Hermetic Encapsulation Technique Developed for the IBM Z-Server Multi-Chip Module

The large MCM developed to package the main processor unit used in the IBM z9 Server makes use of a novel sealing design that imparts many desirable characteristics to the module assembly process, performance and reliability. These packages consist of a large ceramic chip carrier encapsulated using a copper cooling cap and a metal sealing ring. The sealing technique not only provides the hermetic environment needed to protect the non-underfilled devices contained within the module, but also allows for easy rework of the assembly. The seal used can withstand the thermally induced stresses and strains driven by the thermal expansion coefficient mismatch between the carrier and the cap. Depending on the system requirements or application, it can do this and reliably maintain the level of hermeticity needed to protect the encapsulated devices over a thousand or more thermal cycles. In addition to this, the seal and module design must compensate for mechanical tolerances of the carrier and devices that affect the assembled condition of the module. In the z-Server module design these considerations, as well as thermal performance factors, are all taken into account. This paper will cover the various aspects of the module design, focusing on the novel application of the hermetic seal employed. The seal will be described and its design parameters will be discussed. Seal, component and module level qualification testing that is performed to insure that the assembly meets the package reliability requirements will be presented.