Marek Tucan

Influence of mechanical stress and temperature aging on a change of electrical connection resistance

This article deals with the mechanical stress and the temperature aging of the lead and lead free soldered joints. Soldered joints have influence on a total quality of electronic assembly. Mechanical and temperature tests are mainly used for resistance evaluation of components, devices and whole products against stress. This stress takes effect during function, transport and storage of the devices. Mechanical stress and temperature aging can change electrical properties of the soldered joints, e.g. electrical resistance. Even a small change of the electrical resistance could cause the malfunction of the electrical equipment. This is especially in the case of electrical equipment with high current density. In this article, the changes of the electrical resistance of mechanically stressed and temperature aged soldered joints (lead Sn62Pb36Ag2 and lead free Sn95.5Ag4Cu0.5) are compared.

Mechanical stress tests of SMT attachments

This paper covers experiments conducted on the specimens of different lead-free SMT assembly technologies. This research aimed specifically at the properties of lead-free solders and Electrically Conductive Adhesives (ECA) subjected to mechanical stress, applied as cyclic bending of experimental circuit boards. The experiment performed in past showed different properties of both technologies and their influence on the reliability of electronic devices under mechanical stress. These experiments are closely connected with the influence of thermal-induced mechanical stress on the reliability of electronic circuits.

Viscosity of Polymer Paste Materials for Electronics

The article is focused on properties of polymer paste materials these were applied in electronics production. Viscosity of such materials is investigated in particular. Samples of electrically conductive adhesives and electrically conductive pastes for screen printing were chosen for experiment. Viscosity of such materials is influenced on properties and quality final thick film structure. Viscosity was measured by method using the rotational viscometer. The dependence of viscosity on temperature was analyzed.

Tin coating of connectors - reliability risk for electrical equipment

The EC directive 2002/95/ES (RoHS) limited (with few exceptions) the use of lead in electronic production, especially in solders and surface finishes. This brought significant changes in their reliability. The electroplated tin coating is currently among the most widely used surface finishes in the electronic industry, appearing everywhere from the coating of solder pads on SMD devices to corrosion prevention coating on large containers. The tin is usually deposited galvanically, in a layer 10mm thick, often with a yet thinner interlayer between the tin and base material. The most common base materials are copper, brass and bronze. Usually the connector or component lead is cut, bent or otherwise shaped either before the coating or after it. This combination of materials and mechanical stress can lead to appearance of one important risk factor: tin whiskers. Whiskers are very thin and very long crystals of pure metal (usually tin, but silver and other whiskers were observed as well), growing from the surface of the coating. The whiskers are, of course, electrically conductive, their rate of growth is very fast and they can easily breach even organic protective coatings. This naturally presents a significant risk of short-circuit formation, either by the whisker growing between two contacts or by the fragile and thin crystals being broken off the surface. All in all, tin whiskers can pose a serious problem from the point of view of long-term reliability of electronic devices [2].

Homogeneity of joints made of electrically conductive adhesive

Reliability of conductive joints manufactured using electrically conductive adhesives (ECAs) depends mainly on their internal homogeneity. During tests of the reliability of ECAs, a significant incidence of internal inhomogeneities was observed in the joints made by one and two-component adhesives. The reason for the formation of bubbles in the ECAs joints are manufacturer recommended procedures used prior to the preparation of adhesives, processes for deposition of adhesive and the curing process of adhesive. This article presents the results of experiments aimed at research on the causes and consequences of inhomogeneity of ECA joints, and specification of procedures to minimize them. Tested samples were created by one-component (AX 20) and two-component (AX 12 LVT) adhesive, where on the two-component adhesive was studied also the influence of date of the expiration. Homogeneity of the joints was studied both by optical analysis of thin sections and by measuring the shear strength. Test results show that the inhomogeneity is greatest in one-component adhesive. Due to changes in viscosity during the curing process, the one-component adhesive gets under the mounting component. In extreme case this can cause even a short circuit. A similar phenomenon also occurs in two-component adhesives, though on a smaller scale.

Post-curing behavior of two-component Electrically Conductive Adhesive

Electrically Conductive Adhesives (ECAs) are already widely used in the electronic industry, often deployed even in adverse conditions. Because of this, a series of accelerated climatic experiments was taken. During these tests, an interesting behavior of the two-component ECAs was observed: a quick, sharp drop of electrical resistance in a short time after the curing of the ECA. This indicated a possible problem with the stability of two-component ECAs and thus it mandated further research. Its results are published in this paper and seem to support the validity of the original finding - and suggest a possible risk when using certain conductive adhesives.