Rails, Rings and Straps

A power grid is built from three kinds of metal shapes. Rails run along every standard cell row and touch the power pins of the cells. Straps run across the core on the upper layers and feed the rails. A ring can surround the core or a macro and tie the straps together at the edges. This article describes each part, with the real numbers from the power grid of the 16x16 multiplier in sky130.

7 min read | Beginner

What You'll Learn

• Describe rails, straps, rings and via stacks and what each one does
• Explain why rails are on met1 and straps on the upper layers
• Read the pitch, width and offset of a strap pattern
• Decide when a core ring is worth adding

The Grid of the Multiplier

This is the bottom-left part of the multiplier's power grid. VDD is red, VSS is blue. Three layers carry power:

The core has 56 rows, so there are 57 rail positions: each rail between two rows is shared by both, because every second row is flipped. VDD and VSS alternate from row to row.

Rails

Every standard cell has its VPWR pin along its top edge and its VGND pin along its bottom edge (or the other way round in a flipped row), drawn on met1 across the full cell width. When cells sit side by side in a row, these pins join into one continuous line. The rail is a met1 wire laid exactly over that line, along the full row. That is why OpenROAD calls them follow-pins: they follow the cell pins.

Rails are thin, 0.48 um, the same width as the cell pins, and met1 has a sheet resistance of 0.125 ohm per square. A rail 155 um long has a resistance of about 0.125 x 155 / 0.48 = 40 ohm from end to end. Fed only from its ends, the cells in the middle would see a large drop. Rails therefore need to be fed often, along their length, from above.

Straps

Straps are wide wires on the upper layers that run across the core and connect to the rails at every crossing. In the multiplier:

• met4 straps run vertically, every 27.14 um for each net, 1.6 um wide. They cross every rail.
• met5 straps run horizontally, every 27.2 um, also 1.6 um wide, and connect to the met4 straps where they cross.

VDD and VSS straps alternate. The first VSS strap on met4 sits near x = 16.91 um and the first VDD strap at 30.48 um; the next ones follow at 44.05 and 57.62 um. Each net repeats every 27.14 um, and the two nets are 13.57 um apart, half a pitch.

The upper layers are used because they are thicker. met4 has 0.047 ohm per square, less than half of met1, and met5 has 0.0285. A 1.6 um wide met4 strap 150 um long has about 4.4 ohm, against 40 ohm for a met1 rail of the same length.

Via Stacks

A met4 strap cannot touch a met1 rail directly. At each crossing, a stack of vias connects met1 to met2 (via), met2 to met3 (via2) and met3 to met4 (via3). In the multiplier there is one stack wherever a strap crosses a rail of the same net: 28 VDD rails times 5 VDD straps gives 140 stacks for VDD, and 29 times 6 gives 174 for VSS.

Vias are resistors too, and each one may only carry a limited current. A stack is usually an array of several vias side by side, sized to the width of the rail. Electromigration shows the current through these stacks.

Rings

A ring is a pair of wide wires, one VDD and one VSS, around the core, on two layers: here met4 for the vertical sides and met5 for the horizontal ones, each 2.0 um wide. The straps are extended to the ring, so every strap is tied to its neighbours at both ends.

A ring is useful when power enters at the edge of the block: in a wire-bonded chip, where the supply pads sit around the die, or for a block inside a larger chip that is fed from the sides. It also gives a convenient place to connect macros and the grid of the surrounding chip.

For the multiplier, adding the ring changed the worst static IR drop from 1.31 mV to 1.33 mV, practically nothing. The analysis feeds power into the top layer of the grid across the whole core, as a flip-chip package would, so the current never has to pass through the ring. Whether a ring helps depends on where the power comes from.

Pitch, Width and Offset

Three numbers define a set of straps:

• Width: wider straps have less resistance and carry more current, but take more routing tracks.
• Pitch: the distance between straps of the same net. A smaller pitch means the rails are fed more often, so the drop is lower.
• Offset: where the first strap sits, measured from the core edge. It is usually chosen so that the straps do not land on the edge itself.

Every strap takes routing resources from the signals. A 1.6 um met4 strap, with its spacing, blocks two to three met4 tracks (pitch 0.92 um), so the VDD and VSS straps together take about 5 of the 29 tracks in every 27 um, close to 17 % of met4. The power grid designer looks for the lowest strap density that still meets the IR drop and electromigration targets. IR Drop: Static and Dynamic measures how the pitch and width change the drop.

Running It in ChipVerify Lab

Run Place and Route on the multiplier and read the PDN messages in the log, then open the layout and switch on only met1, met4 and met5. Compare what you see with the table above: are there straps on met4 and met5, or only rails? At the time of writing the lab's default script builds the met1 rails only; with PRO you can add the strap commands from Building a PDN in OpenROAD to a custom script.

Common Mistakes