||As the semiconductor manufacturing industry continue to develop high-density integrated circuits （IC）, many studies of 2.5D and 3D IC packaging technology are being carried out. The copper pillar bump bonding process has become the mean means of fabricating 2.5D IC structures. Recently, cold joint defects have been detected in copper pillar welding. Currently the industry believes that structural warping by heating is likely to be the main cause of the formation of defects.|
This work establishes a two-dimensional model of the 2.5D IC by using the finite element method to simulate the impact of bonding with warping on welding process, and determines the range of bonding forces when copper pillars debond and generate cracks, using the debonding setting in ANSYS. Finally, the impact of an IC on warping and the range of bonding forces is studied by changing the thickness of the die layer, the material structure of the copper pillars, and the reflow temperature.
The results of the numerical simulation model that is used in this research, which does not consider the alignment show that when the bonding force exceeded 285MPa, no crack appeared. When the bonding was less than 50MPa, debonding occurred. Compared with 50um thickness of the die, when the thickness was changed to 100 and 150um, the critical bonding forces of crack were increased by 14 and 30%, respectively, and the warpages were increased 13.53 and 25.05%, respectively. Adding a nickel layer to the copper pillars directly reduced the overall warpage and increased critical bonding force. For example, the layer improved the critical bonding force of type 3 by 8.77%, reduced the warpage by 0.76%. Adding the gold layer reduced the stress concentration in the solder that was caused by the nickel layer, reducing the critical bonding force; the type 5 had a 4.84% lower bonding force that the type 3, which had no gold layer. Compared with 100oC of reflow temperature, when the temperature was changed to 150, 200, and 240oC, the critical bonding forces were increased by 31.71, 36.59 ,and 39.02%, respectively, and the warpages were increased approximately linearly with temperature by 46.47, 74.83 and 77.89%, respectively.