EPROM sealant causing shorts

[Slater]

I have some EPROMs which show some short circuits between adjacent
pins. I got rid of the shorts by scraping the white sealant with a
needle and now they program and read correctly.
Should I put something to seal the chips again? What can I use?

 

[Dave Platt]

I have some EPROMs which show some short circuits between adjacent
pins. I got rid of the shorts by scraping the white sealant with a
needle and now they program and read correctly.
Should I put something to seal the chips again? What can I use?

Was this a white sealant on the chips themselves? or on the PCB?

In either case... there are quite a few "conformal coatings" that you
can use to protect PCBs and chips against moisture infiltration...
acrylics, urethanes, epoxies, silicones, etc. In a lot of situations
they are quite unnecessary.

It's possible that the current leakage which shows up as a short isn't
due to the sealant itself (if those are ceramic-case EEPROMs, then the
sealant is also probably a ceramic) but is due to some sort of surface
contamination... dirt, dust, smoke, etc. which has built up on the
chips over the years.

If the sealant is something that was gooped on the chips and PCB...
then, yeah, it may well have gone conductive over the years, and is
best removed. It's probably not necessary to *seal* the chips, but
may have simply been gooped on them to make sure they didn't work free
from the sockets due to vibration or thermal cycling.

If you're concerned about those EEPROMs, you might want to scrub them
down thoroughly with a toothbrush and some nonconductive solvent
(electronics-grade isopropyl alcohol, or denatured alcohol for
example) and then dry them thoroughly (e.g. shake off all liquid,
air-dry for a few minutes, and put them in a 140-degree oven for a
couple of hours). My guess would be that this is all you'll need to
do. For example, this is the recommended cleaning procedure for the
old CDV-715 ionization-chamber radiation detectors, which are
sensitive to leakage currents down well below 1 nanoampere at a
50-volt potential.

If you want to try protecting the chips or the board, you can spray on
a conformal coating... but you have to make sure it doesn't get into
places where you don't want it (e.g. into sockets, between the socket
pins and the IC pins) or it may make matters much worse!

For some small PCBs that needed moisture protection (high voltage,
leakage-sensitive circuits) I've done the "scrub and oven-dry" steps,
and then given the board two or three very light spray-coast of clear
Krylon acrylic... cheap and effective.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Jeff Liebermann]

On Tue, 16 Oct 2012 22:45:12 -0700, dplatt@radagast.org (Dave Platt)
wrote:

I have some EPROMs which show some short circuits between adjacent
pins. I got rid of the shorts by scraping the white sealant with a
needle and now they program and read correctly.
Should I put something to seal the chips again? What can I use?

In either case... there are quite a few "conformal coatings" that you
can use to protect PCBs and chips against moisture infiltration...
acrylics, urethanes, epoxies, silicones, etc. In a lot of situations
they are quite unnecessary.

Approximately how many ohms is the "short circuit"?

If there's a conformal coating, it always has a phosphorescent dye
mixed in to allow for inspection. Where there's no blue or green
glow, the coating needs to be touched up. Find one of the cheap UV
LED flashlights, and inspect the EPROM for the dye.
<http://www.youtube.com/watch?v=38V-xLSZW74>

If coated, try dipping the EPROM in some nasty solvent, such as paint
thinner, acetone, xylene, or trichlorehtylene and see if anything
appears that looks like a coating. If you want to go for broke, try
paint stripper (methylene chloride) which will surely remove any
coating. Scrape off the mess with a wire brush. Oh yeah, wear
protective gloves and do it outside so you don't breath the chemicals.

In the distant past, I had to deal with a product that was rather
static sensitive. Management didn't want to deal with a redesign, so
I stupidly suggested that we spray it with conductive anti-static
spray and seal it with acrylic spray conformal coating. I think it
was about 100 Kohms/square. The problem was that the resistivity was
not stable, and would change with coating thickness, temperature,
humidity, handling, and the position of the moon. I eventually did
the redesign. Hopefully, wherever the EPROM's originated, did not
have the same bad idea.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558