5
\$\begingroup\$

I am designing an RF board in Altium where due to the dielectric selected and its thickness, the trace width for a 50 ohm characteristic impedance is thicker than some of the SMD component pads. What is the best method for routing a larger RF trace to a small SMD pad to minimize the impedance mismatch?

I am currently using the teardrop tool to taper the line down gradually to the smaller width (see below)

enter image description here

\$\endgroup\$
  • 2
    \$\begingroup\$ I'm not an RF engineer, is a sudden change in width a bad thing in RF? With a teardrop you have a longer path with the wrong impedance, so a step change closer to the pad would reduce that. \$\endgroup\$ – Arsenal Oct 4 '18 at 12:00
  • 1
    \$\begingroup\$ I think that is wrong. For RF, you want to keep the impedance correct for as much of the circuit as possible. So, right up to the SMD part, then a small bit of a trace that is narrow enough to connect things together. \$\endgroup\$ – JRE Oct 4 '18 at 12:19
  • 1
    \$\begingroup\$ What is your frequency of interest? \$\endgroup\$ – filo Oct 4 '18 at 12:38
  • 1
    \$\begingroup\$ Can you make the dielectric layer thinner? Use a coplanar geometry instead of microstrip? \$\endgroup\$ – The Photon Oct 4 '18 at 14:54
  • 3
    \$\begingroup\$ You may or may not have a problem. The IC package will have inductance, the bond wire will have inductance, the IC With metallization will have inductance; there is parasitic capacitance between leads on the IC package, the bond pad has parallel-plate capacitance to the substrate, etc. Use a network analyzer and examine the S11. With some series R dampening, you may not have any problem. \$\endgroup\$ – analogsystemsrf Oct 5 '18 at 4:28
1
\$\begingroup\$

You have several approaches you can use: First if this a relative low frequency (below 500MHz) you can stay at 50ohms as close as possible and then narrow the line with narrow taper. Second if the frequency is higher you can narrow the RF layer to the extent that the 50 ohms line will be narrow as the pad. Third, you can also use grounded coplanar line and keep the 50ohms by narrowing the gap of the ground. If non of these options don't work for you, you can use more complex grounding that i can send you per your request.

\$\endgroup\$

Your Answer

By clicking "Post Your Answer", you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.