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This document addresses application and reliability
characteristics for quartz crystal devices and Surface Acoustic Wave (SAW)
devices. Information pertaining to
military-grade quartz crystal units may be found in MIL-PRF-3098, “General
Specification for Quartz Crystal Units” and MIL-PRF-55310, “General
Specification for Crystal Controlled Oscillators”. Complete listings of the
military specifications are available from DSCC
(Defense Supply Center Columbus) 3990 E. Broad St., Columbus, Ohio,
Quartz crystals have a number of applications and are
employed in a variety of electronic circuits and systems. Quartz is the piezoelectric material used in
most applications. Typical applications
for piezoelectric crystals include frequency control (oscillators), delay lines,
timing, and transducers.
Surface Acoustic Wave (SAW) devices are advanced electronic
components used primarily in high frequency signal processing applications. Typical uses for SAW devices include band
pass filters and delay lines.
Crystals come in the many
different packages, depending upon the application of the device. Surface mount technology is employed for both
ceramic and plastic packages, and through-hole mounted devices are also
available. Oscillator devices are also
available in ceramic and plastic J-leaded configurations and in ceramic leadless
chip form. Oscillators are also
available in through-hole configurations in 14, 8, and 6 pin dual in-line
packages. Recommend that oscillators are
sealed in glass, metal, or ceramic packages, and hermetic packages are utilized
where appropriate. Recommend that no
adhesive or polymeric materials are used for package lid attachment or seal, and
flux is not used in the final sealing process.
Also, recommend that sufficient distance be maintained between the lid
seal and any glass-to-metal seal to preclude damage or seal degradation when a
welding process is used for final lid seal.
The packaging requirements
outlined in MIL-PRF-3098 are recommended for general crystal units and the
packaging requirements in MIL-PRF-55310 are recommended for crystal oscillator
units. SAW devices can also be purchased in a variety of package styles. Metal can, dual in-line package, and flat
pack styles are typical outlines that are used.
Failure Mechanisms and Anomalies
It is estimated that about 90% of quartz crystal
failures are open circuits. The
remaining 10% of failures occur when electrical contact remains but there is a
lack of oscillation due to the loss of the piezoelectric phenomena
characteristic with the crystal structure.
Electrical parameters of
piezoelectric crystals are deteriorated by excessive driving current or from
high voltages that cause mechanical stress and movement to be generated in the
crystal plate. When the voltage is
excessive, mechanical forces cause motion in excess of the elastic limit of the
crystal. This results in crystal
fracture, such as a lifted platelet.
Such fractures, when occurring in sufficient number, will cause enough
change to the operating electrical characteristics for the crystal to go out of
specification or to cease operation entirely.
For crystal units employed in systems for the intended
purpose of frequency control, a crystal may experience a change in its frequency
over of time. This phenomenon is known
as aging, or drift. One of the primary
causes for this is mass transfer to or from the resonator surfaces due to
absorption and release of contamination.
Another factor is the stress relief within the mounting structure or at
the interface between the quartz and the electrodes. The aging rate for crystals tends to be
greatest when the unit is new, as stabilization tends to occur as time
passes. Temperature changes experienced
by crystals can initiate the beginning of a new aging cycle. The aging temperature evaluation techniques
outlined in MIL-PRF-3098 may be used as a guideline in addressing aging
considerations for crystal units.
reliability prediction models for quartz crystal and SAW devices. For quartz crystals, the part failure rate is
based upon the base failure rate, the quality factor of the part, and the
environmental factor (the environment in which the device is used). The base failure rate is a function of the
frequency (in MHz) of the crystal unit.
For SAW devices, the part failure rate is a function of the part quality
factor and the environmental factor.
1. Derating Requirements
Surface Acoustic Wave (SAW) Devices
|Input Power from Max Limit (F > 500 MHz)
|Input Power from Max Limit (F
< 500 MHz)
Standard crystal units do
not require any special derating. The
specified maximum and minimum parameters given in the specification should be
used as limiting factors. Surface
Acoustic Wave (SAW) devices should be derated in accordance with Table 1. The degree of derating is
dependent on the frequency at which the device operates. At frequencies of about 500 MHz, the derating
requirements are relaxed.
