View - Micro Systems Engineering

Jens Müller, Jürgen Pohlner,
Dieter Schwanke
Micro Systems Engineering GmbH
Schlegelweg 17
D-95180 Berg/Germany
Phone: +49 9293 78-0
Fax: +49 9293 78 42
jmueller@mse.biotronik-erlangen.de
www.mse-microelectronics.de
Development and Evaluation of Hermetic Ceramic
Microwave Packages for Space Applications
Authors:
Günter Reppe
RHe Microsystems GmbH
Heidestrasse 70
D-01454 Radeberg/Germany
Phone: +49 3528 4199-50
Fax: +49 3528 4199-99
guenter.reppe@rhe.de
www.rhe.de
Abstract
Heiko Thust, Ruben Perrone
Technische Universität Ilmenau
Microperipheric Group
D-98684 Ilmenau/Germany
Phone: +49 3677 69-2605
Fax: +49 3677 69-1204
Heiko.Thust@tu-ilmenau.de
www.tu-ilmenau.de
Within the KERAMIS project (funded by the German Ministry of Education and Technology) a consortium of six
companies, research institutes and universities is developing ceramic based microwave packages for satellite
applications. Several interface concepts like Ball Grid Array (BGA), Land Grid Array (LGA) and peripheral wire
bond packages are among the candidates to realize microwave compatible connections to other modules or to the
mother board. From a technology point of view the focus is put on fine line resolution and hermeticity. The latter is
predominately realized by multilayer LTCC substrates sealed with metal or ceramic lids.
Based on the specific function of the module, packages with and without metal heat sinks are considered. Therefore,
the LTCC substrate itself needs to be hermetic. Several thermal via and transition line configurations have been
investigated in the start phase of the project. Practical results will be demonstrated and design rules are derived.
The test matrix contains also lead free materials for frame and heat sink soldering to be compliant with the
European “Restriction of the use of Hazardous Substances” (RoHS). First results of the performance of microwave
transitions up to 50 GHz will be shown.
In addition to the main topic of the paper results of thin film processing for resistors will be given in the paper.
Key words: LTCC, Microwave Packages, Thermal Design, Lead Free, Thin Film, Fine Line Thick Film
Introduction
The objective of the funded program is to
develop packages for space applications fulfilling the
following functions and properties:
Impedance matched interconnections and
shielding,
Embedding microwave components (divider,
filters etc.),
Gas-hermeticity,
Interfaces to active and passive components
(wire bonding, soldering, gluing),
Good thermal conductivity.
The package itself will be mounted on a
printed circuit board with a metal core to transfer the
heat from power modules assembled. Regarding the
thermal management, two cases need to be
considered. For low or medium power dissipation no
heat sink is necessary on the package. Larger power
dissipation requires an attached metal plate acting as a
heat spreader and heat sink.
Thermal Design Options in LTCC
LTCC materials offer thermal conductivities
in a range from 2 to 5 W/m*K. This seems to be high
compared to organic boards which have a thermal
conductivity of about 0.2 W/m*K. However, it is still
not sufficient for the thermal management of a variety
of applications. Typical RF-power amplifiers have a
power dissipation of about 2-5 Watts. In order to
prevent overheating the MMIC, the substrate
underneath should provide the highest thermal
conductivity possible. Additionally, the substrate
thickness should be as small as possible to reduce the
thermal path to the module heat sink. Thick- or thin

film alumina have a better performance regarding
thermal conductivity (λ ≈ 24 W/m*K). Their weakness
is, however, the hermeticity of vias through the
substrate.
Thermal vias in LTCC are able to improve
the integral value of the thermal conductivity.
Depending on the array formation (diameter of vias
and pitch), values comparable to alumina or even
better can be achieved [1][2].
Fig. 1 shows the build-up structure of a
MMIC-package mounted on a FR-4 board with
thermal vias and an embedded metal core. Thermal
vias in the LTCC conduct the heat to the package
heatsink which act as a heat spreader. The latter is
glued or soldered to the copper pad on the FR-4. It
should be mentioned that the bonding methods for
die, heat sink and package is important for the
thermal management as well [3].
Fig. 1: Build-up structure for a MMIC in a LTCC package assembled on a FR-4 board with metal core
Hermeticity Test Coupon Design and Procedure
Since hermeticity is a required function of
the package, the thermal options need to be evaluated
in more detail. The material system selected was
DuPont 951AX. Due to the space application
requirements, the Au conductor system was selected
for the test. Vias in LTCC have a high metal content
(for good conductivity) and a very low glass content
as bonding agent. Therefore, bonding to the glassceramic
walls is not fully achieved. Increasing the via
diameter (typical for thermal vias) increases the risk
for a potential leakage. For a single layer with thermal
vias it is doubtful whether full hermeticity can be
realized.
In the study, six different design cases were
considered (Fig. 2). Packages might be required with
(case a and d) or without heat spreader attached. The
other options are two versus four layer designs and
thermal vias with or without diameter variation
between layers.
Fig. 2: Design options considered for the
hermeticity test pattern
The via diameter used are 250 µm and
130 µm after firing. Furthermore, two ways of
feeding signals into the package were considered.
Fig. 3 shows a schematic cross section with RF
feeding lines from package pads to the inner cavity.
The cavity wall is used for the cover assembly which
can be realized by a metal lid, kovar frame + lid,
metal cover or metallized plastic cover. The reasons
to investigate the two configurations were to verify
the hermeticity (leakage across the line type b) as
well as the electrical performance.

