Piezos are a bulk ceramic with metal electrodes plated on.  The ceramic
is polarized through those electrodes.  Once the plating is gone it is
going to be very hard to connect to them.
The plating may be silver or aluminum.  If it is aluminum you can make
contact by a spring contact or conductive epoxy.  You can also use a
conductive ink pen though the result is not as durable and needs some
other glue for physical strength.
If the plating is silver you can solder to it.  You want "silver
bearing" solder.  This is mostly tin/lead with about 2% silver.  It is
not "silver solder" which takes too much heat.  Regular tin/lead solder
will dissolve the silver off the ceramic.  Sliver bearing solder is
already saturated with silver so it doesn't steal silver from the
ceramic.
You want to minimize heating of the ceramic as that depolarizes it.
Make sure everything is clean and work quickly with a hot iron.  Also
the piezo will develop some voltage as it gets hot.  On large disks this
can be enough to shock you into dropping your soldering iron in your
lap, so be careful.

Sherpa Doug

> -----Original Message-----
> From: Jinx [mailto:joecolquitt@CLEAR.NET.NZ]
> Sent: Wednesday, July 18, 2001 6:16 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]:Soldering piezos
>
>
> I've got a few piezo discs to rework and need to find the best
> way to attach a wire to the ceramic centre. The brass outer
> ring is OK but solder (60:40 5-core) won't stick to the ceramic.
> Any suggestions ? I could stick the wire down with aluminium
> tape or silcone but that's not an ohmic bond and I expect it
> would fail pretty quickly
>
> --
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>
>

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