Properties of adhesive bonds exposed to the static and dynamic mechanical load

Two types of mechanical load have been applied: static load and dynamic one. The load has been induced by a definite deflection of a PCB after mounting of the resistors with zero resistance by the use of electrically conductive adhesives. The static load has been induced by the constant deflection, the dynamic load by the periodical deflection of the board. The frequency of 1 Hz has been used for generation of the dynamic load. The loading with the static load has been carried out using special fixtures. Three test boards assembled with resistors have been mounted in a fixture, where they have been deflected. The fixture has been fabricated with sufficient robustness and resistance against climatic conditions, in which the joints have been investigated. After measurements in normal conditions the fixtures with the boards have been aged. The ageing has been carried out at three types of climatic conditions: at the increased temperature, at the combination of increased temperature and humidity and at the increased humidity. It has been found that the static load has, in dependence on its magnitude, significant influence on nonlinearity and noise of the bonds; influence of this load on resistance of the bonds has been substantially lower. The influence of the dynamic load has been investigated at normal climatic conditions. The dynamic load has caused changes of basic electrical parameters of the bonds in dependence on the number of deflections. The higher number of deflections has been applied, the higher changes of parameters have been found. Similarly like in investigation of influence of the static load on properties of bonds, the changes of resistance have been lower in comparison with the changes of next two investigated parameters. The main reason of changes caused by mechanical load is creating of microcracks, which influence mechanisms of conductivity of the adhesive, especially its noise and nonlinearity.