Fig. 3: RF-transitions into the package cavity
The substrate was manufactured using
standard processing conditions of LTCC. Outer layers
(frame and back side) of the four layer substrate were
structured by postfiring using two different solderable
pastes (AuPtPd and Au-brazing). The former is
intended for SnAgCu-soldering. The Au-brazing
system provides optimum leaching conditions even at
higher soldering temperatures as required in
Au80Sn20-soldering. Fig. 4 shows the sites used for
hermeticity and RF-testing on the multi-purpose
substrate.
Fig. 4: Hermeticity Test Coupon
The die bond pad size is 3 mm by 3 mm. For
future thermal assessment, dc-bias lines are routed
from the outside to the inner cavity.
Kovar frames with Ni/Au plating were
mounted on the substrate surface in the first assembly
step. For series manufacturing both the heat sink and
the frame can be soldered in one step. However, for
the evaluation it was necessary to check the
hermeticity without the heat sink attached. Solder
preforms were prepared by laser cutting. A Kester
high viscosity flux for lead free soldering was used to
activate the joints and to keep the frames in place. For
SnAgCu soldering a standard reflow oven (BTU
Pyramax) was used as an alternative to vacuum
soldering. After frame soldering the flux residues
were cleaned in an IPA-based solution with ultrasonic
support. Helium fine leak testing was performed
upside-down (Kovar frame on a rubber seal)
according to MIL-Std. 883F, Method 1014.11, test
condition A4 (leak rate < 10 -8 atm cc/s).
Heat sinks made of Molybdenum were
soldered during the second phase of assembly in a
vacuum process (gas phase activation). Fine leak
testing was done again to monitor any hermeticity
changes. Fig. 5 shows an example of a test package
after heat sink soldering.
Fig. 5: Test package with Kovar frame and Mo
heat sink
Finally, several parts were X-rayed for solder
joint evaluation.
Hermeticity Test Results
No He-leak problems could be assigned to
the solder joints after optimizing the soldering
parameters. Both SnAgCu and AuSn solder behaved
similar. Major problems were related to the stack of
vias with large diameter. Table 1 contains the results
of all test groups. On the 2-layer design, 80% of the
stacked via packages (250 µm) were not hermetic.
Even with two more layers, still 60% showed failures.
The concept of alternating the via diameter yielded
better results. The failure rate could be reduced to
20% on the 2-layer design and down to zero for four
layers. The position of the feeding lines had no
impact on hermeticity. After heat sink soldering, all
previously defective modules passed the He fine leak
test.

Heatsink Via stack Type Failure [%]
w/o
w/o
2 layer
4 layer
Stripline (Fig. 3a) 0
Coplanar line (Fig. 3b) 0
Constant via diameter: 250 µm (Fig. 2b) 80
Altern. via diameter: 250/130µm (Fig. 2c) 20
Constant via diameter: 250 µm (Fig. 2e) 60
Altern. via diameter: 250/130µm (Fig. 2f) 0
with all types all types (Fig. 2a/d) 0
X-ray analysis revealed no major problems
with heat sink soldering. Heat sinks soldered with
AuSn had small voids (less than 10%) whereas the
SnAgCu solder was almost void free (Fig. 6 and 7).
Fig. 6: X-ray picture of a heat sink soldered with
AuSn (small voids visible)
Fig. 7: X-ray picture of a heat sink soldered with
SnAgCu
Table 1: He fine leak test results
Package RF Performance
The two line configurations (Fig. 3) needed
to be optimized for best wave impedance control [4].
Critical areas are the transitions underneath the wall
to enter the inner cavity. The dielectric cover causes
an impedance step towards lower values. Therefore, a
line width reduction is mandatory for compensation.
Vias, surrounding the lines for shielding purposes and
ground connection, play an important role for the
wide band behaviour of the transition. Their location,
size and pitch determine the maximum possible
frequency with low return loss. Fig. 8 and 9 depict the
structure of the transition for both line types.
CST Microwave Studio was used to simulate
and optimize the transitions.
Fig. 8: Structural model of the stripline
configuration
The outer connection was designed as a
grounded coplanar line allowing direct contacting
with microwave probes (250 µm pitch). The test
packages were measured with a network analyser and
Z-Probes from SÜSS MicroTech up to 50 GHz
between the two outer connections (4 line transitions).
Measurement results are shown in Fig. 10.

