ENGINEERING ANSWER
The standard published process Dk is 3.48 ±0.05 at 10 GHz and 23°C. The published design Dk is 3.66, derived using a different method over an 8–40 GHz test basis. These values describe different measurements, not competing material grades.
What to decide before release
Use a design value suited to the transmission-line geometry and frequency, then correlate it with measured coupons. Do not select 3.48 merely because it has a tighter-looking tolerance. A change in assumed Dk changes both impedance and electrical length.
Checks to put in your design brief
- Separate process and design Dk in the drawing
- Record frequency and test method
- Retune using measured line data
Important distinction
The 4 mil laminate has a separate process-Dk footnote. Never apply the headline value to every thickness without checking it.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.