boring leakage tests

Boiling them in the pasta pot will work too. ;) It'll make the pasta taste a bit like balsamic vinegar too!

Cheers

Phil Hobbs

Pease said to use a domestic dishwasher! I tried that and it seemed to work visually well but I didn't measure leakage resistivity.

piglet

He was writing back in the phosphate days, though--I doubt the newer detergents will work well. Alconox maybe?

Cheers

Phil Hobbs

That was for cleaning. I want to take a bunch of soldered boards (with various no-clean fluxes) and expose them to 100% humidity for a couple of days, and re-measure leakage.

On a D25 connector layout, leakage between rows of alternate pins is very roughly 3e13 ohms. If I rub it with greasy and salty fingerprints (many Fritos died for science) and stash at 100% humidity overnight, it drops to about 1e12. Cleaning with qtips and acetone gets it back to 3e13 range.

He specifically mentioned Calgonite. One MSDS I see for it lists surfactants, sodium carbonate, and sodium silicate - no phosphates (although I imagine there could have been some, pre-2010... seems likely in fact).

Since Alconox still contains tetrasodium diphosphate it might well work better than today's commercial automatic dishwasher detergents.

Add saltpeter to a cup of water till it won't absorb any more. Stick in airtight container with your sample.

RL

Bubble Bandit: "All the phosphates."

Seems like a puddle of water in the bottom of a bucket will get to

You have too much capacitance in the connecting cable. Use the J105 Unity Gain output on the back panel as a guard. It follows the input signal to within 10ppm, cancelling the cable capacitance.

Connect the cable shield to the J105 output and the test circuit ground, and the center conductor to the test point.

Current time:

I = C * dv/dt dt = (C * dv)/I set I = 1e-14 Amp, C = 10 pf, dv = 1 V dt = (10e-12 * 1) / 1e-14 = 1,000 seconds = 16.66 minutes

with dv = 0, dt = 0

See X1 Output (J105) on page 10:

It's about an inch long, a 1/4 w resistor in free air actually. The capacitance is mostly the pads on the PCB, and likely stuff inside the meter. There's about 6 pF between the test points on the D25 pattern.

Our main use for leaded 1/4 w resistors is as a source for short bits of wire.

Possibly I could bootstrap the PCB ground plane. But just the meter (Keithley 610C) with nothing connected looks like it will take a good chunk of an hour to pin the needle at 1e14 ohms full scale.

Cool instrument, though.

A lot of seals that will last forever in clean water start to leak as soon as there is an osmotic gradient involved.

CH

Yes.

Alconox if by hand, but AlcoJet if a dishwasher is used. Alconox foams too much.

One can also still get phosphate detergents - they are now sold for boiling deep fat fryers out.

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Available on Amazon.

Joe Gwinn

Mine is much faster than that. You might hit eBay for another unit.

Cheers

Phil Hobbs

Dissolving stuff (usually some salt) in water changes the vapor pressure and thus the relative humidity in the air above the water. Making the solution saturated makes it easy to make, you just dump in enough so some doesn't dissolve, and there are tables of salt solutions to maintain most any desired relative humidity. These are useful to directly maintain some relative humidity, and to check the calibration of a humidity meter (sodium chloride results in 75% which is popular for meter calibration). At 25C a saturated solution of potassium nitrate will result in a relative humidity of about 94%. The problem with these and with pure water is how long they take to reach equilibrium. I'm assuming the benefit over pure water is that the humidity will be more constant over small temperature changes which would make your measurements more precise. One TLDR reference is:

There's your problem. I estimated 10pf, but 6pf is reasonable. The stuff inside the meter is cancelled with the J105 output. dv=0, so dt=0.

Just enclose the stray capacitance in the shielded guard, and your test time will go down proportionally.

RTFM. J105 is provided to set this time to zero.

I looked it up. At high humidity, the salt mixture will suck water out of the air, namely reduce humidity. I want maximum humidity, like

An inch of water in a bucket is sure easy.

But it doesn't give you the thrill and risk of spattering salt water on your board and ruining your test :-). I wasn't advocating for using a saturated salt solution, just explaining what they do. Stand some large sponges up in the bucket so the bottom part is submerged and the rest is exposed to air to maximize your evaporation surface area. Starting with hot water will speed things up, too.

As other posters have pointed out, it's relatively easy to perform reproducable tests at a controlled RH.

If you use a shallow glass container, in the bottom of your sealed environment. you'll get a good surface area and easy visual confirmation of both liquids and solids in the saturated solution, through whatever peep-hole is available.

You want a high humidity, non-condensing environment. Condensation will spoil your test.

Having spent thousands of dollars on environmental chambers, it is a pleasure to find one test that anyone can do, yes?

RL

Is there a tube in that thing? I use a Hiportronics teraohm meter, and it's uh real sensitive to everything. I get the feeling even the binding posts hurt the sensitivity some too. The whole ground vs guard planes on the PCB and terminals are weird too.