Journal of Adhesion Science and Technology, Vol.13, No.6, 679-693, 1999
Pressure-dependent conduction behavior of various particles for conductive adhesive applications
The efficiency of electrical conduction in particle-filled conductive adhesives largely depends on the interparticle conduction, In order to gain insight into the pressure-dependent conduction behavior with particles of different sizes, shapes, and types, the effects of external pressure on the filler resistance were measured by the four-point probe method using different conductive fillers. The following types of particles were used: Ni powder, Ni flake, Ag powder, Ni filament, magnetite spindles, and Cu particles. Non-filament particle size was in the range 0.7-44 mu m. The filaments were 20 mu m in diameter and 160 or 260 mu m in length. The particle treatment procedures investigated included silver coating using different methods, and the use of acid solutions including H3PO4 HF and HCl to etch and remove the surface oxide layer. Resistivity levels were measured using a nonconductive hollow cylinder plunger device developed in our laboratories. The results of our work showed that when the pressure was increased by ten-fold from 0.7 to 7 MPa. the resistivity of some of the particles decreased substantially (e.g. 45-fold in the case of magnetite spindles, 20-fold in the case of 9 mu m nickel powder). For some other particles, however, the change was not as significant (e.g. 50% reduction for 4-7 mu m silver powder). As a result, 1-5 mu m nickel Rakes, as well as 260 mu m nickel filaments of 20 mu m diameter, coated with 0.033 M AgNO3 had lower resistivities at 7 MPa compared with the resistivity of the 4-7 mu m silver powder at 0.7 MPa. Etching of 6-7 mu m copper particles with 20% H(3)PO4 solution yielded 9.42 m Omega cm resistivity for these particles at 7 MPa. Our experimental results also indicated that the HCl etch procedure increased the packing efficiency in Ni particles of three-dimensional nature (i.e. powder and filaments) and also reduced the tunnel resistivity.