I use a 32.768 kHz SMD crystal (datasheet) for a MCU. Here is the layout part of the crystal.
Here is a view from an actual PCB
I mounted the components by hand-soldering, and produced about 30 modules.
Most modules' crystal didn't start working at the beginning, but I solved it by re-soldering the crystal by applying a little bit more solder under it. The interesting thing is that when I touch a pin of the crystal by a tweezers, the crystal starts working. Here is an an oscilloscope view of the crystal's signal.
The glitch is where I touched the path by a tweezers, and then the oscillation starts.
Another fact is that some of board's crystal work if I touch to the C451 cap's path, some of them start working if I touch to C441 cap's path (e.g., if it works by touching the C451 cap's path, it does not work if I touch to C441 cap's path).
This made me doubt if it is related with the solder under the crystal (maybe uneven contact surface or another reason that I can't think of). Or if it is not a purely solder related problem, as I sometimes needed to perform re-soldering process several times till the crystal problem is solved. On the above PCB view, the extra solder sticks out from side of the crystal (no short circuit to another pad or case of the crystal), there should be for sure a connection between the crystal's pin and the PCB pad, but the problem still remains.
Another issue is that I experienced on 4 boards that they work after I re-apply solder under them, but when I test them the next day, the crystal has the same issue.
Question.1 Has anyone experienced similar problem or could think of what could be the actual problem?
Question.2 The boards will be on the field, I have a concern that they will work here but have problem when the customer needs to use them. How I test them is to start the modules couples times at a day and observe if there is any failure and spread this test to a week long. Is there any method/technique to test (or get indication) if the crystal will probably work fine in the near feature
- I inspected the PCBs by a microscope that there is no short circuit between any trace or there is no any solder connection from the case of the crystal to any path
- On the problematic boards, I re-placed the crystal with the ones that I removed from the board that works OK, therefore it should not be a component problem
- I cleaned the flux residues on PCB but it does not change the result
- I did search if there is a special procedure/technique to solder such type of SMD crystals but couldn't find any related information.
EDIT I tried placing a different capacitor values but didn't help.
EDIT2 Here is the gerber view of the reference design of TI, as it is an optional crystal, it is connected through 0 ohm resistors(R451, R441)