Hi
This week I was selecting SMD caps for a design.
I was looking at low temp coeff, and begun thinking...

Suddenly i realized, in theory, temp coeff of a PCB mounted cap is probably=
 not the same as for a cap freely suspended.

Because, by temperature the cap expands mechanically, and also, the cirquit=
 board expands.
If they do not expand equally when by itself, then the cap will be=
 stretched/compressed when soldered to the PCD and temperature changes.

This stretching is bound to change the capacitance, because stretching a=
 capacitor makes the dielectric area larger, and at the same time thinner,=
 both lead to increased capacitance.

So actual temp coeff of the cap depends on PCB material...
However i guess that glass fibre reinforced boards have same temperature=
 elongation as ceramic.  (as glass temperature elongaiton is approx same as=
 concrete (and iron, BTW))

Well, how sensitive this is i don=B4t know.

But, plastic film capacitors are probably another issue.  Plastic materials =
 have far more temperature elongation than glass, so do aluminium (the=
 electrodes)


Any thoughts?


Maybe use a cap for strain sensing?  ;) =20
Probably temp coeff is in the way however...


BTW, the same thinking applies to SMD resistors.  As ceramic and glass=
 reinforced PCB probably have the sam temp coeff i guess there is no=
 problem.  But i=B4d expect resistor value increase with temperature for a=
 SMD R on a non-glass PCB.


/Morgan=20

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