An investigation of stable bonding for Au-Al solid-phase diffusion bonding techniques

By using a chip that has Au bumps and a substrate that has only a varying Al film thickness, initial bonding strength is made constant in Au-Al solid-phase diffusion bonding. On the other hand, by changing the Au-Al intermetallic compound formed during bonding, a relationship is obtained between the formed intermetallic compound and bonding reliability. Samples obtained when installing ICs under the same conditions on substrates with Al film thicknesses of 350 nm and 1000 nm were left for 1000 hours at 125 degrees C and the bonding strength and connection resistance were measured. Immediately after bonding, there was no meaningful difference between them. However, after 1000 hours, reduced bonding strength and increased connection resistance were observed in the sample whose Al film thickness was 1000 nm whereas a stable connection was obtained in the sample whose Al film thickness was 350 nm. The difference in reliability in a high-temperature environment results from differences in the Au-Al intermetallic compounds formed at the time of bonding and by the existence of unreacted Al. In the sample whose Al film thickness was 350 nm, there was no Al at the junction at the time of bonding, so that the final product was predominantly Au4Al. On the other hand, in the sample whose Al film thickness was 1000 nm, Al existed at the junction, so that several Au-Al intermetallic compounds were formed which subsequently degrade by diffusion reactions at high temperature. (C) 1999 Scripta Technica.