Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old June 28th 20, 11:15 AM posted to sci.electronics.repair
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Default Bad chip resistors?



Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


..... Phil

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Old June 28th 20, 02:11 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On Sun, 28 Jun 2020 03:15:16 -0700 (PDT), Phil Allison
wrote:



Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


.... Phil


As formation of sulphides is a surface phenomenon, I doubt
it plays a role in properly reflow-soldered junctions.

Mechanical stress can.

Needle-pointed probes are quicker, though possibly painfull
when left lying around. Detachable ones make sense, if they
don't get lost.

RL
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Old June 28th 20, 06:07 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On 06/28/2020 05:15 AM, Phil Allison wrote:


Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


.... Phil


I spent some time during my career as an electronics tech in the
petrochem industry. Lots of corrosive substances and vapors that will
kill electronics. It was common practice that after a board was
assembled and tested, it was bathed in a clear epoxy coat to completely
seal it.

-John
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Old June 28th 20, 08:53 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On Sunday, June 28, 2020 at 1:07:15 PM UTC-4, John Crane wrote:
On 06/28/2020 05:15 AM, Phil Allison wrote:


Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


.... Phil


I spent some time during my career as an electronics tech in the
petrochem industry. Lots of corrosive substances and vapors that will
kill electronics. It was common practice that after a board was
assembled and tested, it was bathed in a clear epoxy coat to completely
seal it.

-John


It's called a conformal coating and they are a number of different types depending on the environmental conditions one is trying to safeguard the electronics with.
I think Phil's question is more along the lines of the failure modes of the SMD resistors or interaction with the board material.
I have not heard of the failure mode Phil has described. IIRC, the sulphiding will form an insulation layer, so poking with needle probes is necessary to get good readings. I have not read anything about how the sulphiding changes the resistor characteristics. Sorry.
The only thing I am aware of, which is remotely related to this is the tin 'whisker' phenomenon which shorts out components. Caused by when tin (especially electroplated tin) is used as a final finish.

Could hot air reflowing the areas where the resistors are located help? or have the resistors themselves really undergone the change?
Sorry I could not be of more help
J
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Old June 28th 20, 09:19 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On 28/06/2020 8:15 pm, Phil Allison wrote:


Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


**If I have a weird fault, I always suspect high value (100k) or low
value (100 Ohms) resistors. Both sometimes fail for different reasons.
That said, mostly I find such faults confined to cracked carbon resistors.

--
Trevor Wilson
www.rageaudio.com.au

--
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Old June 28th 20, 09:23 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On 28/06/2020 8:15 pm, Phil Allison wrote:


Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


**Also: Get yourself a pair of these:

https://www.wagneronline.com.au/4mm-...-test/6762/fl/

Pierces insulation (and fingers!) easily.


--
Trevor Wilson
www.rageaudio.com.au

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

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Old June 28th 20, 09:40 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On Sunday, June 28, 2020 at 6:15:19 AM UTC-4, Phil Allison wrote:
Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


.... Phil


The pointer to the tool is interesting to me. It beats keeping an old style pencil eraser around, which tends to dry out over time...speaking of that.....
What is the material in this pen? fiberglass? and does it dry up or change hardness in any way?
Thanks
J
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Old June 29th 20, 12:08 AM posted to sci.electronics.repair
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Default Bad chip resistors?

On Sunday, June 28, 2020 at 5:15:19 AM UTC-5, Phil Allison wrote:
Hi,

recently came across an SMD board with lots of chip resistors (0805) and similar size ceramic caps. There was mild surface corrosion on the ends of each resistor - enough to prevent a DMM from reading the value until cleaned with a "fibre pen". For the puzzled, it's an essential servicing tool.

https://www.ebay.com.au/itm/Fibregla...-/310094191492

The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.

Any insights?


.... Phil


Not unusual for high value resistors to fail open. This was also true for leaded resistors. The same environmental conditions that cause the surface corrosion contribute to the resistor failures.
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Old June 29th 20, 03:04 AM posted to sci.electronics.repair
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Default Bad chip resistors?


Terry Schwartz wrote:
====================


Not unusual for high value resistors to fail open.


** Well they don't fail short very often - but high rates of such in new looking equipment is very unusual.


This was also true for leaded resistors.


** Only those with lots of DC across them, not the case here.


The same environmental conditions that cause the surface corrosion
contribute to the resistor failures.


** Yeah - I did figure that.

But chip resistors are unusually vulnerable cos of exposed silver.

...... Phil





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