Repairing a Rusted Chip

[Bashstreet]

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.

Tin do not rust 

[stj]

yes it does.
you must be pretty young to have never seen a rusty tin can before they switched to aluminium.
even so, you should have seen rusted pins on leadfree soldering that was dumped outdoors.

[helius]

Tin is an expensive metal. When canning was originally developed in the early 1800s, steel was rolled with tin to make tinplate. The cans were not ever made of solid tin—that is absurd.

If you really believe that cans were made from solid tin, you should go out and buy all the razor blades you can, since they say they are platinum.

[T3sl4co1l]

Tin corrodes as well as any other metal, though it's not usually called "rust".  Tin is rarely used pure, almost always as a plating.   Copper has an oxide that may be confused with rust.

Tim

[james_s]

As far as I know, "rust" refers only to iron oxide, but oxygen is a very reactive element and most metals will oxidize readily. The biggest difference is in the properties of the oxide. Iron oxide crumbles and falls off exposing fresh iron which then oxidizes and this is what happens when something rusts away. Aluminum oxidizes more readily than iron but aluminum oxide sticks quite firmly to the surface forming a barrier that protects the metal below. I don't know as much about the properties of tin oxides but I know they will form.

[Bashstreet]

yes it does.
you must be pretty young to have never seen a rusty tin can before they switched to aluminium.
even so, you should have seen rusted pins on leadfree soldering that was dumped outdoors.

No Tin do not Rust

The cans are not pure tin it would be too expensive and pure tin is too soft.
They are just tin plated soon as tin gets knocked, way to the iron/steel is exposed and they rust out 

[stj]

so why does lead-free soldering rust???

[james_s]

If it rusts then it must have iron in it. I don't recall ever seeing solder rust but I suppose there could be an alloy with iron, or maybe the flux resembles rust?

[Bashstreet]

so why does lead-free soldering rust???

No idea ? why would anyone add iron on solder.
I have never witnessed such solder nor seen it sold 

[KL27x]

I would guess the iron is in the chips. And that additionally, these boards were soldered with a water soluble flux containing ammonium/zinc chloride. It's probably the steel in the lead frame producing the rust, and the epoxy case of the IC was not adequate to seal it from chloride ions and air getting in there. So tinning can't cover the steel and stop the rust, because it is leaking out from where the pins come out of the epoxy. It's maybe older encasing tech or some failure in processing that damaged the epoxy.

If OP could not buy new chips, I would remove them all, clean the flux residue thoroughly, and dunk them in any acid but HCl to eat away any rust. Rinse in water and dry with alcohol. Dunno how many chips that is. It's certainly a lot of work. But mechanically removing the visible rust probably can't stop the source of the problem. This probably can't either, but it theoretically has a chance... of slowing future problem. If the chips are already acting funny, it's probably too late. It's not just presence of rust you can see, but there is maybe rust in the chip pushing on and breaking the wire bonds.

If you can buy new chips I bet you have no problem in the future, provided you are using halide-free flux. I would wash the FPC thoroughly in hot water and a brush before putting on the new chips. I imagine they would be a horror show on the inside, already, by the time rust is pushed out of the chip.

* scratch all that.
If you have device(s) that still work, I'd clean what I can see with a fiberglass brush, then coat thoroughly with some suitable lacquer/paint to slow any more oxygen from getting into the chip.

I have some PCB I soldered with plumber's flux. Maybe this will happen to them in 30 years. 

[PKTKS]

Nobody said the most plain solution which BTW I have done
a lot with decent results.

I just clean what is possible (the  just metallic dust part) with ISOPROPANOL
As the chip may be very fragile a soft brush in a strong solution should do.

Then if not wasted completely I would cover the parts with SOLDER MASK (UV curable)
with some thin tip a portable UV.

The really wasted parts I would consider a JUMPER  -  really not that hard w/today
tools and optical assist. And BTW I would use LEADED solder.

LEAD-FREE profile is just too complicated to bond that crappy left overs.

It usually works cheap and easy
Paul

[Bashstreet]

Nobody said the most plain solution which BTW I have done
a lot with decent results.

