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LMC662/LMC6001 Moisture causing output changes.

Mauricio Ortiz
Prodigy 10 points
Other Parts Discussed in Thread: LMC6001, LMC662

I am currently using LMC662/LMC6001 on a high impedance circuit and are getting changes in results once humidity is introduce to the PCB. Any suggestions on coating or set-up as to eliminate or reduce changes due to moisture

  • 0
    TI__Guru 62586 points

    Hello Mauricio,

    How low are the currents? uA? pA? fA? aA?

    First, see the following article:

    http://www.edn.com/design/analog/4368681/Design-femtoampere-circuits-with-low-leakage-part-one

    http://www.edn.com/design/analog/4375459/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

    http://www.edn.com/design/analog/4395651/Design-femtoampere-circuits-with-low-leakage---Part-3--Low-current-design-techniques

    If you are going to run low current traces on a board, then you must use guards around the sensitive traces/nodes.Sub pA currents are possible with common FR4 PCB if properly guarded.

    You can also bury the traces inside the board using multiple layers with guards on each side (trace on layer 2 with wider guard traces on layer 1 and 3). Treat the sensitive trace like a stripline with the guards as the "ground" traces. Side-leakage can be reduced by placing a row of vias (connected to the guard traces) along the sides of the trace, but that is an extreme. Basically, use techniques borrowed from RF to surround the trace within a guard.

    Normally the solder resist does a decent job sealing against moisture, but gaps in the resist due to un-plated holes or the edge of the board can allow moisture to absorb into the board. Make the component pads slightly oversize and overlap the solder resist so that the pad edges are sealed. If you can monitor the circuit while in the chamber, the amount of time it takes for the leakage to build may give you a clue if it is a surface or absorbed leakage.

    Paraffin wax can also be used. But for the lowest current, you cannot rely on the coating. The entire sensitive portion of the circuit should be in a sealed container with moisture control (with a desiccant like silica gel). This could be as simple as a metal shield over the sensitive circuit with solder-sealed edges (and with no holes and make sure the vias are are tented or sealed).

    I am not claiming to be an expert on the various conformal coatings, but it looks like you want a urethane or silicone based coating, as these seem to have the highest volume resistivity and highest dielectric strength. Hopefully, someone in the audience a bit more familiar with them will offer some advice. Otherwise, consult with the conformal coating manufacturers and run some experiments. Be sure to flex and temp-cycle the boards.

    My concern would be what happens as the coating ages - at least for the fist 30-90 days as it finally cures - and does it cause stress during that time that may cause a low level charge to develop during that time.

    Regards,