Can not cleaning boards after reflow soldering result in signal drift?

Discussion in 'The Projects Forum' started by tsgregoryv, Nov 11, 2011.

  1. tsgregoryv

    Thread Starter New Member

    Nov 11, 2011
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    I have previously prototyped a circuit board containing a mixture of surface mount chips and through-hole resistors/caps. I hand-soldered this original board and it experiences minimal voltage drift.

    In order to speed things up, I started using reflow soldering for the surface mount components and doing the through-hole components by hand. I did not clean the boards after the reflow soldering, although I did use a stencil to apply the solder paste. The only difference between the original board and the reflow boards is the addition of a final gain stage on the pcb.

    This new board experiences a large amount of signal drift while it is powered on. I have tried cooling the board while it is powered on, and this appears to reduce the drift, however, the new board does not heat up anymore than the older one. When I say there is a fair amount of drift, I mean that I can physically see the signal rising to rail on my o-scope. (~300 mV increase over a few minutes on a signal that is around 1.5Vpp).

    Could this signal drift be from moisture in the flux? The through-hole components are rather tightly packed on the board, and the main area where the drift is present is on a high - order filter (with G=10). The filter chip has a pitch of 0.65. I can't really see what else the issue could be.

    Any help would be much appreciated.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Couldn't the cause be the additional gain stage?. Are you making comparison measurements between boards at the same point?

    Even if the board temperature doesn't rise much, the temperature rise of a single component, even if it's only a few degrees through self-heating, can have a huge effect on errors.
    If you have cooling spray at hand you can find out if the problem is related to temperature or not and what component(s) are causing the problem.
    I suggest you post your circuit diagram and a picture of the PCB you made.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yes, we need to see a schematic of the circuit that drifts.
     
  4. tsgregoryv

    Thread Starter New Member

    Nov 11, 2011
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    I'm away from my computer with the schematic on it, but I will post it when I get back.

    Yes, I am making comparisons from the same locations on each board. The gain stage is the final stage of the circuit and the comparisons that I am making are earlier on.

    The part that is most confusing is that the temperature increase of the board is similar (although, I haven't quantified it yet) to the board that works.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    There is always the piezo effect on surface mount devices as they heat and cool.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Does it occur in components other than capacitors and intentional piezo devices?
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This could simply be due to a faulty component, or incorrect assembly. One possible "nasty" might be a reversed electrolytic capacitor - if your circuit has any.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    All semiconductors do it, some badly.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    There is no question that leakage (or creepage) across the surface on a contaminated PCB can cause drift in high impedance circuits. It might be expedient to, rather than wondering whether the contamination could be the problem, go ahead and carefully clean the board and see if the drift is reduced.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    BTW, the 'cleaning" requires distilled water since most of the problem is from sodium from finger touch and solvents won't remove it. A water pic with DI water to shoot the board works pretty well. Then oven bake at about 150F and avoid fingering the boards.
     
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