Design and Material
A quartz crystal acts as a stable mechanical resonator
and determines the frequency generated in an oscillator circuit. The properties of a crystal device depend
significantly upon the angles at which the wafers are cut from the mother
crystal. In order to cover a wide range
of frequencies, different cuts are used. Table 2 summarizes the significance of these cuts of the crystal material on device
performance. Recommend the specification
be consulted as to the type of material used for a given
2. Common Crystal Material
common), BT, SC (high precision applications)
As with any device, materials
should be used which enable the crystal units to meet defined performance
requirements. Recommend the guidelines
provided in MIL-PRF-3098 and MIL-PRF-55310 be consulted for guidance on proper
material management and usage.
Facility Assessment and Quality
Crystal units should be
manufactured in such a manner as to ensure that quality is uniform and free from
any defects that would adversely affect life, serviceability, or
appearance. The interior of the crystal
unit should not contain flux, particles, residue, or other foreign or
undesirable materials. In addition,
fractures or cracks should be avoided.
Electrode abrasion and crystal element exposure to a halogen vapor should
not be present. Recommend all crystals
be subjected to the qualification and group tests outlined in MIL-PRF-3098
(crystal units) and MIL-PRF-55310 (crystal oscillators) to meet
the highest quality requirements.
However, tests and evaluations may be tailored as needed to assure the
highest quality possible for a given crystal application (dependent upon the
environment in which it is used).
Most crystal manufacturers employ a sampling plan to
control the quality of the product.
Sampling plans vary among manufacturers. Crystals manufactured to MIL-PRF-3098 and
MIL-PRF-55310 require a specific number of samples be evaluated and that a
certain number of defects not be exceeded.
Recommend that these performance specifications be consulted to determine
adequate sampling inspections and recommended sample sizes.
Crystal units and SAW devices
should be purchased from a manufacturing line employing quality controls and a
monitoring system of critical processes. The Component Evaluation System
outlined in Appendix B of MIL-PRF-55310 is recommended for the purchase of
quartz oscillators. In particular, for
crystal oscillators, a demonstrated control of the wire bonding process is
considered a critical process in manufacturing control. Such a process control
program should, for example, include a process machine/operator evaluation, the
destructive evaluation of test samples, elimination of non-conforming process
machines, a detailed corrective action program, the presence of identifiable
information records (e.g., to each machine, operator, work shift, and test
date), a wire bonding strength test plan, and a lot sample bond strength test
plan. Appendix B of MIL-PRF-55310
provides a guideline for the implementation of such a program.
Additional qualification testing
or screening by the user is dependent upon the specific application for which
the device is to be employed. Examples
of qualification and screening plans are outlined in MIL-PRF-3098 and
MIL-PRF-55310 and should be considered for the assessment of crystal units and
crystal oscillators. Product
specifications should be consulted for the recommended screening and
qualification provisions for SAW devices.
Handling and Storage Precautions
The ESD sensitivity of crystals
depends on the particular packaging style and tolerance, with tight tolerance
units considered more sensitive. As a
whole, crystals are not considered particularly ESD sensitive, most can be
treated as Class 3 ESD sensitive.
However, some Surface Acoustic Wave (SAW) devices are considered Class 2
and even Class 1 sensitive, susceptible to ESD voltages of less than 1,000
volts. ESD damage in crystals usually causes only operational degradation, but
further ESD damage to a crystal can result in catastrophic failure.
The use of crystal devices and SAW units manufactured to
military specifications is preferred.
The use of commercially manufactured product may be acceptable only if
appropriate consideration is given to factors such as packaging, quality,
screening, qualification and other factors, e.g. the application and environment
in which the product is used. These factors will affect the reliability of the