Study Of Environment-protecting Conductive Pastes

Conductive pastes, including the sintering conductive pastes and the curing electrically conductive adhesives (conductive ink), are the key materials to the electronic components'encapsulation, electrodes, and connections. Conventional firing conductive pastes, usually with large amount of toxic materials such as lead, are harmful to environment. The curing electrically conductive adhesives also contain some harmful materials and are very costly. Now, the European Union's RoHS instruction has been issued and implemented, the development and the application of the environmental conductive pastes are imperative.Based on theoretical study of the conductive pastes, the conductive paste material systems, including the sintering conductive pastes and the curing electrically conductive adhesives, were designed. And also the formulations, processing, and the properties of conductive pastes were researched. The low temperature lead-free firing conductive pastes and the low cost curing electrically conductive adhesives have been developed. With addition of a suitable amount of Li2O, NaF, Al2O3, and ZnO, the low metling temperature and lead-free Bi-B-Si glass system was investigated. The effects of Li2O, NaF, Al2O3, and ZnO on the soften point and melting point of glass was studied in details. Some explanations about the effects of Li2O and NaF on the soften point and melting point of glass frits were given out. At the same time, based on the experimental and theoretical analysis, the optimized composition of the glass frits was given out (wt%): (Li2O+NaF): (Al2O3+ZnO): (B2O3+Bi2O3+SiO2+TiO2)=11.5:16.0:72.5. By using this formulation, the low temperature and lead-free glass frits with soften point below 500℃and melting point below 560℃was successfully prepared. The XRD pattern of this glass frits showed it's amorphous and very stable. The environment friendly conductive pastes containing this glass frits exhibited good electrical characteristics and matched the requirements of specified applications. The effects of sintering temperature and glass frits content on the adhesion strength and resistivity of electrodes were studied. When the glass frits content was 5wt% and sintered at 580℃/10min, the adhesion strength of electrodes was 13.32N/mm2 and the resistivity was 7mΩ/□. The relationship between the glass content and the solderability was also investigated, and the results showed that the solderability was good when the glass content was no more than 5wt%.The reliability of PTC electrodes were tested through water-boiling experiment and voltage with-standing method, and the results showed the cut off phenomenon of electrodes didn't occur and the voltage with-standing ability of PTC electrodes was over 600V. The harmful materials were also analyzed by using ICP technology, and the results showed the conductive paste was completely environmental friend.The effects of silver-plated copper content, curing agents, thinner, additives, curing time, and curing temperature on the adhesion strength and resistivity were researched in details. The results showed that when the silver-plated copper, curing agents, thinner, and additives were(wt%)55,37.8, 4,3and 0.2 respectively, the adhesion strength was 8.72N/mm2, and the resistivity was 0.125Ω.cm after cured at 120℃/30min. The shelf life of this adhesive was 7 days.