Goldilocks failures: Not too soft, not too hard

It is well known that circuits fail when one or more of the constituent components fails, due -for example- to phenomena such as electromigration in wires. Such hard failures, typically due to topological changes in circuit connectivity, are treated distinctly from soft failures which could be due to components drifting out of spec over time. However, in certain types of circuits, such as memory, the distinction between soft and hard failures is not clearly defined. The primary cause of the blurring between these two phenomena is manufacturing variability, which can make a topologically correct circuit behave as if it had a short or an open. This paper will show the linkage between these two failure types, and show how increasing variability in future technologies will likely exacerbate this problem further.