Flip Chip testing with a capacitive coupled probe chip.

Abstract

Testing integrated circuits that employ an area array of I/O presents unique challenges because the face of the chip is not visible for probing. On chips that use perimeter bond pads the face of the chip is exposed, so signals on the wiring in the top layer metal may be probed while the chip is in operation.

This is not possible when the face of the chip is hidden. This work proposes a way to probe test points on the top layer metal of chips that use area I/O. The method works by attaching the chip to a specially designed probe chip instead of the normal packaging. Metal pads on the top layer of the probe chip correspond to lines on the top layer of the chip being tested. These points form a capacitive coupling between the chips, letting the probe chip read the signals at the test points. This leaves the original chip largely unchanged, and allows critical signals to be probed.

The geometry of the test points is examined and evaluated using a field solver for their potential to couple between the chips. A square section of metal roughly 6 mm on a side provides 1 fF coupling capacitance, enough for a receiver on the probe to reproduce the signal.

The work continues with the design of a receiver circuit to amplify the small input from the test points. The receiver employs a differential amplifier followed by an inverter to amplify the signal without excessive loading at the input. Simulations of the receiver demonstrate its ability to recreate the signal. Additional simulations measure the performance of the receiver under varying conditions, and explore the operational characteristics.

This work also describes the design of a four issue superscalar microprocessor that was used as a reference for explorations of systems design for multichip modules (MCMs). This work focused on the chip testing aspect of area array I/O chips used in an MCM. Other work investigated partitioning, routing, and other system design issues.

Finally, the work gives an outline of the CAD tool setup created for use at N. C. State University. The design kit created supports research as a vehicle for creating chips, and for integrating research CAD algorithms.