Is oxide on surface of RF conductor problem?

[fonograph]

Lead,tin,copper... they all oxidize,the oxide is insulator.At RF frequencies,skin depth might be only 1 micron deep.Now,ofcourse,if have 4 GHz signal with 1 micron skin depth,and 1 micron thick conductor gets oxidized 1 micron deep,thats obviously a problem.

What if same scenario happens but the conductor is 10 micron thick,so when its oxidized 1 micron deep,we are left with 9 microns of unoxidized conductor.Becose the skin depth is only 1 micron,in this case,the oxidation should not cause any problems becose the RF signal will just travel deeper inside,under the oxidized surface layer and no extra resistivity will be measured,right?

My question is,can the surface oxidation/corrosion by countered by making thicker conductors that still allow enough thickness compared to skin depth such that there is always enough unaffected conductor to conduct RF signals even despite surface oxidation?

Or is oxidation big problem no matter how thick conductor is,any surface oxidation or corrosion is bad,in some way,to the RF signal?

[DC1MC]

Well, it is really simple, the oxide is reducing the actual cross section of the conductor, it's like someone shaved a bit of the metal, for low freqs doesn't matter too much, until the oxidation goes really deep, for hi freqs it really does, since it produces a lot of side effects, not only reducing the cross section of the conductor, but the oxide itself has a different dielectric characteristic compared to the air and this it's going to change impedances and tunning.
So up to few hundred megs, the normal surface oxidation is no (significant) problem, afterwards it starts to matter.

 Cheers,
 DC1MC

[DC1MC]

There is no resistivity on hi freqs, just impedance , if you have your metal so thin that the normal surface oxide will matter, than is too thin. For hi freqs, usually the bigest problem is not the DC resistance but change of the impedance and q factor of the circuits, not to mention the resonance frequency. this why there a attempts to protect the conduits. Of course as you've mentioned the thicker the metal, the less influence has the oxidation, but for many applications this solution is not possible (strip antennas and friends). Better put a 4 microns layer of gold on it and be done  .

 Cheers,
 DC1MC

[fonograph]

With all due respect,I mean,your da microwave wizard and I am noob but...  I think there is resistivity on high frequencies.Sure impedance becomes much more important,that doesnt mean resistance disappears.

You said (wrote) that the surface oxide layer will mess up the impedance becose of its dielectric constant,but is that even really realistic problem under 10 Ghz? At 10 Ghz the wavelenght is 3 cm,if we consider the slow down due to dielectric constant of 2.2 in ptfe pcb,then the wavelenght is 2 cm long,even at 10 micron deep oxidation,thats 0.5% compared to wavelenght.Copper oxide have 18 dielectric constant,in case like this,it wouldnt be problem,would it?

[CJay]

My understanding of this is probably simplistic but adding an insulator to the surface of a conductor has the effect of changing the velocity factor, at HF a thin layer of oxide has little to no effect, a mm of PVC will make a measurable difference.

I assume the effect will be linear and as such a layer of insulation (oxide) will have an effect that increases proportionally with frequency (as well as other effects coming into play) so by the time we're in the cm bands oxidation will have a significant effect?

[DC1MC]

Well, there IS a reason why the strip line and PCB antennas from 2,4GHz up are covered with gold, mainly for the fact that the gold will conduct better, and stopping the oxidation will not hurt either.
And sadly I'm far removed from a microwave wizard  .

 Cheers,
 DC1MC

[fonograph]

By the way,how do they coat the PCBs for microwave frequencies? I read that you cant just coat gold on copper becose it will dissolve into copper over time.I read that they must put nickel layer between them but that sucks becose nickel is magnetic so at microwave frequencies it really faks it up.

Do they put like,thicker layer of gold on top so not all gold gets dissolved? Or do they make the copper layer so thin that not much gold can be dissolved into it?

[In Vacuo Veritas]

https://www.pcdandf.com/pcdesign/index.php/editorial/menu-features/12498-rf-circuits-1803

[qno]

Good RF connectors are silver player. Silver oxide is An excellent conductor. Silver turnsters black when oxidized.

[fonograph]

Good RF connectors are silver player. Silver oxide is An excellent conductor. Silver turnsters black when oxidized.

Correct me if I am wrong but I believe silver is noble metal,it doesnt oxidize in normal atmosferic conditions.That silver tarnish is actually silver sulfide which is neither conductor or insulator,its semiconductor with 1.08 eV bandgap.

