The document discusses optimization of the manufacturing process for temperature controlled crystal oscillator (TCXO) designs to improve reliability. It identifies key areas for improvement including the substrate assembly, electrical connections between components, and solder joints between components. Process variables in thick film printing, solder application and reflow, and component tuning were evaluated to minimize variations and improve reliability over the lifetime of the product. Accelerated testing was used to validate reliability improvements implemented through process optimization.

1. Manufacturing Optimization for
Improved Reliability in TCXO Oscillator
Designs
Michael Logue
mlogue@mtronpti.com
Production Engineering, MtronPTI, Yankton, SD/USA
…
Robert Cremins
cremir@us.ibm.com
Procurement Engineering, IBM, Hopewell Junction,
NY/USA
Abstract:
The reliability of Temperature Controlled Crystal Oscillator
designs can be improved through process optimization and
controlling key process variables. Industry demands for a
highly reliable TCXO combines the need for the frequency
accuracy of complex oscillator designs with long-term
reliability. A project intended to improve the long-term
reliability focused on key process steps of oscillator thick
film substrate assembly including design layout, solder
application, solder reflow, and component circuit tuning.
The project starting point was an oscillator design
statistically capable of meeting the customer’s
requirements. Design layouts were evaluated to minimize
unbalanced or abnormal component stresses. The solder
application and solder reflow process points were closely
evaluated through a Design Of Experiments to provide the
most robust construction. Component circuit tuning is a
common practice in TCXO and OCXO products. Because
of the manual nature of component selecting it is difficult
to maintain quality control. Focused reliability testing,
along with accelerated life testing verified or identified
product and process improvements. Optimization was
measured through a quantifiable increase of performance
during qualification tests, accelerated life test, test to
failure, and ultimately field performance. Selecting the
correct test method to measure meaningful reliability
improvement was a key element in the project. Test to
failure and accelerated life tests identified reliability
optimization opportunities on parts that successfully passed
all required qualification tests.
Introduction:
As is often the case, a customer may have a timing
application requirement which cannot be easily solutioned
with any standard, catalog oscillator component offered by
any supplier. Many of these situations may involve
relatively low volumes but have extreme value to the
customer due to their criticality in the ongoing functionality
of the end product application. In these cases, suppliers
willing to pursue such opportunities must first analyze the
customer requirements and then compare them to both their
current technology capabilities and the flexibility of their
manufacturing processes. If an existing technology and its
associated manufacturing process are both capable and
flexible enough, it is both technologically possible and
economically feasible for both the customer and the
supplier to work together to develop and optimize an
application specific solution.
In the subject of this paper, the end customer required a
Temperature Compensated Crystal Oscillator (TCXO) with
customized specifications and an extremely high level of
reliability performance over a multi-year period.
TCXO Design overview:
Common TCXO designs consist of a thermistor network
with thermal characteristics approximately equal but
opposite in temperature coefficient of the crystal. The
thermistor network provides a voltage change creating a
capacitance variation when combined with a varactor. The
changing crystal load capacitance pulls the frequency
oscillation of the crystal. The thermistor and varactor make
up the TCXO temperature correcting circuit. Each crystal
will require an individually tuned thermistor network. A
well understood and highly stable thermal environment is
required for accurate circuit measurement and tuning
during manufacturing. As a result a TCXOs’ performance
capability is closely tied to the capability of the
manufacturing process to measure oscillator circuit
variations over temperature and tune each individual device
to the electrical values required.
The first step in determining the feasibility of providing a
solution to a customer’s requirement for an oscillator with
nonstandard functionality and a high level of reliability is
selecting the optimum overall design platform. The
intrinsic reliability of an oscillator is based upon its
fundamental design. (Christiansen, 1996, p. 6.1) As is the
case with any oscillator, the analysis begins with the
frequency and frequency tolerance over temperature
required by the customer. The tighter the frequency
requirement, the more complex the oscillator design
typically is. In this case, a pre-existing hybrid microcircuit
technology and manufacturing process employing discrete
subcomponents mounted on a thick film substrate was
chosen as the design platform.
Once a product family has been found that can theoretically
support the customer’s requirements, the design platform
may need to be adjusted to fit the specific application.
