Composition and Form: How Feedthrough Design Affects Package Hermeticity

Ceramic-insulated feedthroughs have been in use in microelectronic packages for decades. Their track record is generally good, but not unblemished. This is because ceramic insulators tend to crack under stress - the stress of assembly temperatures, thermal shock screening and fine leak pressurization. The cracks often propagate to surface termination, forming leak paths and rendering afflicted packages non-hermetic. Package manufacturers have long battled this phenomenon and developed a number of creative designs that attempt to prevent, minimize and/or contain this inherent vulnerability. Yet it persists. To successfully prevent this undesired effect, the factors causal to its origination must be identified, understood and eliminated. That is to say, the effects of the constituent components or parts – the package wall, insulator, conductor and brazes – in summation and subtraction, in function and failure, on the integrity and reliability of the whole must be understood in order to design a package robust to thermal and pressural excursions. The optimal design will consist of components with complimentary properties of composition and form; properties that in interaction with one another multiplicatively enhance package robustness and reliability. Any other design will be vulnerable to failure. Herein, the effects of the component properties of composition and form on the structural integrity, reliability and hermeticity of metal cases employing ceramic insulators will be discussed, the primary factors causal to contemporary feedthrough hermeticity failures will be introduced in the form of a simplified force model, an optimization case study will be presented in brief, and currently utilized design features will be objectively compared and assessed.