WP Shielding RF Components at the Package Level.fm

Transcription

AN RFMD® WHITE PAPER
RFMD.
®
Shielding RF Components at the
Package Level
Scott Morris and Eric Schonthal
Key Concepts Discussed:
• Pros and cons of competing shielding techniques.
• Test methods to determine best shielding techniques.
• Integrated Plated Shield Technology is shaping microwave application requirements.
RF MICRO DEVICES®, RFMD®, Optimum Technology Matching®, and PowerStar® are trademarks of RFMD, LLC. All other trade names, trademarks and registered trademarks are the property of their respective
owners. ©2009, RF Micro Devices, Inc.
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7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at (+1) 336-678-5570 or [email protected].
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Shielding RF Components at the Package Level
Contents
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Industry Standard: Can Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Painting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Testing: Which Conformal Method is Best . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Establishing a Baseline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Package Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Plating Alone Passes all Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
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List of Figures
Figure 1. Can Shield Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2. Typical process flow used for stamped and formed shields (cans) . . . . . . . . . . . . . . . . . . . . 5
Figure 3. Illustration of the conformal shield and grounding of the module . . . . . . . . . . . . . . . . . . . . . 5
Figure 4. Process flow for painting application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 5. Process flow for plating application. Sub-dice step occurs prior to chemical treatments . . 6
Figure 6. Examples of plated shield and non-plated parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 7. The Cu/Ni plating material after application on top of mold compound . . . . . . . . . . . . . . . . 7
Figure 8. Process flow used for conformal shielding as a batch process . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 9. Two different cross hatch test methods used to check the adhesion of the conformal materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 10. Measurement data of different shielding methods. All data is normalized to the non-shielded reference part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 11. Initial (left) and sub-diced (right) laminate prior to conformal material application . . . . . . 9
Figure 12. Subdiced samples after plating, before final singulation . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 13. RFMD’s MicroShield integrated RF shielding technology . . . . . . . . . . . . . . . . . . . . . . . . . 11
List of Tables
Table 1.Environmental Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Table 2.Reliability Tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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Introduction
Because original equipment manufacturers (OEMs) want
to bring new products to market quickly, common
platforms are used whenever possible to reduce costs
and development time. While standardized individual
components can be used in multiple product designs to
defray costs, predicting electrical interference between
various designs can be difficult or impossible. Even
though electromagnetic shielding is necessary in radio
frequency/microwave applications, and is often a
requirement from the customer, it is often retrofitted to
the design of electronic components. This approach can
cost designers in terms of form and performance. A shield
implemented at the customer level can change the
performance of the design through electromagnetic
coupling between its components and/or the shield. This
effect causes delays as the customer and OEM go back
and forth tweaking the design and the shield to achieve
the desired results. If the shield is treated as an integral
part of the design, however, the end product performs
better, is less expensive, and is faster to produce. It is the
goal of this paper to survey techniques for implementing
electromagnetic shielding and contrast them with an eye
towards best performance and manufacturing efficiency.
In general, can shields take up more PCB real estate and
are taller and heavier than the other newer shielding
methods.
Can shields must also be designed ad hoc per
application, so there can be high costs associated with
making adjustments to stamping dies. Variation in the
specifications for these different shield designs demands
extra inventory space, which can increase cost of
production exponentially.
Figure 1. Can Shield Example
Industry Standard: Can Shields
Metal “can” shields are widely used in wireless devices.
Can shields are made by stamping and forming a piece of
conductive metal, usually steel, to a specified size and
shape. Typically the shield shape is a rectangle with
connection points made of solder or conductive epoxy
around the outside edges.
Can shields produce excellent results in terms of
eliminating interference inside, outside, and between
components (spurious noise), and are found in many cell
phones, MP3 players, and PDAs.
These shields lend themselves to high-volume production
and are so common that they benefit from much industry
design and production expertise, as well as existing highspeed stamping equipment to support efficient
manufacturing.
The drawbacks to can shields are significant, however,
and grow continually more prohibitive as requirements for
wireless devices trend ever smaller. As the designs
become smaller, making can shields becomes more
difficult, and less room is left for the necessary grounding
points and other keep-out areas of the design. This is
especially problematic when multiple components on a
printed circuit board (PCB) must be individually shielded.
