Download 0.1 pF – 180 pF, 1% Tolerance 0402 RF Capacitor

0.1 pF – 180 pF, 1% Tolerance
0402 RF Capacitor
Capacitance Stability from 1 MHz to Several GHz*
* Patent Pending
Haim Goldberger
Vishay Intertechnology, Inc.
Phone: +972 (3) 5570966
[email protected]
Tony Troianello
Vishay Intertechnology, Inc.
Phone: +1 714-573-0150
[email protected]
Abstract
Silicon-based technology has been identified as ideal for use in high-precision capacitors for high -frequency
applications such as wireless communications devices, smart cards, etc. Development criteria include the following:
• use of widely accepted advanced technologies with long history of reliability;
• simple, mass processing for high precision at low cost;
• unlimited material supplies available throughout the world;
• process stability for repeatable high yield of tight tolerances with no trimming necessary;
• product configuration that would solve major board assembly problems;
• low profile for smart card applications;
• product characteristics that would yield higher capacitance density, higher capacitance values per volume;
• product performance that would assure lower parasitic inductance, lower or equivalent ESR, higher selfresonant frequencies, and tighter tolerances than previous capacitance technologies.
This paper will discuss the technologies involved and present data comparing a new silicon-based capacitor to other
high-frequency capacitor technologies presently used.
I. Design Objectives and Construction Details:
In the last decade or two, semiconductor production has
been greatly refined in materials, processes,
producibility, cost reduction, repeatability, reliability,
and simplicity. In contrast, there have been few
significant innovative advances in passive components.
So the search for a significant advance in a high Q, high
frequency capacitor centered on the semiconductor
processes instead of the traditional capacitor processes.
The resultant product yields a true RF capacitor with a
ten-fold increase in capacitance density based on simple,
low cost, high reliability semiconductor processes, and
on materials economically available throughout the
world.
Photo 1: HPC 0402 integrated silicon RF
Capacitor with single-sided contacts.
Solder
Passivation
Conductor
TABLE 2.
Process Yields & Capacitance Density
vs. Technology, offered by leading manufacturers of
precision RF capacitors.
SiO2
Si
Fig. 1
Technology
Fig. 1 shows a cross-sectional drawing of the HPC0402
capacitor. In this configuration, the high-conductivity
silicon forms the base plate of the capacitor. A
proprietary process forms a void-free silicon dioxide
dielectric that is very thin (in the order of 0.05 to 3
microns depending upon capacitance), very uniform
(variation of less than 0.4% across a wafer for 10pF), and
very repeatable (less than 1% variation from lot to lot.)
This makes it possible to produce tight tolerance
capacitors without trimming. This is highly desirable
because trimming changes the high frequency
characteristics of the capacitor. That is, the same
capacitance value, in the same capacitance tolerance, in
the same capacitor model, from the same capacitor
manufacture will have different RF performance from lot
to lot.
Tables 1 and 2 compare the integrated silicon capacitor to
the other leading RF capacitor technologies.
TABLE 1. Capacitance vs. Size vs. Technology, as
offered by leading manufacturers of precision RF
capacitors.
Process stability
w/o trimming
w/o sorting
HPC
LTCC
ThinFilm
Semiconductor
MLCC (LTCC)
Thin Film
+/-2%
+/-15%
+/-9%
Max Capacitance values
Up tp 200pF
Up to 15pF
Up to 22pF
Capacitance density
0.27pF/mil2
0.02pF/mil2
0.03pF/mil2
II.
Configuration/ Mechanical Effects:
The wraparound terminations of the standard chip
configuration reduce the resistance in the terminations,
but they add inductance at the same time. They also
require more board space than necessary. An 0402 size
chip requires 25 to 45% greater length than the actual
chip for the mounting pads, depending on the
manufacturer’s recommendations. Additionally, there is
a tendency for wraparound terminations to tip up on one
end during board assembly when the solder at each end
either melts or solidifies unevenly, an effect known as
“tombstoning”, see Fig 2.
M1
LTCC - 0402
LTCC - 0603
LTCC - 0805
T3
ThinFilm - 0402
T1
ThinFilm - 0603
T2
ThinFilm - 0805
ThinFilm - 1210
HPC0402
0
50
100
150
200
Capacitance (pF)
HPC Silicon Capacitor, Si/SiO2 integrated process.
Thin Film Capacitor, deposited SiO2 on ceramic.
LTCC , multi-layer chip capacitor
250
T1 = Surface tension of liquid solder
T2 = Tack force of solder paste.
T3 = Weight of the chip.
M1= Moment lifts chip upright.
Fig. 2
termination on the same pad. The corrective action
increased the SRF by 0.08 Ghz, approx 4%.
Tombstoning requires costly off-line testing and rework
to correct the problems. If only 0.5 % of the mounted
components exhibit this behavior, and there are 10
components per board, then approximately 5% of the
boards will need rework.
The HPC0402 single-side contact configuration was
developed to eliminate the tombstone effect in board
assembly by placing both contact pads only on the
bottom surface of the chip (See Fig 5).
Sometimes the chip is not completely lifted at one end
but, though still with a good electrical and mechanical
connection, is positioned with one end raised higher than
the other, see fig. 3.
