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Aluminium capacitor rotation after soldering

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#73829

Aluminium capacitor rotation after soldering | 22 April, 2015

We'have been working on an issue regarding aluminium capacitor placement in PCB. In fact, we have twisted (shifted) capacitors. Through root causes analysis, we confirmed that this phenomenon is happening during reflow process (no issue on placement or soldering). Have you any idea what are factors in reflow process taht could move the capacitor knowing that it's the only component on the PCB presenting this issue?

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#73830

Aluminium capacitor rotation after soldering | 22 April, 2015

Hi Myriam,

I think we may need some more information. Is the Electrolytic (Aluminum) Capacitor placed more than once per board? If so, are all locations twisting? This would typically be caused by a thermal imbalance that could be design induced. (uneven copper conductors or traces that would prevent even heating of the solder joint) I've also seen excessive air turbulence shift parts during reflow. How often is the shifting, or twisting occurring? Have you tried turning the PCBs 180 degrees prior to reflow? I would also recommend running a thermal profiler through the oven with thermocouples attached to each side of the capacitor. This should help you determine whether or not you have a thermal imbalance contributing to your problem. Please let us know the results.

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mbb

#73831

Aluminium capacitor rotation after soldering | 22 April, 2015

Thank you for answering,

Yes, we have 3 different capacitors for each daughter board with 4 boards/PCB. Another information that could help that we have the issue randomly on boards (not a specific one). The occurance of the issue is random. yes, we tried turning the PCB by 180° prior to reflow but the issue persists.

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emeto

#73832

Aluminium capacitor rotation after soldering | 22 April, 2015

I assume you are running Lead paste and that is why your parts are moving during reflow. Most probably these caps have leads that are bended in random directions(look at some parts from the bottom and observe how symmetrical the leads are). In addition if the pad is designed a little bit wider that increases the problem you are seeing. What happens is that in the reflow zone of your profile the part will try to center the lead to the pad where the center mass point is. In case of bended leads the part will come out crooked.

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#73833

Aluminium capacitor rotation after soldering | 22 April, 2015

We see this from time to time. Usually, one of the leads of an electrolytic is attached to a ground plane, and the other to a trace. The ground plane acts as a big heat sink and reflows significantly later than the trace.

If you have glue in your line, gluing helps significantly. Slowing down your oven or raising your soak temperature can help if your board can stand it. If you have control of the design thermals instead of going straight to the plane can help too.

Or if you have one of those moronic pad designs that is 3 or four pad widths across, start by reducing that.

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