Alloyed Thick Film Gold Conductor for High-Reliability High-Yield Wire Bonding

Abstract

The widespread use of ultrasonic aluminum wire bonding and the increasing use of automatic thermosonic gold wire bonding in thick-film hybrid microelectronic circuitry has prompted the develolpment of materials optimized for these specialized bonding techniques. A new reduced thickness ("thin") thick-film gold conductor, developed to give the high reliability and high yield required for both interconnection methods, is described. The advantages of a "thin" printing thick-film gold conductor in providing improved bonding yields in automatic thermosonic gold wire bonding have been described previously. However, in applications requiring ultrasonic aluminum wire bonding and elevated temperature burn-in or life tests, solid-state reactions occur at wire bond/thick-film interfaces that degrade bond integrity. The changes in these reactions and in interconnection performance when an alloying element is added to the reduced thickness thick-film conductor is focused on. The ultrasonic aluminum wire bond performance of the new reduced thickness thick-film gold conductor containing a small amount of an alloying element is compared to that of a similar thin printing gold conductor developed for automatic gold wire bonding and to a conventional thick-film gold containing a different alloying element. It is shown that for extended storage (to 1000 hours) at 150°C both alloyed compositions exhibit a failure mode different from that of the "pure" gold material and higher pull strength retention. Aged wire bonds on the unalloyed control exhibit mean pull strengths of 4 grams and fail at the thick-film conductor/aluminum wire interface. Aged wire bonds on both alloyed conductors exhibit mean pull strengths above 6 grams and fail in the aluminum wire at the heel of the bond. The effect of elevated temperature storage on wire bond resistance has also been determined. Wire bonds on unalloyed controls exhibit large increases in electrical resistance, while the electrical resistance of wire bonds on alloyed metallizations remains constant. Initial bondability is of prime importance when considering either bonding yield or how well a new gold can be retrofitted into an existing process. The reduced thickness alloyed gold is shown to have greater bonding latitude than the conventional alloyed metallization in the formation of ultrasonic aluminum wire bonds. The differences in ultrasonic aluminum wire bond performance of the gold conductors were explored through scanning electron microscopy and X-ray analysis of the interface metallurgy. Mechanistic aspects are described.