Odd Transistor behaviour after soldering

Discussion in 'General Electronics Chat' started by mikeysela, Feb 16, 2011.

  1. mikeysela

    Thread Starter Member

    Jul 24, 2010
    87
    1
    Hey guys,

    I soldered a bunch of these circuits (see attachment).

    When i tested the circuits on a breadboard, the transistor behaved as expected, and only shorted when i passed current through their base using an Arduino digital output. However, after I soldered a bunch of these, there was some small resistance (0.5k or so) between the base and the source and another resistance between the drain and the base. This lead to essentially having a very small resistance between the power and ground, which shorted my entire circuit out essentially. How could this have happened?

    Thanks guys,
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That FET is good for 60 V & 500 ma, which seems safe. However, the data sheet shows it fully on with Vgs = 10 volts. The voltage out of the micro was certainly less than that. The device was in a conducting region where it got hot and died.

    The cure is to use a logic level FET. If your Uno is a 5 volt micro, you might use a VN10LP.
     
  3. mikeysela

    Thread Starter Member

    Jul 24, 2010
    87
    1
    So did the soldering have any effect on this?

    When i tested these BS170s on a breadboard, everything worked just fine and the transistors were acting as good switches. Would soldering the logic level FETs have any difference with this?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can easily fry components if you heat them up too much, or too long.

    It's best if you use a heat sink when soldering to keep most of the heat away from the component body. Hemostats (a type of locking clamp used in surgery; looks sort of like very thin-nosed scissors) are a great tool to use for this kind of thing.

    If you're new to soldering, it's a good idea to practice on some cheap perfboards with various "junk-box" type components.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Another way in which soldering can damage components is static discharge. If the soldering iron and the operator are not at the same potential as the device being soldered, a discharge through the device may cause failure. FETs are rather sensitive to this because the gate insulation has a very high resistance, and may not tolerate much voltage, typically only a few tens of Volts.

    You should use full antistatic precautions when handling semiconductors so that you and your tools are all at the same potential as the work-piece.

    For your safety, only ever use personal grounding equipment (e.g. wrist-strap) including a series resistance of order 1Megohm.
     
  6. mikeysela

    Thread Starter Member

    Jul 24, 2010
    87
    1
    thanks for the responses guys. Do i also want to solder all the connecting wires first and then the bigger components such as LEDs and transistors?
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Soldering the most sensitive things in last makes some sense, but if you are doing things right it should not be all that critical.

    It's difficult to judge what you are doing from a distance, but if you are using small devices like BS170 in SOT-23, soldering them would require good equipment and some skill, as well as really good eyesight! A TO-92 or other through-hole type might be a better bet for hand soldering.

    That said, it should be possible to solder even quite small devices with enough practice. An occasional failure might occur as a result of hand soldering, but if you are consistently frying them something must be wrong.

    Having the board good and clean, and preferably pre-tinned will help. You should aim to make the joints quickly, but don't dither. If you do have to use a very small device, having it secured in place before soldering is half of the battle. You really do not want the thing sticking to the soldering iron!
     
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