21-08-2012, 03:09 PM
Thermal Assessment of RF Integrated LTCC Front End Module (FEM)
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ABSTRACT
The thermal performance of Front End Module
(FEM) incorporating Low Temperature Co-fired
Ceramic (LTCC) substrate is investigated. An Infrared
Microscope System was used to measure device surface
temperature with both RF and DC power at various duty
cycles (25 to 100%). The maximum junction
temperature (~112°C) occurs at the second stage. By
powering the module with DC only, the comparison
between numerical and experimental data indicates
good agreement, with less than 10% difference in the
peak temperature values.
When replacing the common 2-layer organic
substrate with a 14-layer LTCC substrate and silver
paste metallization, the peak junction temperature
reaches 130.1°C, ~51% higher than before. However,
by increasing the silver paste thermal conductivity from
90 to 350 W/mK, a significant drop in peak
temperatures occurs, indicating the impact on module's
overall thermal performance. The top metal layer
thickness (10 vs. 30 microns) only contributed to 5-8%
changes in peak junction temperature.
INTRODUCTION
Projected increases in interconnect density and
requirements for higher clock rates for digital and
microwave electronics defined a need in the early 1990
for improvements in traditional microelectronic
packaging. The package, together with the interconnect
board plus discrete components complicate the
assembly, increasing the volume as well as the weight.
A new technology could integrate these three functions
and reduce the size and assembly complexity with
concurrent improvements in cost and reliability.
Requirements for reduced volumes for personal
electronic systems for communication are the main
drivers for the new technology. Low Temperature
Cofired Ceramic is a potential solution for achieving a
newly integrated packaging technology from a
combination of thick-film and low-temperature co-fired
dielectrics [1,2]. It offers a competitive edge in the
integrated passive components over other MCM
technologies.
COMPARISON BETWEEN ORGANIC AND
LTCC MODULES
Baseline Organic Substrate Module
A case with organic substrate is considered for
comparison purposes. As shown in Fig. 1, the
Integrated Circuit (IC) is a 3-stage, dual-band PA placed
on a 75 mm thick GaAs die (2.025 x 1.925mm). Six
power amplifier MESFET (Metal Semiconductor Field
Effect Transistors) stages are placed on the die: 3 for
low-band operation and 3 for high-band operation. The
low/high-band stages are powered separately. Total
power dissipation on the die for low and high-band
operation is 5.1 and 4.8 W. Dimensions, power levels
and power densities are provided in Table 1. The die
contains 80mm diameter solid gold (Au) vias (13 total),
connecting MESFETs source pads to backside
metallization (plated gold with a total thickness of 3
mm). Gold bond pads (100 x 100 mm size and 25 mm
thickness) are located on the die periphery.
CONCLUSIONS
The junction temperature for the PA with LTCC
substrate and silver paste metallization is 130.1°C,
~51% higher compared to the baseline case with 2-layer
organic substrate. Increasing the metal thermal
conductivity from 90 (silver paste) to 150, 250, and 350
W/mK, a significant drop in peak temperatures occurs.
The thickness of the top metal layer (10 vs. 30 microns)
contributes 5-8% changes in peak junction temperature.
The 14 PTH vias placed under the die play an
important role in removing heat from the die through
the substrate.