The present invention relates to stacking integrated circuit (IC) chips, particularly to stacking IC chips having surface pads rerouted to form pads on the side of the chips, and more particularly to attaching the side pads of such stacked chips to one another and/or to a circuit board via metal lines on a flexible plastic tape by an anisotropically conducting adhesive.
Packaging integrated circuits in three dimensions (3-D) remains an important area of endeavor in the multichip module community. Many technologies have been proposed and demonstrated in the past decade. Impressive solid-state recorders featuring 3-D stacks as integrated memory components in a high performance multichip module system have been demonstrated, and very high density 3-D stacks (&gt;40 chips/6 mm thick stack) have been produced.
A major drawback of a full implementation of any of the various 3-D technologies has been the cost of the packaging. The need for simpler, lower cost options to current technologies is the driving force behind the research and development in the high density IC packaging arena.
Among the various efforts to simplify packaging of stacked IC chips or via was the rerouting of the pads on the surface of the chips to form pads on the side of the chips, whereafter the chips can be stacked on top of each other, with needed interconnections being made readily accessible due to the rerouted pads on the chip sides. This pad rerouting approach is described and claimed in copending U.S. application Ser. No. 08/387,495 filed Feb. 13, 1995, entitled "3-D Laser Patterning Process," and Ser. No. 08/733,854 filed Oct. 18, 1996, entitled "L-Connect Routing of Die Surface Pads to the Die Edge for Stacking in a 3-D Array," each assigned to the assignee of the instant application. The stacked chips were connected to one another and to a circuit board by solder connections.
The present invention utilizes the pad rerouting approach of the above-referenced applications, whereby the chips can be stacked on top of each other after the pad reroute, and then connects the stacked chips to each other and/or to a circuit board using a flexible plastic tape having metal lines and an anisotropically conducting adhesive (ACA). The pads on the side of a chip are connected to metal lines on the flexible plastic tape (flex) by the ACA. Metal lines on the flex are likewise connected to pads of other chips in the stack and/or to the circuit board. The flex can accommodate dimensional variation in the alignment of chips in the stack; bonding of the ACA is accomplished at low temperature and is otherwise simpler and less expensive than solder bonding; and the chips can be bonded to the ACA all at once if the sides of the chips are substantially coplanar.