PIERS Proceedings, Moscow, Russia, August 18–21, 2009
642
Quad Flat Non-lead Package Characterization and Circuit Modeling
M. Sigalov1,2,3 , D. Regev2 , E. Kabatsky3 , and R. Shavit1
1
Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev
Beer-Sheva 84105, Israel
2
Elipse-RFIC Array Devices, Kfar Neter 40593, Israel
3
Department Electrical and Electronics Engineering, Sami Shamoon College of Engineering
Beer-Sheva 84100, Israel
Abstract— The increasing demand for faster data transmission rate and higher capacity channels is pushing the communication systems toward higher microwave frequencies with more complex radio frequency integrated circuits (RFIC). At such high frequencies the electrical parasitic
effects of the packages become significant and degrade the RFIC’s performance. One way to
improve the functionality and performance of the RFIC devices is to develop accurate broadband
electrical circuit models of the packages for optimization purpose. The objective of this work is
to investigate the characteristics of a chip scale (CS) quad flat no-lead (QFN) package electrical
circuit model. This objective is obtained by utilizing three-dimensional electromagnetic (EM)
numerical simulation results and extraction of the equivalent circuit parasitic elements from a
model based on the physical structure of the package.
1. INTRODUCTION
The QFN is a CS leadless package where electrical contact to the printed circuit board (PCB) is
made by soldering (by conductive solder) the leads on the bottom surface of the package to the
PCB, instead of using the conventional leads. The bottom-lead package structure is able to provide
good electrical interconnections to the PCB. The exposed die paddle on the bottom of the chip
efficiently conducts the heat to the PCB and provides a stable ground for the bonds. The small
size and weight with excellent thermal and electrical performance [1, 2] make the QFN package an
ideal choice for handheld portable applications such as cell phones or any other application where
size, weight and package performance are required.
In this work, our attention was concentrated on the characterization and circuit modeling of a
fully encapsulated plasticmolded QFN package. The equivalent wideband (up to 25 GHz) electrical
circuit model of the package that accounts for the high-frequency parasitic effects, such as resonance,
coupling, and frequency-dependent losses was developed The parasitic elements of the equivalent
circuit models have been extracted from the 3D electromagnetic (EM) simulation.
2. QFN CHARACTERIZATION AND CIRCUIT MODELING
An interconnect path of the package can be electrically modeled by different equivalent circuits
models like: A single lumped element connected to a transmission line with fixed impedance and
time delay, a lumped element section, or by a distributed element circuit with per unit length
parameters. The choice of a model depends on the electrical length of interconnects. A package
interconnects path, is considered to be “electrically short” if, at the highest operating frequency
of interest, the interconnect length is physically shorter than approximately one-tenth of the wavelength [3]. Otherwise, it is “electrically long”. Therefore, an interconnect can be modeled using
a lumped element circuit if it is shown to be “electrically short”, while distributed models are
required to model “electrically long” interconnects. To investigate individual components of the
package, e.g., wirebond, lead, pad, a distributed model that models each impedance discontinuity
along the propagation path is required. Depending on the complexity of the model used, the electrical modeling requires the calculation of self-capacitances, inductances, resistances, and mutual
couplings for use in the equivalent circuit.
In Fig. 1, one can observe the cross-section view of the QFN package (one bondwire) and its
two ports equivalent Π circuit model [2, 4].
To calculate the CL , Cpg , Rw and Lw values of the model one can use the following expressions:
Im(Y11 + Y12 )
Im(Y22 + Y21 )
and Cpg =
ω
ω
Im(Y12 )
Re(Y12 )
=
and Rw =
2
2
ω (Re(Y12 ) + Im(Y12 ) )
(Re(Y12 )2 + Im(Y12 )2 )
CL =
(1)
Lw
(2)
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 643
(a)
(b)
Figure 1: (a) Cross-section view of the QFN package, (b) equivalent Π circuit model of the two ports QFN
package (CL = CLP + CLG capacitance of the lead to the ground, Cpg capacitance of the pad to ground, Rw
and Lw resistance and inductance of the bond wire).
(a)
(b)
Figure 2: (a) Cross-section view of the QFN package, (b) equivalent high frequency model of the two ports
QFN package.
These expressions are derived from the two port network analysis of the equivalent circuit shown
in Fig. 1(b). The Y parameters can be obtained from an EM simulator or measurements [5].
The model shown in Fig. 1(b) is a simple model that gives good correlation to the measurements
or EM simulation results up to 6 GHz [2]. For higher frequencies the sizes of leads are comparable
to one–tenth of the EM field wavelength. So, there is a need to consider the distributed currents on
the lead surfaces. These distributed currents lead to additional inductance effects. Consequently,
a more complex (high frequency) model is necessary for the two port QFN characterization.
In Fig. 2, it is shown the cross-section view of the QFN package and the proposed upgraded
(high frequency) equivalent Π circuit model.