I just clean what is possible (the  just metallic dust part) with ISOPROPANOL
As the chip may be very fragile a soft brush in a strong solution should do.

Has been recommended by many.

Then if not wasted completely I would cover the parts with SOLDER MASK (UV curable)
with some thin tip a portable UV.

Unless the rust is neutralized it will continue corroding under the solder mask as it has bonded with sufficient oxygen.

The really wasted parts I would consider a JUMPER  -  really not that hard w/today
tools and optical assist. And BTW I would use LEADED solder.

LEAD-FREE profile is just too complicated to bond that crappy left overs.

Using leaded solder to rework lead free boards is wrong way to go.
Use lead free solder to repair lead free boards 

Cheers.

[helius]

A device made in 1986 is very unlikely to use lead-free anything.
But in general, unless there is a very good reason, rework should use leaded solder no matter what the factory used. It makes better, more consistent joints, removes the risk of tin whiskers, and avoids unnecessary thermal stress to the board and components.

[VK5RC]

Have those leads corroded right through? - looking at the photo - it appears as though something is joining the pins - perhaps rust but other oxides possible - do you have access to a fibreglass pencil brush? If you do, wear gloves, sealing safety goggles and work outdoors - little bits of fibreglass in you fingers take weeks to come out (don't ask) - can't imagine what would happen if you got some in your eye.

[PKTKS]

Once properly covered the amount of corrosion will be very low.

If you manage to clean enough a still usable part the protection cover will last
last enough to not worry for a long period.

The tip on LEAD-FREE is just to make that sure that by some convenient
you pick the wrong solder.  LEAD-FREE today is as common as LEADED

IMHO  for hand soldering BENCH REPAIR - LEADED just works better
LEAD-FREE or paste are very specifics.

In that case I would do all in LEADED with a very very minimum temp
to avoid stressing the corroded parts. They will get worst (worse?!) with the heat...

Paul

[Bashstreet]

A device made in 1986 is very unlikely to use lead-free anything.
But in general, unless there is a very good reason, rework should use leaded solder no matter what the factory used. It makes better, more consistent joints, removes the risk of tin whiskers, and avoids unnecessary thermal stress to the board and components.

It is better repair lead free boards with lead free solder as it is not good idea to mix alloys and different melting temperatures can cause bonding issues.

You can repair lead free boards with leaded but as guide it is better use same alloys.

Modern lead free solders are very good and there is not need to continue using leaded solders except to repair very old equipment or in special circumstances where leaded solder will be required.

Some people seem be very fond of their leaded solders that's fine but times move on and no point get too nostalgic about solder

[james_s]

I haven't found a lead-free solder that works as well as the standard tin-lead stuff. With good old 60:40 rosin core I get nice clean shiny joints every time.

[KL27x]

There are several very different lead-free alloys. Without knowing what alloy and what flux has been used on a board, do u still rather use any random lead free? Im asking, not judging. I am jot voting, here, but I have been reworking lead free boards with leaded, and I'm curious, now.

Through the years, several new products have hit the market. Is there even a standard lead free alloy, today? With leaded you are talking a few percent here, maybe a dash of silver. pretty much it.

[PKTKS]

I haven't found a lead-free solder that works as well as the standard tin-lead stuff. With good old 60:40 rosin core I get nice clean shiny joints every time.

Neither do I.

For repairing rusted or oxidized parts LEAD-FREE just can't do it

A good tin bond requires plain LEADED and 99% cases I use a flux
proper to OXIDIZED MATERIAL - not the regular "normal" flux.

The thing i use has several drawbacks:
- contains AMONIA - very crappy smoke fume
- it is HIGHLY CONDUCTIVE and if not properly cleaned it will left conductive residues.

But it works like nothing more. It justifies the drawbacks.
Once applied it will  VANISH all corroded material and BOND the LEAD SOLDER almost
perfectly in minimum time. It works but there are drawbacks using the stuff....
Normally this flux is used in HIGH THERMAL mass chassis and parts. Not micro-soldering

But i haven't found anything that works near close that ...
for rusted oxidized parts. No matter how bad the rust
the crappy flux vanish and bond the tin like magic.