CRYSTAL FAILURE
CRYSTAL FAILURE
(OP)
Hello, I currently have a problem with a crystal used on one of our PCBs. When the board is first mounted in the unit a faulted circuit indicator)it is tested. Once it passes the tests an LED is glued to the enclosure that the PCB is mounted in using an ultraviolet flood lamp. The enclosure is then potted with an epoxy resin and the enclosure is sealed using an ultrasonic welding process. Somewhere between the initial test and the final test the crystal that is used for the microcontroller fails. Does anyone have any suggestions? Would one company make a stronger crystal than the other? The failure rate is about 5%.





RE: CRYSTAL FAILURE
Often crystals are precoated with some silicon (electrical type!) and then potting occurs giving the crystal some "space".
You never said - SMD or thruhole?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: CRYSTAL FAILURE
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: CRYSTAL FAILURE
If it is a leaded crystal, you can try changing it's orientation so it is subjected to the ultrasonic welders vibration through another axis. If that doesn't work, you may have to go to a ceramic resonator - that is if it will have enough accuracy for your application.
RE: CRYSTAL FAILURE
Would there be a way to protect the crystal without changing the welding process?
RE: CRYSTAL FAILURE
A) If the crystal is a full height HC49 package, try the low profile version.
B) Try standing the crystal slightly off the board.
C) Bend the crystal over (change it's orientation).
D) Secure the crystal with RTV or wrap it in foam tape before potting so it is not as rigidly mechanically coupled to the potting.
E) Reduce the ultrasonic welder amplitude and duration cycle time.
F) Combination of the above.
G) Check out the ultrasonic web sites (Dukane, etc) for app notes. Give them a call. They have probably encountered the problem before.
RE: CRYSTAL FAILURE
What frequency is the crystal? Welders typically run at 20-50 kHz, which is a really unfriendly frequency for 32.768 kHz tuning fork crystals. If that's what you are using, you might have better luck with a 1+ MHz non-tuning-fork part.
"The failure rate is about 5%."
You mean the immediate catastrophic failure rate is about 5%. Many of the remaining units are probably running with greatly reduced margin and will fail in service.
RE: CRYSTAL FAILURE
The 5% failure rate is an immediate failure.
RE: CRYSTAL FAILURE
RE: CRYSTAL FAILURE
Ceramic resonators don't have as much inital tolerance, and have more temperature drift than crystals. However, for a microprocessor clock they are fine as long as there are no critically-timed loops or measurements being made by the processor. You will need to check with the design engineers of the module.
RE: CRYSTAL FAILURE
Warren