Microscrubbing: An alternative method for 3D thermocompression bonding CuCu bumps and high bump density devices with low force, time and temperature

Requirements for thermocompression bonding (TCB) successfully are dependent on the material as well as the area required to join. One of the paths for 3D integration is bonding Cu to Cu bumps or TSV's to bumps. A second path is by integrating fine pitch high density array of bumps, which may equate to >37000 bumps in a 8×8mm die size. Both require a significant amount of pressure as well as temperature before a good bond is achieve. For Cu to Cu bonding, temperatures >300°C is required as well as a compressive force that is needed to overcome the yield point of the metal, causing plastic deformation. On fine pitch high density array of bumps, a massive amount of force is needed to overcome the area of metal to bond. For instance a device with 37000 bumps (40μm pitch) would require at least 121kg of force to successfully bond. In this paper, we take a look on an alternative method of thermocompression bonding where we define, explore and characterize microscrubbing as an added process step during the bond. Experiments comparing differences between the standard TCB and the alternative method will also be explained. The experiment set involves die to die (D2D) stacking and is also applicable to D2W stacking. Experiments are done considering units with underfill using no-flow types. We discuss key understanding on the significant difference and improvements which microscrubbing contributes to the whole bonding process. It also touches on possible effects to bond quality and underfill reactions. In summary this paper covers tool parameters and material behavior during the thermocompression stacking process, exploring microscrubbing as an alternative method to direct TCB. The goal of the paper is to facilitate fundamental learning's and improvements on 3D stacking as a whole by exploring alternative methods.