Compliant, copper based chip to substrate connections

The nature of the copper electroless bonding of copper pillars to substrates with different size gaps distances was studied. The direct pillar-to-pillar bonding of two copper pillars with narrow gap (<20 µm) separation showed occurred preferable at the overlapp region of the pillars. The bonding between two copper pads with large gaps (>100 µm) showed that a metal connection occurred at the preferred locations, however, the growth of copper was associated with spurious plating. The bond strength of the two copper pillars by the electroless plating process was at least 170 MPa (shear force divided by the contact area) after annealing at 180°C. A computational simulation of stress evolution in a silicon die connected to a organic substrate by copper pillars was performed using a half-GPD (generalized plain deformation) model to evaluate the thermomechanical strain. The model showed that the maximµm stress is less than 175 MPa, and the use of polymer collars around the copper pillar decreases the value to <115 MPa.