A study of electrically conductive adhesives as a manufacturing solder alternative

Soldering processes using tin/lead solder are standard interconnection technologies for electronic manufacturing. These processes are currently under threat from the Waste Electrical and Electronic Equipment and the Restriction of Hazardous Substances (RoHS) environmental directives, issued by the European Union in 2000. These directives explain that solder is to be free from lead by 2006, as lead has been recognized by the European Union as an environmentally harmful material. One solder alternative that has been investigated by the electronics industry is the area of electrically conductive adhesives (ECAs). This paper outlines the electrical and mechanical analysis of two isotropic conductive adhesives where the main properties of joint resistance and adhesive strength were examined before and after different environmental treatments. Joint resistance was measured with a four-probe tester and adhesive strength was examined with the use of shear testing. Cross-sectional and scanning electron microscopy analyses were used to determine problems such as oxidation and moisture absorption that may have an affect on the adhesive properties of the connection. An experimental design was carried out to examine the adhesives on a standard production line. Taguchi analysis was used to determine the build parameters required to produce an optimal adhesive joint. The results show that although ECAs can pass a functionality test, the electrical properties of ECAs are inferior to that of solder with ECAs having a high joint resistance. They also exhibit poor mechanical strength when shear or drop tested and illustrate that current visual inspection techniques used to examine solder joints are not applicable to conductive adhesives.