Thermal Relief in RF paths - Page 1
Forum section: electronics / rf-microwave

author: nabnab
author_rank: Newbie
timestamp: May 21, 2019, 09:20:18 am
content: Hi,I'm designing a RF path with a LNA and SAW, the SAW has 4 pins in GND. I use this kind of connection to link it to the GND. The loops of these thermal relief doesn't disturb the RF?Please see attached the image.Thanks in advance

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2019, 04:08:27 pm
content: What frequency? How much isolation do you need?That will tell you how much trace length you can have to ground, and thus whether the slots and necks are acceptable or not.Below a GHz it's hard to justify not using thermals.Tim

author: ejeffrey
author_rank: Super Contributor
timestamp: May 21, 2019, 08:01:41 pm
content: Here is a paper I have used for evaluating inductance of thermals:https://www.cedm.be/system/files/public/library/publications/2013-09_EMC-Europe_Modeling-the-Low-and-High-Frequency-Impedance-of-Thermal-Reliefs.pdfIt is mostly for traditional wagon wheel thermals in through-hole parts like connectors, but should provide something to compare with for surface mount components.The worst case they evaluated is about 200 pH for a single bridge, 100 pH for 2. The latter would correspond to about half an ohm reactance at 1 GHz. Not something you would generally worry about at that point.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: May 22, 2019, 06:54:35 am
content: Unless your hand soldering parts, why would you have thermal reliefs on SMT pads?

author: in3otd
author_rank: Contributor
timestamp: May 22, 2019, 07:22:29 am
content: As said, it all depends; 0.5is 40 dB down from 50so if a final rejection of that order is ok, then it's not a problem.The OP said the SAW filter has 4 ground pins but I see only three with a good connection to ground; if the fourth ground pin is the one in the upper left corner, I'd add a gound via nearby just to be on the safe side.

author: Berni
author_rank: Super Contributor
timestamp: May 22, 2019, 07:30:35 am
content: Quote from: D3f1ant on May 22, 2019, 06:54:35 amTo avoid this:

author: nabnab
author_rank: Newbie
timestamp: May 22, 2019, 09:20:43 am
content: Thanks for all your answers and commentsThe frequency is around 868MHz and the needed isolation is 40dB.I have added a via close to the 4th pin, it's an oversight.in3otd, I dont undertand what's the link between 0.5Ohm and 40dB?, could you explain more please.Thanks for all

author: in3otd
author_rank: Contributor
timestamp: May 22, 2019, 09:51:42 am
content: it was just a rough calculation: if the filter is shunting the signals in the rejection bands to ground and the ground connection has a 0.5impedance, across that impedance you'll have only about 40 dB attenuation due to the voltage division between the 50source and the 0.5to ground, \$20 \log_{10}\frac{0.5}{50}\$ (ok, if load is also 50then it's 6 dB less). Of course things are more complex than this, it was just to show that even a small impedance can be important if you need a large final rejection.Edit: corrected \$\LaTeX\$ stuff

author: ejeffrey
author_rank: Super Contributor
timestamp: May 22, 2019, 05:41:34 pm
content: That's a reasonable worst case scenario, but I wouldn't take it to seriously from a quantitative standpoint. Even for a simple third order LCL tee filter it doesn't take into account the rising impedance of the series elements that will be much higher than 50 ohm impedance in the stop band. I simulated a third order lowpass filter with a parasitic 100 pH inducance on the ground connection and the stop band limit was more like 60 dB. In a multi-stage filter where each stage has its own ground connection or in a SAW filter where the input and output have separate ground connections the limit is much much lower as you get multiple chances to shunt the rejected signal to ground. That extra inductance will create high frequency resonances that can allow transmission at frequencies much above your passband (maybe 10-20 GHz)I tried a simple SONNET simulation of a pad like this and also found numbers in the 100 pH range, but even with a continuous ground plane on 3 sides the inductance only dropped to 65 pH. So basically the added inductance from the thermal relief is smaller than the inductance of the pad and the port. I could have probably come up with a better simulation, but I think the point is that unless you are carefully simulating every part of the design, other parasitic effects will likely be larger than the thermal relief.

author: coppercone2
author_rank: Super Contributor
timestamp: May 23, 2019, 04:06:39 am
content: I always wanted to apply ceramic heat sinks to something