78M6612 Hardware Design Guidelines
AN_6612_007
20
Rev 2
6.3 Other Considerations
6.3.1 Solder Flux, Condensation and Other Conducting Materials
A common cause of crystal oscillator malfunction is the buildup of contaminants on the PCB. PCB
contaminants such as flux, humidity, finger prints etc. can create a high-impedance path from one of the
oscillator pins to GND or the V3P3 supply preventing oscillator startup. To overcome this problem, check
for contaminant accumulation between the crystal leads and beneath the 78M6612 package. Board
cleanliness is most critical when using water soluble solder paste.
6.3.2 QFN Substrate Pad
The 789M6612 does not dissipate much heat. Therefore, the size of the underside substrate PCB pad
does not have to be equal to the package pad dimensions. A PCB pad dimension of 50% of the package
pad dimension minimizes flux residue under the device and eliminates solder shorts due to excessive
solder paste from a large pad surface.
6.3.3 Soldering Process
An inappropriate soldering profile can cause excessive stress on the components. Improper handling of
the crystal may cause the fracture of the hermetic seal. This would allow moisture and other contaminants
to infiltrate the case, causing sporadic operation or complete failure. Excessive temperatures or excessive
exposure time to high temperatures due to an inappropriate soldering profile can also damage the crystal.
6.3.4 Oscillator Start-up Time
In general, the startup time for a low-frequency crystal oscillator is longer than for at high-frequency
crystal. For a 32.768 kHz crystal, startup should be within the range of 200 – 400 ms. Startup time
exceeding 700 milliseconds is most likely an indicator that the crystal is having trouble starting at all, and
that the value chosen for the capacitors C1 and C2 does not meet the crystal manufacturer’s
requirements or there other causes such as contaminants (solder flux etc.).
Crystals require a certain amount of power to start into a stable oscillation pattern. Since the power
supplied to the crystal is going to be a function of the power supply, oscillator start-up times are going to
be strongly affected by the rise time of the power supply. Another factor is that a power supply with very
sharp rise times will act like an impulse to the crystal, causing it to start faster when compared to using a
power supply with a very slow rise time. Some crystal characteristics that affect startup time:
•
High Q-factor crystal oscillators typically start slower than crystal oscillators with higher frequency
tolerance.
•
Crystal with low load capacitance typically start faster than crystals requiring high load capacitance.
•
Crystals with low ESR start more quickly than high ESR crystals.
•
Oscillators with high OA (Oscillation Allowance) start faster than low OA crystal oscillators.
To increase the drive strength and obtain a shorter startup time, it is possible to place a resistor between
XOUT and the V3P3 supply.
The value of the resistor should be 2.5 MΩ or higher, as shown in Figure 23.