[fonograph]

https://www.pcdandf.com/pcdesign/index.php/editorial/menu-features/12498-rf-circuits-1803

Thanks for that link,that was honestly one of best electronic engineering articles I have ever read,very clearly written and packed with tons of usefull information.

[firehopper]

Good RF connectors are silver player. Silver oxide is An excellent conductor. Silver turnsters black when oxidized.

Correct me if I am wrong but I believe silver is noble metal,it doesnt oxidize in normal atmosferic conditions.That silver tarnish is actually silver sulfide which is neither conductor or insulator,its semiconductor with 1.08 eV bandgap.

silver does oxidize in normal conditions. its called tarnish

[fonograph]

Good RF connectors are silver player. Silver oxide is An excellent conductor. Silver turnsters black when oxidized.

Correct me if I am wrong but I believe silver is noble metal,it doesnt oxidize in normal atmosferic conditions.That silver tarnish is actually silver sulfide which is neither conductor or insulator,its semiconductor with 1.08 eV bandgap.

silver does oxidize in normal conditions. its called tarnish

I think what you call silver is really a silver copper alloy,most "silver" isnt really pure silver.The mixed silver does oxidize,but its the copper and other stuff inside it,not the silver itself.Pure silver is highly oxidation resistant,its the hydrogen sulfide which is in air in small amounts that causes pure silver to tarnish,its not an oxide.

From wikipedia "Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish, although it may tarnish with oxygen over time."

[T3sl4co1l]

Skin depth depends on resistivity.  A micron of insulating oxide has infinite skin depth, and current continues on to the actual surface of the metal.

I always find it amusing when people talk about -- oft quoted but always unsupported -- reasoning, like silver oxide being conductive.  (Of course, silver doesn't oxidize in air, it sulfidates.)  No, a lossy material (moderate resistivity) is always more lossy than an un-lossy material (very high or very low resistivity).

More important is having connectors that properly wipe, so that metal-oxide contact is shorted out by metal-metal contact.

You can still get some higher order effects, like THD/IMD due to nonlinear oxides.  Bare nickel plating is notorious for this.

Tim

[fonograph]

You can still get some higher order effects, like THD/IMD due to nonlinear oxides.  Bare nickel plating is notorious for this.

Can you please tell (write) me more about these non-linearities? I was reading on some RF connector manufacturer sites and I read something about some kind of nonlinearity/distortion being created at the point of contact.I also read something about these surface oxide acting like diode or rectifier,distorting signal.Any word of wisdom or even better,some link to great article would be welcome.

[TheUnnamedNewbie]

There is no resistivity on hi freqs, just impedance , if you have your metal so thin that the normal surface oxide will matter, than is too thin. For hi freqs, usually the bigest problem is not the DC resistance but change of the impedance and q factor of the circuits, not to mention the resonance frequency. this why there a attempts to protect the conduits.

What?

At high frequencies resistance still very much matters, as this resistance is what causes the loss. Surface oxide can increase the loss of the traveling wave for a number of reasons - A less perfect surface finish of the conductive layer, which increases losses (this is why rolled copper is less lossy than plated copper), a conductive 'alloy' of oxide and non-oxidized material that carries a lot of current due to skin-effect, again increasing loss. I think for many applications at 'low' (low being a few GHz) the oxide layer acting as dielectric will have negligible impact on impedance since it is so tiny compared to the wavelength. What does impact it is it's effect on the resistance component of the conductors and that can increase the impedance. (Remember that a basic transmission line model starts with series R and L, with parallel G and C - in most cases we ignore R and G, but if R becomes significant, it will start impacting the impedance).

Of course as you've mentioned the thicker the metal, the less influence has the oxidation, but for many applications this solution is not possible (strip antennas and friends). Better put a 4 microns layer of gold on it and be done  .

I'm not sure this is true either - at a certain point, most of your energy will be at the surface anyways due to skin effect. Making things much thicker past this point doesn't really help, and so the impact of oxidation will be almost the same.

Well, there IS a reason why the strip line and PCB antennas from 2,4GHz up are covered with gold, mainly for the fact that the gold will conduct better, and stopping the oxidation will not hurt either.
And sadly I'm far removed from a microwave wizard  .

 Cheers,
 DC1MC

This is also not true.

The reason starts out with soldermask. I copy this from a stackexchange answer I wrote a while back:

Soldermask is applied as a liquid. As such, its thickness may not be as well controlled and predicable as the thickness of the substrate and conductor layers. In addition, it may have an unpredictable profile - how does it "flow" in between the traces? All of this means that you cannot accurately model the impact of the solder mask on your line, and cannot predict the impedance of the trace.

