Author Topic: Repairing a Rusted Chip  (Read 3428 times)

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Offline tpw_rules

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Repairing a Rusted Chip
« on: August 14, 2018, 07:14:42 pm »
I've got this vintage laptop (Toshiba T3100) with a Matsushita gas-plasma display MD400F640PD2. Unfortunately, there must have been some screwup at the factory and now most of them are rusting. Specifically, the plasma driver ICs get rusty pins which stop conducting, causing lines on the screen.

Given that replacing the screen or the chips does not seem to be especially feasible, is there any way to repair the ones I've already got? Fortunately, the rust is on the chips and not really on the flex board itself. I've considered dremeling the sides with the legs to remove them and expose fresh leadframe, but I'm not sure where to go from there. Pictures of some of the chips are attached.
 

Offline helius

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Re: Repairing a Rusted Chip
« Reply #1 on: August 14, 2018, 07:19:54 pm »
Neither the packages nor the FPC should be ferrous in nature. What is the liquid in the picture? Are there electrolytic caps leaking from up above? Is anything in the assembly glued?
 

Offline tpw_rules

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Re: Repairing a Rusted Chip
« Reply #2 on: August 14, 2018, 07:41:22 pm »
Neither the packages nor the FPC should be ferrous in nature.
Fair enough. But it's been corroded to the point that the lead metal has been eaten away in several areas.

What is the liquid in the picture? Are there electrolytic caps leaking from up above? Is anything in the assembly glued?

I'm not sure regarding the shininess. It's not liquid, it's some kind of hard conformal coating or lacquer. There are no leaking electrolytics. There is glue in the assembly, but it's to keep the board folded over. You can see the black foam remains of the glue pads on the left edge.

Whatever happened to mine, it happens to all of them. I've even seen threads on this very forum about their screens getting lines, from what I assume is the same problem. All the ones on ebay have lines as well. Everything else in the machine is rust-free. Rest assured I didn't dig this out of a rainforest and wonder why there was rust; I'm convinced there was some kind of incorrect process during manufacture (maybe corroding flux) or design (the entire module sits at -200V) that has doomed all of these screens.
 

Offline JFJ

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Re: Repairing a Rusted Chip
« Reply #3 on: August 14, 2018, 10:24:31 pm »
Neither the packages nor the FPC should be ferrous in nature...

Copper IC leads are often iron plated - prior to receiving their outer plating of nickel (or other non-ferrous finish). That is why many IC's can be picked-up with a magnet:

 

Offline Paul Moir

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Re: Repairing a Rusted Chip
« Reply #4 on: August 14, 2018, 10:30:30 pm »
I'm sure you'd be able to grind into the plastic a bit to expose a bit of lead, then solder on a wirewrap wire.  I used to do that all the time as a kid when I accidentally broke leads off.
 

Online james_s

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Re: Repairing a Rusted Chip
« Reply #5 on: August 15, 2018, 12:07:29 am »
I've used some stuff called Evaporust to clean various things with success. Never tried it on an IC though.
 

Offline helius

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Re: Repairing a Rusted Chip
« Reply #6 on: August 15, 2018, 12:32:26 am »
Whatever happened to mine, it happens to all of them. I've even seen threads on this very forum about their screens getting lines, from what I assume is the same problem. All the ones on ebay have lines as well. Everything else in the machine is rust-free. Rest assured I didn't dig this out of a rainforest and wonder why there was rust; I'm convinced there was some kind of incorrect process during manufacture (maybe corroding flux) or design (the entire module sits at -200V) that has doomed all of these screens.
I wonder if there is an electrolytic process at work between adjacent pins? It looks like the conformal coating either failed or was inconsistently applied. It may have been required to prevent electric creepage at such high voltages.
Another issue could be electromigration inside the flex PCB, causing CAFs that subsequently corrode the pins—I can't see under the chips.
Either way it looks like a challenging repair.
 