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Figure 2. Typical process flow used for stamped and formed shields (cans)
The Wave of the Future: Conformal
Shields
Because of the limitations of can shields described
above, new and ingenious methods of electromagnetic
shielding were conceived. To augment standardization
between platforms, shielding was done at the package
level. This improved the overall module performance with
a minimal increase in component size.
This new approach is known as conformal shielding.
Multi-chip modules are typically produced with either
organic laminate substrates or are built using metal leadframe technologies. In either case, the components are
mounted on the laminate and then overmolded for
environmental protection. To shield the individual
components, contact must be made to the grounding in
each individual part, either by mold tooling, or by
mechanically removing the mold compound. Once the
ground is exposed, the conformal material can be
applied.
Conformal shielding is a batch process that fits into the
back end of standard module processing flows. And the
three most promising conformal shielding techniques,
sputtering, painting, and plating, were tested to
determine which offered the most benefit with regard to
fit, form, and function. Each process is described below
along with a description of its advantages and
disadvantages.
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Figure 3. Illustration of the conformal shield and
grounding of the module
M o d u le
l
A pplica tion PC B
Sputtering
In the sputtering process, a thin layer of copper (Cu) and
either stainless steel or nickel (Ni) is applied to the
outside surface, using a physical vapor deposition (PVD)
magnetron machine in a vacuum chamber. The goal is to
get the target (the compound which will become the
shield) to release atoms inside the chamber and then
coat the part. To get the compound to release atoms, it is
excited by ions in a plasma environment.
The resulting finish is thin when compared to the
underlying mold compound materials, usually around
1µm to 2µm. But this process can be repeated to create
multiple metal stacks to achieve the average needed
thickness. Backside masking is not needed, but the
component must be mounted with the edges sealed.
Since this is a line of sight process, the top of the
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component piece will be sputtered. The vertical surface
coverage is half of the top surface.
Sputtering is relatively easy to implement with off-theshelf and/or specialty equipment. There are multiple
vendors that use this technology in cosmetic application,
but the capital facilities and equipment costs are high.
Sputtering can also present problems in terms of
thickness variation and adhesion.
Painting
Conductive paints are not new to industrial applications.
For many years, the aerospace industry has used these
paints to protect vital components from spurious noise.
The goal in using the painting process for the component
is to create an epoxy coating impregnated with metal
flakes. Ideally these flakes are silver (Ag), though
sometimes Ag/Cu, or other metals are used. The process
begins by suspending the metal flakes in a solvent
solution. The paint is sprayed over the targeted area, and
the solvent evaporates as the coating dries.
Painting is an easy application process, but the surface
preparation is more involved. The parts must first be
masked to keep critical areas protected and then
cleaned with methyl ethyl ketone (MEK) or isopropyl
alcohol (IPA). To make identical application repeatable, a
robot is required. The parts must then be cured at 250ºC
to eliminate volatile organic compounds (VOCs).
Advantages of using conductive paint are:
• smaller product form and grounding pad
• comparatively very low cost to implement
• ready availability of painting systems on the market
Plating
Plating on plastics is a very common process in multiple
industries, especially in the PCB industry. Implementing
this technique at the package level, however, is an
innovative, application. This technique separates itself
from those mentioned previously because the grounding
footprint required is much lower than any can shield
solution. The piece price cost is also the lowest of all the
processes examined.
The plating process requires the module surface to be
prepared during a roughening step which helps the
subsequent steps adhere to the module. Specifically, the
mold compound is chemically roughened to promote the
adhesion of an electroless Cu layer. This initial thin Cu
seed layer is used to carry current for the actual shield
layer, made of electrolytic Cu and Ni. The Cu is critical for
shielding performance, while the Ni is used as an
environmental protectant and for cosmetic purposes.
Sample preparation is a critical process. At the package
level, the process begins with masking the back and/or
front of the strip to prevent plating on functional areas.
Masking can be done with tape or a fixture, and is an
important operation in the electroless plating process
because all exposed areas are plated during this step.
Electroless plating is the most critical step in ensuring the
robustness of the shield. Even though plating on plastics
is common, implementing this process at the package
level requires extensive development. The correct process
times, chemicals, and steps all require careful planning.
It should be noted that the manufacturing floor space
required in plating is large compared with other solutions.
The significant facilities necessary will raise initial costs.
