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smd precision resistor "paint layer" peeled off while soldering *** advice needed ***


Rogerio O
 

Dear all,
I was doing a A5 board repair of a Tek2465BCT I bought some time ago.
This is a late unit, serial # B0577746 and the A5 board is the 671-0965-05.
I inspected the board when I bought the unit and there were no signs of corrosion, so I left it as received for quite a long time.
It seems the caps have been replaced ( I can't say for sure) but the board was not cleaned well enough so the corrosion started.

Near the ADC (U2101) the only pads "missing" are the ones for C2520, but I had to remover and solder again some SMD components in this area.
While doing this the "paint layer" of R2012 (RES,FXD,FILM:10KOHM,0.1%,0.125W,TC=T9) fell off, exposing the tiny traces.

I could not find a direct replacement for it so I am thinking of leaving it there and apply a layer of some isolating material (PCB varnish, for example).

My question is: Is it possible the the varnish would react with the exposed traces, changing the component characteristics?

|Is it better leave the component as it is? Any other coating material suggested?

Thank you in advance,
Roger


Chuck Harris
 

As long as you cleaned the board of all of the corrosive (to copper)
residue, any solvent based paint would be fine. This is just a slow
computer, and as such it is all either dc or digital.

Also, look at the solder joints on the 4 capacitors. They should be
shiny as a mirror. If they are milky, or white, the capacitors have
not been replaced. If you touch one of the capacitor's solder joints
with the tip of a soldering iron, and you get the stench of boiled
radiator and rotten fish, there is electrolyte present.

-Chuck Harris

Rogerio O wrote:

Dear all,
I was doing a A5 board repair of a Tek2465BCT I bought some time ago.
This is a late unit, serial # B0577746 and the A5 board is the 671-0965-05.
I inspected the board when I bought the unit and there were no signs of corrosion, so I left it as received for quite a long time.
It seems the caps have been replaced ( I can't say for sure) but the board was not cleaned well enough so the corrosion started.

Near the ADC (U2101) the only pads "missing" are the ones for C2520, but I had to remover and solder again some SMD components in this area.
While doing this the "paint layer" of R2012 (RES,FXD,FILM:10KOHM,0.1%,0.125W,TC=T9) fell off, exposing the tiny traces.

I could not find a direct replacement for it so I am thinking of leaving it there and apply a layer of some isolating material (PCB varnish, for example).

My question is: Is it possible the the varnish would react with the exposed traces, changing the component characteristics?

|Is it better leave the component as it is? Any other coating material suggested?

Thank you in advance,
Roger







Rogerio O
 

Chuck,
Thank you for your reply.
Yes I could smell this while working on the components around the place where the are located.
Is it necessary to repair each and every solder joint that is not shinny?
I have washed these parts of the board with tap water and than with IPA.
I am letting the board dry while I wait for the replacement parts.
Roger


Chuck Harris
 

Hi Roger,

The electrolyte is water soluble. Use hot water, dish detergent
and a soft toothbrush or paintbrush.

Put some electrical tape on the tops of trimmers and dip switches,
and scrub away.

The shine is an indication of corrosion. It tells you that the
electrolyte has been there... so you should scrub there.

When the board is rinsed, get the loose water off, and put it in
a warm oven... usually the oven light is more than hot enough...
for overnight.

-Chuck Harris

Rogerio O wrote:

Chuck,
Thank you for your reply.
Yes I could smell this while working on the components around the place where the are located.
Is it necessary to repair each and every solder joint that is not shinny?
I have washed these parts of the board with tap water and than with IPA.
I am letting the board dry while I wait for the replacement parts.
Roger






Chuck Harris
 

Lost shine... aka milky white solder joints...

-Chuck Harris

Chuck Harris wrote:

Hi Roger,

The electrolyte is water soluble. Use hot water, dish detergent
and a soft toothbrush or paintbrush.

Put some electrical tape on the tops of trimmers and dip switches,
and scrub away.

The shine is an indication of corrosion. It tells you that the
electrolyte has been there... so you should scrub there.

When the board is rinsed, get the loose water off, and put it in
a warm oven... usually the oven light is more than hot enough...
for overnight.

-Chuck Harris

Rogerio O wrote:
Chuck,
Thank you for your reply.
Yes I could smell this while working on the components around the place where the are located.
Is it necessary to repair each and every solder joint that is not shinny?
I have washed these parts of the board with tap water and than with IPA.
I am letting the board dry while I wait for the replacement parts.
Roger










Richard in Edenton NC
 

Chuck, would 99% isopropyl alcohol do the same thing, but without having to put it in a oven overnight? Also I assume that the electrolyte (that leaked from the caps) being heated is the cause of the smell like rotten fish? Good to know. Thanks for sharing

73 Richard W4MCD


Chuck Harris
 

In short no, 99% IPA won't even touch the electrolyte,
except for that which gets removed by physical scrubbing.