T2
T1
T2
T1 = weight of chip.
T2 = Tack force of solder paste.
Fig 5
Fig 3.
In high frequency applications the added inductance of
the raised connection significantly affects the high
frequency performance of the mounted capacitor.
1000
1500
2000
3000
0.02
A
-25
0.048
-20
0.02
B
-15
0.048
S21(dB)
-10
2500
0.02
500
0.07
0
-5 0
Instead of a force tipping the chip upward, all forces act
to draw the chip down tight against the board,
eliminating the tombstone effect, and assuring the best
high frequency performance. The silicon-based singlesided contact configuration also requires less spacing
between chips; shorter traces result in lower resistance
and inductance in the circuit. Additionally, the flatter
surface and the more uniform edges of the silicon,
compared to a LTCC ceramic 0402 chip, means less
dropped components and less shifting of position at
pickup, so pick and place equipment can be operated at a
faster speed with less skewed devices and less rework.
-30
-35
-40
0.02
Fig. 4
0.06
Frequency (MHz)
Measured on Agilent E5071A
A
Fig. 4 illustrates the difference in performance. Curve A
shows the high frequency performance of a 12 pf, +/- 2
% capacitor with wraparound terminations, with one end
intentionally raised only 15 mils and its SRF at 2.13 Ghz.
Curve B shows the high frequency performance of the
same chip after rework to reduce the length of the solder
B
Fig 6
C
III.
High Frequency Performance, Comparison by
Technology:
Among the chief design objectives in the development of
the integrated silicon capacitor were high Q, high selfresonance frequency (SRF), and a high degree of
capacitance stability.
The following tables compare the resultant HPC
capacitor’s RF performance against the other leading RF
capacitor technologies.
Figures 7 through 9 show that the integrated silicon
capacitor exhibits significantly greater capacitance vs.
frequency stability than the other RF capacitors available.
Cap(pF) vs. Freq. (MHz)
10pF
22
HPC
ThinFilm
LTCC
20
Cap (pF)
18
16
14
12
10
8
1
1000
2000
Freq. (MHz)
3000
Measured on Agilent 4287A + 16197A
Fig. 8 Capacitance vs. Frequency 10pF, 0402 size
Cap(pF) vs. Freq. (MHz)
47pF
Cap (pF)
The attached drawing (Fig. 6-A) shows the 0402 pad
design recommendations from several major suppliers of
high frequency chip capacitors. Each manufacturer’s
chip is shown mounted on his own recommended pads.
Fig 6-B also shows how the HPC0402 would fit on these
same pads. It can be seen that the HPC can be
immediately used on boards already laid out to
accommodate the conventional 0402 chip with
wraparound terminations. However, the board space
required for the HPC0402 is 25% to 45% less than that
needed for the conventional 0402 chip, see Fig_6-C. A
board designed specifically to mount the HPC0402
would save a considerable amount of space, but more
importantly, it would produce better high frequency
performance. That is, components can be placed closer
together, board traces can be shorter, trace resistance can
be decreased, parasitic inductance/capacitance can be
decreased, and the resulting high frequency performance
would be much better.
140
120
100
80
60
40
20
0
HPC; not avialable in other technologies
1
1000
2000
Freq. (MHz)
3000
Measured on Agilent 4287A + 16197A
Cap(pF) vs. Freq. (MHz)
3.9pF
Cap (pF)
6
Fig. 9 Capacitance vs. Frequency, 47 pf, 0402 size
HPC
ThinFilm
LTCC
5
Figures 10 through 12 show that the integrated silicon
capacitor has a Q as good as, or better than, any other RF
capacitor technology available in the 0402 size.
4
3
1
1000
2000
Freq. (MHz)
3000
Measured on Agilent 4287A + 16197A
Fig. 7 Capacitance vs Frequency, 3.9pF, 0402 size
Q vs. Freq.
3.9pF
1000
HPC
ThinFilm
LTCC
800
600
400
200
0
1
1000
2000
3000
Freq. (MHz)
Measured on Agilent 4287A + 16197A
Q Factor
Fig.10 Q vs. Frequency, 3.9pF, 0402 size
Q vs. Freq.
10pF
1000
900
800
700
600
500
400
300
200
100
0
HPC
ThinFilm
LTCC
1
1000
2000
3000
Freq. (MHz)
Measured on Agilent 4287A + 16197A
Fig.11 Q vs. Frequency, 10pF, 0402 size
Q vs. Freq.
47pF
500
The most formidable problem in cell phone design is in
the difficulty of matching the output impedance of the
power amplifier to the input of the antennae. Typically,
the designer is able to calculate and predict the
capacitance values in this circuit only 50-70% of the
time. The exact values must then be determined in the
lab. Often, particular capacitors are characterized at the
circuit frequency, with capacitors tested at 1 MHz. For
example, a circuit that requires 13 pF at 2.7GHz uses a
capacitor that tests 8.2 pf at 1 MHz but has been
characterized as being 13 pF at working frequency. The
capacitor has been characterized to fit within a
frequency-dependent envelope, broadening towards the
phone’s operating frequency. The designer fits the
characterized capacitor into the circuit and selects the
capacitance values that work best.
