Is this a protective coating on PCB ?!

Hi,

Currently i'm fixing a device which seems to have problems because of moisture !
The device is Elster EK220 gas volume corrector which was installed in a humid weather area.
This type of problem is seen many times in installed EK220 devices.

The problem is:
It seems one or more of the arrow keys on the device are locked to pressed state. so the device display doesn't get turned off and the device gets malfunctions.
I tested some points on the device, the problem is not mechanical (keys are not locked in pressed state mechanically) but 3 of 4 input pins of the MCU which reads arrow input keys used a high value 470K pull up resistor and a 10nF bypass cap to GND. when i measured voltage on these pins, i saw some pins are in 0 state (when key is press input is pushed to GND) ... The device is ultra low power, the whole circuit consumes micro amperes during functioning. a small leakage current can lead to malfunction. i guess the problem is because of moisture which leads to current leakage so the high value pull up resistor can't compensate it. this leads to reading 0 state in the input pin and MCU senses as key is pressed.
When i heated the pull up resistor and the bypass cap by soldering iron, the problem solved but not completely stable.
It seems there is a coating on the PCB, you can see it in the picture, inside the left orange area (no1) in the pic, the coating is visible. in the right orange area (no2), i have cleaned it by a dry cleaner spray and seems the coating is almost removed.
The device has worked about 5 years, now i just don't know if it is a protective coating ? or created by moisture and dirties ...
I have read in an application note (maybe TI) even if you use no clean solder for ultra low power boards, you must clean the whole board after soldering ...



1- Do you think it is a protective coating on PCB ?
2- Do you recommend to use Ultrasonic cleaner for the whole board ?

3- I don't know why elster used pull up resistors on 3 of 4 input pins only ! the other pin has 10nF bypass cap to GND only. any Idea ? (as i checked the pin is not connected to any other component in the board ... in fact i guess it's a mistake by them !

2- Do you recommend to use Ultrasonic cleaner for the whole board ?

Click to expand...

I worked a while ago in a repair center where we often received mainboards of equipment subjected in the field to an environment heavily impregnated with saturated particles in the air, more precisely, sugar diluted in the ambient humidity within a filler room of a bottling machinery. Some boards came with unequivocal evidence of the presence of the dirties, while others did not, but the defect pattern strongly suggested this possibility. Then, as standard procedure, we did ultrasonic cleaning with isopropyl alcohol in all of them, irespective of being visibly dirty or not; but we took care to remove components that were not water resistant desoldering prior to submerge, and in a last finishing, we applied a lacquer for extra protection (covering contact surfaces of connectors, to avoid make them become electrically isolated), which gave excellent results. What I mean, in summary, is that depending on the environment in which the equipment was used and the time exposed to these conditions, a revision like this is quite recommended, but you must be very careful in this process, as mentioned above. Anyway, as a simplest solution (or attempt), aplication of dry air jet may clean a lot, but should also be careful to not rip off components of the board.