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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

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    \$\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
    Commented Oct 4, 2018 at 12:00
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    \$\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
    Commented Oct 4, 2018 at 12:19
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    \$\begingroup\$ What is your frequency of interest? \$\endgroup\$
    – filo
    Commented Oct 4, 2018 at 12:38
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    \$\begingroup\$ Can you make the dielectric layer thinner? Use a coplanar geometry instead of microstrip? \$\endgroup\$
    – The Photon
    Commented Oct 4, 2018 at 14:54
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    \$\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
    Commented Oct 5, 2018 at 4:28

1 Answer 1

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You have several approaches you can use:

First, if this a relative low frequency (below 500 MHz) you can stay at 50 ohms 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 50 ohms by narrowing the gap of the ground.

If none of these options work for you, you can use more complex grounding that I can send you per your request.

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  • \$\begingroup\$ I'm not the question author but I would love to see those more complex method I'm searching for the past 3 hours :) \$\endgroup\$
    – Wireless Learning
    Commented Jan 31, 2022 at 19:00

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