Chip stacking with dies of different sizes
Chip stacking is a method of attaching chips vertically to increase efficiency and make better use of the available space. The vertical assemblies can involve chips of the same size or different sizes. When the chips are different in size, they are stacked offset one on top of another; usually three or four die. They are connected to each other typically by a film adhesive, and electrically, by wire bonding or flip chip methods. The stack is then connected to the substrate in the same manner. If the chips are the same size, invariably they have a staggered arrangement. The stacking connections again are made by using adhesives; along with wire bonding or flip chip techniques as mentioned before for final connections to each other or to the base substrate. These are illustrated in the drawing below.
Chip stacking with dies of the same size
New developments for connecting 3D devices are by TSV or “through silicon vias”. This involves actually creating a separate pathway through the “stack” by chemically etching, or using lasers or other techniques. The vias are typically narrow-a couple of thousandths-and extend through the stack. A thin metal layer is then deposited to insure an electrical connection. The TSV method may eventually overtake the more traditional methods of chip stacking but this is a little bit more down the road as the process’s becomes more cost effective.