Several prototype builds may be required to verify the
functionality of the design concept. In parallel with the
prototype builds, the design team will also complete a
computer simulation. “A large number of variables affect
oscillator operation, and if the performance is inadequate,
the apprentice is uncertain about a solution. Although
much literature exists on the subject of oscillators,
references typically address specific oscillator types. A
fundamental understanding of the concepts is all too often
buried in pages of equations.” (Rhea, 1990, p.xi-xii).
TCXO Manufacturing Process overview: Hybrid
microcircuit technologies commonly consist of thick-film
printed on a ceramic substrate. A standard thick film
process consists of screen-printing thick film paste onto the

2. f(t)
substrate, drying the substrate then firing the substrate
through a high temperature furnace. A major advantage of
this design platform is the usage of thick film resistors
which can be laser trimmed to precise values of +/-1.0%.
Precise resistors, tuned to a specific value, increase a
TCXO design’s capability and ultimately increase the
flexibility of the design platform to provide a variety of
customizable solutions.
Process optimization is defined as achieving control by
centering the process on the target output. Controlling key
variables tightens the standard deviation of the output
generated by a process. A better-targeted process with less
deviation produces a more predictable, repeatable and
reliable product. Thick film and component assembly
process variables can easily impact the internal circuit
performance of the TCXO and therefore must be identified
and well understood. A baseline must be established and
continuous, in-line measurements need to be implemented
to monitor and control process manufacturing variations as
they occur.
TCXO Reliability Overview:
Reliability is defined as the ability of a device to operate as
intended under a defined set of conditions. Failure is used
to describe a device that does not perform as intended.
Reliability is measured as the probability that a device will
perform satisfactorily for the predetermined time. A
product’s failure rate can be broken down into three basic
periods referenced to time; early failure period (infant
mortality), intrinsic (random) failure period, and wear out
failure period. The failure rate when charted verses time is
commonly called the bathtub curve.
“The general approach to reliability for electronic systems
is to minimize early failures by emphasizing factory test
and inspection and to prevent wear-out failures by
replacing short-lived parts. Consequently, the useful life
period characterized by stress-related failures is the most
important period and the one (period) which design
attention is primarily addressed.” (Christiansen, 1996,
p.6.5)
Manufacturing defects are generally the main cause of
undesirable infant mortality failures and a major concern to
end customers. “Mechanical defects such as weak wire
bonds, poor pad adhesion, defective sub-components and
partially cracked or chipped ceramics constitute a
significant portion of infant mortality failures.”
(Christiansen, 1996, p. 6.42). Electro-static Discharge
(ESD) induced damage, which may also result from
manufacturing or handling errors, can potentially cause
latent defects which may manifest themselves as either
early life failures or wear-out later in the products life.
Ideally, a highly reliable product must have a very low
early life failure rate together with a wear-out level that
manifests itself only well after the end products’ anticipated
life is over. To insure the overall reliability of a TCXO,
production and assembly techniques as well as design and
testing methodologies must be assessed. The reliability of
an oscillator is commonly lower than the intrinsic level
because of faulty procedures during manufacturing.
(Christiansen, 1996, p. 6.1) Therefore, manufacturing must
conduct a thorough process review to eliminate faulty
procedures.
Material and sub-component selection must also be
evaluated for individual reliability performance under the
specific TCXO circuit conditions. Independent
components require the probability of system failure to be
calculated as independent events. As the oscillator circuit
becomes more complex with the addition of the
temperature correcting circuit, the reliability of the system
can decrease. “Even though the individual components of a
system might have high reliabilities, the system as a whole
can have considerably less reliability because all
components that are in series must function. As the
number of components in a series increases, the system
reliability decreases.” (Stevenson, 2005, p.156).
TCXO Qualification:
As mentioned earlier, the customer required that a very
high level of sustained reliability over a multi-year lifetime
be demonstrated. The customer therefore defined a series of
complex and detailed qualification procedures in order to
detect the presence of any failure mechanism, wear-out
phenomenon, manufacturing process weakness or other
concern, which could potentially have, even a small,
negative impact on this requirement. The results of these
procedures were jointly analyzed by both the customer and
the supplier. Both teams then jointly identified
improvements to both the TCXO design and the associated
manufacturing process that will be covered later in this
paper. Once implemented by the supplier and optimized
via ‘design of experiments’, selected portions of these
qualification procedures were later repeated to validate the
effectiveness of the improvements.