Offline coromonadalix

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Re: Repairing a Rusted Chip
« Reply #7 on: August 15, 2018, 12:39:23 am »
I would try to slip a fine needle to remove anything between the ic legs / pins, try to very gently brush them with an horse hair tooth brush, just to remove rust excess, and try an power up, if its fine, i would coat them with an colloid clear to cut the oxygen ...

my 2 cents
 

Offline T3sl4co1l

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Re: Repairing a Rusted Chip
« Reply #8 on: August 15, 2018, 12:42:01 am »
More likely the lead frame is steel (or invar, especially in the case of ceramic packages?), then copper and/or tin plated.  Dunno how common that is versus solid copper.

Copper can also corrode with a brown oxide, being Cu2O (cuprous oxide, the mineral cuprite).

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Offline Rasz

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Re: Repairing a Rusted Chip
« Reply #9 on: August 15, 2018, 03:04:39 am »
ultrasonic cleaner would be able to shake most of that crap off, except its all connected to the glass in your case?

I would leave mechanical solutions (scraping with needles etc) for last, those are prone to irreversible damage.
Removing those ICs from flex wouldnt hurt either, but that would depend on your hotair skills.

Maybe start with vinegar on a cotton bud, it will eat al the rust, but monitor the process and remember to clean it off :)
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Offline kizmit99

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Re: Repairing a Rusted Chip
« Reply #10 on: August 15, 2018, 03:32:01 pm »
I've used some stuff called Evaporust to clean various things with success. Never tried it on an IC though.

Evaporust is great for removing only the rust and not affecting the base metal.  I've never tried it on any electronics, but have used it many times on machined (steel) parts and other than removing the pre-existing rust it did no further damage to any of the parts (doesn't etch the surface or anything like that).  I've also never seen it damage plastic, so I would expect it might be worth a try on these (even without removing the chips from the flex-board).
 

Online Gyro

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Re: Repairing a Rusted Chip
« Reply #11 on: August 15, 2018, 04:11:36 pm »
Remember that plastic packages are not hermetic, anything you put on the leads is going to find its way to the [Edit: probably Aluminium] bond wires and the Silicon at some point. I would go carefully with anything chemical. Stick to mild things and remove any moisture afterwards with IPA. Mechanically removing as much contaminated 'crust', dry, as you can is probably going to be less contaminating than letting it get back into solution.
« Last Edit: August 15, 2018, 04:16:04 pm by Gyro »
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Online james_s

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Re: Repairing a Rusted Chip
« Reply #12 on: August 15, 2018, 04:20:53 pm »
That's an interesting point, has anyone done testing to see if an IC is affected by long term liquid immersion? I've certainly washed plenty of boards, and I've seen old nasty crusty ICs that still worked. I've also seen plenty of bad ICs though unfortunately it's rarely possible (with my resources) to determine the cause of failure.
 

Offline Bashstreet

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Re: Repairing a Rusted Chip
« Reply #13 on: August 15, 2018, 06:13:21 pm »
It is best first just clean everything with alcohol and toothbrush to see the real damage under the surface rust.
It might be many pins will be beyond repair and you might as well stop there.

If there is hope and if you can bodge pins that have gone you need neutralize further damage from happening.
Vinegar is one option but it is corrosive and not really useful in this case as it can do damage.
Better use rust converters that convert the rust and prevent further damage (If legs are ferrous)
You can get rust converters from most hardware shops.


 

Online Gyro

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Re: Repairing a Rusted Chip
« Reply #14 on: August 15, 2018, 06:18:53 pm »
That's an interesting point, has anyone done testing to see if an IC is affected by long term liquid immersion? I've certainly washed plenty of boards, and I've seen old nasty crusty ICs that still worked. I've also seen plenty of bad ICs though unfortunately it's rarely possible (with my resources) to determine the cause of failure.