However, the
capacitor supplier has a 9-15% process variation,
depending upon technology and must therefore trim/sort
capacitors to value to supply the +/- 1-5% tolerances
typically used in the power amplifier impedance
matching circuit. When the capacitors are trimmed their
plate areas are changed and the capacitors no longer have
the same frequency characteristics.
Similarly, the
dielectric thickness changes from lot to lot, adding to the
causes for non-uniform frequency performance. These
variations not only make it necessary for the designer to
select the workable values in the lab, they virtually assure
that the production phones will operate differently from
the ones produced in the lab. A capacitor with greater
capacitance vs. frequency stability, and with no trimming
to value, would greatly reduce, if not totally eliminate,
this problem.
HPC; not avialable in other technologies
400
Q Factor
Typical Applications:
A.
Process uniformity affects RF performance,
impedance matching.
Capacitance vs. Frequency
17.00
300
16.00
LTCC + ThinFilm
8.2± 0.1pF
15.00
200
100
0
1
1000
2000
Freq. (MHz)
Measured on Agilent 4287A + 16197A
3000
Capacitance (pF)
Q Factor
IV.
14.00
HPC 12pF ±1%
13.00
12.00
11.00
10.00
HPC 8.2pF ±0.1
9.00
8.00
0
Fig. 12 Q vs. Frequency, 47pF, 0402 size
500
1000
Measured on Agilent 4287A + 16197A
1500
2000
2500
3000
3500
Frequency (MHz)
Fig 13
Figure 13 shows the frequency variation of a sampling of
LTCC, Thin Film SiO2, and HPC integrated silicon
capacitors. In this case the user had to start with LTCC
and Thin Film capacitors of 8.2 pf to have a 13 pf
capacitor at the working frequency of 2.7 Ghz. Because
of the greater capacitor vs. frequency stability, the user
would use a 12 pf HPC0402 for the same application. It
can be noted in the graph that the LTCC and the Thin
Film capacitors fall within a spread of about 10% at 2.7
Ghz, while the integrated silicon capacitor falls within a
spread of only 1%. Normally, the designer starts with a
capacitor tolerance of 1 % to hold the capacitance
envelope to 10% at the working frequency. Broadening
the initial tolerance broadens the envelope as well.
Substitution of the HPC capacitor, with its much tighter
envelope at 2.7 Ghz, allows the designer all of three new
options:
1. Tighten the circuit tolerance at working
frequency,
2. Extend the working frequency to higher
frequencies with no expansion of the
capacitance envelope.
3. Start with a much more economical initial 5% or
even 10% tolerance. Using a 5% tolerance
instead of a 1% tolerance RF capacitor reduces
the capacitor cost by 50%, a significant savings
made possible by the frequency-stability and
lot-to-lot uniformity of the integrated silicon
capacitor.
B.
Low Profile, High Capacitance to Size Ratio,
Contactless Smart Cards.
A typical contactless Smart Card is comprised of an
ASIC or microprocessor, a capacitor, and an antennae
mounted on a flexible plastic card. When triggered by an
interrogating signal the smart card transmits its stored
data. . Because the card is flexible and the capacitor is
very thin, less than 0.3mm, the mounted capacitor must
be kept to the smallest possible board area to avoid
component breakage. Thus, 0402 size capacitors are
desired for system reliability. Moreover, designers tend
to increase antennae turns in order to avoid the need for a
tuning capacitor. Conversely, the number of turns should
be decreased to optimum value to improve the working
range of the card. In order to achieve the desired
resonance frequency, a capacitance value of 100pF to
180pF should be selected. But for these capacitance
values an 0805/0603 size chip would be needed, and this
reduces reliability due to greater susceptibility to bending
damage. The low profile HPC, with capacitance up to
180 pf in the 0402 size, allows the production of Smart
Card with the highest reliability and greater working
range.
Summary:
Table 3 summarizes the comparisons among the different
RF capacitor technologies and Table 4 lists the main
advantages of the integrated silicon capacitor.
Table 3. Comparison by Technology, Key Features of
RF Capacitors.
Capacitance tolerance
Capacitance stability vs. frequency
Capacitance variation (batch to batch)
High capacitance capability
Resistance
Q
HPC
Very Good
Very Good
Very Good
High
Good
Good
LTCC
Good
Good
Low
Poor
Very Good
Good
ThinFilm
Good
Good
Good
Poor
Very Good
Good
Table 4.
Main Advantages of the HPC0402
Integrated Silicon Capacitor.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Capacitance stability per frequency
10 fold capacitance for the same size
10 fold capacitance density
Low profile
Ultra high SRF
Low parasitic inductance
High Q
Low ESR
Tight tolerance
No tombstone effect
Eliminates board assembly problems
Use directly on existing pads
Reduce board size 25-45 %.
Cost effective
In summary, the new high HPC0402 integrated silicon
capacitor provides the highest capacitance in an 0402
size, exhibits greater capacitance stability at higher
frequencies, enables designers to improve high frequency
performance at the board level as well as at the
component level, and reduces assembly cost at the same
time.