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I am designing a PCB in EAGLE. In this PCB, the major heat sources are my SMD/SMT resistors.

I learned that a general principle to dissipate heat from PCB components, is to place thermal vias directly at the bottom of high-power components ( reference: https://www.pcbcart.com/article/content/pcb-theraml-design.html ).

However, I noticed that the EAGLE footprint of all of my SMT/SMD resistors, which I downloaded from Ultra Librarian (https://www.ultralibrarian.com/), come with a "tRestrict" layer at the center, in between the two pads. As an example, the photo below shows a ERJ-UP3F2201V (Panasonic) resistor on my board; all of the resistors on my board (all are from Panasonic; all of their EAGLE footprints were downloaded from Ultra Librarian) share this pattern of footprint.

"tRestrict" is a layer that indicates areas where copper will not be poured. (https://www.autodesk.com/products/eagle/blog/every-layer-explained-autodesk-eagle/).

My questions are the following:

(1) What is the purpose(s) for having a tRestrict layer below a SMT/SMD resistor? I am new to PCB design, and I assume that it would be ideal to dissipate heat from major heat-generating components, such as a resistor. However, the lack of a copper plane below the resistor appears to be against this intuition.

(2) If (1) is indeed justified, i.e. it is important / necessary to omit the copper layer below the SMD/SMT resistors, would it still be useful to install thermal vias below the resistor (i.e., inside the tRestrict region)? Without the heat-conducting copper, I do not see how these would help by much, but thought that I would still ask!

A rectangular tRestrict layer at the center of all my SMT/SMD resistors downloaded from Ultra Librarian

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You don't have bare copper beneath a surface mount resistor because there is a possibility that it will form a voltage breakdown path with the body of the resistor. If you want to remove heat use bigger solder pads. Given that resistors are used to develop voltage across them (in many applications) you don't want to reduce the voltage withstand capability of a resistor by offering a potential path for voltages to break down through the PCB copper either.

If heat is a problem, use bigger sized resistors and more copper around the solder pads.

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  • \$\begingroup\$ Thanks! However, I plan to pour a ground copper plane surrounding the two pads (i.e. surrounding the entire resistor) though, as advised by most PCB board layout tutorials (e.g, learn.sparkfun.com/tutorials/using-eagle-board-layout ). Is the chance of having a voltage breakdown any lower, through the surrounding copper plane, compared to a copper plane right beneath the resistor? \$\endgroup\$ Aug 15 at 5:09
  • \$\begingroup\$ I talked about bare copper in my answer and, it is bare copper that offers the best chance of removing heat but, it comes at the expense of being uninsulated. \$\endgroup\$
    – Andy aka
    Aug 15 at 8:58
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In addition to Andy's answer, the only point of contact between the resistor and the PCB is the pads.

Much more thermal energy is going to couple through the solder connections than via radiation, through the solder mask (insulator), and into anything underneath it.

If removing that heat is problematic, you have several additional options:

  • Forced air cooling.
  • Vent slots milled into the PCB.
  • Redesign of circuit to use less power.
  • AIN (Aluminum Nitride) thermal sinks. Looks like a resistor but only thermally conductive.
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