I'm not sure about long term liquid immersion, but there have certainly been lots of plastic package humidity reliability studies, eg. http://web.cecs.pdx.edu/~cgshirl/Glenns%20Publications/ASQ%20Webinar%20Package%20Reliability%20CGS%20Rev%203.pdf


P.S. Most manufacturer sites have some information, but this mostly relates to package cracking due to boiling of absorbed moisture in non-dessicated parts during reflow.
« Last Edit: August 15, 2018, 06:23:56 pm by Gyro »
Chris

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Offline helius

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Re: Repairing a Rusted Chip
« Reply #15 on: August 15, 2018, 06:44:31 pm »
Inside the package are the lead frame, the gold bond wires, and the chip itself. Gold is pretty hard to damage. The chip is passivated with what is essentially glass and is also quite resistant to all but the most caustic chemicals. The pads to which the wires are bonded are not passivated: they are made of aluminum or aluminum oxide.
The most susceptible thing inside the package is the lead frame, which as mentioned earlier may be ferrous. The references I was able to find state that the lead frame is either a copper stamping, or a photoetched coupon of Alloy 42 (58% iron, 42% nickel).
« Last Edit: August 15, 2018, 06:48:15 pm by helius »
 

Online james_s

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Re: Repairing a Rusted Chip
« Reply #16 on: August 15, 2018, 07:49:27 pm »
I recently came across a handful of very old ICs, as in late 70s which had been stored in ESD safe foam. Unfortunately the foam had broken down and seemed to have attacked the leads, they were covered in black oxide and rust. I tried brushing off the bits of rotted foam and several of the pins fell off too. Not being super rare valuable parts I threw them away, whatever the pins were made of certainly seemed to be ferrous.
 

Offline coromonadalix

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Re: Repairing a Rusted Chip
« Reply #17 on: August 15, 2018, 08:34:51 pm »
I would try to clean the leads on place without trying to de-solder them, you may add thermal stress in them, they may break ... as i wrote earlier, if it still work, add an sealing rosin with high dielectric isolation on top of them ..

Trying to remove them with hot air may work, but the flex pcb maybe damaged in the process ??? add kapton tape as a shield ?
 

Online james_s

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Re: Repairing a Rusted Chip
« Reply #18 on: August 15, 2018, 09:12:59 pm »
I agree, I would also avoid desoldering them unless absolutely necessary. I think I would try Evaporust first, maybe lay the board chips-down in a glass cake pan and pour in just enough Evaporust to immerse the PCB. It shouldn't hurt the plasma panel to get wet but no reason to tempt fate.
 

Offline amyk

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Re: Repairing a Rusted Chip
« Reply #19 on: August 16, 2018, 02:15:18 am »
What is the marking on those chips?

Maybe there are replacements or equivalents available.
 

Offline helius

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Re: Repairing a Rusted Chip
« Reply #20 on: August 16, 2018, 02:35:00 am »
You can find them for sale by chipmongers with minimum quantities of 40. Group buy opportunity, perhaps.
 
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Offline JacobPilsen

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Re: Repairing a Rusted Chip
« Reply #21 on: August 16, 2018, 04:44:26 pm »
but the flex pcb maybe damaged in the process ??? add kapton tape as a shield ?
Flex PCB is made of Kapton.
 

Offline stj

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Re: Repairing a Rusted Chip
« Reply #22 on: August 16, 2018, 05:59:14 pm »
what a mess, that's what happens when pcb's are left covered in flux.

the tin-plating / tin in the solder has rusted because moisture has become trapped in the flux.
 

Offline metrologist

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Re: Repairing a Rusted Chip
« Reply #23 on: August 16, 2018, 06:21:43 pm »
I need to see cleaned pins, immediately.  :scared:
 

Offline coromonadalix

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Re: Repairing a Rusted Chip
« Reply #24 on: August 16, 2018, 09:45:37 pm »
loll   we do use very strong flux at my job, we have at least 24-36 hours max to remove it, afterwards it will attack the parts and the pcb plating, because it's an eco friendly flux we can flush the drain  lolll
 


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