Looking for electrically-conductive paste

Question

I apologize in advance if this question is a shopping question.

I am having problems with noisy connection problems on a metal shield that covers a very sensitive VCO in a professional wireless audio transmitter.

This transmitter contains two metal cans that are covered with removable metal shields. The shields have metal fingers that securely grip the metal sides of the cans. One can contains the VCO that both sets the frequency of the transmitter and provides FM modulation, the other can contains the PA stages that feed the antenna.

The problem that I am having is that there is significant "rub" noise when the body of the transmitter is flexed. The noise level ranges from insignificant (better than -80dB) all the way to completely unacceptable (-30dB or worse).

I've tracked the problem down to the metal shield that covers the VCO. If I remove the shield cover, the noise disappears completely. Re-tensioning the metal fingers that grip the shield can makes the problem go away for months or even years but oxidation rears its ugly head and the noise eventually returns.

Both the shield can and cover plate appear to be tin plated. They both accept standard 63/37 tin/lead solder very readily and there isn't ever any visible oxidation or corrosion present when I disassemble these units.

I'm not able to solder the cover to the shield can because there is a trim capacitor inside the shield can that is used to tune the VCO when it becomes necessary to tune the transmitter to a different frequency range. This happens infrequently but is necessary on occasion.

I'd like to try adding an electrically-conductive paste to the interface where the shield cover metal fingers grip the shield can. I'm looking for recommendations.

I'm aware of both Cool-Amp silver powder as well as the Cool-Amp Electrolube silver-bearing conductive paste but I know that I've come across other similar products in the past. I simply can't recall what they were.

I have used Noalox when doing electrical work that involved aluminium wire and cable but I don't think that it is what I'm looking for here - I want something with better electrical conductivity.

I'm aware of Aremco-Bond 614 as suggested in Question 86040 but it appears that Aremco makes only electrically-conductive adhesives.

I also saw NO-OX-ID mentioned in Question 38965 and plan to investigate it further.

I'd like to find several products that I can compare to each other to find which one provides the best performance.

[Edit]

The mechanical noise that I am hearing is more of a "scratching" sound than anything else. I'm well aware of this noise from other work that I do - one of the types of professional intercom system that I work with uses a special type of radio repeater with 2-way radios for mobile intercom users. The repeater is special in that the base transmitter is in constant transmit mode and is also tied into the wired intercom system. This allows the 2-way radio users to hear all of the chatter on the intercom circuit. Any one radio user can jump into the conversation at any time and everyone on the circuit (both wired and wireless) hears that chatter.

Because the field strength of the base transmitter is so strong near the antenna, one will hear a scratching sound simply by rubbing two pieces of metal together near one of the walkie-talkies that is listening to that open-squelch carrier. It is that exact sound that I get when the wireless body-pack transmitter is flexed.

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One suggestion was to put a hole in the shield top, then solder the top to the can. I am reluctant to do that - there are other trims inside that can that may need to be tweaked at some point in time.

Another suggestion was to use a braid to connect one corner of the shield top to the can. That won't fix this problem - the shield top does make good connection to the can. The problem is that this connection slides when the PCB is flexed (twisted).

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A different form of this problem came up with the newer Lectrosonics SM-series of body-pack transmitters. These are high-power (250 mW) wireless microphone transmitters that run from 1 or 2 "AA" cells. These transmitters are extremely power-hungry and will drain a fresh set of cells in a relatively-short time period.

Lectrosonics found that the thumb-screw that holds the battery cover in place was causing significant power loss. They found that a tiny dab of silver-bearing grease on the screw threads can increase battery life by as much as 50%.

Larry Fisher - Lectrosonics

The silver-bearing paste they use is from 2psi - tech specs and this material is also on my list of substances to try.

Answer 1

Here's an article on the instructables.com site that explains some ways to make your own conductive glue, which might as well be called liquid solder.

[http://www.instructables.com/id/Make-Conductive-Glue-and-Glue-a-Circuit/][1]

But I have to tell you that after reading your problem, I think you may be barking up the wrong tree. As you said, you are dealing with a very sensitive VCO, and it seems to me that the metal shield is interacting with that oscillator in much the same way as an electret (capacitor) microphone is used to change audio into a usable electric signal. If that is indeed the case, I don't think improving the conductivity of ground reference to the shield will help. In fact, sometimes the opposite can be more effective. For example, if the shield (can) were connected to ground through a capacitor instead of directly, it might better serve to block the frequencies you'd associate with vibration (scratching) while still acting as an effective shield at the frequency of the VCO. Just a thought.

Beyond that, you may have to resort to some mechanical solutions. For example, in magnetic guitar pickups, you WANT magnetic interaction with th strings, but don't want vibration to become part of the signal, which is difficult considering magnetic pickups are often enclosed in a metal shield too. In that case, manufacturers literally soak the entire pickup and shield assembly in molten wax, and then let it cool at a slow rate. Or, if you can make the metal can larger, spaced farther away from the sensitive VCO components, it should serve to lower the effective capacitance, and at least minimize the pickup problem.

I know this isn't much of an answer and I hope you don't vote me down. But I've dealt with things like this before, even torn out some hair over it, and feel obligated to warn you what you might be up against.