The following are the high lights of these qualification
procedures.
First, the customer required that several ‘production ready’
samples be submitted for a ‘construction analysis’ report.
A construction analysis involves a complete decapsulation,
dissection, physical analysis and general reverse
engineering of the oscillator component. All critical
dimensions, interfaces and materials are analyzed,
photographed and/or documented via assorted analytical
time
Intrinsic
failure
Wear-out
failure
Early
failure

3. means such as EDX, X-ray, cross-sectioning and extremely
high power magnification equipment (SEM). In addition,
the level of workmanship and process consistency both
within the component and between components made via
the same manufacturing process is readily evident and can
therefore be assessed. The final report, which can be
shared between customer and supplier, includes this
assessment as well as identification of potential concerns.
The personnel assigned by the customer to perform this
work and prepare the final report, ideally have a wealth of
historical experience, which they can bring to each
analysis. The report has an additional, continuing value in
that it provides for a common baseline understanding of the
component structure, which enables both customer and
supplier to have a knowledgeable dialog on both potential
weaknesses and proposed improvements.
As already discussed, the reliability of a component is
largely a function of the manufacturing process which
produces it. A component with a high degree of
interconnect complexity will require a large number of
process steps to produce. The greater the number of
process steps, the greater the potential for error or
weakness. Due to the complexity of this TCXO, the
customer felt strongly that the qualification process needed
to include a full review and assessment of the
manufacturing process. This was performed via on-site
audit visits by customer process and material personnel
with significant expertise and experience. During these
visits, general capability, operator certification and quality
control practices of the suppliers’ factory are first discussed
in detail. This is then followed by an in depth review and
explanation, on paper, of the specific manufacturing
process steps used to produce the TCXO in question to
insure a common understanding of the purpose and details
of each step. The customer is then physically taken to and
shown the work area and/or manufacturing tool set
associated with each process step in sequence starting with
incoming receipt of raw materials and finishing with final
TCXO test, pack and ship. At each manufacturing step,
process control methods and resultant metrics are reviewed,
general cleanliness and organization are assessed and ‘on-
the-spot’ interviews are conducted with operators to
determine their capability and understanding of their
respective processes. Supplier manufacturing, design and
product engineering personnel from both management and
working level ranks participate together with the customer
during each stage of this process which can take several
days to complete. At the conclusion of the visit, the
customer compiles their findings, observations and
concerns into a presentation given directly and immediately
to the supplier management and staff. Findings are broken
in to major concerns, minor concerns, recommendations
and items requiring follow-on explanation. Soon after the
visit, the customer provides the supplier with a detailed,
written report, which includes all the findings. As with the
construction analysis report, the audit report provides for a
common vehicle between supplier and customer to
communicate and track corrective actions and
improvements to the manufacturing process.
The last portion of the qualification procedure that we will
discuss is the actual reliability testing typically referred to
as ‘life’ or ‘accelerated stress’ testing. Accelerated testing
is effective in detecting the existence of and measuring the
level of assorted failure mechanisms within the TCXO
throughout its’ anticipated life. By increasing the operating
temperature and/or operating voltage/current on test
samples, acceleration can be achieved and the devices’
anticipated life cycle can be simulated. A proper reliability
test matrix is designed to detect the presence of and
measure the level of the various early life failure and/or
wear-out mechanisms, which might be present in the
TCXO. As with manufacturing process steps, a device
with numerous interconnects, materials and sub-
components could be susceptible to a wide variety of
mechanisms. Therefore the testing must be designed to
detect a wide variety of mechanisms. Another factor in
defining the test matrix is the equipment and test
capabilities available. As is often the case, due to the need
to have appropriate device level test equipment to
periodically test electrical parameters of devices under
stress, supplier resources were used for the majority of the
testing.
