AVR-Chat

I'm putting together a pcb with a Phillips PCF8563 Real Time Clock.  I have
an Abracon 32.768 cylinder type crystal for it.  I made the pcb pads exactly
as suggested in the data sheet for the crystal, which has one long rectangle
immediately under the cylinder part.  I just finished soldering the two
"tongs" onto their pads and everything went fine, but it just occurred to me
am I supposed to solder the can to the big pad under it also?

This is the first time I've dealt with a crystal like that.  Please excuse
the dumb question if it is one.

Thanks, Steve
 

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--- In AVR-Chat@yahoogroups.com, "Steven Hodge" <stevehodge@...> wrote:
> am I supposed to solder the can to the big pad under it also?

I dont know the particulay crystal but most 32k watch xtals are in a 
tiny cylindrical metal case. If it is one like that then do not 
attempt to solder it. It may be aluminium anyway which will be next to 
impossible to solder. The heat will likely damage the crystal. The big 
pad is probably to stop you putting any other pcb tracks under the can 
which would short circuit them.
My suggestion is to use a blob of light glue (e.g. somthing out of a 
tube for general purpose office/home use) to hold the body to the pad 
to prevent mechanical vibration. Don't use anything like bathroom 
sealant which is nice and rubbery but contains an acid which could 
cause damage.
USe something you can peel off with a knife when you want to re-use 
the crystal for the Mk II pcb...

luck,

I believe you're right; don't attempt to solder the crystal case.

I have used a dab of RTV silicone sealant to hold crystals in place if I was
concerned about vibration. Also, a tiny piece of double-sided adhesive foam
rubber will hold things like that down indefinitely. Just make sure any
oil/grease/dirt are removed from both surfaces (I use rubbing alcohol)
before putting on the tape.

Zack

On 12/31/08, George <buggiesmith@gmail.com> wrote:
>
>   --- In AVR-Chat@yahoogroups.com <AVR-Chat%40yahoogroups.com>, "Steven
> Hodge" <stevehodge@...> wrote:
> > am I supposed to solder the can to the big pad under it also?
>
> I dont know the particulay crystal but most 32k watch xtals are in a
> tiny cylindrical metal case. If it is one like that then do not
> attempt to solder it. It may be aluminium anyway which will be next to
> impossible to solder. The heat will likely damage the crystal. The big
> pad is probably to stop you putting any other pcb tracks under the can
> which would short circuit them.
> My suggestion is to use a blob of light glue (e.g. somthing out of a
> tube for general purpose office/home use) to hold the body to the pad
> to prevent mechanical vibration. Don't use anything like bathroom
> sealant which is nice and rubbery but contains an acid which could
> cause damage.
> USe something you can peel off with a knife when you want to re-use
> the crystal for the Mk II pcb...
>
> luck,
>
>  
>


[Non-text portions of this message have been removed]

--- In AVR-Chat@yahoogroups.com, "Tim Gilbert" <tim@...> wrote:
>
> Hate to disagree but we always solder the case down to the board. 
(That is why the pad is on the board.) 

No problem Tim  - we can remember one customer that wanted to do that. 
For most others we provided a couple of drilled (grounded) pads so 
they could put a wire loop over the can.

I found this message on a quick web search but admit to not knowing 
anything much about the internal construction:-

"The crystal is commonly mounted within the can by the help of a 
polymeric glue. Heating up the can by soldering can damage the glue, 
resulting in a drastical increase of mechanical dampening of crystal. 
The consequence can be a shift of frequency or even a refuse of start-
up. So, normally, you should not solder the can. "

luck,

Since this is "Mk I" and won't be subject to vibration I think I'll skip the
glue.  But on "Mk II" I'll use some epoxy.   Thanks for the tip.  Steve

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of George
Sent: Wednesday, December 31, 2008 1:25 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Re: crystal mounting

 

--- In AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> ,
"Steven Hodge" <stevehodge@...> wrote:
> am I supposed to solder the can to the big pad under it also?

I dont know the particulay crystal but most 32k watch xtals are in a 
tiny cylindrical metal case. If it is one like that then do not 
attempt to solder it. It may be aluminium anyway which will be next to 
impossible to solder. The heat will likely damage the crystal. The big 
pad is probably to stop you putting any other pcb tracks under the can 
which would short circuit them.
My suggestion is to use a blob of light glue (e.g. somthing out of a 
tube for general purpose office/home use) to hold the body to the pad 
to prevent mechanical vibration. Don't use anything like bathroom 
sealant which is nice and rubbery but contains an acid which could 
cause damage.
USe something you can peel off with a knife when you want to re-use 
the crystal for the Mk II pcb...

luck,



[Non-text portions of this message have been removed]

Hate to disagree but we always solder the case down to the board. (That is why the pad is on the board.)  I've never seen an aluminum case; might be RoHS which makes is a bit harder to solder even with Pb/Sn solder, just use more flux.

We've also moved away from this case since our pick and place machine doesn't like them.  There are lots of other case styles that are easier to use.

Regards,

Tim Gilbert
JEM Innovation Inc.
303-926-9053 (office)
303-437-4342 (cell)
720-890-8582 (fax)
www.jeminnovation.com
www.pdksolutions.com

  ----- Original Message ----- 
  From: George 
  To: AVR-Chat@yahoogroups.com 
  Sent: Wednesday, December 31, 2008 2:25 AM
  Subject: [AVR-Chat] Re: crystal mounting


  --- In AVR-Chat@yahoogroups.com, "Steven Hodge" <stevehodge@...> wrote:
  > am I supposed to solder the can to the big pad under it also?

  I dont know the particulay crystal but most 32k watch xtals are in a 
  tiny cylindrical metal case. If it is one like that then do not 
  attempt to solder it. It may be aluminium anyway which will be next to 
  impossible to solder. The heat will likely damage the crystal. The big 
  pad is probably to stop you putting any other pcb tracks under the can 
  which would short circuit them.
  My suggestion is to use a blob of light glue (e.g. somthing out of a 
  tube for general purpose office/home use) to hold the body to the pad 
  to prevent mechanical vibration. Don't use anything like bathroom 
  sealant which is nice and rubbery but contains an acid which could 
  cause damage.
  USe something you can peel off with a knife when you want to re-use 
  the crystal for the Mk II pcb...

  luck,



   

[Non-text portions of this message have been removed]

The idea that all crystals are the same should be taken out back,
shot, burned, and the ashes should be scattered.

It is common practice to ground the shield can of crystals, but there
are certainly some that aren't designed for this, and even some that
aren't capable of being soldered. Some have a third lead so that the
can doesn't have to be directly soldered to.

It's up to the designer to select the proper crystal for the
application, which includes reading and understanding the specs.

Then we can talk about drive levels, and loading capacitance.