What's a "thermal jumper" do, anyway?

I’ve always been interested in simple-looking components which solve well-defined, clear, bounded problems. One carpentry example I encountered and used many years ago is formally known as a hanger bolt, Figure 1.

One end has a wood-screw thread and other has a machine screw for a nut or threaded fitting. It’s the mechanical “interface” between a wooden element such as a table leg and a metal mounting bracket.

(Tops) The schematic of the hanger bolt shows it interfaces a wood-screw thread with a machine-screw thread; (below) the hanger bolt allows a wooden furniture element to be connected to a metal fitting.
Figure 1. (Tops) The schematic of the hanger bolt shows it interfaces a wood-screw
thread with a machine-screw thread; (below) the hanger bolt allows a wooden
furniture element to be connected to a metal fitting.

There’s even a specialized version that features a reversed (left-hand) thread on the machine-screw side, used for suspending construction wiring or metal assemblies from wood. These reverse-thread hanger bolts solve a subtle problem, where the continuous rotation of an assembly would cause a standard right-hand threaded fastener to unscrew, while a left-hand fastener would remain securely in place.

There are also clever electrical components, of course. Given the number of years I’ve been “hanging around” electronic comments, circuits, and systems, I thought I was somewhat familiar with, or at least aware of, just about all of these, especially those related to management and removal of heat. I’ve had a long affinity for heat sinks, Figure 2, as well as heat pipes (yes, I know that sounds weird). They do one thing, they do it well, they’re reliable, they don’t push back, and they don’t need software, initialization, attention, or periodic upgrades.

Figure 2. Three of the heat sinks I have collected over the years: (left) slip-on “wings” for a TO-5 can transistor; (middle) heat
sink designed for the Intel Pentium II from the late 1990s; (right) a large heat sink for a power-converter module.

Imagine my surprise when I saw a press release (Ref. 1) from Stackpole Electronics, Inc. (SEI) for a component whose name and function were new to me: the “surface-mount thermal jumper resistor”, or simply “thermal jumper”, Figure 3. The word “resistor” definitely had me confused there, so I clicked over to the data sheet (Ref. 2) but found that it had all the facts related to ratings, size, and so on, but did not have the “story” on applications.

The thermal jumper is very plain and gives no hint as to its function.
Figure 3. The thermal jumper is very plain and gives no hint as to its function.

Next step was a quick Google search and, not surprisingly, saw several pages of links to clothing outerwear thermal jumpers designed to keep you warm in cool but not cold weather. Eventually, I reached a page of technical links when I saw this entry from another component vendor (Vishay), which stated it clearly: “a thermal jumper allows the connecting of high-power devices to heat sinks without grounding or otherwise electrically connecting the devices.”

OK, now it made sense, or at least started to do so.

The thermal jumper uses an aluminum-nitride (AIN) substrate with high thermal conductivity to provide a low (not zero) path for thermal energy (heat) to get away from its source to a nearby heat sink of some type. At the same time, it offers a high insulation resistance between its electrical terminals.

This jumper is the thermal analog to a zero-ohm resistor. As that name indicates, the zero-ohm device looks like a conventional resistor but is actually a short circuit. It’s used as a machine-insertable jumper to work around PC board-layout challenges (especially on single-sided boards), as a placeholder when a board has multiple configurations, or to obscure circuit specifics by camouflaging some details.

I still wasn’t sure about how to actually use this component, but an application video (Ref. 3) from Vishay showed how it functions as a tiny bridge from a resistor as heat source to a nearby PCB copper area functioning as a heat sink, Figure 4.

The test arrangement has a one-watt resistor without heat sinking on the left side, and an identical resistor but with thermal jumper and PC-board copper as heat sink on the right side.
Figure 4. The test arrangement has a one-watt resistor without heat sinking on the left side, and an identical
resistor but with thermal jumper and PC-board copper as heat sink on the right side.

Using a Fluke thermal imager, the video showed the resistor without thermal jumper was at about 140 °C (Figure 5) while the one with the jumper and the modest heat-sink area was at 100 °C, a significant 40 °C difference (of course, the difference is also a function of the size the associated PCB copper acting as a heat sink).

The left-right temperature differential between resistor was about 40 °C.
Figure 5. The left-right temperature differential between resistor was about 40 °C.

This thermal jumper is an effective way to solve a specific class of problems. Of course, although it is simple in appearance and function, it is not. It takes engineers, production specialists, material experts, and people skilled in many other disciplines to make it happen and do so in volume production.

Have you even found a small, unassuming passive or active electrical or mechanical component that is simple and clever, and at same time solves a pesky problem? Did it “save the day” and resolve a problem that was causing you to lose sleep, to use a cliché?

References

  1. “TMJ Thermal Jumpers Help Lower Temperatures for High Power Supplies”
  2. “TMJ Series Surface Mount Thermal Jumper Chip Resistor”
  3. “ThermaWick Thermal Jumper Demo”

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