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LMV601: input bias current too high after reflow

Lukasz Wroblewski
Prodigy 30 points
Part Number: LMV601

Hello,

We use the LMV601 as an amplifier for signal coming from a pH probe.

When soldered by hand on our prototypes, it works great. However when it comes out from our desktop reflow oven, the impedance is too low to handle correctly the voltage generated by the pH probe which has an output impedance of about 250 MOhm.

It looks like after reflow, the 20 fA of input bias current stated on the datasheet is not relevant anymore.

Other parts works perfectly and the temperature of the oven is good. The peak temperature is 245 deg. C for only 15 seconds. The reflow profile is compatible with Level-1-260C-UNLIM.

We can't test the input current as we don't have the right equipment for such low values.

Is it a known issue ? Is there a solution other than soldering this part by hand ?

Thanks !

  • 0
    TI__Guru*** 149776 points

    Lukasz,

    Does this happen on some boards or all of them? Perhaps there is residue present after the oven solder process; have you tried cleaning and drying the boards?

  • 0 in reply to Ron Michallick
    Prodigy 30 points

    Thanks for the answer.

    It happens on each and every reflowed board.

    After swapping the LMV601 from a reflowed board to brand new one, the same problem appears on the component.

    Handsoldered: when applying a voltage on the input, it keeps it indefinitely when the voltage source is removed.

    Reflowed: After removing the voltage source, the voltage on the input pin fades in 4 to 10 seconds totally.

    We will try to use leaded solder paste with a less agressive reflow profile next time.

  • 0 in reply to Lukasz Wroblewski
    Guru 140200 points

    Hi Lukasz,

    from my own experience I would say that it is nearly impossible to get professional results from a simple or even self-built reflow oven when it comes to lead-free soldering.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 30 points

    Yes, I guess you're right. but there is still a way to see if it's an oven issue or not:

    I will double check the temperature accuracy of the thermocouple and place it directly on the component as I have only one of this op amp per board. Then I will reflow only this board. The temp curve will reflect the temperature of the op amp and we will see if it will work or not.

    We've tested 3 different desktop reflow ovens (mainly chinese with IR heater) and none of them have had a correct performance except for the oven we built ourselves with a Controleo 3 control board. There were no issues at all for the last 18 months! Let's see if there is a problem with it.

  • 0 in reply to Lukasz Wroblewski
    Guru 140200 points

    Hi Lukasz,

    the problem is that the IR "lamps" of oven create an inhomogenious temperatur distribution within the oven. We made tests with small soldering paste drops which we distributed evenly on a test PCB. It came out that some of the soldering paste drops melt while others did not, even not after considerably increasing the temperature. Another issue is that the components unevenly heat up by the IR radiation. The more they absorb the IR radiation the hotter they become.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru*** 149776 points

    Lukasz,

    In the experiment where the op amp from a leakier board is soldered to a fresh board, be sure to thoroughly clean the op amp to see if the leakage is on the outside of the op amp package or somehow got inside the op amp.