What are the main challenges of using multi-Vt cells in timing optimization?

Biggest challenge is balancing leakage and timing. When you use multi-Vt cells, mixing high-Vt and low-Vt, it’s not easy to get the right trade-off. If you push too many low-Vt cells, you get timing closure but leakage power goes up.

We usually struggle with process variations when using multi-Vt cells. A path might look fine in one corner, but due to variations in Vt, timing may fail in another corner. So, while multi-Vt is powerful, managing these variations is the real challenge.