As with the construction analysis, all test cells consisted of
production level samples made, where possible, from
different manufacturing runs. The following table
describes the major reliability stress tests performed on the
TCXO:

4. Focused Improvement Areas:
Through the use of these qualification procedures, the
following items within the TCXO product design and its
associated manufacturing process were discovered and
selected for focused improvement:
• The TCXO construction employs a two sided
substrate, mounted on a metal can header base
assembly. The bottom of the substrate is
connected to the header base via non-conductive
epoxy, which also acts to electrically and
physically isolate the header from the substrate.
The general internal construction of the device is
shown below. Elevated temperature and voltage
life testing produced several frequency shift
failures late in the test. The root cause was
determined to be a high temperature wearout of
the non-conductive epoxy and substrate
glassivation, which resulted in a leakage path
from header to substrate through the non-
conductive epoxy and glassivation in areas of
minimal spacing between base and substrate.
The construction analysis report and audit
confirmed that variations were possible in the
amount of applied non-conductive epoxy and
associated header to substrate spacing. Applying
an industry standard acceleration model for
dielectric breakdown indicated that the failure
rate adder for this mechanism under normal use
conditions was extremely small however
improvements were possible.
• The integrity of the electrical connections
between the header I/O pins and the substrate is a
function of the material used to make the
contacts, the method applied and the amount of
contact area. Also, as is evident in the depiction
of the device construction shown, the integrity of
these connections is largely dependent on the
same non-conductive epoxy connection discussed
above which provides the basic mechanical
strength to connect the header to the substrate. It
was determined that variations in this header to
substrate non-conductive epoxy bond or
excessive customer handling after shipment could
result in cracking or degradation at one or more
of the I/O contact points which in turn could
cause intermittent TCXO output failures. This
mechanism could possibly escape initial
customer test and cause an intermittent ‘no
oscillation output’ field failure later in life.
• The TCXO contains a number of discrete
components mounted via tin-lead based solder on
to silver palladium runs on a ceramic substrate.
TEST TYPE TEST CONDITIONS
Preconditioning Assorted temperature and
environmental extremes to simulate
transportation, storage and attachment
of the component to the next level
assembly. This includes temperature
cycling, humidity soaking, flux
application and temperature shocking
to simulate solder assembly. This
procedure is performed on all stress
test samples prior to the start of test.
High
Temperature
Operating Life
Tests
Several test cells are used with
different applied voltages varying from
nominal to max Vcc and temperatures
varying from 85 to 125 degrees C. All
samples are fully loaded. Tests are
conducted for 2000 hours and all
samples are extracted periodically to
measure and record electrical
parametrics.
Temperature
Cycling
Samples are unbiased and put in a
temperature cycling chamber. Several
test cells are used with temperature
deltas varying from 100 to 165 degrees
C and different ramp rates and/or
number of cycles/per hour. A
minimum of 1000 cycles is required
but tests with smaller temperature
deltas are extended longer to better
understand wearout mechanisms.
Temperature
Shock
Samples are unbiased and put in a
temperature shock chamber. Several
test cells are used with temperature
deltas larger than that of temperature
cycling. Ramp rates and/or number of
cycles/per hour are also more
aggressive than that of temperature
cycling. A minimum of 100 cycles is
required but tests are again extended to
better understand wearout
mechanisms.
Accelerated
Aging
Samples are unbiased and put in a
temperature chamber. Again several
test cells are used with temperatures
varying from 85 to 125 degrees C
Drop Shock &
Mechanical
Vibration
Assorted drop and mechanical tests
based largely on MIL standards with
minor changes to address customer
application specific concerns
HEADER
CERAMIC
SUBSTRATE
Thickfilm
i it
Solder
CRYSTAL
Pin
Epoxy
Pin

5. It is well known in such situations that the tin
within the solder composition can to be absorbed
into the silver and possibly degrade the integrity
of the solder joints involved. Further, the
application of heat accelerates this absorption and
degradation process. This phenomenon is
unavoidable without radically changing the
materials involved. It is however considered
benign if proper process controls and associated
temperatures are well controlled and resultant
parts are properly tested. In the case of this
TCXO, there are numerous manufacturing
process and reliability test steps involving heat
application, which could accelerate this
mechanism. Construction analysis and
accelerated testing indicated the presence of this
mechanism at varying levels. As discussed, this
mechanism is a concern since it causes degraded
solder joints which can manifest themselves as
intermittent electrical failures. Yet, some level of
this mechanism is unavoidable. The challenge
was to optimize the device construction and
manufacturing process to minimize this
phenomenon and, as an additional precaution,
establish appropriate screening methods to detect
worst case situations and prevent them from
being shipped to the customer.
