Re: Soldering surface mount components

From: Clarence (no_at_No.com)
Date: 12/04/04 

Date: Sat, 04 Dec 2004 02:44:58 GMT

"Ken Smith" wrote
> Clarence wrote
> >"Ken Smith" <kensmith@green.rahul.net> wrote in message
> >> Clarence <no@No.com> wrote:
> >> [...]
> >> >If your aware of any other piezoelectric materials please list them. I
can
> >> >think of many applications for a low cost piezoelectric operated
mechanical
> >> >device.
> >>
> >> Both X7R and X5R are slightly piezoelectric. Neither is good enough to be
> >> put to practical use as an actuator. If you put about 500mA at 2KHz
> >> through a 2.2uF X7R cap mounted on a PCB you get just enough motion that
> >> you can hear it.
> >
> >500mA is a rather substantial level of power for that value. It takes 39.3
> >Volts sinusoidal directly across the cap.
>
> I didn't suggest it was a good idea to put that much AC through the poor
> thing. I just said that it screams if you do.
>
> BTW: 1/( 2 * PI * F * C ) = 36.17 on my slide rule

Did you allow for ESR? I did!

> 0.5 * 36.17 = 18.08 Then we have to add a little for the ESR.
>
> Also the ATC capacitors are rated for several Amps at 30MHz.

Which is NOT 2Khz!

> [...]
> >As little as a reliable motion of 0.002 for a 100V bias would be useful.
> >Assume a bar shape for maximum deflection.
>
> A lot depends on what you call reliable. If you want cheap use a plastic
> piezoelectric element. There are many makers of piezoelectric ceramic
> elements. If you are doing something like pushing a grating to tune a
> laser, don't use the plastic stuff. It is a good thermometer.

I'm looking for a pump impeller an implanted medical application.

> >So how much motion do you get at 100KHz? The entire unit is 0.180 long,
0.120
> >wide, and 0.080 high (max) with tolerances of +/- 0.010 Inches. The cap
> >element is coated with a resilient encapsulate.
>
> I've never been interested enough to measure it.

I Goggled, and found no published studies which gave numbers. Only that the
motion was detected.

> >The AC component is 30VRMS max and the DC is 80V.
> >I find nothing that confirms the property, nor the magnitude of the motion.

I

> Try google. I'll bet you will find something from AVX's web site that
> confirms the effect and says words like "but our do it less".
>
> >BTW, NASA, and FAA do not provide any warnings on these devices.
>
> I'm not surprised about the FAA.

The FAA is very stringent about any condition which might cause a fair or spark
on a board. For instance "Gold caps" often used to maintain a RTD are not
acceptable since they fail (rarely) by out gassing with a flame.

> NASA does surprise me a bit. They wouldn't allow PVC parts to be used in
> the U2/ER2 because of the fume risk to the pilot. This even applies to
> the parts that are at outdoors pressure.

I used many of these in a satellite program.

> >How about the X5V material?
> I think all high K materials do it to some degree.

Very small Degree!

These got clipped from my last mail:

Per AVX:
Effects of Mechanical Stress – High "K" dielectric ceramic capacitors
exhibit 'some' 'low level' piezoelectric reactions under 'mechanical
stress.' As a general statement, the piezoelectric output is higher,
the higher the dielectric constant of the ceramic. 'It is desirable to
investigate this effect before using high "K" dielectrics as coupling
capacitors in extremely low level applications.'

Also found:
Piezoelectric Effect. One minor drawback for X7R dielectrics has been
their sceptibility to 'piezoelectric-induced stresses.' Although this
effect is 'marginal' and 'may be neglected for case sizes smaller than
the 2220,' for larger capacitors it can lead to catastrophic failures
caused by cracking. Modifying the dielectric composition to avoid any
piezoelectric effects within the range of operating frequencies may
skirt this problem.
Whereas the standard X7R material shows piezoelectric noise, this is
almost absent from the new ceramic. These measurements were made
under a 300-V bias at 90°C. Note this effect is not present in film
or aluminum-electrolytic capacitors. With this new development,
ceramics now offer a viable alternative to film and electrolytic
capacitors in the large case sizes.

Relevant Pages

  • Re: Soldering surface mount components
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