From: Humphreys, David (david.humphreys_at_honeywell.com)
Date: Thu Nov 18 2004 - 06:44:27 PST
> -----Original Message-----
> From: newtontalk-bounce_at_newtontalk.net
> [mailto:newtontalk-bounce_at_newtontalk.net] On Behalf Of Woody Smith
> Sent: Tuesday, November 16, 2004 9:30 PM
> To: newtontalk_at_newtontalk.net
> Subject: [NTLK] eMate acceleration
> Has anyone had success in removing the crystal. If so please
> share any tips leading to success. I am now considering a
> small rotary grinder to cut the crystal away from the plastic spacer.
Wow! I'm not sure how many rules you broke doing this but it's a lot.
First of all, this should not be attempted with the kind of tools you get at Radioshaft.
The crystal is a surface mounted device. It was originally attached by screening some solder paste on the pads, placing
the crystal on top and passing the board through a reflow oven.
As the board passes through the oven, the temperature slowly rises according to a set profile and the solder paste
starts to 'reflow' or melt. When the solder joint is formed, the temperature is ramped down and the solder solidifies.
A soldering iron is NOT a good substitute for this process. It is a point source of heat that cannot be accurately
controlled. Yes you can spend money on a temperature controlled iron but however accurately you control the tip, the
surrounding area is at a big temperature differential. This will cause all sorts of problems. Different materials have
different coefficients of expansion. This means that applying heat to one end of, say a ceramic capacitor, will cause it
to expand at different rates along its' length and ultimately crack. You may not notice this but it has altered the
characteristics of that part permanently.
The crystal is the same. Excessive heat will cause the quartz material inside to crack. At best, this will change the
frequency at worst it will not work.
To repair surface mounted devices in the field one needs to have an SMD rework station.
Basically, it is a hot air gun with different shaped nozzles. Temperature and air flow is controllable. In addition, a
hot air bath is used underneath the board to bring the substrate (board) near to reflow temperature. An even temperature
across the board reduces the possibility of cracking. The directed flow from the hot air pencil is then all that is
needed to slightly raise the component local temperature past reflow and the part can be removed.
This is how you remove the big Cirrus chip without lead or board damage.
Don't use soldering irons for anything other than through hole devices and decorative wood burning.
For reference, the adhesive you saw was actually adhesive. That is how the copper is attached to the board. Prolonged
heat will soften it and allow the pad to lift away.
Using a soldering iron to heat the pad results in incomplete solder reflow under the part and this combined with a
pulling force when trying to remove the part will very likely result in pad dislocation.
The amount of heat required to reflow the solder when the rest of the substrate is at room temperature is much greater
than the amount of heat necessary to damage either the board or part or both.
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