Resistance, polymer electrical properties contamination

Even with resistant polymers, the surfaces of products can be contaminated by microorganisms if they become roughened or pitted, and they then remain moist for long periods. Also, they become discoloured if constituent additives are vulnerable. Other symptoms include black or pink staining, odour, and changes in electrical or mechanical properties. Affected plastics products attract and retain dirt more than unaffected ones. 

By nature, polymers are insulating materials and some of them are employed without any added fillers. However, most applications require either electrical or thermal conductivity or both. According to the final use, thermal stabiUty and resistance to aggressive environment can be important criteria to select a particular type of polymer. In fact, few adhesives have been specifically developed for the electronic industry. Instead, most materials were preexisting compositions that have been tested and incorporated in production processes. Specific requirements such as ionic contamination, water absorption, and better thermal stability appeared later. Major chemical companies and semiconductor manufacturers, in particular in the USA and Japan, launched variants of these polymers fulfilling the demand for organic materials with enhanced properties. 

The conductive polymer is usually composed of dispersed electrically conductive particles such as carbon black or metal in a durable polymer matrix such as silicone rubber. As it is possible to obtain conductivity and elasticity in simple materials, this is useful for high density micro-sized electrical contacts as connector or as key-board switch. However, in spite of the excellent properties, it is difficult to obtain low electrical resistivity. In order to approach this problem, performances of the contacts between the polymer surface and metal were studied from the view point of surface contamination. The electrical conduction mechanism and a method to get low resistivity were also studied. The results show that the surface of the polymers containing carbon black shows contamination resistance properties because of their chemical stability. The conduction mechanism changes with particle content. For large content, electric current flows through particles contacting each other and for small content, the current passes through the gap between particles by Schottky conduction. Thus, as the resistance consists of particle resistance, constriction resistance and gap resistance, the resistance can be controlled by particle material, their content and size. 

---