Quartz Crystal Units
n Precautions for Use
Please read the following precautions regarding correct use of
NDK’ s crystal units and to ensure optimum performance over a
long time.
1MΩ
※
1. To ensure good electrical performance
C1
(For circuit constants,
refer to p. 7 of the material.)
C2
Fig. 11 Example of Crystal
Oscillation Circuit
− A simple test to check an oscillation circuit −
Insert a fixed resistor corresponding to the desired oscillation
margin to a crystal unit in series. (See the position with the * mark
in Fig. 11) Then, switch on & off several times. Make sure that
oscillation starts each time without any delay. (In this case,
because of the series connection, oscillation frequency is not the
same as nominal frequency.) In this test, if oscillation does not
start, there is a delay or oscillation is unstable, it can be assumed
that the amplitude condition mentioned before is not sufficiently
satisfied, and the composition of the oscillation circuit is wrong
and requires improvement. If oscillation starts easily and is stable,
then, remove the inserted fixed resistance and use the circuit.
1-3 Drive level of a crystal unit
Table 1 in p. 25 shows the mechanical oscillation modes of a
crystal unit. However, without some limitation on the mechanical
vibrations of a crystal unit, continuity of frequency may be lost at
specific temperatures, or the effective resistance of the crystal unit
may increase; therefore, use the crystal unit at an appropriate
drive level.
When high frequency stability is required for such applications as
mobile communications, it is recommended for use in the range
between 10 µW and 100 µW.
1-4 Frequency / temperature characteristics
Frequency / temperature characteristics of a crystal unit used
alone are different from those of a unit installed as an oscillator. If
the standards for frequency / temperature characteristics of
oscillation circuits are narrowed, some circuits may not meet the
standards. This is because not only crystal units but also
oscillation circuits have temperature / frequency characteristics. In
such cases, it will be necessary to carefully check the frequency /
temperature characteristics of the oscillation circuit to be used,
and then place an order for a crystal unit with frequency /
temperature characteristics capable of correcting the difference
(see Fig. 12).
If more strict specifications are required, we recommend that you
use a temperature-compensated crystal oscillator. Refer to our
technical data sheet on crystal oscillators.
4
×10-6
Frequency change rate
1-1 Crystal Oscillation Circuit
Crystal units are passive products like resistors and condensers.
Therefore, in order to ensure a rapid start-up of oscillation and to
obtain the required stable precise oscillation frequency, it is
essential that the optimum oscillation circuit conditions are taken
into consideration.
Please refer to typical oscillation circuits listed on pages 28 to 30.
The oscillation frequency of a crystal unit is determined by load
capacitance (CL) and the crystal unit’ s own equivalent constants.
Although the values are fixed by the circuit, with regard to the
circuit constants given in the examples, due to differences in the
type of IC or transistor used, or different wiring patterns, the
characteristics may be different.
Load capacitance for a basic oscillation circuit shown in Fig. 11
can be roughly found by using the following formula.
CL = {C1 C2 / (C1 + C2)} + CS + CIC
CS: Stay capacitance, CIC: IC's input/output capacitance
CL: Load capacitance, C1 = C2 = Capacitor which is connected
For example, when CS = 2 pF, CIC = 4 pF, C1 = C2 = 20 pF,
calculated load capacitance (CL) gives CL = 16 pF. In such a case, it
is essential to use a crystal unit with a center frequency designed to
oscillate with CL = 16 pF.
1-2 Oscillation circuit, oscillation margin and check method
Fig. 9 (p. 27) shows a crystal unit and oscillation circuit when
oscillation has started and reached a stable condition. This
indicates a series circuit with negative resistance –R and load
capacitance CL. The crystal unit side becomes equivalent to a
series circuit with the effective inductance, X = ωLe, and the
effective resistance Re (corresponding to R1 in p. 6). In this case,
it is necessary to satisfy the following conditions simultaneously
for oscillation.
(1) Phase condition:ωLe = 1 / ωCL = 0
(2) Amplitude condition: Re ≤ | −R | (−R is negative resistance)
(1) Phase condition fixes the oscillation frequency, and this is
determined by load capacitance CL as mentioned above.
(2) The correct amplitude condition is essential to obtain a stable
oscillation frequency for start-up and to ensure continuous
oscillation.
It is necessary to design the circuit so that the absolute value of
the negative resistance (−R) of the circuit is sufficiently higher
than the effective resistance (Re) at the time of start-up. The
higher the negative resistance, the higher the performance, i.e.
the greater the oscillation margin of the oscillation circuit.
(Oscillation margin) = | −R | − (Re)
Although the required oscillation margin value is significantly
affected by the choice of product application, environmental
conditions, frequency or crystal’ s model name and
characteristics, common minimum values are 300 to 3000 Ω.
Check and ensure the oscillation margin, if not the crystal unit will
not function as a crystal unit in the oscillation circuit.
R1
3
2
Oscillator
1
0
Oscillation circuit
-1
-2
Crystal unit
-3
0
25
50
℃
Fig. 12 Influence Exerted
on Frequency / Temperature Characteristics by a Circuit
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Quartz Crystal Units
n Precautions for Use
2. To satisfy functional performance
requirements
According to the particular internal structure, the inside of the
holder of a crystal unit is evacuated or filled with inert gas to
maintain its characteristics.
2-1 Mounting of surface-mount type crystal unit
(1) Severe temperature change
Under prolonged and repeated severe temperature changes
solder may crack; this is caused by expansion due to the
different temperature coefficients of the print wire board material
and surface-mount type crystal unit ceramic package.
