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MSP430 32kHz Crystal Failure

Mike Gallagher
Prodigy 40 points
Other Parts Discussed in Thread: MSP430F235

I am having an issue with a 32KHz crystal failing due to extend periods of high moisture. The PCB that the crystal is attached to is operating in a ventilated enclosure that is outdoors. This leaves the crystal exposed to high humidity and dew. Putting the PCB into a waterproof enclosure is not an option due to an on-board barometric pressure sensor that needs to be vented to atmospheric conditions.

The crystal being used is by Abracon Corporation P/N: ABS07-32.768KHZ-T, the microprocessor is MSP430F235. The crystal is the only part of the board that is having issues with the moisture.
I have tried various types of conformal coatings to deal with the high humidity and dew. Some have worked better then others, but given enough time, they all have failed. The conformal coatings have typically lasted for about a month in the Seattle area before the crystal stops oscillating. Luckily the PCB and crystal fully recovers and become functional after going through a drying oven.

Can anyone recommend a conformal coating that will work for years under a high humidity and dew environment that can be applied over the crystal and associated pads, traces, and pins. I am also open to alternative solutions to solve the issue.

Thanks

Mike

  • 0
    Guru 46710 points

    Did you try 3M Novec? If you have plenty of power then other solution would be to make some kind of regulated heater on the board area where crystal is so you can (hopefully) push dew point away. Or use SMD oscillator package containing 32KHz fork _and_ oscillator inside.

  • 0
    Expert 2740 points

    Another possible cause is flux residue on poorly cleaned boards. Water soluable fluxes must be removed after soldering.

    Peter

  • 0 in reply to Ilmars
    Prodigy 40 points

    The 3M Novec looks very promising. I am in the process of ordering a sample to try it out.

    Unfortunately I do not have any spare power for heaters, the system is battery and solar panel powered.

    I would like to stay with a crystal and not go with an oscillator due to cost restricts on the product. I do agree that an oscillator would most likely solve my problem. I will use an oscillator as an last ditch effort, if I can not protect the crystal from moisture. 

  • 0 in reply to Peter Dvorak
    Prodigy 40 points

    Our contract manufacturer uses no-clean solder and flux. I am not sure if the products are water soluble or not.

    I guess you can never have a boards to clean, I will ask them to wash the boards in all future builds.

  • 0 in reply to Mike Gallagher
    Guru 46710 points

    Mike Gallagher said:
    I am not sure if the products are water soluble or not.

    You better ask for extensive materials info, check yourself. Also test it! - Wet/rinse board  with small amount of distilled water, measure water resistance (before)/after.

    Mike Gallagher said:
    I will ask them to wash the boards in all future builds.

    Good idea. However don't rely on clean boards because water in the air is rarely clean, in polluted cities it is little acidic, in areas near sea/ocean it is salty.

    BTW before applying 3M Novec, boards shall be rinsed in special degreasing solution.

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