Electrical conduction of adhesive joints in microwave applications

The electrical conduction of adhesive joints is provided by the metal content, and high conductivity requires high metallic content. Isotropic electrically conductive adhesives are usually manufactured with filler concentration sufficiently greater than the percolation threshold to guarantee low resistance with allowance for manufacturing tolerances. However, we have found that it is not sufficient in the case of microwave applications since, due to the skin effect, only a thin layer of adhesive plays an important role in current conduction. Therefore, it is necessary to secure the proper arrangement of filler particles on the adhesive joint surface. In this work, instead of measuring adhesive joint conductivity, we have measured the quality factor of the resonant circuit, which was made as a stripline circuit, including the measured joints. It was found that quality factor did not increase if the blend of silver flakes and heavy silver (powder) was used as a filler. In meantime, in the same filler mixture, the adhesive conductivity has increased in the DC range, as expected. It means that not the filler volume content but eventual particle disorder on the joint surface is of the highest importance in the case of solder replacement with electrically conductive adhesives in microwave electronics.