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I am looking at a PCB that has tiny glass beads on the input and output legs of an EMI filter. The middle pin goes to GND.

What is the purpose of the glass beads?

Couldn't get the focus quite right but here is a picture.

Glass Beads on Filter Legs

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  • \$\begingroup\$ The beads below the yellow parts might be spacers to hold them off the board...not sure why. \$\endgroup\$ – gbarry Apr 30 '15 at 6:54
  • \$\begingroup\$ To protect the device from heat of soldering? And/or give a consistent distance, where PCB layout is important, e.g. where track resistance/inductance/capacitance is highly optimised? \$\endgroup\$ – CL22 Apr 30 '15 at 10:04
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The beads are entirely a mechanical feature, and they are rather important.

They ensure that there is enough lead length between the PCB and the filters. The filters are brittle. If you don't provide sufficient lead length, the thermal strains from cycling the board temperature couple too tightly to the ceramic substrate of the filter. This could fatigue the substrate itself, but most usually will fatigue the pin attachments first, leading to "flaky" behavior that's hard to diagnose.

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