We are using low power external crystal 32768Hz with Rf: 10Mohm, Rs: 4.7kohm, load capacitances 18pF and 12pF. Actually it works without any problems under normal conditions. We lived some problems after mass productions with our crystal circuit. Most of them didn't work (doesn't start-up). To modify them, we unpluged the MCU, cleaned the PCB and soldered the same MCU again. They start working.
So, I wonder if there is a way to improve moisture and flux immunity of our low power 32768 xtal circuit.
MCU is LC60 and we have to use that High Gain Oscilator(HGO) is disabled for our application.