• In line with the above challenge is the difficulty
associated with defining and controlling the
processes associated with solder dispensing and
reflowing. The amount of solder and the reflow
temperature will affect not only the amount of tin
absorption but the overall integrity of all the
internal solder joints. Unfortunately, due to the
detrimental impact of heat on the components
within the TCXO and the high performance
customer specifications, a traditional enclosed
reflow oven with a well controlled temperature
profile could not be used. Instead, open air
equipment, which focused heat on the bottom of
the substrate while minimizing heat on the top
side components was required. Although
necessary, this process methodology created
additional variables, which needed to be assessed
and controlled. The customer audit identified this
process step and its associated inspection areas as
a major concern and focus area for improvement.
Design & Assembly Enhancements:
The two-sided TCXO construction requires the mounting
of a ceramic substrate on a metal header assembly. The
bottom side of the circuit, with the base conductor metal of
a silver palladium compound, is connected via non-
conductive epoxy to the metal header base. Reliability
testing along with failure mode and effects analysis
provided an opportunity to improve this design. High
Temperature Operating Life tests, employing elevated
temperature, voltage and current proved to be effective in
detecting a measurable, potential wear-out mechanism.
Between one and two thousand hours of testing at 125
degree C., several minor frequency shift failures were
encountered. Electrical analysis determined that the
frequency shift was caused by a low resistance leakage path
between the bottom of the substrate and the metal header.
Subsequent physical analysis concluded that the leakage
path resulted from a combination of wear-out of the non-
conductive epoxy and substrate glassivation materials
together with manufacturing variation in the physical
distance between header and substrate. The net effect was
a change in the oscillators’ output frequency which
obviously could have an adverse impact on the customer.
The usage specifications of both the non-conductive epoxy
and the glassivation were reviewed and discussions were
held with the suppliers of both materials. Unfortunately,
the capability and performance of each material at the
stress temperatures applied was unclear. Therefore it was
difficult to determine a proper acceleration factor and
assess the impact of this mechanism over the life of the
TCXO. By using a standard arrehenius model, plotting the
time-to-fail distribution and varying the activation energy
(Ea), the customer estimated the range of potential impact
of this failure mechanism as follows:
Time Interval
(in hours)
Interval Failure
rate with Ea=1.0ev
Interval Failure
rate with Ea=0.5ev
0-8620 0 72 ppm/thousand
hours
8620-20K 0 293 ppm/thousand
hours
20-40K 0 527 ppm/thousand
hours
40-60K 0 694 ppm/thousand
hours
60-100K 0.4 ppm/thousand
hours
800 ppm/thousand
hours
Although potentially insignificant, the uncertainty involved
in modeling this mechanism was enough to warrant
improvement in the design construction and associated
manufacturing process.
To minimize the likelihood of substrate to metal header
shorting, the initial thought was to modify the TCXO
construction by increasing and guaranteeing a fixed,
consistent distance between the substrate and metal header
base with the use of physical assembly aids inserted
between substrate and header during manufacturing.
However, the increased distance adversely affected the
adhesive strength of the non-conductive epoxy connection.
The strength of this adhesion had already been identified as
a concern since the mechanical integrity of the I/O pins
connection to the substrate is completely dependent on the
non-conductive epoxy adhesion. Changing the composition
and application of the I/O pin to substrate connection from
conductive epoxy to solder and eliminating the non-
conductive epoxy all together was considered and

6. experimented with in an attempt to increase the strength of
these connections. Although the soldered pins
demonstrated increased mechanical strength, the high
temperatures associated with any type of additional
soldering process proved detrimental to the nearby
components and associated solder joints already placed on
the substrate. Temperature cycling stress testing proved
that the additional heat treatment further aggravated the tin
absorption and associated solder joint integrity. As a result,
the conductive epoxy connections had to be retained.
