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SMD Jumpers: Bridging the Gap

Key Takeaways

  • A brief overview of the major applications of SMD jumpers.

  • The advantages SMD jumpers can offer over through-hole.

  •  use of TheSMD jumpers in thermal circuits.

Close-up of PCB

Most people’s earliest experience with electronics, hobbyist or professional, likely includes working with jumpers. Often paired with a breadboard, these double-sided connectors are wrapped in insulation and are an almost universal introduction to the basics of network and circuit analysis. By plugging in some conductive wire and a few through-hole devices, it is easy to gain a rudimentary understanding of the marvels of modern electronics. Coupled with tactile feedback comparable to the building block toys of childhood, it is a very enticing method of welcoming soon-to-be engineers and designers into the world of circuit design. 

However, jumpers exist in many forms beyond the tangled rainbow of wires running back and forth across a breadboard. SMD jumpers find significant use in PCB design for a number of purposes both shared and unique with wire jumpers. For locations and designs where through-holes would not be an option, SMD jumpers present an alternative design implementation.

Why Use an SMD Jumper?

The role of an SMD jumper may be mysterious at first. Through-hole jumper usage is often a far more flexible tool at nearly any stage of design iterations, from proof-of-concept to prototyping. However, there are times when a through-hole option is not a possibility without a massive restructuring of the extant design. In these cases, an SMD jumper might provide a feasible design solution. Jumpers typically serve one of two purposes:

  • During prototyping and development, jumpers can act as an optional connection while testing or debugging certain circuitry. They may be represented on a schematic with a 0-ohm resistor and listed in a BOM as a DNI (do not install) component. 
  • To enable or toggle on-board settings in the case of a motherboard or other programmable circuit board. This could include boot settings or toggling primary and secondary drives, such as the systems found in the most antiquated PATA architecture.

To further expound on the first point, the modularity aspect of an SMD jumper cannot be understated. Jumpers can drastically expedite testing by allowing entire circuitry blocks to be readily isolated and then quickly reintegrated with a simple desolder. 

This flexibility in testing also extends to the manufacturing side. Consider the situation where a circuit requires a particular resistance at the load; while the simulation is sophisticated, it cannot entirely account for all the quirks of the physical world. A resistor with low, but nontrivial resistance may be needed in the application, but it is possible the inherent parasitics of the components provide the necessary value. In this case, the layout can provide pads (space permitting) for both an SMD jumper as well as the expected nonzero resistor value in a parallel resistance network. Now, the fabrication part number stays the same; that is, the fabrication part number does not change, and companies simplify their platform while expanding inventory. At the assembly stage, production runs can choose whether to include the resistor, jumper, or both. 

Much like standard circuit analysis, a jumper’s position may be called out as on/closed or open/off. The positions describe the number of pins the jumper is covering - 2 or more in the closed position and 1 or less in the open position. 

Through-Hole vs. SMD

Through-hole jumper connections can offer range, length, and flexibility, which allows them to easily span large distances of the board. With a smaller connection area, it is also far easier to run the jumper through dense areas of the board that may otherwise be troublesome to add additional density to, component or otherwise. 

While the range of motion is significantly compromised compared to a through-hole jumper, SMD jumpers still have options to navigate dense designs. A common SMD style appears as a square arch with rounded corners. By allowing significant clearance beneath, the trace can easily navigate the above components as well as copper features on the board without invoking significant coupling with the top layer. Further, high-current traces that would exceed the current-carrying capacity of a common-gauge wire can still receive the benefits of a jumper through surface mount technology (SMT). While space must be accounted for with the solderable pad areas, this also notably allows for the crossing of two same-layer outer signals without a layer transition. In essence, a jumper performs as a trace on a bonus layer of the board in exceptionally dense configurations.

SMD Jumpers Provide Thermal Routing

Jumpers may also play a perfectly thermal role. In dense, thin designs where space is at a premium, a thermal jumper may be of use to draw heat away from high-power areas such as switching regulators and other power circuitry.  A thermal SMD jumper offers some advantages, primarily that its low ℃/W rating means it can effectively transfer a large amount of heat away from the generating source before dropping down to an internal ground layer by thermal vias for dissipation. Where this finds the greatest use is for power components that do not have a thermal pad connected to the ground and therefore require electrical isolation from the internal power and return planes. By interfacing between the heat-generating component and the ground plane, an SMD jumper can transform a large copper pour on an outer layer into a much more compact feature, opening up more routing and via channels for dense designs while improving thermal efficiency.

No matter what role SMD jumpers serve on your board, you want to make sure your toolset is well-equipped to handle the integration at all design levels. Cadence’s PCB design and analysis software provide a comprehensive, highly collaborative design suite that is built to handle the multitudes of the current design world. 

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