From Fig. 2, one can see that the lead high frequency inductance effects can be modeled as a
serial inductance LL1 at the input of port 1, a serial inductance LLG to the capacitance CLG , a
serial inductance LL2 to the capacitance CLP , a mutual inductance ML1w and a mutual inductance
ML2w . The inductance LP G is due to the distributed currents on the paddle surface.
The simplified two port model shown in Fig. 2 is not sufficient to model the package frequency
performance when coupling effects to the neighboring bondwires need to be considered. In Fig. 3,
it is shown such an improved four port circuit model taking in consideration, the coupling between
the bondwires.
CL1L2 — is the mutual capacitance between two adjacent leads and bondwires. CP 1P 2 — is the
mutual capacitance between two adjacent die pads. Mw1w2 — is the mutual inductance between
two adjacent bond wires. M 1L1L2 , M 2L1L2 , and M 3L1L2 represent the mutual inductances between
two adjacent leads.
3. SIMULATION AND OPTIMIZATION RESULTS
In this work, we used Ansoft HFSS EM simulator for modeling and characterization of a QFN-48
structure with 48 bottom leads The package size is 7 × 7 × 0.2 mm. The size of the paddle is
5.6 × 56 mm. The lead pitch is 0.5 mm and the lead dimensions are 0.23 × 0.6 × 0.2 mm. The size
of the die pads is 50 × 5 µm. The diameter of the gold bond wires is 1.2 mil and the length of the
PIERS Proceedings, Moscow, Russia, August 18–21, 2009
644
Figure 3: Equivalent high frequency model of the four ports QFN package.
(a)
(b)
Figure 4: QFN-48 structures: (a) One bondwire simulation (two ports), (b) two bondwires simulation (four
ports).
bond wires is 1.8 mm. The dielectric constant of the molding compound is 3.7.
Geometric models of the simulated one and two bondwires QFN48 structures are shown in
Figs. 4(a) and 4(b), respectively.
From Fig. 4, one can notice that the geometry simulated by HFSS includes board, package, die
and bond wires. The package is surrounded by an air volume which has, on its external surface, a
radiation boundary condition. This condition allows the EM field to radiate freely in the space. On
the lower surface of the board a perfectly electric conductor (PCB ground) is chosen. This plane
is a common reference both for the ports of the leads, directly connected to it, and for the ports
on the pads connected to it through the die paddle.
In the next two figures, it is shown the comparison between the S parameters obtained from
the full EM HFSS simulation of the two (Fig. 5) and four (Fig. 6) ports QFN-48 structures and
the S parameters obtained from the equivalent circuit model presented in Fig. 2(b) and Fig. 3,
correspondingly.
One can observe that the S parameters of the improved high frequency equivalent models presented in Fig. 2(b) and Fig. 3 yield very good correlation with the HFSS results up to 25 GHz.
The final equivalent circuit (Fig. 3) parameters values are based on the parameters extracted
from the EM simulation and after circuit optimization. The optimization was performed with RF
Designer of the ADS circuit simulation program from Agilent Company. The extracted parasitic
parameters of the equivalent circuit models of the QFN-48 package are listed in Table 1.
Due to the small values the mutual inductances M 1Lw , M 2Lw , and M 3Lw they don’t affect the
performance of the equivalent circuit and can be neglected.
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 645
Figure 5: Comparison between the S parameters obtained from the full EM HFSS simulation of the two
ports QFN-48 structure and the S parameters obtained from the high frequency equivalent circuit model
presented in Fig. 2(b).
Figure 6: Comparison between the S parameters obtained from the full EM HFSS simulation of the full four
ports QFN-48 structure and the S parameters obtained from the high frequency equivalent circuit model
presented in Fig. 3.
PIERS Proceedings, Moscow, Russia, August 18–21, 2009
646
Table 1: Extracted parasitic elements of the equivalent circuit model of the QFN package.
CLG
38.3 fF
LL2
0.24 nH
M 1Lw
0.008 nH
CLP
106.5 fF
LLG
0.01 nH
M 2Lw
0.496 nH
Cpg
41.4 fF
LP G
0.1 nH
M 1L1L2
0.008 nH
CL1L2
20.2 fF
Lw
1.26 nH
M 2L1L2
0.008 nH
LL1
0.15 nH
Mw1w2
0.066 nH
M 3L1L2
0.008 nH
4. CONCLUSION
In this study, the equivalent wideband electrical circuit model of a fully encapsulated plastic-molded
QFN package that accounts for the high-frequency parasitic effects was developed. The parasitic
elements of the equivalent circuit models were extracted from the 3D EM simulations. It was
shown that the developed model gives a very good correlation to the results obtained from the EM
simulation up to 25 GHz. The equivalent circuit model developed in this work can be used in RFIC
circuit level simulators for high level system simulations up to 25 GHz.
ACKNOWLEDGMENT
This work was supported by the ELTA Systems Ltd. a group and subsidiary of Israel Aerospace
Industries.
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