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  Microchip Technology Semiconductor Electronic Components Datasheet  

AN830 Datasheet

RFID Tag and COB Development Guide

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AN830 pdf
AN830
RFID Tag and COB Development Guide with Microchip’s RFID Devices
Author: Youbok Lee, Ph.D.
Microchip Technology Inc.
INTRODUCTION
A passive RFID tag contains an RFID integrated circuit
(IC), resonant capacitor (C), and antenna (L), as shown
in Figure 1. The antenna and cap acitor form a parallel
LC resonant circuit. The LC circuit must be tuned to the
reader’s carrier frequency for maximum performance
(read range).
The two most common antenna types for RFID tagging
applications are: (a) wire-wound coil and (b) etched (or
printed/stamped) spiral inductor on a dielectric sub-
strate. The antenna types are typically determined by
carrier frequency , t ag’s p ackage type, performance,
and assembly cost factors. For example, low frequency
(< 400 kHz) tags need a few mH of induct ance. This
inductance is achieved with a few hundreds of turns of
wire. This kind of induct ance cannot be obtained eco-
nomically with etched antenna, but with a wire-wound
antenna. However , medium frequency (4 - 30 MHz)
tags need a few uH of inductance. This inductance can
be achieved with a few turns of wire or etched (or
printed/stamped) spiral inductor on dielectric substrate.
After the antenna type is chosen, the next step is to
attach the silicon device to the antenna. There are two
basic methods for the device att achment: (a) using a
chip-on-board (COB) or (b) direct die attachment to the
antenna. The COB is commonly used for wire-wound
antennas and the direct die attachment is for the
etched (printed/stamped) antenna types.
The COB is made by packaging a r esonant capacitor
and an RFID device together in the same p ackage. It
has two external terminals for antenna atat chment. The
inductance of the antenna is determined by the COB’ s
resonant cap acitor value and the reader ’s carrier
frequency. The antenna is att ached to the COB‘s two
external terminals by welding or soldering. Because
most of the COBs are used for ISO cards which need
to meet the ISO card st andard thickness (0.76 mm)
specification, typical thickness of the COB is approxi-
mately 0.4 mm. Although the CO B p ackage is
designed to protect the internal silicon device during
the card lamination process which involves mechanical
pressure with hot temperature, care is needed to pre-
vent mechanical cracks on the device. The two popular
COB package types are IOA2(MOA2) from IST in T ai-
wan and World II from HEI Inc. in the USA.
Since the direct die att achment reduces a step for
making the COB package, it is widely used for low cost
and high volume applications such as smart labels. The
direct die att achment can be achieved with two dif fer-
ent methods: (a) wire bonding or (b) flip-chip with
bumped die. For the flip-chip, it needs a special bump-
ing on the die’s bond pads. Typically the bump material
is made of gold with approximately 25 um of height.
The flip-chip assembly process att aches the bumped
area to the antenna tr aces. Several bumping and flip
chip assembly methods are available for RFID t ags.
The wire bonding method needs a relatively simple
process for the die attachment. The die is directly wire-
bonded to the antenna, and covers the wire bonded
area with a black colored epoxy glob top. For small vol-
ume pr oduction, the wir e-bonding method is still les s
expensive than using the flip-chip process. However, it
is less efficient for high volume production. The flip-chip
method is preferred for high volume production.
The read range of an RFID at g is greatly afected by the
tag’s size, tuning, circuit Q, de vice’s power consump-
tion and data modulation depth. The tag’s size must be
chosen depending on it s application and cost con-
straints. Tags must be tuned precisely to the reader ’s
carrier frequency for long range applications. Since the
tag’s antenna circuit consists of a combination of L and
C components, the tolerance of the component often
causes the variation in the read range between t ags.
Once the induct ance is designed, it s tolerance is typi-
cally within 1 ~ 2%. Therefore, a t ag’s tuning variation
is mostly due to the capacitance tolerance. The capac-
itance used for the antenna circuit or COB must be
chosen carefully. For example, the tolerance must be
kept within ~5% and the capacitor’s Q factor should be
greater than 100 at the operating frequency to maxi-
mize the read r ange performanc e. T he internal
resonant cap acitors of the MCRF451/452/455 and
MCRF360 devices are made with silicon oxide. Their
tolerance is approximately less than 5% for the devices
in the same wafer and within ~10% from dif ferent
wafers. Their Q factor is greater than 100 at
13.56 MHz. The cap acitance tolerance result s in
variations in the read range between tags. Therefore, if
the read range variation (about 10%) is a concern due
to the internal capacitor’s tolerance, the MCRF450 and
MCRF355 can be used with an external cap acitor that
has a smaller tolerance (within 2~5%).
2002 Microchip Technology Inc.
DS00830B-page 1
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  Microchip Technology Semiconductor Electronic Components Datasheet  