The electrolyte is a water soluble mix of some sort of
glycol compound and a salt that doesn't corrode aluminum,
or aluminum oxide.... but does a job on copper. Water
and detergent is what you need to remove it.

If you don't want to dry the board over night in an oven,
blow it off with compressed air, and put it in a convection
oven at 50C for 2 hours. I use my environmental chamber.

As a drier, IPA has to be able to reach the nooks and crannies
that the water gets to, and it has to be able to move the
water through evaporation... mostly what happens is the
alcohol evaporates, and leaves the deep water behind.

You need to get the water out from under the IC's, and other
packages that almost touch the board, but not quite.

Watch the temperature! Cooking isn't exactly good for some
parts... especially the NVRAM.

And, yes, the electrolyte is a glycol based mix, and the
glycol gives the dead fish smell when it is burned with
your soldering iron.

-Chuck Harris

Richard in Edenton NC via groups.io wrote:

Chuck, would 99% isopropyl alcohol do the same thing, but without having to put it in a oven overnight? Also I assume that the electrolyte (that leaked from the caps) being heated is the cause of the smell like rotten fish? Good to know. Thanks for sharing

73 Richard W4MCD






Richard in Edenton NC
 

Thanks Chuck

73 Richard W4MCD


Ed Breya
 

If the over-coating on these SMDs has floated or burned off, then it's likely due to being undercut by the corrosive, leaked electrolyte. This stuff can travel considerable distance by capillary action on the surfaces. From your description of the resistors affected, they are fairly high precision, metal film types. The paint flaking off indicates the metal films have been compromised by the corrosion, so may be out of spec - they would tend to be higher in resistance due to missing some of the metal. Try to measure the values in-circuit, as accurately as possible, considering the associated circuitry.

If you get everything properly cleaned up and working again, you can measure the operating voltages in that section, and assess whether the resistors are still OK. If good, just leave them alone.

Ed


Chuck Harris
 

Oh, I missed that it was the resistor's paint. They are toast,
and must be replaced. I thought he was talking about the solder
mask paint.

-Chuck Harris

Ed Breya via groups.io wrote:

If the over-coating on these SMDs has floated or burned off, then it's likely due to being undercut by the corrosive, leaked electrolyte. This stuff can travel considerable distance by capillary action on the surfaces. From your description of the resistors affected, they are fairly high precision, metal film types. The paint flaking off indicates the metal films have been compromised by the corrosion, so may be out of spec - they would tend to be higher in resistance due to missing some of the metal. Try to measure the values in-circuit, as accurately as possible, considering the associated circuitry.

If you get everything properly cleaned up and working again, you can measure the operating voltages in that section, and assess whether the resistors are still OK. If good, just leave them alone.

Ed






Rogerio O
 

Thank you all for the replies.
I am not sure of what to use as a replacement part.
I have ordered the KOA RN732ETTD1002B25 part from Mouser, which was the closest to the spec I could find.
Is it adequate? Any other better replacement?
Do I have to replace both R2012 and R2013 or just the one that has been compromised?
Roger


Chuck Harris
 

I would make both resistors the same... besides, you can't buy
just one SMD resistor.

-Chuck Harris

Rogerio O wrote:

Thank you all for the replies.
I am not sure of what to use as a replacement part.
I have ordered the KOA RN732ETTD1002B25 part from Mouser, which was the closest to the spec I could find.
Is it adequate? Any other better replacement?
Do I have to replace both R2012 and R2013 or just the one that has been compromised?
Roger






Rogerio O
 

I forgot to ask what is the temperature coefficient of these resistors?
In the parts list it is listed as TC=T9.
Thank you


Chuck Harris
 

The 2465 family lists them as follows:

2465: 10K film, 0.5%, 1/8W
2465A: 10K, 0.5%, 0.2W, TC=T2, MFR PNO=CR8 20 DYE 10K0
2465B: 10K, 0.5%, 0.2W, TC=T2, MFR PNO=CCF501D10001D, Dale Electronics.

Dale shows:
T-2 = +/-50ppm/K
T-9 = +/-25ppm/K
T-10 = +/-15ppm/K
T-13 = +/-10ppm/K

Since the TCR is uncontrolled as to direction, they are going for as
close to zero TC as they could afford, and the circuit could appreciate.

-Chuck Harris

Rogerio O wrote:

I forgot to ask what is the temperature coefficient of these resistors?
In the parts list it is listed as TC=T9.
Thank you






Rogerio O
 

Thank you Chuck.
The parts have just arrived and I will post the results after replacing the damaged parts