Leaky circuit boards

Try baking it, maybe 80C overnight. Phenolic can absorb moisture.

Phenolic is paper-based, and absorbs moisture easily, especially if left in a humid environment. I suggest a bake in a low-temp oven for a day or two.

If it has absorbed any oil, a soak in a 99% isopropyl (or if it's deeply contaminated, lacquer thinner) bath for a week or two, followed by the oven bake could help to reduce the leakage.

When you get the leakage to acceptable level, a conformal coating is well advised.

Cheers, Dave M

Tim Wescott wrote:

Exhaust does include water. Plus it is hot. That and the nitric acid from burning nitromethane.

On Sat, 28 Nov 2015 16:14:04 -0500, "Tom Miller" Gave us:

One doesn't need to bake overnight though.

A couple hours at 60C should do it.

With the aid of a vacuum, we used to use a mere 30 minutes for very high HV and never had a moisture problem.

A couple hours should be good. Depends on the age and thickness of the hygroscopic media in question.

Or throw it away and use a fresh piece of FR4.

BTW, most alcohol you buy has water in it. Isopropyl alcohol is typically somewhere between 9 and 30% water. So cleaning with this will add water to your board.

Is it worth redoing the board with FR4? But even that may have a similar problem since the surface contamination can cause the same problem. Can you get the board out of the exhaust path by enclosing it?

I don't have much experience with phenolic boards, but I have found leakage on FR4 boards, especially those without solder mask. My standard cleaning procedure is to scrub thoroughly with alcohol using a stiff bristle brush, to dissolve and loosen solder flux, and then follow that with hot water and detergent scrubbing, to remove ionic contaminants such as salt. Rinse with clean hot water, and then dry using a heat gun until all traces of water are gone. Sometimes water will get trapped under IC sockets and connectors, so those may need extra attention.

Baking is also a good idea, and a follow-up with conformal coating will help. I prefer acrylic, because it can be removed with solvent, and silicone can be removed mechanically if needed. Epoxy is very good, but difficult to remove if rework is needed.

Paul

I'm kinda thinking along those lines -- the Bakelite board did a nice job of proving the concept, but given the size I can get three boards from OshPark for $5, which is hard to beat.

Every time it's happened it's been several days since the last flight -- I fly on weekends and I needed to have my nose rubbed in the problem before I looked to that board to be the problem.

I think my cells may be bad _now_, but as a consequence of the draining, not the cause.

I'm thinking of redoing the board in FR4. I've mostly fixed the exhaust path problem -- the cowling had been open, but once I realized that it was getting serious exhaust in there and not just the light sprinkling that it gets during ground testing, I shut that hole.

Did you do the math? If the cell is fully charged, it will take months to discharge it with a 50 kohm load. Are you saying you left the cells discharged until the next weekend and the 50 kohm load put the hurtin' on them during that time?

Even 1% water is plenty of contamination when poured on the board. But worse is the surface contamination from the exhaust which is worse than anything the board will do. You said you got 1 Mohm once you cleaned the surface. How could that be a problem? It would take *years* for that to drain a 150 mAHr cell.

Try putting the board in a ziplock bag and I think your problem will be fixed if it already isn't. I expect you don't need to do anything with the board material, that's not really the problem unless the very slight remaining leakage of the board is causing the circuit to drain the battery through amplification. If the board material isn't the problem, changing it won't fix anything.

BTW, I envy you flying model planes. I tried it as a kid and my friends always wrecked them. I should get (or build) a model sea plane and fly it at my lake house. :)

Scotch brite pads is excellent for the initial scrub.

Jamie

I use a small vacuum chamber for home-made boards too. The advantage over baking is that it won't turn oil based solvents to gum. It's nice for pulling the bubbles out of epoxy too.

Hmm, two-stroke exhaust is still burning on the way out...could it have become hot enough to carbonize the board a bit?

I used to call them "cheese boards" because they would break so easily, so fragile, like cheese. Also they were so cheesy in looks and price.

YES!!!!!!!!!! Or better yet, get the fiberglass boards available Amazon; inexpensive as all heck, and a wide variety of sizes.

Real Bakelite is far superior but almost non-existent these daze (do you have a stash from the '40s?).