Figure 5. Process flow for plating application. Sub-dice
step occurs prior to chemical treatments
The main disadvantages are:
• short duration of effective shielding life
• harmful waste products
• difficult/time-consuming preparation, in other words,
product masking
• thickness variation over the total surface area
Figure 4. Process flow for painting application
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Figure 6. Examples of plated shield and non-plated parts
Figure 7. The Cu/Ni plating material after application on
top of mold compound
Figure 8. Process flow used for conformal shielding as a batch process
Testing: Which Conformal Method is Best
The advantages of smaller, more economical conformal
shielding applied at the package level over less elegant
can shields retrofitted and reworked to fit an existing
design have been explored and addressed. Advantages
and disadvantages of three competing conformal
shielding techniques have also been explained and
examined. But to decide which method to use, hard data
derived from rigorous testing is required. These tests have
been performed and are presented below.
Establishing a Baseline
In order to select the best material to use for shielding, a
baseline must be established as a control for the
experiment. A passive vehicle was chosen, which allowed
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for a wide range of measurements in a controlled
environment. Rather than rely on the narrow radiation of
a single part, the measurements taken were an average
of multiple parts to allow statistically valid results to be
gathered with a single test.
To ensure the passive vehicle baseline was accurate, test
setup was scrutinized with great attention to detail.
Spurious noise in the test setup would distort the data
and render the test useless. For that reason, the entire
surface of the evaluation PCB was shielded with copper,
which was then connected to ground. In fact, all
connections to the board were required to be shielded.
This setup prevented unintended radiation that would
skew the results and make them ineffective.
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Test Setup
The test apparatus setup was rather simple: one signal
generator, a Gigahertz Transverse ElectroMagnetic
(GTEM) test chamber, and an electromagnetic
compatibility (EMC) analyzer. The signal generator was
used to create a known signal which was applied to the
part via an open trace. The EMC analyzer read the signal
so the amount of radiation could be detected. The
purpose of the GTEM test chamber was to keep out
signals from other sources. An off-the-shelf software
system was used to run the measurement system. After
all measurements were taken, the relative attenuation
was calculated. For this application, the goal was 15dB of
attenuation or better.
Using this test setup, measurements were made on the
passive test vehicle comparing plating, metal
impregnated paints, sputtering, and discrete cans. The
baseline device measured was chosen to be an
unshielded part. The shielded parts were then measured
and compared against this reference point. The
difference was defined as the shield attenuation. The
tests were made in a frequency span of 400MHz to
12GHz in 200MHz increments. The results presented in
Figure 10 show that, on average, the plated shield yielded
the best overall performance.
Package Reliability
Another critical part of choosing a shielding method, and
therefore testing competing techniques is the reliability of
the package. As a device’s footprint becomes smaller, the
need for robust packaging solutions becomes more
critical. To meet RFMD requirements, the integrated
shield must withstand various Joint Electron Device
Engineering Council (JEDEC) standards package reliability
tests such as reflow, MSL, temperature cycling, salt spray,
and others. During process development, reflow ovens
were utilized to speed the development cycle. Parts were
subjected to three passes at 260ºC.The failure criteria
included delamination of the shield and degradation of
shield attenuation. If delamination occurs the shield is
considered a failure. A tape test (cross hatching the
shield, applying tape and pulling the tape vertically) was
conducted to assess the adhesion of the plating. If any
blistering occurs, the shield will peel off, resulting in the
part's failure.
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Figure 9. Two different cross hatch test methods used to
check the adhesion of the conformal materials
Plating Alone Passes all Tests
Multiple experiments were performed to find the “sweet
spot” for each material process, thus defining the limit for
the given process. For high-volume manufacturing, these
process limits must be determined. The only material that
was able to pass all manufacturing and reliability tests
was plating. The data from testing can shields was
omitted from the study because, though they are very
effective shields, the overall cost restrictions in
production and the increase in the size of the package
render them undesirable.
As an added benefit, these reliability tests can also be
used to inform best practices in the production process.
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.
Figure 10. Measurement data of different shielding methods. All data is normalized to the non-shielded reference part
.
Figure 11. Initial molded (left) and sub-diced (right) laminate prior to conformal material application
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Figure 12. Subdiced samples after plating, before final singulation
Conclusion
Conformal shielding techniques were deemed the best for
shielding and several different shielding techniques such
as painting, sputtering, and plating were explored. The
only technology to pass all reliability requirements was
plating. From an electrical performance standpoint, all
techniques were acceptable, but of all the available
conformal processes, plating has the durability and
repeatability needed in a high volume manufacturing
environment.
