Bondability of Cu wire on Cu substrate with Sn plating by ultrasonic

Abstract

This study evaluated both the joint strength of copper wire on a copper substrate with tin plating and the joint reliability of copper wire bonding after heat treatment. The suitable tin thickness and bonding conditions, which are stage temperature, wire bonding power and bonding time, were chosen by the peel test after copper wire bonding. Tin thickness of 10 m showed a high bonding rate under the conditions of stage temperature 373 K, bonding power 500–700 mW and bonding time 30 50 ms. Before heat treatment, the peel strength of the copper wire on the copper substrate with tin plating conditions was weaker than that of gold wire on a gold substrate. After heat treatment for more than 70 h at 298 K, the peel strength of the copper wire became higher than that of the gold wire and twice as high as the initial bonding strength. The tin layer remained between the copper wire and copper substrate before heat treatment. When the samples were held at 298 K, tin reacted with copper and turned into a Cu–Sn intermetallic compound. Upon completion of this reaction at 298 K for over 70 h, the soft tin layer between the copper wire and copper substrate disappeared. Therefore, the peel strength of copper wire after heat treatment increased. These results were observed by scanning electron microscope images of the interface between the copper wire and copper substrate before and after heat treatment.

Keywords::

• wire bonding
• peel strength
• ultrasonic
• Cu–Sn
• heat treatment
• Cu wire
• tin plating

Acknowledgements

We would like to express our gratitude to the FIB and SEM staff of the Stanford Nanocharacterization Laboratory of Stanford University for their help with FIB and SEM examinations and to the Mark Stephen Oliver, Alex Wei-Heng Hsing, Taek Soo Kim and other students of the Dauskardt Lab., Department of Materials Science and Engineering, Stanford University for their help with AFM measurement and peel tests.