Why would you use a zero-ohm resistor?

If you had never heard it before, the term “zero-ohm resistor” might seem at best like an oxymoron or, at worst, some sort of joke to play on a new or naïve circuit designer. After all, designers are under constant pressure to simplify their designs and reduce the bill of materials (BOM), so why would they add something that does nothing?

But the zero-ohm resistor (0 Ω, but almost always spelled out in full) is not a joke at all. For proof, consider that during a recheck stock check, major distributor Digi-Key had 150,000 of one version in inventory. Clearly, they are needed and in demand.

So, why would a designer even want to use one or more? What can it add to a circuit that wouldn’t be better served by getting rid of it, thus saving cost and board space? (Incidentally, this would be a good question to ask an interviewee for a circuit-layout position — even if they don’t know the answers, how they speculated would give insight into their thinking!)

There are at least three very specific cases where this apparently “useless” component makes sense. Two are related to design, test, and manufacturing, while the third…let’s just say it has little to do with those factors.

1) PC board layout

Let’s start with an older reason, which is still viable. About fifty to sixty years ago, in the early days of this new thing called “printed circuit boards,” our now-ubiquitous FR4 glass-epoxy board with copper cladding on two sides did not exist. Instead, those earliest boards were made of pressed phenolic paper with copper on only one side. Components were inserted by hand, which was feasible since most of them were large, such as vacuum-tube sockets, discrete transistors, passives, transformers, and connectors.

It took real skill to lay out a board’s wiring using traces on one side only — no CAD layout tools back then — and there were times when it was simply impossible. The solution was simple enough: wire jumpers were added to “bridge over” areas and enable connection between two other traces.

Figure 1. This single-sided phenolic pc board from a 2010 microwave oven contains the power supplies (low and high voltage), AC transformer, and power devices; note the use of top-side jumpers along the top edge about one-third of the way from the left side. (Image: Low Price Mart)

Subsequently, as machine-based insertion of components became the standard in manufacturing, the basic wire jumper was replaced with a discrete, through-hole, leaded zero-ohm resistor to serve the same function.

These low-cost single-sided phenolic boards with zero-ohm jumpers and resistors are still in use. Cheap toys and even modern appliances, such as coffee machines or microwave ovens, use one-sided phenolic boards when possible to minimize cost, or when there are larger components to mount, such as transformers. These boards use zero-ohm devices as bridges to solve topology problems, as seen in Figure 1.

2) Circuit and board layout flexibility

Zero-ohm resistors still have a place even with our modern two-sided and multilayer FR-4 board design and surface mount (SMT) components. There may be cases where the layout routing is so complicated that some path connections simply cannot be completed. The solution is to “buy” an extra layer for a few cents in that critical local spot via a zero-ohm resistor.

These resistors can also ease the interruption of a circuit’s interconnection and signal-flow path. They can be used to create complete electrical separation between a board’s subcircuits, a condition which may be needed for debug and test; obviously, it is easier to unsolder/solder even a tiny SMT zero-ohm resistor than to cut and then try to restore a hair-thin PCB trace. They can also be used to “short out” circuit functions, such as extra filter stages that are not needed in all configurations, or may have to be disabled for test and calibration cycles.

Another use is for these resistors to enable a single PC board layout to be tailored to different configurations even after the board is populated and soldered. In the simplest case, consider a signal path that needs either zero ohms or 100 ohms in a damping or snubber circuit, with the correct value determined by the specifics of the load that the product will be driving.

The board can be laid out to accommodate a single resistor of either zero ohms or 100 ohms, with the appropriate value put on the BOM for that assembly run, or even placed and soldered by hand. Alternatively, the circuit and PC board can be designed with both the zero-ohm and 10-ohm resistors in parallel, and the zero-ohm one removed if 100 ohms is the correct value.

This is a more cost-effective approach than creating two slightly different board layouts, one having the resistor in place and one laid out without any resistor. However, it is cheaper, smarter, and yields better inventory management to have just one board and insert/remove the zero-ohm resistor as needed.

3) Through a thin veil over the schematic

Finally, there’s a less obvious rationale for zero-ohm resistors: to complicate and conceal a circuit’s function and confuse someone trying to trace out and thus reverse-engineer a design.

I again encountered these resistors on a major project, where I was asked to review the schematic diagram of a product that had been shipping for a few years. I looked at the schematic and the bill of materials (BOM) and saw about a half-dozen zero-ohm resistors serving no functional or test-related reason.

The project manager told me the reason: it was their standard practice to put these at various places on the board to confuse anyone trying to reverse-engineer the design from a physical unit. In fact, to make this reverse-engineering more difficult, the zero-ohm components had no markings and were specially ordered in different colors, so it would seem that they were different devices. Pretty clever, I thought.

While this was more commonplace in the early days of simpler single-sided boards with mostly analog circuitry, it is still done for lower-density areas such as power functions. When someone is sussing out a circuit diagram this way, the first step is to trace out the schematic, followed by trying to identify the various components and their roles; there are even apps that scan a board’s top and bottom to create a schematic. Slipping in a few zero-ohm resistors makes that second step more complicated.

Zero ohms, multiple packages

Figure 2. Zero-ohm resistors are available as leaded through-hole components (top) as well as small surface-mount (SMT) devices (bottom). (Image: PCBWay)

Zero-ohm resistors are available as both single and multiple units, in leaded and SMT versions, as shown in Figure 2. This SMT version is in standard 0603 (1.5 × 0.8 mm/0.06 × 0.03 inches). Both certainly look like any other non-zero-ohm leaded or two-lead SMT resistors.

For situations where multiple zero-ohm resistors are needed in proximity to each other, the Panasonic EXB-28VR000X array with four resistors in a standard 0804 package is available, as seen in Figure 3.

Figure 3. Panasonic’s EXB-28VR000X is a four-resistor zero-ohm array in a standard 0804 package size. (Image: Panasonic Industry)

Interestingly, zero-ohm resistors have two unusual attributes in their specifications:

First, they do not have a tolerance specification. That number is normally called out as plus-or-minus some percentage of the nominal value of the resistor, which is meaningless at zero ohms.

Second, these jumpers do have a maximum power rating, which seems unnecessary as their dissipation is defined by I2R, while here it is 0 ohms. But even a zero-ohm resistor is not perfect: most do specify a maximum actual resistance, such as 50 milliohms, and a maximum current, to limit power dissipation to a rated maximum.

Summary

The zero-ohm resistor is an excellent example of a component whose function seems unnecessary at first, and perhaps even useless. However, it actually offers well-defined solutions in multiple ways to designers who are aware of it and understand how it can help solve circuit and layout problems at very low cost and with no or minimal complications. For those reasons, vendors offer them in various configurations, and they are stocked in large quantities.