RFMD’s development of conformal plated self-shielding,
called MicroShieldTM Integrated RF Shielding technology,
is one of the most revolutionary module technologies to
be introduced in the last 10 years. It offers a true
competitive advantage for customers wanting to reduce
their final solution size. It also has a direct benefit to bill
of material costs since fewer components are needed per
application. There are many other positive aspects to the
design that benefit the customer:
• ability to fix the final design of the component with
shield in place
In addition, self-shielded components are used within
specification at the original design house, without the
need for iterative tuning requiring customer involvement.
This differs from most applications where the shield is
applied post design, and therefore needs multiple PCB
(customer/ design house) design spins to arrive at a final
solution. This adds critical time to ever-shrinking product
cycles. Another benefit of eliminating the can shield is
that the designer realizes a 20% to 30% savings in PCB
real estate. Beyond footprint reduction, the overall height
is also reduced by utilizing .010mm of plating instead of
the typical 2mm standoff needed for can shields. The
final advantage is the simplification of reworking the
module (if needed). Since there is no shield to remove,
rework becomes much easier. This novel concept of
providing EMI shielding at the package level provides
improvements in form factor, ease of use, and lower cost
compared to traditional shielding approaches. The
development and implementation of this type of selfshielding technique, package-level plating, has been
effectively proven. RFMD has shipped over 250 million
units with a MicroShield process yield above 99.8%.
• faster total time to market (TTM)
• lower cost (50% to 75% savings) than the industrystandard can shield solution
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Figure 13. RFMD’s MicroShield integrated RF shielding technology
Figure 14. PCB real estate space savings with RFMD’s MicroShield
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Table 1. Reliability Tests
TEST
TEST CONDITIONS
EXPLANATION
STANDARD
Low Temp
Storage
for 96hours @ -40°C
Parts must pass electrically and mechanical
damage i.e. cracking, delamination, etc. should
not occur from the effect of time and temperature
IEC 68-2-1 Aa
Moisture
Sensitivity
Moisture Absorption
Perform CSAM and electrical test. Testing to be
performed to target level ±1 level
REL-30-1011
Temperature/
Humidity Test
With Biasing
85°C/85% RH, 5volts,
1000hours
Perform DC/RF electrical testing before and after
test. Precondition to MSL Level 3
REL-30-1011
Fatigue/Flex
Bend test
Force of 5N, 1mm bend,
10cycles for 10seconds
To see any cracks/electrical discontinuity with the
capacitor layer
IEC-68-2-21
Temperature
Cycling
-40°C +125°C, 1000cycles
1000 cycles – precondition to MSL Level 3
REL-30-1019
High Temperature
Storage
150°C, 1000hours
Parts must pass electrically and mechanical
damage i.e. cracking, delamination, etc. should
not occur from the effect of time and temperature
JESD22A103
ESD
50, 100, 250, 500, 1000
volts zap 3 devices per
voltage
To determine the ESD sensitivity of the device
JESD22A114
JESD22A115
JESD22C101
HTOLDC
125°C for 1000 hours with
readpoints @168hours,
500hours, 1000hours.
To accelerate the life, to identify unexpected
failure modes, to continuously validate wearout
parameters and to predict the fit value.
REL-30-015
REL-30-1016
Drop Shock
Drop from 1.8m (6ft.) onto
concrete 3 times on all 6
different sides
Pass electrical and mechanical tests
IEC 68-2-27 Ea
Table 2. Environmental Testing
Paint
Sputter
Plating
Tape Test (x-hatch)
Pre-Reflow
Fail
Fail
Pass
Incoming 3x Reflow 260°C
Fail
Fail
Pass
Tape Test (x-hatch)
Post-Reflow
Fail
Fail
Pass
MSL3, 3x Reflow 260°C,
J-STD-020
Fail
Fail
Pass
Salt Spray Test MI 883
condition A
Pass
TC 1000 Cycles
JESD22-A104
Pass
Pass
Pass
Temperature/Humidity
Rel-3-1000
Pass
Pass
Pass
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WP Shielding RF Components at the Package Level.fm

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