hm_fa_da
Full Member level 5
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 !
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 !