This is even important on any distributed element filter or microwave component such as a directional coupler, resonator, power combiner, etc. In these cases, a very small shift in the ϵeff of the system will potentially shift the center frequency out of the band of interest.

With high-performance RF substrates we can get very accurate models, provided we know very precisely the etch profile of the process. The unpredictable nature of solder mask ruins this.

So, we don't want solder mask on top of the transmission lines. But copper oxidizes a lot. As a result we want to add a layer to stop this from happening. On normal boards there are a number of ways of doing this - I myself am not an expert. Most of them I think are called "HASL" (hot air solder leveling). I believe this involves some tin alloy being plated onto the board. But the problem is that the thickness of the layer is unpredictable - not a problem for normal PCB work, but unacceptable for the precise tolerances we have on our transmission lines. So instead, we tend to use much thinner platings on those PCBs, usually Hard gold (electroplated gold over electroless nickel), ENIG (Electroless Nickel, Immersion Gold) or ENIPIG/ENEPIG (electroless nickel, immersion/electroless Palladium, immersion gold) - the difference between these has to do with wirebond capabilities. These platings are very, very thin - a few micrometers at best, and tend to have much better surface finish.

However, all of these finished has higher loss than pure copper, as can be seen in this paper: https://ece.uwaterloo.ca/~oramahi/IEEE-TADVP-Surface-Feb2008.pdf or this magazine: http://iconnect007.uberflip.com/i/586473-pcbd-oct2015/53?m4=. The argument that they use these finishes for lower loss is a myth spread by people who assume that gold being a better conductor than copper must mean putting it on top of copper gives us lower losses - in most cases the layer of gold is tiny (less than a micrometer thick) and the nickel needed to support it is much thicker (a few micrometers), and nickel is a much worse conductor. In addition, surface finish is hurt. The reason they use gold is because gold doesn't form an oxide layer, unlike most other plating, which form a thin, passivating oxide (=oxide layer that is not penetrable by oxygen, and as a result protects the underlying material from oxidizing further).



To put some stuff into context, I work with very high frequencies (tens to hundreds of GHz), so I don't know how some of this stuff translates to more 'common' frequencies in the <6 GHz applications.

[DC1MC]

Hi TUN, this is really fascinating stuff, thanks for such a nice organized answer with an overview of the current technologies and research, just two extra bits:

- when I telling the resistance/impedance thing, it was just to point that above some freqs there is no more pure resistance.

- you seem to be an actual microwave expert, so just a personal curiosity, what about those pated ceramic antennas, I seem to find them lately in phones and 5GHz capable routers, are they directly plated (gold over ceramic) or they use the same multimetal technology (Cu + Ni + Au) ?

 Cheers,
 DC1MC

[fonograph]

My guess is the ceramic antenas are either due to higher dielectric constant of ceramic so it radiates more easily or due to thermals becose it have higher thermal conductivity.

[TheUnnamedNewbie]

- you seem to be an actual microwave expert, so just a personal curiosity, what about those pated ceramic antennas, I seem to find them lately in phones and 5GHz capable routers, are they directly plated (gold over ceramic) or they use the same multimetal technology (Cu + Ni + Au) ?

I'm not really an expert on all things microwave (I wouldn't call me much of an expert to begin with really), and antenna design for those bands is not something I have experience with. I think they are also based on copper, but I could be wrong. I know it is possible to do them with just gold, but that stuff quickly gets really, and I mean really expensive (some guys at the research lab I work at have done ceramic substrate PCBs, and their order of 10 pieces, cost more than my grandmother's car). Keep in mind though these are PCBs that have 10 um holes and stuff, and the manufacturing is more similar to IC manufacturing than the process of making your 2 dollar prototype board in China.

The reason they use those ceramics in antenna modules is likely because the high dielectric constant makes an antenna shorter for a certain frequency and/or the low loss tangent of the dielectric makes them efficient. They might also have very predictable dielectric behaviour that does not change much with respect to frequency and processing.

The thing about those antennas (I believe, again, no actual experience here) is that they allow people to get very high performance antennas with some form of directivity. Antenna design is an art with quite a bit of handwaving, more than anything else in microwave engineering, and it takes a lot of expertise and expensive software to do it. Using module antennas gives people the ability to get a very predictable antenna pattern, without having to go to quite omni-directional antennas such as half-wavelength dipoles (which in addition to not being as directive must generally be mounted outside of the enclosure).