Leaky FR4

[markr14850]

Is anyone noticing voltage leakage on FR4 boards?

I'm building a buffered effects loop (d-lator) using 1/16" black FR4 (McMaster part # 85345K621).

I was measuring voltages, and was detecting DC on a grid. After unsoldering a big chunk of the circuit trying to track down all the logical places that this could come from, I wound up with the case where a lone unconnected eyelet was showing a few volts to ground.

Hunting around some more, I'm able to get 6 volts to ground if I put my test lead on the edge of the power supply board within a few inches of the B+ connections. Not touching any metal - just the edge of the board!

The McMaster catalog page lists this material as "excellent electrical insulator". Well, not for me. These boards are headed for the garbage.

Note: I'm not seeing any measurable resistance with my meter. It seems like the breakdown is related to the "high" voltage (~370V) of my power supply.

Anyone else seeing any strange behavior like this?

Thanks,
--Mark

[Structo]

I haven't used that board but it sounds reminiscent of the old Tweed syndrome or whatever they called it.

The old fishpaper boards used to go conductive from moisture absorption.

[Aurora]

FR4 is often referred to as Flame Retardent fiberglass boards, although the FR4 designation is related to the degree of flame retardant.
I see yours is garolite, which should also be OK. Normally this stuff is used with considerably higher voltages than you have, so there must be another cause. If you prod your DVM probes directly into the board a few tenths apart, you should see reading lower than 1 meg for this to happen.
Yours is black garolite...but I can't imagine the colour is from any conductive filler.....

[markr14850]

I played with this a little more this morning. There doesn't seem to be any leakage through the main faces of the board, only along the edges. I'm able to repeatably measure greater than 50 volts on the edge right next to an eyelet with 360v.

[gearhead]

Ruh roh, I got the same stuff from McMaster (in tan) for a build that's 80% complete.

Please keep us informed.

Do you have any leftover pieces to see if they conduct?

[gearhead]

Went to McMaster-Carr, and they cite the Electric Insulator performance characterisitc for their Grade G-10/FR4 Garolite is:

"Effectively resists flow of electricity. Commonly used in housing or mounting of electrical components. Generally resists at least 500 volts per .001 in. of thickness"