Architecture implications of pads as a scarce resource

Due to non-ideal technology scaling, delivering a stable supply voltage is increasingly challenging. Furthermore, competition for limited chip interface resources (i.e., C4 pads) between power supply and I/O, and the loss of such resources to electromigration, means that constructing a power delivery network (PDN) that satisfies noise margins without compromising performance is and will remain a critical problem for architects and circuit designers alike. Simple guardbanding will no longer work, as the consequent performance penalty will grow with technology scaling. In this paper, we develop a pre-RTL PDN model, VoltSpot, for the purpose of studying the performance and noise tradeoffs among power supply and I/O pad allocation, the effectiveness of noise mitigation techniques, and the consequent implications of electromigration-induced PDN pad failure. Our simulations demonstrate that, despite their integral role in the PDN, power/ground pads can be aggressively reduced (by conversion into I/O pads) to their electromigration limit with minimal performance impact from extra voltage noise - provided the system implements a suitable noise-mitigation strategy. The key observation is that even though reducing power/ground pads significantly increases the number of voltage emergencies, the average noise amplitude increase is small. Overall, we can triple I/O bandwidth while maintaining target lifetimes and incurring only 1.5% slowdown.