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Inside of an automotive ECU I noticed that all three crystals are mounted about 1/4" above the PCB. What is the advantage of doing this versus mounting the case flush with the board? This seems like it could lead to vibration cracking the solder joints over time. Is there a thermal issue - crystals don't really dissipate much heat? Manufacturing constraint? Some of the capacitors are mounted in a similar way, although this looks more normal to me.

Crystal mounted above PCB

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    \$\begingroup\$ Quartz crystals are vibration sensitive. The basis of their function is piezo electric after all. So are most ceramic caps. I'm not sure if that's the reason why they are mounted that way though. \$\endgroup\$ – DKNguyen Apr 30 at 3:42
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The bent leads set the component height during wave solder. It does four things:

During wave soldering:

  • retains the component in the board with spring tension
  • limits the solder wicking to just the pad
  • provides thermal relief to improve soldering

After soldering:

  • provides extra lead length to reduce mechanical stress on the component.

The extra lead helps prevent cracking at the lead/component junction in high-vibration environments by spreading the flex over the lead, rather than concentrating flex at the component itself.

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One reason for mounting a 'through-hole' quartz crystal away from the PCB could be to avoid solder shorts between the pads and the can. Another reason could be to prevent heat damaging the glass seals as well as the crystal itself, during soldering.

Likewise the reason for raised mounting of capacitors could be to avoid damage due to heat, during soldering.

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Look at all other TH components, they are all above the PCB. This way solder flows through the hole. Otherwise, it could only stick to the pad outside the hole on the bottom side, which would eventually lead to cracks and disconnection.

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