AN830 Datasheet

RFID Tag and COB Development Guide

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AN830 pdf
AN830
The modulation transistor of the 13.56 MHz devices is
placed between antenna B and V SS. This transistor
creates a junction cap acitance between the two p ads.
This junction cap acitance is relatively lossy comp ared
to the on-board cap acitors. However , the MCRF450,
MCRF451, MCRF455 and MCRF360 are not af fected
by the junction cap acitance. For the MCRF452, this
lossy junction capacitance is in parallel with its second
50 pF internal resonant capacitor. The resulting loaded
circuit Q of the MCRF452 is about 5~10% lower than
that of the other devices.
The cap acitance or induct ance also can be trimmed
within a few percent using proper tuning mechanism.
Various tag design assistance and tuning methods that
are the subject of pending patent applications are avail-
able from Microchip Technology Inc.
The Q factor of a t ag’s antenna circuit is primarily
governed by the antenna’ s resistance. Therefore, the
antenna must be designed to have minimum resisat nce
within a given physical constraint. The antenna resis-
tance becomes smaller with thicker gauge wire or
etched metallic traces with a wider trace width. Etched
antennas with a four-turn spiral inductor on an ISO card
sized dimension can easily be made to be less than
1 ohm with a proper dimensional choice. In this case,
the unloaded Q (antenna only) can be greater than
100. The loaded Q (antenna with device) needs to be
greater than 50 for long range applications.
Microchip’s 13.56 MHz devices consume less than
200 µW of power during reading. This is about 20 times
less than other similar devices available from competi-
tors in the industry today . This reduced power
consumption means there will be more available power
for backscattering (re-radiation), which result s in a
longer read range.
In conventional RFID tags, data is sent by damping and
undamping the antenna voltage. However, for a high Q
circuit, it is very dif ficult to damp the volt age with high
speed (high data rate). Therefore, it is dif ficult to send
data with 100% AM modulation with the conventional
method. To overcome this problem, Microchip’s current
13.56 MHz devices are designed to shift the circuit’ s
tuning fr equency ins tead of damping the c oil v oltage
directly. This can be achieved by shorting and un-short-
ing one element in the resonant antenna circuit.
Microchip’s devices have a modulation transistor
between antenna B and V SS. The frequency tuning
element should be placed in p arallel with the modula-
tion transistor (antenna B and V SS). The modulation
transistor shorts the frequency switching element when
it turns on (sending dat a “Hi”), and releases when it
turns of f (sending dat a “Lo”). When the switching
element is released, the circuit tunes to the reader ’s
carrier frequency causing the circuit to develop
maximum volt age. When the switching element is
shorted, the circuit tunes away from the carrier
frequency, therefore, developing less volt age. The
reader monitors the changes in the t ag’s coil volt age,
and reconstruct s the modulation dat a. Refer to
Microchip’s application Note AN707 (DS00707) for
more details of this feature.
The de vice req uires three co nnection p oints to the
external antenna circuit: antenna A, antenna B, and
VSS. This is in order to switch the resonant frequency
(tuned and detuned) by shorting and un-shorting the
element between antenna B and VSS. In the MCRF452,
the antenna B is internally connected to the second
internal 50 pF capacitor between antenna B and V SS.
See Figure 1 for various external c ircuit configurations
for each device.
The resonant frequency of the tag is determined by the
LC component combination between antenna A and
VSS. The circuit must be tuned precisely to the reade’rs
carrier frequency for best read range performance.
Microchip’s 13.56 MHz devices are designed to send
data with 100% modulation with the appropriate exter-
nal circuit configuration. The modulation depth is deter-
mined how much the tag’s coil voltage is changed when
it sends dat a from “Hi” to “ Lo” or vice versa. In the
13.56 MHz devices, this is directly related to the sep a-
ration between tuned and detuned frequencies. The
detuned frequency is the result of shorting the
frequency switching component between the antenna
B and VSS. This component value is typically optimized
between one-third to one-half the total L or C value. For
example, three turns between antenna A and antenna
B, and one turn between antenna B and V SS for a t ag
made of a four-turn spiral inductor . If the shorting
element (between antenna B and V SS) is a cap acitor,
the same value of the cap acitor can be chosen for the
element between antenna A and B for simplicity.
The COB for MCRF355 and MCRF450 include two
identical 68 pF cap acitors in series, externally to the
device. The cap acitor C1 is connected between
antenna A and B, and C2 is between antenna B and
VSS.
The MCRF452 COB does not require external cap aci-
tors since the device has two internal capacitors.
The MCRF452 is the best choice for the COB in a
sense of unit COB production cost. However , for the
highest performance, the MCRF450 is recommended.
Various circuit configurations for each device are
shown in Figure 1.
DS00830B-page 2
2002 Microchip Technology Inc.


Part Number AN830
Description RFID Tag and COB Development Guide
Maker Microchip
Total Page 26 Pages
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