Electro-migration Of Silver Alloy Wire With Its Application On Bonding

With the development of integrated circuits (IC), wire bondingtechnology has been the main method in current industry. As the bondingwire material,Gold wire has been widely used in semiconductor industry.However, silver alloy wire has been researched to replace gold wire andbeen used in industry in order to cost down. It’s shown that bondability ofsilver alloy wire could meet the industry standard. However, silver alloy wirehas risks in reliability: silver would migrate under high temperature andcurrent density.The main content of this paper is divided into two parts:electromigration resistance of silver alloy wire and migration mechanism.The conclusion of electromigration resistance of silver alloy wire is asbelow:1) Through comparing changes of wire surface morphology before andafter electromigration under the condition of250℃and5.04mA/μm2,it’s shown that silver alloy wire has more poor electro-migration resistance than gold wire.2) Through measuring changes of wire resistance by KI-Tool, it’sshown that resistance of gold wire doesn’t have obvious changes.However, under the accelerated condition, resistance of silver alloywire changes significantly during the migration process: changes flatin early time and open circuit occurs at some point.3) In order to research how temperatures and electrical stress affect theelectromigration, comparative trials are conducted with varioustemperatures (240~280℃) and different electrical stress(4.48~5.6mA/μm2). Temperature and electrical stress both acceleratethe occurrence of EM and lessen the median time to failure.4) Direction of electromigration can be changed by changing theelectric field direction. And for the silver alloy wire, ions migratefrom the anode to the cathode under high temperature and currentdensity.The conclusion of migration mechanism of silver alloy wire is as below:1) It’s found that black spots occur on the wire surface under highermagnification observation and it’s identied to be silver oxide. By themethod of wire surface line scanning to measure the curve of thesilver content, the results show that the surface of the silver ions of wire surface migrates from the anode to the cathode.2) Two methods have been conducted to analyze the microstructure.The results show that cracks occur in the section of wire “neck” andgrain size has changed. Under the high temperature and electric field,the small grains migrate from the anode where the cracks occure tothe cathode in which grains grow larger, even the “bamboo” grainstructure is formed throughout the wire.