Ales Duraj

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.

Diagnostic Tools for Evaluation of the Quality of the Anisotropic Conductive Adhesive Joints

Isotropic and anisotropic electrically conductive adhesives (ICAs and AC As) are materials for special applications in electronic assembly. Quality of these adhesive joints is evaluated according of their electrical and mechanical properties. This paper is focused on evaluation of quality of Anisotropic Conductive Adhesives. Especially dependence of electrical resistance, noise and nonlinearity on bonding pressure is the main purpose of this research. Effect of curing (bonding) pressure ofACA was also analyzed with scanning electron microscope (SEM). Also influence of three different types of surface finishes (Cu, Cu+Sn, Cu+Ni/Au) of printed circuit boards has been examined and then evaluated effects on measured parameters.

Failure mode and effects analysis of a process of reflow lead-free soldering

There are many methods usable for production process optimization. The paper describes the use of failure mode and effects analysis (FMEA of a process type) for the process of lead-free soldering improvement. The schedule of FMEA has been set. Then a flow chart of a process has been drawn and critical failures including their consequences for the final product have been described. Criteria for significance of failures characterization have been defined and have been used for evaluation of significance of failures. Diagnostic methods for critical failures have been proposed and corrective actions have been realized.

Analysis and prediction of electrical contact resistance for anisotropic conductive adhesives

Anisotropic conductive adhesives (ACAs) and anisotropic conductive films (ACFs) are promising solder alternatives. These materials are preferable for fine pitch and ultra-fine pitch connections in electronics assembly. Quality of these interconnections is usually evaluated with measurement of electrical resistance. This paper describes few simple models for prediction of electrical contact resistance in ACAs interconnects. Nevertheless in these models is usually neglected effect of thin insulating layer between conductive particles and conductive tracks on substrate. Therefore two different methods for evaluation of electron tunneling resistance are analyzed and implement into models for prediction of electrical contact resistance of ACAs/ACFs. Extended models are directly compared with experimental results. Also effect of bonding pressure and different materials of conductive tracks is analyzed.

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.

Equipment for measurement of nonlinearity of nominally linear components

Nonlinearity of a nominally linear component is a significant parameter, which informs about the difference of and investigated component from an ideal one. The higher is quality of the nominally linear component; the lower is level of its nonlinearity. Nonlinearity is caused by inhomogenities and unstable barriers in the component, which can, after some time, cause failure of the component. Nonlinearity is also a significant technical parameter of components, which work with more signals of different levels. Nonlinearity contributes here to generation of intermodulation distortion of current signals. It has been necessary, for our research, to investigate nonlinearity of adhesive joints. Because there is not equipment for measurement of nonlinearity of low-impedance components on the market, it has been necessary to construct it. Equipment is based on evaluation of intermodulation distortion, which is generated by the nonlinear component, when two sinusoidal signals are fed into it. Equipment makes a measurement of nonlinearity of 2. to 5. order possible. Measured level of nonlinearity is -170 dB with respect to the actuating signal. Maximum power of the actuating signal is 2 VA, the sensitivity of an indicator 50 nV. Some examples of measurements of nonlinearity of adhesive joints are shown.

Frequency Dependence of Impedance of Adhesive Joints

Electrically conductive adhesives have, in comparison with solders, significantly higher non-homogeneity of the structure, and therefore stability of electrical as well as mechanical properties of adhesive joints is lower than soldered ones. The impedance of the joints has been measured in wide frequency range (20 Hz to 1 MHz and 300 MHz to 3GHz). Measurements have been carried out by two methods: impedance in the range 20 Hz to 1MHz has been measured by LCR meter HP 4284A. A special test board for assembly of 1 jumper (of the type 1206) by electrically conductive adhesive has been fabricated for this measurement. The measurements at the frequencies 300 MHz and higher have been carried out using a shielded high-Q triplate line. Specimens have been prepared on Teflon boards, a scalar analyzer Rohde Scwartz ESPI has been used for evaluation. Very low changes of impedances of the joints have been found in the range of 20 Hz to 1 MHz. Typical courses of real and imaginary components of joints impedances in the frequency range 300 MHz to 3 GHz are as follows: the real components have grown in consequence of a skin-effect and have dominated. The imaginary components have been very low. The task for the future research is to explain decrease of the resistance of the joints with growing frequency for two types of adhesives

Relationships between noise and nonlinearity of adhesive conductive joints and joints resistance

Noise and nonlinearity of the voltage vs. current characteristic of conductive adhesive joints were examined in dependence on the value of the joint resistance. Changes in the resistance were caused by different types of climatic ageing. Two types of adhesives, AX12EV and ELPOX SC515 (both Amepox, Poland) based on bis-phenol epoxy matrix and silver filler were used for formation of the adhesive bonds by adhesive assembly of 0R0 resistors of the type 1206. Noise measurement was carried out using a lock-in amplifier; nonlinearity was measured by a modulation method and the resistance using a four-point method. It was found that the course of dependence of noise on the joint resistance differs from the course of dependence of nonlinearity. It was also found that the relative changes of noise are mostly higher than relative changes of the resistance and that the relative changes of nonlinearity are mostly higher than relative changes of noise. It means that the ageing of the adhesive joints, especially in climate with higher humidity, can change nonlinearity of the joint dramatically and increase probability of dis-function of the device, especially if adhesive joining is used in high and ultra-high frequency area.

The contact resistance at interface of two different thick film layers

The work is focused on analysis of interface of two different thick film layers. Contact resistance between conductive a resistive layers was measured and analyzed. Contact resistance is influenced by technology of deposition of layers, by conditions of curing, by materials and surface quality of substrate etc. Three point methods is used for measuring of contact resistance. This parameter was analyzed in dependence on geometry of contact area. Polymer thick film pastes were used for preparation of samples.

Statistical Inspection of a Workplace for Adhesive Assembly

Quality of a workplace for adhesive assembly has been examined using acceptance sampling. The goal of the work has been to find our whether dispensing of electrically conductive adhesive makes forming of adhesive joints with sufficient reliability and repeatability possible. Adhesive joints have been fabricated by adhesive assembly of jumpers. Four types of adhesives, one of an acrylate type and three of an epoxy type with Ag filler have been used for testing. Joints resistances have been measured by a four-point method. Acceptance sampling by attributes has been used for statistical inspection. Acceptance plan has been calculated. It has been found that the workplace does not give results with sufficient quality.