Evolution of engineering change and repair technology in high performance multichip modules at IBM

In multichip modules (MCM's), engineering changes (EC) are required for both repair of defective chip to chip connections within the module, as well as modification of electrical connections for module performance optimization. In IBM's thermal conduction module (TCM) technology, used in previous generation bipolar devices, 100% EC capability was implemented by connecting each signal input/output (I/O) pad on the chip through a C4 solder connection on the module to a top surface EC pad. The EC pad is connected to an internal net via a delete strap. New connections can be made between chips on the MCM by first laser deleting the internal nets, then bonding discrete EC wires, and routing these to desired locations on the top surface of the module. The drawbacks of this design approach were threefold: 1) EC pads took up valuable real estate around each chip site; 2) near end coupled noise among long lengths of discrete EC wires on the top surface of the module was prohibitive; 3) insulated discrete wires used to make EC connections required unique wire bond and routing tools. With the recent use of complementary metal-oxide-semiconductor (CMOS) chips in IBM's latest generation of mainframe machines, EC design has been modified to accommodate chips with a much higher number of signal I/Os. Using the previous design methodology of connecting each signal C4 to an EC pad, a large area of the top surface of the module would be required for EC features. This would force increased chip-to-chip wiring length and impact module performance. In addition, larger size MCM's would be required, driving up cost. The new EC approach utilizes top surface thin film wiring in the X and Y directions, which is not pre-connected to any signal C4 pads. The approach used to make desired EC: connections is described. New processes were developed to make micro-connections to customize an EC connection. CMOS based MCM's have more than 5x the signal I/Os per chip compared to bipolar devices. As a result of the evolution in EC technology, CMOS chip based MCM's have been successfully designed,, built, tested and debugged quickly. They are being used in IBM's latest generation mainframe machines.