If such conditions are anticipated and to avoid such problems,
please contact us beforehand for temperature conditions, etc.
(2) Shock from automatic mounting
Please take note, during automatic mounting, such processes
as adsorption, chucking, or mounting to the circuit board, may
administer too great a mechanical shock to the crystal unit, and
the electrical characteristics may change or deteriorate.
(3) Stress caused by bending the PC board
If after a crystal unit is soldered to the PC board, the board is
bent, the mechanical stress may cause the soldered part to peel
away or the crystal unit package to crack.
(4) Grounding terminal
If the crystal unit is provided with a grounding terminal, be sure
to solder it to GND or to the power supply terminal. If it is not
grounded, the correct frequency may not be obtained.
2-2 Soldering and ultrasonic cleaning
The soldering temperature conditions of a crystal unit are
designed so as to allow the simultaneous processing of other
electronic components, but depending upon the product type
the conditions may be subject to restrictions. Confirm the
conditions prior to use. Basically, the ultrasonic cleaning of flux
is allowed, but in some cases, the resonance with the oscillation
frequency of the ultrasonic wave cleaner might cause the
characteristics of the crystal unit to deteriorate. Please check all
conditions before cleaning.
2-4 Mounting a lead-mount type crystal unit
(1)Mount a crystal unit on the PC board so that the height of the
unit is lower than those of other parts; this will prevent the
holder-base glass from breakage caused by shocks given from
the upper side. Breakage of the glass may affect the airtight
seal causing a deterioration of performance.
(2) When mounting a lead-mount type crystal unit in contact
with a PC board, the distance between the holes on the PC
board should equal the distance between the terminals of the
crystal unit to be mounted.
The slightest error in pitch may cause cracks in the glass
section of the crystal unit holder.
(3) When mounting a lead-mount type crystal unit, we
recommend that the unit should make contact with the PC
board and be soldered in such a way as to prevent fatigue and
breakage of the leads due to mechanical resonance (see Fig.
13).
PC board
Fig. 13 Crystal Unit Mounting Method
(4) After installation of a crystal unit on a PC board, moving the
unit as shown in Fig. 14 causes the holder-base glass to crack
resulting in the deterioration of characteristics. Do not move the
crystal unit in this way.
×
Crystal unit
2-3 Effect of corrosive materials
When a crystal unit contacts salt or corrosive materials or is
exposed for long periods to certain substances in the
atmosphere such as chloride or sulfide-based gases, this may
cause a serious flaw such as the package losing its airtight seal
due to corrosion.
Exercise great care when selecting an adhesive or potting agent
to be used at the perimeter of a crystal unit.
PC board
Fig. 14 Care to be Taken After Mounting a Crystal Unit
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Crystal Units
for Use
■ Precautions
■ XXXXXXXX
3. Reflow Soldering
The figure below shows the standards for reflow soldering
temperature profiles of surface-mount type crystal units.
● Examples of soldering conditions
10s max
260±5℃
Temperature
230℃
150 to 180℃
90±30s
30s
max
30±10s
Time
●Soldering conditions
Peak-temperature: 260±5 °C Max. 10 seconds
Heating conditions: Min 230 °C 30 ±10 seconds
Warm-up rate: Max. 3 °C/second
Cool-down rate: Max. 6 °C/second
Preheating conditions: 150 to 180 °C 90 ±30 seconds
Precautions
Never use these products under any conditions that exceed the
following limits; such use may cause the product’s characteristics to
deteriorate or the product may break.
Heat Resistance of SMD Crystal Products
[Reflow Soldering Heat Resistance]
Peak-temperature: 265 °C, 10 sec.
Heating conditions: Min. 230 °C, 40 sec.
Warm-up rate: 3 °C/second
Cool-down rate: 6 °C/second
Preheating conditions: 150 to 180 °C, 120 seconds
Number of reflow passes: 2
[Manual Soldering Heat Resistance]
Use condition: Apply 400 °C soldering iron to product terminal
electrode for 4 seconds.
Number of applications: 2
(1) Glass-sealed product
When using a soldering iron for soldering glass-sealed
products, apply the iron tip below the sealed part to prevent
the iron touching the sealed glass part
(if the iron tip touches the glass part, the glass may melt and
the inner airtight seal may be destroyed).
(2) Au/Sn-sealed product
Do not touch the tip of a soldering iron to the sealed part of
an Au/Sn-sealed product.
(The iron tip may melt the sealant and break the airtight seal.)
In addition, if possible, it is recommended that this product it
to be mounted with reflow without using a soldering iron or an
air-heater.
In purpose of reworking crystal unit, during removing from
the board or module, or removing module from board, any
excessive heat may melt the Au/Sn sealant, resulting in the
deterioration of characteristics or the breaking of the airtight
seal. Therefore, please handle this product with particular
care to the above precautions. However, in case an air-heater
is need to be used, do not exceed below heating conditions.
Air-heater temperature: 280 °C, time: 10 seconds
Heat Resistance of Crystal Products other than SMD
[Reflow Soldering Heat Resistance]
Soldering temperature: 265 °C, 10 sec.
Number of flow applications: 2
[Manual Soldering Heat Resistance]
Use conditions: Apply 400 °C soldering iron to product
terminal electrode for 4 seconds.
Number of applications: 2
4. PC boards
When mounting SMD crystal devices on glass epoxy boards, FR-4
(JIS:GE) is recommended; it is hard to crack during soldering.
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