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I bought two of these trimmers from Digikey to tune a resonant circuit, but the screw is covered with silicone. I dug out the silicone with an xacto so I could turn the screw. Why is the silicone there? Seems counter-productive.

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    \$\begingroup\$ Do you think you could get an in-focus photo and crop it? 98% of your photo is useless background. \$\endgroup\$
    – Transistor
    Aug 25, 2016 at 19:25
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    \$\begingroup\$ I suspect it's sealant to prevent contamination during solder reflow and flux washing operations. \$\endgroup\$ Aug 25, 2016 at 19:31
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    \$\begingroup\$ @BSEE - Don't you mean "silicone", not "silicon"? Silicon is what circuit components are made of. \$\endgroup\$
    – Bort
    Aug 25, 2016 at 19:34

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Many trim caps cannot be cleaned. Murata, for example, recommends against cleaning for many of their products:

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but they do have some (eg. TZB4 Series) with a 'cover film'.

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Recommended procedure after mounting and cleaning:

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In Digikey's photo you can see what looks like silicone (rather than the typical polyimide cover film one would expect from the name).

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Silicone is good for heat and water resistance such as Thermal Reflow Oven and Aqua solder flux cleaning making it suitable for a variety of assembly processes. It also stabilizes the position from vibration.

I suspect it should be left in.

I haven't tried but isn't this just an outside layer sealant that can be operated with the trimmer tool? Does it affect pF readings?

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