Electrically Conductive Adhesives for Medical Electronics

ABSTRACT Electrically conductive adhesives have been used in the assembly of medical electronics for many years particularly for component attach on hybrid ceramic substrates where reduced size and weight have been primary application objectives. Trends in electronic assembly including lead-free solders, disposal electronic applications, and increased demand for size/weight reduction each drive the development of new high performance adhesives to support assembly of medical electronics. The development of stable and reliable low temperature curing, fast cure, and tin compatible electrically conductive adhesives substantially increases the opportunities to medical product manufacturers to reduce cost and meet performance objectives for new product designs by improving manufacturing processes and availing the use of lower cost materials and components. The following paper addresses the issues relating to electronic assembly using conductive adhesives and explores the opportunities available to medical device designers to reduce cost and maintain performance by introducing advancements in electrically conductive adhesives for component attach. Electrically conductive adhesives (ECA) have traditionally been used in hybrid applications. These applications typically use noble metallisations such as gold and silver palladium on ceramic substrates and noble metal finished components. The major limitation of ECA has been their instability on common electronic metals such as copper and tin and therefore requires the use of these expensive noble metal finished metallizations. The unstable contact resistance of ECA on copper and tin is due to electrochemical corrosion of these metals under elevated temperature and humidity conditions. The moisture allows for the completion of the galvanic cell, which then causes the anode or lower electrochemical potential metal (copper, tin or lead) to corrode. The oxide formed during this process is non-conductive and creates a dielectric layer between the adhesive and metal. Based on the above fundamental understandings, Emerson & Cuming has developed new formulas which exhibit exceptional electrical and mechanical stability on previously unstable metals. These materials will drive the use of tin finished components on hybrid applications allowing a substantial cost benefit in the assembly of electronic devices as well as enable component attach of tin terminated devices on low-cost substrates. The high temperature exposure inherent with lead-free solder, as well as for more traditional SnPb-solder alloys, predicates the use of more costly substrates for circuit assembly. The availability of low temperature cure and snap cure (less than 60 seconds) electrically conductive adhesives enables medical device manufactures greater options in material selection to optimize performance, production output and cost particularly for disposable medical device applications. The following sections introduce several issues related to the use of ECA for component attach: Contact resistance (instability) of different components, Processing affects on single joint contact resistance (SJCR) stability, Adhesion strength, Temperature and humidity testing, Thermal cycle testing, and Fast cure and low temperature cure. Each section summarizes an issue and provides data supporting the resolution of the issue achieved by the latest in ECA product development. To avoid any misunderstanding, it is worthwhile to clarify that the products referred to herein are not one single product. Adhesive performance is highly dependent upon the specific materials being bonded. Accordingly, this paper seeks to inform of the technology capability and variety of products available using this technology to address specific assembly requirements.