Working with a high-speed PCB design at 1.6 GHz on a 12-layer board with matched trace routing. The operating temperature range is -10°C to +50°C.
I’m trying to understand how much the dielectric constant (Dk) of the PCB material might vary over this temperature range, and whether that variation could impact signal integrity at these frequencies.
The material used is TU-768, but its datasheet doesn’t provide much info about how Dk behaves with temperature. In general, how much does Dk change with temperature for materials like this? Is it typically within 1–2%, or can it vary more?
1 Like
Great question and it’s smart that you’re thinking about Dk variation with temperature at 1.6 GHz. TU-768 is a mid-tier material, and while it performs decently at RF, its Dk stability over temperature isn’t as tight as more premium materials like Megtron 6 or Isola I-Speed.
For TU-768 specifically, it’s common to see the Dk shift in the range of 1–2% over a -10°C to +50°C range sometimes a bit more depending on frequency and resin content. While that may not sound like much, even a 1% shift in Dk can cause timing skew or impedance mismatches in tightly matched differential pairs especially if your design is margin sensitive.
If the datasheet doesn’t include Dk vs. temperature, you might consider reaching out directly to the manufacturer or looking at IPC TM-650 test data if available. Alternatively, you could build in some margin in your stackup or simulate using worst case Dk values across the temperature range to see how it impacts performance.
At 1.6 GHz, you’re on the edge where this matters so it’s worth a closer look. If you’re seeing tight eye diagrams or crosstalk issues, material stability could be one of the culprits.
1 Like
You might find it helpful to look at the thermal coefficient of dielectric constant (TCDk), which quantifies how much the Dk changes with temperature. A common value for FR-4 is around 200 ppm/°C. For a 60°C swing (from -10°C to +50°C), that would translate to about 1.2% variation in Dk, assuming linear behavior.
That said, not all FR-4 or FR-4-like materials behave the same. Some vendors have reported that standard FR-4 materials can experience much larger variations, even up to 20% across a 0-70°C range in extreme cases, though that may represent worst-case or poorly controlled materials.
There are low-TCDk alternatives out there. For example, N6000 (an “improved FR-4”) has been shown to exhibit less than 0.3% variation in Dk from 0°C to 60°C.
So for TU-768, if the datasheet lacks detail, it’s worth asking the manufacturer about the TCDk, or estimating performance using worst-case assumptions in your simulation. At 1.6 GHz, these variations could start affecting impedance and delay, especially for matched pairs or tight timing budgets.
1 Like
You might also want to consider that Dk variation with temperature can be anisotropic, that is, it can differ between the Z-axis (through the thickness of the board) and the X-Y plane (along the laminate). This is largely due to the orientation of the glass fibers in the weave, which can cause directional differences in both Dk and its thermal response.
So even if the overall Dk change seems manageable, the directional variation could still influence impedance and timing, especially for signals crossing multiple layers or skew-sensitive differential pairs.
1 Like
Is that just ambient temperature, or do you also need to worry about part of the board being much hotter than another part?
Initially, I was considering only the ambient operating temperature. However, your point highlights an important aspect, localized heating from components can lead to hot spots on the PCB, which may result in non-uniform changes in the Dk across the board.
There are two approaches when dealing with Dk variation in high-speed designs. Either you rely on the manufacturer’s published specs and trust that the material consistently meets them, or you implement a qualification process, including incoming batch testing to verify the material’s suitability for your specific application.
It’s worth noting that even within the FR-4 category, dielectric behavior (including thermal stability) can vary significantly, sometimes by a factor of two, depending on the manufacturer and resin system. This variability makes general assumptions risky, especially in applications like yours operating at 1.6 GHz with tight impedance and timing constraints.
If your design margins are tight or the datasheet is lacking detail, it’s wise to take caution, validate the material through measurement or simulation based on worst-case Dk and temperature variation. That way, you can be confident in your design’s robustness across all operating conditions.