Why would a SMPS high speed Unitrode rectifier stud diode 1N5812 , 20 amp,
fail sitting around in a normal human occupation heated environment for 10
years unused?
Currently reads a stable 6.2 ohms. Replaced with a new one but I intend
passing ever increasing current through the old one to see what happens.
Any other ideas concerning cause or analysis ?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

A Motorola 2N2904A outside the ps had gone B-E ohmic of 77 ohms in the same
timespan of un-use. Would these faults have occured if regularly used , ie
the m/c would have needed repairing twice in 10 years ?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On Fri, 23 Oct 2009 12:48:20 +0100, "N_Cook"
wrote:

Why would a SMPS high speed Unitrode rectifier stud diode 1N5812 , 20 amp,
fail sitting around in a normal human occupation heated environment for 10
years unused?

I'm not an expert on packaging, but it's possible that package leakage
allowed moisture to enter the device. Moisture then rots the aluminum
(or copper?) wire bonds and possibly the surface layers. If you're
curious, a helium leakage test will show if there are any leaks.

Currently reads a stable 6.2 ohms.

Currently *what* is reading 6.2 ohms? Forward resistance? What does
a good 1N5812 read? What's the reverse resistance? Got a curve
tracer?
http://en.wikipedia.org/wiki/Semiconductor_curve_tracer

Replaced with a new one but I intend
passing ever increasing current through the old one to see what happens.

I don't have any info on what's inside the diode, but if it's a bunch
of diodes in parallel, and some of them have opened thanks to internal
corrosion, my guess(tm) is that you'll see something resembling a
good, but much less than 20A rated rectifier.

Any other ideas concerning cause or analysis ?

Yeah, try non-destructive testing before you destroy it. Otherwise,
break it open and look inside before you melt the guts into a blob of
silicon.

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

Jeff Liebermann wrote in message
...
On Fri, 23 Oct 2009 12:48:20 +0100, "N_Cook"
wrote:

Why would a SMPS high speed Unitrode rectifier stud diode 1N5812 , 20
amp,
fail sitting around in a normal human occupation heated environment for
10
years unused?

I'm not an expert on packaging, but it's possible that package leakage
allowed moisture to enter the device. Moisture then rots the aluminum
(or copper?) wire bonds and possibly the surface layers. If you're
curious, a helium leakage test will show if there are any leaks.

Currently reads a stable 6.2 ohms.

Currently *what* is reading 6.2 ohms? Forward resistance? What does
a good 1N5812 read? What's the reverse resistance? Got a curve
tracer?
http://en.wikipedia.org/wiki/Semiconductor_curve_tracer

Replaced with a new one but I intend
passing ever increasing current through the old one to see what happens.

I don't have any info on what's inside the diode, but if it's a bunch
of diodes in parallel, and some of them have opened thanks to internal
corrosion, my guess(tm) is that you'll see something resembling a
good, but much less than 20A rated rectifier.

Any other ideas concerning cause or analysis ?

Yeah, try non-destructive testing before you destroy it. Otherwise,
break it open and look inside before you melt the guts into a blob of
silicon.

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

DVM resistance either way round is 6.2
Ant chance of Beryllium oxide or whatever the serious nasty is , likely to
be inside?
Which plane to go in with a dremmel cutting disc?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On Sat, 24 Oct 2009 07:58:34 +0100, "N_Cook"
wrote:

DVM resistance either way round is 6.2

It's blown. No way was that caused by package leakage or corrosion
which would result in an open circuit. What you have is an amorphous
blob of silicon. It's also a single junction, so my theory about
parallel diodes is all wrong. Sorry(tm).

Ant chance of Beryllium oxide or whatever the serious nasty is , likely to
be inside?

Berylium oxide is white and is used as an insulator. It looks like a
hard ceramic. I looked at the data sheet but no insulator material
was specified. It's a DO-4 case, which methinks is just molded
plastic. If it's white ceramic, don't grind.

Which plane to go in with a dremmel cutting disc?

Circumscribe through the plastic near the metal base. It's a "cup"
which should come off easily. You may need to cut the solder eye off
the anode lead to remove the "cup". Sometimes, the guts is filled
with silicon grease.

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

Jeff Liebermann wrote in message
...
On Sat, 24 Oct 2009 07:58:34 +0100, "N_Cook"
wrote:

DVM resistance either way round is 6.2

It's blown. No way was that caused by package leakage or corrosion
which would result in an open circuit. What you have is an amorphous
blob of silicon. It's also a single junction, so my theory about
parallel diodes is all wrong. Sorry(tm).

Ant chance of Beryllium oxide or whatever the serious nasty is , likely
to
be inside?

Berylium oxide is white and is used as an insulator. It looks like a
hard ceramic. I looked at the data sheet but no insulator material
was specified. It's a DO-4 case, which methinks is just molded
plastic. If it's white ceramic, don't grind.

Which plane to go in with a dremmel cutting disc?

Circumscribe through the plastic near the metal base. It's a "cup"
which should come off easily. You may need to cut the solder eye off
the anode lead to remove the "cup". Sometimes, the guts is filled
with silicon grease.

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

I've never done that before. Dremmelled around the top of the can exposing
plug of ceramic or glass which easily came away. As did the central copper
pin , presumably the joint to the die failed with the Dremmel vibration.
The casing is presumably steel , but no corrossion on the outsde. But on the
inside of the can is about 90 percent covered in brown deposit, presumably
rust. No grease or anything like that inside. Before dremmelling, the
reading had changed to 10 ohm and it still reads 10 ohm between stud and
weld blob on the die. I wil dig out my microscope but I doubt I'd see any
sign of crept silver, a few atoms thick, especially as it is presumably on
the edge of the die. I'd have to grind off the rest of the cap to view the
side of the die.
Certainly no obvious burn marks on the shiny sliver and no smoke/sputter
trails. As this failed in storage, or theoretically at the point of last
switch off , then no such damage expected.
Is ferric/ferrous oxide conductive? if some off that could magnetically
shift and migrate to the die.

Any other suggestions what/how to explore further ? eg fine needle point and
DVM going over the die surface plotting ohmage


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

I've now removed the remaining cap cylinder and unravelled it and the patchy
rust film, or whatever, stops abruptly at where the glass seal was.

Much more interestingly viewing the die bond area to the base with a 30x
microscope. This appears to be a gold metalisation that has been eaten by
snails , if you've ever seen paper that has been in a leaking shed and so
grazed. Roughly linear tracks and then blobs of missing gold that seem to
have shamfered or grazed edges rather than the plating lifting and then
leaving sharp edges. One side of the die seems to have some of this gold as
a sort of powder in a few lines like iron filings over a magnet, rather than
solid metallic tin whisker needles, collecting there. Something seems to
have etched away the gold into a dust and then electrostatically or
magnetically drawn it to the die edge. It still measures 10 ohm so not a
will'o'whisp, frail accumulation . Perhaps I will take some microphotos
before probing the accretion with a needle. These grazing marks are roughly
midway between die bond edge and the edge of the support pillar.

So not an atomic level process of metal migration , so what is the process
that went on here presumably mainly when unpowered and very slow .


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On Oct 24, 4:57*am, Jeff Liebermann wrote:
On Sat, 24 Oct 2009 07:58:34 +0100, "N_Cook"

Ant chance of Beryllium oxide or whatever the serious nasty is , likely to
be inside?

Berylium oxide is white and is used as an insulator. *It looks like a
hard ceramic. *I looked at the data sheet but no insulator material
was specified.

Beryllium oxide is rare, would usually be tinted (I've seen pink and
maybe purple) as a warning.

As others have noted, the likelihood is that the die has melted or
heated enough to rediffuse into a blob without junction or layers.
A 20A rectifier would usually get lots of heating after an initial
fault, and the result is always a blob; nothing to learn there.

"N_Cook" wrote in message
...
Why would a SMPS high speed Unitrode rectifier stud diode 1N5812 , 20 amp,
fail sitting around in a normal human occupation heated environment for 10
years unused?
Currently reads a stable 6.2 ohms. Replaced with a new one but I intend
passing ever increasing current through the old one to see what happens.
Any other ideas concerning cause or analysis ?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/



I suspect tarnish of interior metals, especially if they contain any silver.
I see this sort of thing in Euro capacitors and metal connectors,
fuseholders, and the like. I also have to scrape it off the leads of old
semi's that have been in a parts drawer for years.

Mark Z.

Mark Zacharias wrote in message
...
"N_Cook" wrote in message
...
Why would a SMPS high speed Unitrode rectifier stud diode 1N5812 , 20
amp,
fail sitting around in a normal human occupation heated environment for
10
years unused?
Currently reads a stable 6.2 ohms. Replaced with a new one but I intend
passing ever increasing current through the old one to see what happens.
Any other ideas concerning cause or analysis ?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/



I suspect tarnish of interior metals, especially if they contain any
silver.
I see this sort of thing in Euro capacitors and metal connectors,
fuseholders, and the like. I also have to scrape it off the leads of old
semi's that have been in a parts drawer for years.

Mark Z.


Most oxides, sulphides, carbonates of copper silver lead etc seem to be
insulators except silver oxide. I 've not found AgO conductivity , could it
give 6 ohms over the geometries in a stud diode?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/