To compensate and address the concerns, the amount and
placement of the non-conductive epoxy was modified to
optimize adhesion and strength. The original design used
two separate epoxy bond applications with one at each end
of the header base. Uneven epoxy dispensing and manual
placement allowed for potential unevenness or ‘see-sawing’
within a module. The modified design employed a single
and larger amount of epoxy placed in the center of the base.
This facilitated the use of the physical assembly aids which
in turn insured a consistent and even spacing between the
header base and substrate. Also, the cleanliness
requirements and incoming inspection criteria for the
procured metal header base were tightened to insure an
optimal surface for epoxy adhesion. The header base pin
height requirement was also increased compared to the
standard height to compensate for the higher substrate
mounted position and also provide additional contact area
and increased strength for the conductive epoxy pin
connections.
The modified assembly technique was again tested via the
same High Temperature Operating Life methodology. The
wear-out mechanism, previously detected between one and
two thousand hours of test, was no longer evident. The
increased and consistent distance between header and
substrate proved to be effective.
In depth thermal shock, vibration and drop testing was then
performed to assess the overall mechanical strength of the
modified TCXO construction. To demonstrate the
effectiveness of the design, three groups of test samples
were built. The first group of samples was built with oil
intentionally applied to the metal header to represent
contamination which would negatively impact epoxy
adhesion (labeled ‘C’ for contaminated). The second group
was constructed without any epoxy (labeled ‘N’ for no
epoxy) to simulate the worst case condition of no adhesion.
The third group was constructed normally (labeled ‘G’ for
good). Samples from each group were tested both as
individual components and as part of assemblies after being
attached to printed circuit boards to better simulate
anticipated application conditions and normal stress on the
I/O pins. Loose parts underwent incremental drop testing
from 1 to 36 inches and incremental thermal shocking of 10
cycles starting at -40 to 60 with increasing 10 degree
increments. Card-assembled parts underwent incremental
thermal shock and drop as well as incremental thermal
shock and vibration.
The results, depicted in the table below, demonstrated an
acceptable level of mechanical robustness even in samples
made without the non-conductive epoxy. Failures
occurred primarily due to extreme thermal shocks which
were deemed well beyond anticipated typical usage.
Solder Joint and Assembly process Optimization:
The next challenge was associated with the solder
attachment of the various sub-components within the
TCXO assembly. As discussed, tin within standard solder
composition can to be absorbed into the silver-palladium,
causing brittleness in the contact area and degraded
adhesion of the silver palladium to its ceramic base. This
can manifest itself as a weak or degraded solder joint which
may crack or even open as a result of normal expansion and
contraction of materials and sub-components within the
TCXO. It may also appear as an intermittent electrical
failure which may or may not be screened out during
normal electrical or visual inspections. This absorption
mechanism is further accelerated by any heat application.
Unfortunately, the manufacturing process associated with
this custom TCXO required several heat treatments for
assorted epoxy curing steps well after component soldering.
As a result, this mechanism which could result in either
early life or wear-out failures was targeted for in-depth
analysis.
Although traditional Temperature Cycling stress testing did
not result in any electrical failures, physical analysis of post
stress samples indicated a level of potential variability
which was deemed unacceptable by the customer. A high
magnification cross-sectional photo of a worst case, post
stress solder joint showing degradation and fracturing at the
silver-palladium to ceramic interface is shown below.
A Design of Experiments was conducted to assess the
impact of temperature on the resultant solder joint integrity.
“DOE techniques offer a structured approach for changing
No Fails
1 fail @90C Tmax
1 fail @100CTmax
1 fail @120CTmax
1 fail @130CTmax
No Fails
Thermal-Shock
(-40 to130 C)
No FailsNo FailsG
No Fails1fail @120 C/ 30”
(Failed in drop test)
N
No FailsNo FailsC
PARTS
ON
CARDS
No FailsG
No FailsN
No FailsC
LOOSE
PARTS
Thermal-Shock &Vibration
(-40 to 150 C/ .01 to 1G2 /Hz)
Thermal-Shock &Drop
(-40 to 130 C/ 1to 36”)
Drop
(1 - 36”)
RESULTS
No Fails
1 fail @90C Tmax
1 fail @100CTmax
1 fail @120CTmax
1 fail @130CTmax
No Fails
Thermal-Shock
(-40 to130 C)
No FailsNo FailsG
No Fails1fail @120 C/ 30”
(Failed in drop test)
N
No FailsNo FailsC
PARTS
ON
CARDS
No FailsG
No FailsN
No FailsC
LOOSE
PARTS
Thermal-Shock &Vibration
(-40 to 150 C/ .01 to 1G2 /Hz)
Thermal-Shock &Drop
(-40 to 130 C/ 1to 36”)
Drop
(1 - 36”)
RESULTS

7. many factor settings within a process at once and observing
the data collectively for improvements/degradations.”
