Ions impurities and their influence on exploitation parameters of conductive adhesive joints

Electrically conductive adhesives have been identified as an environmentally friendly alternative to tin/lead solders in electronic packaging applications. Unstable contact resistance on nonnoble metals and poor impact performance are the main limitations of this new technology. One reason for unstable contact resistance can be electrochemical migration. This problem is particularly important for silver filled conductive adhesives. The goal of this paper is to find how ionic impurities influence contact resistance stability during elevated temperature and humidity ageing of conductive adhesive joints. In this work, the influence of two type of ionic impurities (Cl^- and NO₃^-) on contact resistance shift and adhesion changes during climatic tests were investigated. As a test sample, 0 Ω chip resistors were assembled on PCBs with bare Cu, Cu/OSP and NiAu-coated pads. The individual adhesive joint resistance and shearing force as well as peeling force of 0 Ω chip resistors were used as measures of joint quality. The influence of environmental conditions (85%RH/85°C, 21 days) on the joint performance was examined. Ionic impurities in Ag filled adhesives are the source of formation of dendrites or dendrite-like deposits on PCB surfaces and also influence the contact resistance of most adhesive joints during climatic tests. The mechanical properties of joints are not destroyed during the climatic tests