Comparison between thermal performance of silver conductive adhesive and Sn-Ag-Cu solder joints in a medical ultrasound array transducer

The RoHS and WEEE European Directive recently introduced, [1,2], represent focal points for the environmental regulation in the field of electronics industry. While removal of some material of concern has been relatively straightforward, the reduction or quite elimination of Lead (Pb) has caused significant disruption to the electronics manufacturing supply chain [3]. This materials and technological conversion will increase the risk for premature product failures and degraded performances. As known, the Sn-Pb solder joint has over 50 years of manufacturing, field experience and knowledge [4,5,6]; the Pb-free solution, instead, needs accurate studies voted both the characterization of performances and electronic properties of material. To this aim the paper presents preliminary results about new materials for soldering process and, in particular, a comparison between silver conductive adhesive and Sn-Ag-Cu (SAC). As an application, a medical ultrasound array transducer with new soldering materials, between a platelet of piezoelectric and fingers, brass micro-connections between the substrate of piezoelectric material and the printed circuit, is also presented in the paper. Factors, which influence the soldering process, like the thickness of soldering and the polymerization or reflow temperature, the duration of the process, are taken into consideration in order to realise samples for the experimental tests. In order to verify the samples behaviour under thermal stresses, measurements of electrical resistance, parameter chosen like failure pointer were carried out.