Resistor change value

Discussion in 'General Electronics Chat' started by rigers, Oct 15, 2012.

  1. rigers

    Thread Starter New Member

    Jun 14, 2011
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    Hi,

    Has anybody encountered a case where after soldering SMD resistors their value somehow changes. I soldered them on different boards with same design and some came out right, most came out wrong. What's going on?

    Thanks!
     
  2. colinb

    Active Member

    Jun 15, 2011
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    35
    Temperature coefficient of resistance, if they are still hot after soldering?

    What resistor values are you working with? Is it a few ohms, or 100 ohms+? For low ohms, poor soldering joints could be significant (cold joint, solder contaminated with junk from dirty soldering iron, etc.).
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Some cheap soldering irons get way too hot for soldering. My temperature controlled Weller soldering iron never gets too hot.
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. rigers

    Thread Starter New Member

    Jun 14, 2011
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    10k, 5.67k and 590ohms. The 100 ohms actually remain at 100ohms.
    I thought the same thing about the soldering but I only kept the iron there not even second. I was using flux so the solder sticked almost instantly.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Did you measure the resistor before and after soldering?

    Is the change more than the resistor tolerance?
     
  7. colinb

    Active Member

    Jun 15, 2011
    351
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    Are you sure there is no other conductance as part of the circuit to which the resistor has been soldered? If you've installed other components, chances are there is another conductive path that is altering the resistance measurement. Try desoldering the resistor and measure it out of the circuit to confirm whether the resistor has actually changed, or if the circuit is interfering with the measurement.
     
  8. miguelpedroso

    New Member

    Aug 13, 2012
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    To accurately measure resistance, you need to desolder them from the board, you cannot measure the value of a resistance when it is actually part of a circuit.
     
  9. rigers

    Thread Starter New Member

    Jun 14, 2011
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    Yes, it's about half for the 10k. It's weird because on another board of same design it reads 10k.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Are you measuring resistance in-circuit?
    You can only get an accurate reading if you are certain that there are no other components in-circuit that can affect the reading.
     
  11. Austin Clark

    Member

    Dec 28, 2011
    409
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    Heat can definitely permanently change the resistance of resistors. Cutting the resistance in half is pretty extreme, but I wouldn't figure it impossible. Are you sure they went in with the correct resistance? Maybe you've got a few mislabeled or misplaced components.

    Do you know the type or resistors you're using? Size, rating, where you got them, etc;? All of that might help us in understanding the issue as well.
     
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