Could this be the soldering problem

Discussion in 'Tech Talk' started by Hurttt, Oct 20, 2017.

  1. Hurttt

    Hurttt New Member

    As here is a sub forum for tech talk, I wonder whether it is proper to ask you questions about soldering here? I have been having problems with 2N7000 mosfets that perform perfectly on the breadboard, but when moved to a perfboard it start failing. I'm on my third iteration now, replacing all mosfets, between iterations having eliminated possible other sources for failure. On my third attempt I've been probing each separate mosfet for shorts and testing them with the uC between soldering the next. After soldering the third of eight in total, all three suddenly failed. This time they failed having not a shorted gate-drain, as before, but turning the gate ON/OFF has no effect, current now always flows from Drain to Source. I'm quite desperate at this point.
    • I've always been very careful with ESD, always handling the mosfet by the plastic, not the pins. Last run I moved my soldering iron to a grounded socket.
    • Before starting handeling components and soldering, I always ground myself by touching the ground prong on the wall socket.
    • I've reduced soldering temperature to 250 C.
    • For testing purposes I'm switching an 1mA LED, at 24V (I verified the current).
    My setup is as follows: A microcontroller provides 0/5 volt to the gate of the mosfet (100ohm resistor on that). On the Drain, there's the LED with a current limiting resistor. Source is connected to ground. Basically there are no other components.
    1osxD.png
    Since everything is OK on the breadboard, and things start failing after soldering, but never after the first mosfet. I'm fairly confident that something goes wrong during soldering. What could I possibly do wrong here?

    # To be precise, an PCF8547 connected over i2c to an 5V Arduino.
     
    kevino likes this.
  2. KIAB

    KIAB Super Member

    They are very sensitive to electrostatic discharge (ESD), they solder better using a temperature controlled soldering station, the one I have, is variable from 100°C to 450°C.
    Mosfets, look at their spec sheet & see what their soldering temperature.
     
  3. I wasn't aware that these little fellas were that sensitive to either static or heat.

    Hurtt, where did that circuit come from?

    Where does U1 get its 5V supply from - since your main supply is 24V?

    Are you sure your LEDs will fire with less than 1mA going through them (2.1k resistors seems excessive), and is a 100R on the Gate large enough (that's 50mA going through there.

    Have you tested just the MOSFET in situ - eg have the LEDs served by, say, 2.1k resistors (unless that's too low - I don't know what LEDs you are using) and, say, a 10k on the Gate. Then simply trying Hi-ing and Lo-ing the gate directly to the + and - rails in turn to see what the 2N700 has to say.

    Or, could it be due to the LEDs being supplied by 24V, but U1 - and hence the Gate - via some sort of 5V regulator?
     
  4. KIAB

    KIAB Super Member

    You only have a few seconds to solder then, heat them too long & you fry them.
     
    Dr Bodgit likes this.
  5. KIAB

    KIAB Super Member

  6. CGN

    CGN Screwfix Select

    You can always clip a shunt on when soldering. Make sure you have a powerful enough iron so the moment the tip and solder touch the component leg, you get instant solder flow.
     
  7. I suspect this is an electronic design issue rather than a wrecking of the components one.

    Mind you, I was wrong about the value of the Gate resistor - it's a bleedin' FET so presumably no Gate current flows at all (or virtually zero)?
     
    Last edited by a moderator: Oct 20, 2017

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