resistance to corrosion has a conductive adhesive CRM-1033B with an Ag content of 75%. Resistance and voltage during the water drop test. From the measured values, it was determined that the values of resistance and voltage decrease with dendrite growing. X-ray spectroscopy was also performed to determine the chemical elemental composition of dendrites. It was found that the dominant element of the dendrite of solder pastes was Sn. In the case of electrically conductive adhesives, Ag was the main element of the dendrite

Overview of Selected Issues Related to Soldering

The formation of defects and imperfections in the soldering process can have many causes, which primarily include a poorly setup technological process, inappropriate or inappropriately used materials and their combinations, the effect of the surroundings and design errors. This chapter lists some examples of errors that can occur in soldering, while review is devoted to selected defects: non-wettability of the solder pads, dewetting, wrong solder mask design, warpage, head-in-pillow, cracks in the joints, pad cratering, black pad, solder beading, tombstoning, dendrites, voids, flux spattering from the solder paste, popcorning and whiskers

Effect of Recrystallization on β to α-Sn Allotropic Transition in 99.3Sn-0.7Cu wt. % Solder Alloy Inoculated with InSb

The effect of recrystallization of 99.3Sn-0.7Cu wt % solder alloy on the allotropic transition of β to α-Sn (so-called tin pest phenomenon) was investigated. Bulk samples were prepared, and an InSb inoculator was mechanically applied to their surfaces to enhance the transition. Half of the samples were used as the reference material and the other half were annealed at 180 °C for 72 hours, which caused the recrystallization of the alloy. The samples were stored at -10 °C and -20 °C. The -Sn to α-Sn transition was monitored using electrical resistance measurements. The expansion and separation of the tin grains directly during the -Sn to α-Sn transition process were studied using scanning electron microscopy. The recrystallization of the alloy suppresses the tin pest phenomenon considerably since it decreased the number of defects in the crystal structure where heterogeneous nucleation of -Sn to α-Sn transition could occur. In the case of InSb inoculation, the spreading of the transition towards the bulk was as fast as the spreading parallel to the surface of the sample

Effect of Cu Substrate Roughness and Sn Layer Thickness on Whisker Development from Sn Thin-Films

The effect of copper substrate roughness and tin layer thickness were investigated on whisker development in the case of Sn thin-films. Sn was vacuum-evaporated onto unpolished and mechanically polished Cu substrates with 1 µm and 2 μm average layer thicknesses. The samples were stored in room conditions for 60 days. The considerable stress – developed by the rapid intermetallic layer formation – resulted in intensive whisker formation, even in some days after of the layer deposition. The developed whiskers and the layer structure underneath them were investigated with both scanning electron microscopy and ion microscopy. The Sn thin-film deposited onto unpolished Cu substrate produced less but longer whiskers than that deposited onto polished Cu substrate. This phenomenon might be explained by the dependence of IML formation on the surface roughness of substrates. The formation of IML wedges is more likely on rougher Cu substrates than on polished ones. Furthermore, it was found that with the decrease of layer thickness, the development of nodule type whiskers increases due to the easier diffusion of other atoms into the whisker bodies

Numerical investigation on the effect of solder paste rheological behaviour and printing speed on stencil printing

Purpose of this paper was to investigate the effect of different viscosity models (Cross and Al-Ma’aiteh) and different printing speeds on the numerical results (e.g., pressure over stencil) of a numerical model regarding stencil printing. A finite volume model was established for describing the printing process. Two types of viscosity models for non-Newtonian fluid properties were compared. The Cross model was fitted to the measurement results in the initial state of a lead-free solder paste, and the parameters of a Al-Ma’aiteh material model were fitted in the stabilised state of the same paste. Four different printing speeds were also investigated from 20 to 200 mm/s. Noteworthy differences were found in the pressure between utilising the Cross model and the Al-Ma’aiteh viscosity model. The difference in pressure reached 33–34% for both printing speeds of 20 and 70 mm/s, and reached 31% and 27% for the printing speed of 120 and 200 mm/s. The variation in the difference was explained by the increase in the rates of shear by increasing printing speeds. Parameters of viscosity model should be determined for the stabilised state of the solder paste. Neglecting the thixotropic paste nature in the modelling of printing can cause a calculation error of even ~30%. By using the Al-Ma’aiteh viscosity model over the stabilised state of solder pastes can provide more accurate results in the modelling of printing, which is necessary for the effective optimisation of this process, and for eliminating soldering failures in highly integrated electronic devices

Modelling of temperature distribution along PCB thickness in different substrates during reflow

In this paper, analytical modelling of heat distribution along the thickness of different printed circuit board (PCB) substrates is presented according to the 1 D heat transient conduction problem. This paper aims to reveal differences between the substrates and the geometry configurations and elaborate on further application of explicit modelling

The influence of the crystallographic structure of the intermetallic grains on tin whisker growth

In this paper, the relationship between the crystallographic structure of Cu-Sn intermetallic grains and Sn whisker growth was investigated. In order to prevent the influence of the elements in the alloy composition and the effect of the soldering process on the formation of the intermetallic layer, 99.99% pure Sn was vacuum evaporated onto Cu substrates. The Sn layer thickness was sub-micron region (~400nm in average) to reach considerable and rapid compressive stress on the tin layer originated by the intermetallic formation. The samples were stored at room temperature for 1 month. Different types of whiskers (nodule and filament) and the layer structure underneath were studied with a scanning ion microscopy and transmission electron microscopy. It was found that not only the thickness of the intermetallic layer and shape of the intermetallic grains affects the whisker growth but the crystallographic structure of the intermetallic grains as well. The susceptibility of the Sn layer to whisker development is higher in those regions where the intermetallic layer is composed of monocrystalline grains instead of those regions, where it is composed of polycrystalline grains. This effect can be explained by the higher compressive stress generated by the monocrystalline intermetallics compared to the polycrystalline ones

Effect of different thermocouple constructions on heat-level vapour phase soldering profiles

To improve productivity and reach better quality in assembling, measurements and proper process controlling are necessary factors. This article focuses on the monitoring heat-level based Vapour Phase reflow Soldering (VPS), where - as it was found – different thermocouple constructions can affect the preset parameters of the oven and resulting soldering profiles significantly