PCB high-voltage meltdown

"Winfield Hill"

** Suround the BNC with a ring of Ivory.

I'm sure you have just loads of that stuff where you hang out.

With all the other bats.

...... Phil

Can you saw a slit in the board to remove the chance of carbon tracking? That is done a lot in the HV circuits on TVs and video monitors.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Win You didn't say what the pcb material was so I am going to assume FR4. If you clean the boards thoroughly and then bake them out to remove any water that was absorbed, they will be pretty high resistance. Then conformally coat with a silicon based coating and bake out. This should eliminate the problem. Cutting the slit in the board like Michael recommended is also a good idea. I have had problems with leakeage in FR4 boards and it gets much worse with increasing temperature. The conformal coating seems to help. Good luck Jim Stockton

Without seeing a photo of the section it's a tad hard to make concrete suggestions, but I would cut away the bad bits and try to use ceramic insulators/standoffs for repair.

You're running 1100 V on a BNC? You can get away with this sometimes, but you are really pushing things. If the connector solder pins are .1" apart, my only surprise is that SN 1-18 didn't suffer the same fate.

You didn't even CLEAN the flux off of an 1100 V small-clearance circuit? It is possible the PCB itself had enough conductivity due to contamination in it, to break down eventually. But, leave flux on the board and you really invite trouble. And, once it starts, there may be no way to repair it without a Dremel tool.

Yes, I think that would be advisable. Look at the sweep transistors on computer monitors and such. They often make slots in the PCB between the drain or collector trace and the other transistor leads. The slot is a heck of a lot better insulator than even a clean PCB. If you still have 1 KV across .1" of board, then the coating sounds like a must. If you can remove the grounded area, then maybe just that and a thorough cleaning will be sufficient.

I've seen PCB fires in the last vacuum tube gear made with PC boards with plate supplies in the 400 V range. These were in commercial audio gear. On the other hand, I guess PC board materials are probably a lot better today.

I have made small production runs of several photomultiplier tube bases (voltage dividers) in the past, and some of these run 1000 to

1800 V or so. I've never had a fire or breakdown in them. We use SHV connectors for the bias supply to avoid this sort of disaster when going over 1 KV. (We also used the .156" Molex connectors and skipped a pin between gnd and HV. This worked quite well, even in VACUUM!)

Jon

I suppose in a perfect world, with a clean, washed board, low humidity, no dust, low frequencies, and perfect DC balance you can run 10kv per inch. But if it gets muggy, and a little dust gets attracted, or there's some flux and a DC offset, things can get ugly. But you already know that.

The grinding wheel sounds like a good solution. Air is a cheap and relatively good insulator.

Jon, Jon, Jon, we're talking about students here! You don't think I'm going to build 160 amplifiers, with over 150 parts each, myself, do you?

--
 Thanks,
    - Win

I thought BNC was only good for ~600v. I've seen PL259's used beyond that.

Cheers

Yes. Although I can think of cases where 1, 1.5 or even 2kV uses have worked for years on end. But then there's the spectacular failure at 1kV, or less. So, yes, 600 volts sounds about right.

Indeed, with substantial modifications, all the way to 15kV. I've found there are two issues, 1) basic coax-cable dielectric HV withstanding capabilities, which are dependent on pinholes and the like, and 2) issues having to do with the outer woven-shield connector termination of the coax, creating small, high, local electric fields that can disastrously break down at 1/3 to 1/10 of the coax dielectric-breakdown limit.

Well-designed high-voltage connectors deal with the shield termination in an innocuous fashion.

--
 Thanks,
    - Win

On 18 Oct 2006 14:28:53 -0700, Winfield Hill wrote in Msg.

What do you need 160 for these beasts for?

Just curious. robert

*If* the board was cleaned, 25 mils is sufficent to hold off 1250V up to 210C; soldermask is the coating. At 2600V and 20C, surface arcing over the sodermask will occur; cannot say where the voltage "breakdown" is. Helps to severely limit the current (20uA seems to not create carbonized paths in time periods less than 10 seconds). Granted, there are no intermediate layers of copper "in the way" and that the board material we use is not FRxx. Perhaps so-called "no-clean" solder was used, which obviously leaves crap behind. TINSTAAFL.

I thought the HN connector was designed for high voltage coaxial connections? I remember using them in a early '50s RCA TV transmitter.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Anti-proton trapping experiments at CERN; the HV amplifiers drive the trapping electrodes, and apparently there are a lot of them!

--
 Thanks,
    - Win

Manufacturers and suppliers readily publish data on FR4 dielectric strength, usually around 400-500V per mil. But this is only valid for

80 or 100 seconds. I haven't seen a datasheet contain any practical information on operational breakdown performance.

Does anyone have more accurate guidelines for the operational voltage range of FR4 if you want to, for example, avoid any possibility of a failure within 10 years of continuous operation? In electrical printed circuit boards, the breakdown voltage means something only to those few that are worried about transient spikes. I've always wondered why they dont publish specs that would be useful to the other 95% It would be nice to stick to the UL guidelines, but 500V sot-23 transistor pad spacings make a mockery of those. There must be a study out there some where.

There is a related thread at:

A useful trick to decrease the likelyhood of breakdown and arcing is to bond a layer of copper clad Kapton to the bottom side of the PCB, ofcourse this only works for SMD only boards. The Kapton film is copper on one side and 0.1mil teflon on the other (Kapton FN). The teflon makes an excellent thermoplastic adhesive above 280degC and sticks to a PCB nicely. The copper side is grounded which puts a perfect ground plane 2mil below the PCB. If the PCB is thin, this causes the electric field to be directed downward, and due to the dielectric properties, concentrate in the Kapton, thus avoiding breakdown of the FR4. With the addition of some thermal vias, and an aluminium plate, you can also make an excellent heat-spreader or heat-sink attachment, in this case forget the copper layer, use two sided kapton FN and bond direct to the metal backing.

You can also lay the components directly on kapton, but you only get traces on one side. A better option is microwave type, copper clad teflon, which is relatively low-cost and can be processed like 2 layer FR4, except you wont get plated through vias unless you deal with a specialty manufacturer.

/Andrew

Nothing of the sort. Kapton(tm) has no chemical relation to Teflon(tm).

Kapton® Polyimide Film

DuPont? Kapton® is a leader in the high performance films industry, offering over 40 years of diverse products, global technical support and customer service. DuPont has set a high standard in the polyimide film markets with its durability and performance in extreme temperature environments. Kapton® has a unique combination of electrical, thermal, chemical and mechanical properties and retains these properties over a wide range of industrial environments and applications.

From miniaturized electronic components to Mars rover heaters, from high speed locomotive motors to airbag seat sensors, DuPont? Kapton® polyimide films make innovative design solutions possible.

Other than that your post ain't tooo bad.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

ffering

mide

Whats all this advertising? and of course Kapton is not Teflon, did I say it was?

Copper clad Kapton FN is the standard Kapton HN film laminated with a

0=2E1mil layer of teflon on either surface. Its easy to clad one side in copper foil or another metal plate using a standard PCB press. The other exposed teflon coated surface can be used as an excellent adhesive if the bonding temperature is above 280degC.

Ah, last I heard, Kapton is no longer recommended as electrical insulation. Seems a big jet caught on fire due to an electrical short. Which made no sense as the wiring was just 24 volts and insulated with the best Kapton.

After a little experimenting, it was discovered that a small spark could make Kapton conductive, leading to more sparks, and so on.

No, it seems cabin lighting is run off higher voltage (it is often fluorescent) and 120 VAC breaking through into the fuel sensors will explode your 747 no problem.

...

Yes, you did.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller