Metalisation creep?

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

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 *what* is reading 6.2 ohms? Forward resistance? What does a good 1N5812 read? What's the reverse resistance? Got a curve tracer?

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.

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               jeffl@cruzio.com
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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.

amp,

10

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

amp,

10

silver.

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

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).

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.

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     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

to

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

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

Gold devouring nanobot snails beamed in from another dimension?

So it was eaten by snails?

Some pics of all this, 1mm graph paper background

The sides of the cap unfurled, A-A is the distinct cutoff line of the rusted part, the roughness at the ends is my butchery, also clean originally

The green glass filled seal around the anode pin

Its unaffected partner, as far as not being ohmic anyway

View of the gold metalisation over the base metal stub, Os are below some of the largest areas of erosion. I don't know where the missing gold went to , no flakes or obvious dust lying around in the diode when first broken into. The weld for the anode pin is off-centre as can be seen in the pic, is that relevant ?

Edgewise view of the die , showing gold metalisation up to the die but much is eroded away , eg base metal showing between C-C and the most obvious accretion bridging the edge of the die marked with the Vs

looking down on the large gold metalised areas with Bs showing the most obvious bits of base metal, the bright line is the die edge

the wavy gold edge ,near it, is probably metalisation over some underlying pad so presumably at manufacture rather than a later formation

These diodes were stored anode upwards in kit , untouched for 13 years, so something made the dust aggregate and migrate vertically , against gravity

it is now reading 13 ohms

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

The first image of the can unfurled looks like the seal of the can allowed moisture to enter and cause rusting of the interior/inside surface of the can.

If not moisture infiltration, then I would suspect that the metal (or plating/coating material) was incompatible with other materials inside the can.

Another possibility could be that the metal used for the can was specified to be extremely reliable, but the metal alloy/finish was flawed or otherwise contaminated before assembly of the diode.

The design construction technique is flawed IMO if the can isn't more secure than just a pressed-together fit. It might be interesting to see if the military/areospace version of these devices were fabricated by the same method.

Rust is conductive, and I would suspect that the diode failure was going to happen eventually.

So much for long term reliability of heavy duty semiconductors.

-- Cheers, WB .............

I forgot to say these were made in 1977 and the image of the whole die on the hex base is placed on an Al ring for the purpose of photoing

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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.

otherwise

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underlying

so

gravity

Another possible ingress route is via the anode pin. When I cut through it, it was in 2 parts and loose, a pin surrounded by a tube, both of copper and somehow combined somewhere near the solder eyelet end. Maybe cap is assembled after central pin welded to the die, then the cylinder placed over, then glass seal and both copper sections fixed across at the eyelet.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on