(Breyfogle, 2003, p.549). Test vehicles were constructed
and reflowed, first with a constant temperature (210
degrees C.) and varying the reflow time from 5 to 40
seconds, then with a constant time (20 seconds) and
varying temperature. EDAX scans were then performed to
determine material composition and assess deltas in the
level of resultant tin penetration. Results, shown below,
indicate a dramatic increase in the tin penetration level
occurs from a relatively small increase in either reflow time
or temperature. It is therefore clear that the reflow process
as well as any subsequent heat treatment must be tightly
controlled in order to minimize variations in this
mechanism. Obviously, any additional reflows or
component reworking in manufacturing will further
increase the penetration and cause a drop in adhesion. This
underscores the importance of not reworking or replacing
components after the initial reflow.
C o n s t a n t T e m p . ( 2 1 0 C ) C o n s t a n t T im e ( 2 0 s )
2 1 0 C
2 3 0 C
5 s
4 0 s
A g - P d
S n - P b
C e r a m i c
I n t e r f a c e
With this knowledge in hand, solder paste storage, solder
amount, solder application, component mount and solder
reflow processes were all reviewed in depth to drive
consistent and improved process control. Enhancements
were first made in the solder stencil design to reduce solder
application and thickness variation across the substrate.
Along with this, the solder dispense tool calibration
procedures and associated thickness measurement methods
were reviewed and updated where needed. Cleaning and
storage of stencils was also identified as a potential concern
and improvement actions were again taken where
appropriate.
Focus was then placed on the reflow process itself. Reflow
conditions have always been closely tied to product
reliability. Inappropriate reflow process can damage
components or substrate integrity. During solder reflow;
maximum temperature, rate of heating, time spent at each
temperature zone, heat control, cooling rate and belt speed
must all be monitored continuously. As mentioned earlier,
non-conventional open air reflow equipment was required
in the assembly of this TCXO product to minimize
detrimental high temperature impacts on the sub-
component content performance. It was determined that
normal variations in the manufacturing room temperature
and/or air flow could change the reflow conditions seen by
the TCXO. As a result, the supplier modified their physical
assembly area to reduce any variation and implemented
equipment to closely maintain and monitor the air
conditions near and around the reflow equipment. Also,
new reflow profiling vehicles, updated measurement
methodologies and a tighter allowable reflow profile were
all defined and implemented.
The TCXO substrate itself was then modified to improve
the overall robustness of the design. First, the wiring
layout and placement of sub-components was changed with
a careful eye to identify and eliminate any area where
unnecessary mechanical stress could be put on solder joints
due to normal thermal expansion or contraction of materials
during either TCXO manufacturing, next level assembly or
end product life. For example, sharing of a pad between
subcomponents was not allowed. Second, the thickness of
the silver palladium wiring runs on the substrate was
increased at and near any location where solder was to be
applied. This increased the silver material content at the
solder joint and thus decreased the impact of any tin
absorption.
TCXOs and Oven Controlled Crystal Oscillators (OCXO)
often require component ‘selecting’ after the oscillator has
been assembled because of variations in multiple
components. Stacked up tolerances often exceed the design
window for tuning the oscillators to frequency
specification. A tolerance analysis is used to show the
design’s operating window. The tolerance build up of
varying circuit components can lead to an extreme variation
in a critical variable. This variation may start the oscillator
at a frequency outside of the tuning window. Component
‘selecting’ or replacing is used to set the oscillator
frequency back into the tuning window. During component
selecting, typically a previously mounted component is
unsoldered, removed and replaced with an alternate
component of a different electrical value. In
manufacturing, this is often a manual, soldering rework
operation prone to reliability failures. Such failures could
easily be caused by the tin absorption mechanism discussed
earlier and greatly aggravated by the additional focused
heat application associated with solder rework. It is
therefore clear that this component selecting or solder
rework process must be avoided if a high reliability product
is desired. To achieve this, tight tolerance components
with values statistically chosen to center the tuning window
must first be used. Second, a manufacturing process with
high accuracy laser trimming of thick-film resistors with a
wide tuning window is applied. The wide tuning window
allows a larger window of oscillator frequencies to be
tuned. Laser tuned resistors and/or capacitors provide a
clean and efficient method of varying the oscillator circuit’s
electrical characteristics and greatly minimize the need to
select or replace components resulting in a more reliable
TCXO.
