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0603 caps' caps

Discussion in 'Electronic Design' started by oopere, Oct 24, 2008.

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  1. oopere

    oopere Guest

    So, yesterday I was playing building a "dead bug" style 60 MHz
    oscillator. I was obtaining inconsistent signals. Seemed that depending
    on touching cables the measurements changed...

    Again, it turned out to be one of those 0603 capacitors whose metal caps
    tend to break off quite easily, leaving an unreliable contact. Flexing
    the cable to the board flexed the connector which in turn moved the
    metal cap of the capacitor. I re-soldered a new capacitor and... Wait,
    didn't I hear a little "click"? Indeed, the capacitor's cap went off
    again (!) from the strain when the lead solidified.

    It is not the first time having capacitor leads break when prototyping.
    Most of the time, this came from flexing the PCB by an imperceivable
    amount, but until yesterday I had no noticed this happening from the
    solder cooling!

    Pere
     
  2. Rich Grise

    Rich Grise Guest

    It sounds like you're using too much solder or too much heat. Breaking
    just from soldering shouldn't happen. Use the minimum possible amount
    of solder, and solder it quickly.

    Good Luck!
    Rich
     
  3. I used one like this to bypass a divider/reference voltage and got
    quite large errors from the wire moving. It was creating a strain on
    the capacitor body, and I guess modulating the value.
     
  4. oopere

    oopere Guest

    I am using conventional leaded solder (0.3mm diameter). The iron is
    decent, suitable for SMD work, with temperature regulation and so on...

    Should I expect different behavior with _un_leaded solder?

    Pere
     
  5. oopere

    oopere Guest

    As ii pointed out in the original post, I have had _several_ problems
    related to capacitors "breaking". On some prototypes I even had to
    reinforce a PCB (0.8mm, though) with some perpendicular PCB strips to
    avoid the board flexing and ruining capacitors. This has also happened
    on small 1.6mm PCBs.

    I did not note if this was related to the soldering iron temperature. It
    is true that I sometimes regulate the temperature higher than the
    minimum (up to 330 C) to enhance speed but, then, the process is quick.
    But perhaps not so quick as to avoid the small thing to become overheated...

    I have just searched for some information and found this
    http://www.symmetron.ru/suppliers/epcos/ceram/00880098.pdf

    "Manual soldering with a soldering iron is to be avoided, hot-air
    methods are recommended for making repairs."

    So, at which temperature do you regulate your iron when soldering these?

    Pere
     
  6. TheM

    TheM Guest

    If you have enough space, use 2.5mm pitch ceramic caps instead of smd.
    Soldering one end of smd dead bug style means it will break at minimal
    tension. Use smd only where you know there won't be any tension. Not
    on connecting wires, for example.

    M
     
  7. I have never actually had any problems when manually soldering
    capacitors to actual circuit boards. Things can start to come apart if
    you remove again them incorrectly, or if you are not using a proper
    PCB. But I would guess lower temperatures and minimal solder would be
    good. I usually use 0805, so that may be a difference too.
     
  8. Use a 600 degree tip instead of a 700. Perform the solder operation
    within a 2 second window. Refrain from reflowing the solder joint
    whenever possible.

    If you must use the actual smd part to get reliable data for operation,
    you can glue it to an insulator, like a silica transistor heat sink or a
    small piece of PCB material, and solder the wires from it to the other
    circuit parts. If a standard dipped ceramic cap of equiv. value could be
    used to characterize the design, it would be a lot easier on you.

    It isn't the solder cooling that causes the problem, per se. The
    solder process itself almost detaches the terminations on SMD chip
    capacitors, so it is the heat in-flow during hand soldering that does the
    most damage, hence the cooler tip temp suggestion.

    This is why SMD reflow ovens bring up the assembly temp on a gradual
    ramp, then reflow, then ramp down steeper, but not as fast as ambient air
    would.

    You can reduce the issues if you pre-heat the assembly to a couple
    hundred degrees F before soldering as well. It is hardier to solder hot
    parts though, however, less damaging to caps.

    I used to glue a whole bunch of SMD parts
     

  9. Lead free solders are usually a hotter process, so more damage to a cap
    would occur.
     
  10. JosephKK

    JosephKK Guest

    Just one question: Does the failing caps seem to be batch related?
     
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