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Characterization and performance of electrically conductive adhesives for microwave applications

The main advantage of isotropically conductive adhesives used for assembling electronic circuits is lack of lead and other toxic metals in resultant joints. The objective of this paper was to find the best formulation of isotropically conductive adhesive for solder replacement in microwave applications. As a result of the screening experiment the different adhesive formulations were investigated in order to identify the significant factors, which influence the electrical resistance of joints. In these formulations the adhesive base material as well as type of main and additional filler materials were changed. Silver, nickel and graphite were used as a main filler material, whereas soot and heavy silver were filler additives. The adhesive formulations were tested in standard microstrip bandpass filters, which were supplied with an additional gap in the gold strip and bridged by adhesive bonded silver jumper. As the figure-of-merit the quality factor Q and loss L of such a microwave circuit have been investigated. Both the Q-factor and loss of the filter with bonded jumper were measured at the frequency of 3.5 GHz in preliminary experiment and at 3.5 GHz as well as 14 GHz in final experiment. For identifying the best adhesive formulation the experimental design method based on Taguchi techniques for quality engineering has been used. It was stated that to each adhesive, which is prepared on the base of specific type of resin, the strictly defined type of silver filler should be added and strictly defined volume content of the filler should be selected.