Finally, appropriate final screening and test techniques
were established and implemented to detect and eliminate
any potentially weak solder joints prior to customer
shipment. Prior to final frequency tuning, all units are
subjected to 168 hours of 125 degree C. bake followed by
10 temp cycles with a minimal temperature delta of 100

8. degrees C. This not only serves to aggravate and detect
weak solder joints but also to age the on-board crystal.
Also, all completed units undergo temperature step testing
in which all parameters are measured at various
temperatures beginning at the maximum operating
temperature and proceeding to the minimum temperature in
5-10 degree decrements. This sequence was proven to be
effective in detecting weak solder joints under stress since
the high temperature first creates expansion which is
followed closely by contraction as the temperature drops.
The improvements in the solder process, testing and
resultant solder joint integrity were verified using
component shear testing, component pull strength
measurements and extensive reliability testing employing
both temperature cycling and temperature shock stress.
Results indicated that a reliable bond was achieved with
strong adhesion and minimal variability between units.
Conclusion:
Manufacturing optimization begins with a design capable
of meeting the customer’s specification. The adaptation
and modification of a standard design platform for a high
reliability application will require a thorough design and
manufacturing process review to understand any potential
failure mechanism. The manufacturing process has a large
impact on early life failures. Design for manufacturability
improvements along with increased process monitoring and
controls will improve the reliability of a product.
Accelerated stress testing also provides key information for
reliability prediction as well as identifying weak links in a
design or its’ manufacturing assembly methods.
“Reliability prediction, Failure Modes and Effects Analysis
(FMEA), and reliability growth techniques represent
prediction and design evaluation methods that provide a
quantitative measure of how reliably a design will perform.
These techniques help determine where a design can be
improved. Since specified reliability goals are often
contractual requirements which must be met along with
functional performance requirements, these quantitative
evaluations must be applied during the design stage to
guarantee the equipment will function as specified for a
given duration under the operational and environmental
conditions of the intended use.” (Christiansen, 1996,
p.6.11).
Direct participation by the customer in the suppliers’
optimization process is not typically the norm. However, a
close supplier and customer relationship can take advantage
of the technical resources of both companies. In addition to
producing high performance and highly reliable products,
such relationships can result in quicker design cycles as
well as shared responsibility for design and process
changes. The supplier benefits from the experience and can
incorporate resultant improvements into other product
families. The customer will benefit, not only from the
improved resultant product, but from an increased
understanding of the product they are purchasing and the
capability of the supplier who is manufacturing it.
Reference:
Christiansen, Donald (1996). Electronics Engineer’s
Handbook (4th
Ed). McGraw-Hill: Jurgen, Torrero, and
Fink.
Rhea, Randall W. (1990). Oscillator Design & Computer
Simulation. P T R Prentice Hall: Rottino.
Stevenson, William J. (2005). Operation Management (8th
Ed). McGraw-Hill Irwin: Hercher
Viswanadham, Puligandla, & Singh, Pratap (1998).
Failure Modes and Mechanisms in Electronic Packages.
Chapman & Hall:
Breyfogle, Forrest W. III (2003). Implementing Six Sigma:
Smarter Solutions Using Statistical Methods (2nd
Ed). John
Wiley & Sons:
Suhir, Ephraim (2005). Reliability and Accelerated Life
Testing. Semiconductor Packaging, 18.
Acknowledgements:
Special thanks to Pedro Chalco for his participation, test
work, knowledge, guidance and recommendations
throughout the qualification, analysis and improvement
process. Also a special thanks to David Bushong for his
technical guidance and material expertise.