Seba Barto

Optimization of lead-free wave soldering process using taguchi orthogonal arrays

Process of lead-free soldering is deeply monitored starting from July 2006, when RoHS directive was activated. The goal of the work was to analyze influence of 4 control parameters of a process of lead-free soldering on bridging and filling of through holes on a test board. Following control parameters (factors) were taken into account: the solder temperature, the time of a contact between the soldered area and solder, the preheating temperature and flux wettability. Every factor was used in 3 levels. Taguchi orthogonal array of the type L9 was used for process analysis. The analysis showed that bridging is influenced with the preheating temperature and with the time of a contact between solder and a pad dominantly, influence of the solder temperature is minimal. The filling of the through hole depends above all on the preheating temperature; influence of other control parameters is low. It has been also confirmed effectiveness of the use of Taguchi orthogonal array in comparison with full factorial experiments.

Influence of thermal aging on the reliability of electrically conductive adhesives

Quality of electrically conductive adhesives is a main component in electronic packaging. It highly influences the total quality of a product containing such a component. With time and usage, adhesives lose some of the qualities. In some cases, changes in the molecule structure for example, can be responsible for this influence on material characteristics. This process is known as aging. It can badly affect the total electrical conductivity of adhesives and non-linearity, even mechanical properties of adhesive joints can be affected. In research field, aging is applied to test the expected reactions of the most important material parameters of electrically conductive adhesives. It also helps identifying the total operating life of the tested components. This work focuses primarily on analyzing the effect of thermal aging on the resistance of three types of adhesives, one of these adhesives is a two-component type, and the other two are one-component type.

Comparison of different approaches to manufacturing process optimization

There are many process variables, which more or less influence quality of a manufacturing process, and there are many parameters of a product, which constitute its quality. The control of variables of a manufacturing process for achievement of requested quality of a final product is a multi-parametric task. It is paid great effort to find such the way, which might lead to the most rapid solution. The use of full factorial experiments, fractional factorial experiments and Taguchi approach were compared from the point of view of number of experiments requested. It was found that the Taguchi orthogonal arrays are the most efficient method for higher number of process factors and that the full factorial experiments request the highest number of experiments from all methods under test.

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.

Sensitivity of resistance, noise and nonlinearity of conductive adhesive joints to changes in adhesive

Conductive adhesive joints were formed by adhesive assembly of resistors with “zero” resistance on Cu pads. Adhesive with bis-phenol epoxy matrix filled by silver flakes in concentration 75 % b.w. was used for experiments. The test samples were aged at the temperature of 125 °C and the relative humidity of 64 %, in an environment with 95 % relative humidity at the temperature of 24 °C and in an environment having the temperature 85 °C and the relative humidity 85 %. Changes of basic electrical parameters of the joints, the resistance, noise and nonlinearity of the current-voltage characteristic, were examined. It was found that changes of noise and nonlinearity are significantly higher in comparison with the changes of the joint resistance. It is assumed that the reason is that the junction resistance is a result of all the mechanisms of conductivity in adhesive and in the contacts between the adhesive, component and a pad, whereas nonlinearity and noise examine the nonlinear mechanisms only, especially tunneling.

Nonlinear distortion of C/V characteristic — Useful tool for diagnostics of electrically conductive adhesives: Theoretical background, measuring equipment, selected applications

Electrically conductive adhesives are used in electrical engineering to substitute lead free solders for conductive joining of components with low resistivity against heat, for mounting of integrated circuits with fine-pitch and ultra-fine-pitch packages, and for adhesive assembly on flex printed circuit boards. Conductive adhesives are materials composed of insulating matrix and conductive filler, usually silver flakes for adhesives with isotropic electrical conductivity are used. A conductive network whose resistance depends mainly on the resistance between the individual particles of the filler is formed inside the conductive adhesive. The main component of the resistance between the particles is a tunnel resistance. The basic theory of formation of tunnel resistance and the generation of the third harmonic component on such resistance when the tunnel junction is powered by a sinusoidal voltage is described in the article. Two basic principles of measuring of third harmonics, which represents nonlinearity and using of this diagnostics for study of changes in adhesive joints caused by mechanical and climatic ageing are also described.

Using fractional factorial experiments for determination of factors influencing parameters of electrically conductive adhesives

There are many methods used for quality investigation and for optimizing production processes. Such optimization methods help with increasing production effectiveness, decreasing costs and increasing the quality of products. In this paper, fractional factorial experiments are applied to investigate different factors of electrically conductive adhesives modified with silver nanoparticles after the application of mechanical aging and the influence on resistance and non-linearity of joints. The factors considered in the experiment were concentration of silver particles, the size of nanoparticles and the combination of both factors.

Resistance and non-linearity of electrically conductive adhesives aged by thermal, humidity and combined aging

Adhesive joints formed of two types of electrically conductive adhesives have been aged at the higher temperature, higher humidity and a combination of the higher temperature/higher humidity. Correlations between these different types of aging have been calculated for joints resistances and joints nonlinearities. It has been found that correlations are high in the case of ECA #1 and low for ECA #2. In research field, aging is applied to test the expected reactions of the most important material parameters of electrically conductive adhesives. It also helps identifying the total operating life of the tested components.

Climatic resistance of electrically conductive adhesive modified with silver nanoparticles

A part of silver micro-flakes in electrically conductive adhesive based on bis-phenol epoxy matrix was substituted with silver nanoparticles. Formulations with concentrations of 10 %, 20 % and 30 % (by_wt.) of silver nanoballs were prepared. The total concentration of silver in adhesive was 75 % (by_wt.) for every formulation. Adhesive joints of all formulations were formed and aged at the temperature of 120 °C, at the relative humidity of ∼100 %, and at the combination temperature/humidity 80 °C/∼100 % RH for 700 hours. Joints resistances were measured using a four-point method. It was found that substitution of a part of micro-flakes with nanoparticles do not cause significant change in climatic resistance of adhesive joints.

Tolerance design of curing process of electrically conductive adhesives

The work was directed at examination of influence of tolerances of curing process parameters on the tolerances of the resistance and nonlinearity of a current vs. voltage characteristic of adhesive joints. The joints were formed of electrically conductive adhesive with isotropic electrical conductivity of an epoxy type filled with silver flakes. Mathematical models for the resistance and nonlinearity were found using factorial experiments of the type 22. Tolerance design was carried out for these models. It was found that tolerances of the resistance and nonlinearity depend more on the time of curing than on the curing temperature.