High frequency thermosonic flip chip bonding for gold to gold interconnection

In responding the demands to the trend in shrinking package dimensions and shorter assembly cycle time, thermosonic flip chip bonding is highly advocated in the last decade, where gold-to-gold bonds are formed under both thermal and ultrasonic energy. As a matter of fact, much of our knowledge of thermosonic flip chip bonding comes from wirebonding technology, where it is commonly agreed that the flexibility of the process windows in wirebonding decreases with the increase in ultrasonic frequency, even though the bond time and the bond temperature are usually lower under higher ultrasonic frequency. Our main objective is to compare the process windows under different ultrasonic frequencies for thermosonic flip chip application. The interaction effects of bond power and bond force on the die shear strength and package standoff height are elucidated. The effects due to different substrate materials (FR-4 and ceramic substrates) are also included. It is interesting to notice that apparently the vibrating amplitude and thence the bonding frequency play a key role in determining the robustness of the thermosonic flip chip bondings. Based on our findings the process windows of 63kHz and 109kHz bonding on FR-4 and ceramic substrates are established respectively and compared. Unlike some earlier studies on high frequency bonding in wirebonding, the findings in this work suggests that the process window of the 109kHz bonding is in fact wider than the 63kHz bonding on both substrate materials, where rigid ceramic substrates allowed larger process windows for both frequencies bondings than FR-4 substrates.