I am working with a diode barrier that isolates hazardous areas. These barriers are potted in ER2074 epoxy resin and I have been seeing current spikes in the diodes appearing to come from humidity. It was suggested by overseas manufacturer to switch to PX274C epoxy resin and I do not have any expertise in this area to make a decision if this is a good idea. Any suggestions on this topic? thanks
Epoxy Resin Potting Compound
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Resins are a big and complex topic.
I do know, from recent personal experience, that water CAN penetrate FR4-type circuit boards. It typically happens along the surface of the fibers and begins at the exposed board edges. In circuit board material, you can see it as a dielectric constant that varies strongly with frequency in the range of 10KHz to 1MHz. Its called "dielectric adsorption" (in the electrical sense, not the water sense) and is colloquially called "soak".
I do know that some resins are relatively porous, but do not know if this contributes to it all. There should be something in the resin spec sheet about adsorption of water.
Jim
Here is a graph of the leakage current I am seeing on these zener diodes
What is your test circuit where you are measuring this leakage?
btw. zener diodes are rather poor performers for protection purposes. Have you tried using transzorbs?
Are you using any filler for the epoxy mix? You need to make sure that is dry as well. Also, take epoxy curing times into account. Depending on the mix and the size of the container, this can take days.
Its just a reversed bias zener with a dc supply set at 10 volts and a series DAQ to capture the current. most diodes just have a flat line with nanoamps of noise. A few defective diodes have this popcorn type noise with amplitudes in the microamps. I have taken several diodes with this problem, chipped them out of the potting, and they measure fine at room temp. Then I place them in a humidity chamber at 85-95 RH and see the same behavior as before removing them from potting.
I cant recommend a new design using transzorbs at this time so its not an option. All barriers I have seen from the big players like MTL and P&F use diodes.
Can you try with glass encapsulated zeners?
Best solution would be to find why I am getting leakage and fix the problem. I just dont know if manufacturer is making defective parts, if production is damaging parts, if epoxy is causing stress on parts, esd, something else. I was hoping others had possibly seen this same issue in the past.
Frankly, I simply would not trust a zener for that application!
Then, as a fall-back, does this behavior genuinely hurt in your application?
Jim
I can't say I've actually made a zener barrier! I've used a few though. For those that don't know, zener barriers are used in hazardous applications to create an intrinsically safe circuit: that is one that does not have enough voltage or current to create a spark to ignite a flammable atmosphere. You'd die at the price they sell for especially when they have three zener diodes and three resistors per circuit - just in case two fail. These all get nicely potted in resin so the likelyhood of failure is very small. Just to get certification is not a trivial execise nor is it cheap.
The MTL and P&F resin doesn't look like it is loaded with silica
emuler wrote:
Can you try with glass encapsulated zeners?
Maybe the resin is migrating into the epoxy body of the diode and messing things up. Give it a shot.
Can you explain the zero current portion of the scope display you posted? Was the voltage supply off during that period?
Typically these diodes have very small leakage current, a few nano amps. The scale shown is 50uA per division so a few nano amps looks like zero. The supply was on and at a stable voltage the whole time. Nothing changed except the leakage current through the diode.
Then that does not look like leakage to me. Leakage currents do not turn themselves on or off like that; one would expect it to be more or less constant in the short term, or at least more irregular in nature.
My best guess is that the system is picking up RF interference (EMI) from some nearby source, like a kitchen appliance. Or some pump, fan or similar motor that turns on and off at periodic intervals. See if you can track it down.
Can you bypass the input with a decoupling capacitor, say 100nF in parallel with 10nF and see if that makes a difference?
It looks like popcorn noise or burst noise. There are no appliances and its only on specific diodes. When I test several diodes under identical conditions I sometimes see this phnomenon.
new2use wrote:
It looks like popcorn noise or burst noise. There are no appliances and its only on specific diodes. When I test several diodes under identical conditions I sometimes see this phnomenon.
I've used Intrinsic safety barriers before. And yeah most people here would choke to death on their coffee if they knew how much they cost.
The comment I've heard a long time ago from a production engineer was that a lot of diode type packages in particular dislike stress and will also tend to crack when potted. That they show some difference in behavior when you put them in a humidity chamber bothers me, that would be a sign of damage.
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There are no appliances
I said like appliances. Sadly, EMI is all around us; you'll have to go deep underground (abandoned mine?) to get to a quiet spot. A while ago my PC's mouse pointer began jumping around randomly, at irregular intervals. After much head scratching I traced it to my cell phone that I had put on my computer table - whenever it refreshed its connection to the service provider, the mouse would pick up the interference.
Can you log the behaviour for a much longer period of time, say several hours? Especially the diodes that don't show this leakage. This digital noise has me intrigued. Leakage switching between a very low (negligible) value and another, much higher, fairly steady value is something I have never encountered.
I suppose it wouldn't be possible for you to test several identical devices simultaneously?
If you believe humidity (moisture) to be the problem, it may recover if you dry it out. Put it in an oven (preferably environmental chamber) at 80C for a day, this should dry it out. It may still be damaged of course.
Another possible source of interference is light. Those glass packaged diodes show significant more leakage when exposed to light (I don't know if a zener would do the same). If the epoxy is black this is not a problem. We usually encapsulate or paint glass diodes that may see light in use.
Klave
If the components are cracking when the potting compound sets what can I do? Softer material? Protective material over components?
Try changing the resin hardener ratio. A little less hardener should do it. Will take a little longer to set, though.
These come in packages premeasured and its going to be nearly impossible to relay to manufacturer to just use a little less hardener.