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