Designs of Low Stress I/O Pin Attach Structures

Abstract

The recently announced IBM system 390/ES9000 mainframe computers are significantly enhanced by a revolutionary set of packaging materials. Low dielectric constant glass ceramic and high conductivity copper were developed over the last decade and integrated into the pioneering thermal conduction module introduced by IBM in the early I980’s. During the course of the development, one of the major challenges in the glass ceramic multilayer ceramic (MLC) packaging technology is the attachment of I/O pins on the bottom surface metallurgy (BSM) with good reliability. This activity was driven by the stringent demand on limiting the stress of the pin joint so that the combined stresses generated from the braze joint, pin loading, and BSM pad should not exceed the fracture strength of the glass ceramic substrate. To achieve such a goal, improved pin structures as well as low stress joining material were investigated, all of which were designed to reduce the stress of the pin joint. Input/output (I/O) pads made of multilayer thin film metallurgy were designed to replace the traditional cosintered I/O pad for pin brazing. The thin film pads not only offer significantly lower stresses, but when combined with the use of a stress reducing soft metal cushion layer, the high stresses generated from the brazing alloy can be effectively reduced before it can be transmitted to the ceramic substrate. In addition, by modifying the shape of the pin head from the vertical edge to a 45° tapered edge, the pin joint stress can be significantly reduced. The amount of fillet accumulation is reduced and the fillet stress mode is changed and can be more easily absorbed by the soft metal cushion layer using Cu, Al or Au. The thermal expansion coefficient of the pin head material as well as the size of the pin head also play an important role in affecting the pin joint stress. The use of a low modulus, low yield strength soft solder as the pin joining material also reduces the pin joint stress. Solder tends to yield to the stress before it is transmitted to the ceramic substrate, thus greatly alleviating the ceramic cracking concerns. © 1992 IEEE