Why the zero-ohm resistor?

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By Peter Clarke

Passive components – resistors, capacitors, inductors, and connectors – usually don’t get a lot of consideration until late in the design, but they are vital elements of a functioning circuit, especially in the analog and sensor world. Despite all the attention we give to ICs or discrete semiconductors, passive-component vendors still see interesting opportunities for new products and introduce new ones at a fast pace.

Being a curious fellow, I am always interested in the latest updates to what are incorrectly considered “mundane” components. A recent one I saw was a zero-ohm resistor from TT Electronics.

Such “resistors” are not a new development: I first ran into them many years ago working a summer job repairing small electronic gadgets. This was in the days before larger-scale integration, and one circuit board was populated with many single-function ICs, passives, and connectors (all through-hole), plus about half- dozen wire jumpers, some of which were fairly long.

When I asked the manager (an electronic engineer) about this, he explained to me that using these jumpers allowed the manufacturer to “get away” with using low-cost, single-sided PC boards, and they were cheap phenolic material with punched holes rather than a more-expensive FR4 substrate and its more-costly drilled holes. These 0-Ω components functioned as crossovers which allowed for a much-cheaper product, even though they had to be installed by hand (as were the other components, I am fairly sure).

Years later, I again encountered these 0-Ω resistors on a major project, one which was almost entirely surface mount and did use multilayer FR-4 boards. I was somewhat mystified. I looked at the schematic and they were shown called out, and on the BOM, but I saw no electronic reason for them.

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 0-Ω components had no markings, and were special-ordered in different colors, so it would seem that they were different devices. Pretty clever, I thought.

Fast-forward to the present, and we still need these. Despite their functional simplicity, they have real parameters and specifications just like their non-zero-ohm siblings. The TT Electronics LRZ0603 is surface-mount ceramic-chip device rated at 6A despite its tiny SMD 0603 package (1.6 mm × 0.8 mm), and is AEC-Q200 qualified so it can handle high temperatures such as in automotive and industrial applications. Obviously it is not a “perfect” 0-Ω device, in which case there would be zero voltage drop and no concerns about self-heating!

Even its construction is advanced, using thick-film copper conductive element which is screen printed on a 96% alumina substrate, for higher current ratings for a given footprint, and it uses a copper material to achieve low residual resistance coupled with low internal-thermal impedance. Further, the vendor notes that its wrap-around copper terminations have an electroplated nickel barrier and special coating for excellent ‘leach’ resistance and solderability. All of a sudden, this “simple” passive component is not so simple!

Are there any seemingly simple components, either active or passive, which you have encountered and then found to be far more complex and sophisticated that you initially assumed?

Bill Schweber, is an electronics engineer and author who has written for EE Times, was analog editor at EDN and prior to that worked in marketing communications for Analog Devices and was also editor of its technical journal.

This article first appeared on EE Times’ Planet Analog website.

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