EMC Question of the Week: April 24, 2023

Component connections to a circuit board plane through four spokes.

When connecting a solder pad or a connector pin to a solid plane in a circuit board, it is not unusual to make the connection using four spokes as illustrated in the figure. This is called a

  1. thermal relief
  2. strain relief
  3. trace fuse
  4. Bob Smith termination


The best answer is “a.” Thermal reliefs are intended to aid in the soldering process. They prevent the planes from pulling too much heat away from the component pad or pin. Old application notes and texts sometimes advise against using thermal reliefs when making high-frequency connections to a plane. The reason cited is the additional inductance of a connection where the currents are restricted. However, thermal reliefs add negligible inductance compared to the overall connection inductance of a component. There is no need to avoid them. In fact, a poor solder connection is much more likely than a thermal relief to contribute significantly to the connection impedance.

A strain relief is an object or geometry that mechanically keeps a cable in place in the presence of an external force. A trace fuse is a narrow section of trace designed to blow out like a fuse when it carries too much current. A Bob Smith termination is configuration of capacitors and resistors designed to provide a high-frequency, common-mode termination on many Ethernet cables. 

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