S11 [dB]
Fig. 9: Structural model of the coplanar line
configuration
0
-10
-20
-30
-40
-50
-60
Frequency [Hz] [GHz]
0 10 20 30 40 50
Line transition type a. Line transition type b.
Fig. 10: Return loss measurement (S11)
for the both packages
The packages with CPWG-stripline-CPWG
line transitions (type a) show a good impedance
match up to 18 GHz. At frequencies above this value
the reflection coefficient (S11) increases very fast.
The packages with the line transitions type b
show excellent impedance matching up to 50 GHz.
Thin film on LTCC
Thin film structuring on LTCC is a part of
the research program conducted by RHe
Microsystems. Line widths and spaces down to
10 µm have been realized on DP 951 [5]. Since
resistors with small dimensions and high precision are
required for several microwave structures (e.g.
terminations, dividers etc.) thin film resistor materials
on LTCC were examined in detail.
The following two different layer systems
were deposited on LTCC:
1. CrNi / Cu (sputtered) - CuNiAu (electroplated)
for CrNi-resistors
2. TaN / TiW / Au (sputtered) - Au (electroplated)
for Tantalnitride-resistors
CrNi can be used in the first mentioned layer
system both as adhesion layer and as resistance layer.
TaN is not suitable as an adhesion layer and is used as
resistance layer only. Al2O3 substrates were added to
the test matrix as controls.
Sputter conditions were chosen for these
resistance materials according to known Al2O3
parameters to obtain square resistances (Rsq) of 20, 50
and 100 Ω respectively. Different resistor geometries
with length/width-ratios between 1:5 and 500:1 were
produced and examined. The minimum resistor width
was 50 µm.
Fig. 11 shows the relative resistance values
for both CrNi- and TaN-resistors and several Rsq in
comparison to Al2O3-ceramics. A possible
explanation for the lower resistance is the smaller
„micro roughness” of LTCC compared to alumina
[5].
dR/R Al2O3 [%]
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
Resistance related to Al2O3
20 Ohm/sq 50 Ohm/sq 100 Ohm/sq
Fig. 11: Resistance values of CrNi and TaN thin
film resistors on LTCC normalized to Al2O3 for
the length to width ratio of 107:1
No differences were observed between the
different Rsq for TaN-resistors whereas CrNi resistors
with a Rsq of 20 Ω tend to be lower than expected.
The determined hot thermal coefficients of resistance
(HTCR) between + 25°C and + 125°C are even more
interesting. (Fig. 12/13).
It is obvious that CrNi-resistors with Rsq =
20 Ω are not suitable for the use on LTCC. The
negative tempering shift after 6 hours at 250 °C is
another indicator (typical shift is between +5 and
+15%). For 50 Ω/sq and higher values the HTCR is
only 5 ppm/K lower than on Al2O3, and is therefore
acceptable.
CrNi
TaN

HTCR [ppm/K]
70
60
50
40
30
20
10
0
-10
-20
-30
CrNi
20 Ohm/sq 50 Ohm/sq 100 Ohm/sq
Al2O3
LTCC
Fig. 12: HTCR of CrNi on LTCC and alumina for
various resistances (l/b=107:1)
HTCR [ppm/K]
0
-20
-40
-60
-80
-100
-120
TaN
20 Ohm/sq 50 Ohm/sq 100 Ohm/sq
Al2O3
LTCC
Fig. 13: HTCR of TaN on LTCC and alumina for
various resistances (l/b=107:1)
The principally higher thickness of Tantal
Nitride compared with CrNi lead to more stable
conditions. The measured HTCR-values of TaN on
LTCC do not differ from those on Al2O3, (usually
from -90 to -110 ppm /K).
Laser trimming of thin film resistors could
be carried out without any difficulties. Fig. 14 shows
a resistor with the geometry of l = 10,7 mm and b =
100 µm after trimming. Tolerances in the range of
± 1% can be achieved. First stability tests (1000 h @
150 °C) have already indicated positive results.
Fig. 14: Laser trimmed thin film CrNi resistor
Summary and Outlook
Design constraints for hermetic packages
were determined. It has been shown that thermal via
designs have an impact on module hermeticity. Cost
effective lead free solder was successfully used for
sealing LTCC packages. The package designs allow
RF transitions which are capable up to frequencies
over 40 GHz.
Regarding thin film resistors on LTCC it can
be concluded that TaN-resistors are very suited
because of the quite comparable behaviour to
alumina. In general, also CrNi resistors can be applied
on LTCC 951 with the exception of the low resistance
value.
Future work within the project will focus on
the thermal assessment of hermetic packages and the
evaluation of different packaging concepts in terms of
RF and thermal management.
Acknowledgements
The authors would like to thank the German
Government for the financial support. The work
carried out in the project Keramis is funded by the
BMBF under the project number 50 YB 0316.
References
[1] M. Ray Fairchild et al.: LTCC Technologies for Harshenvironment
Automotive Applications Utilizing Soldered
Interconnections, Proceedings of the 37 th International
Symposium on Microelectronics, Nov. 14-18, 2004, Long
Beach, CA.
[2] Dan Amey, Mimi Keating: Thermal Performance of 943
Low Loss Green Tape LTCC with Thermal Via and Internal
Metallization Enhancements, Proceedings of the 2004
Ceramic Interconnect Technology Conference, April 26-28,
2004, Denver/CO, USA.
[3] Yasunari Ukita et al.: Lead free die mount adhesive
using silver nanoparticles applied to power discrete
package, Proceedings of the 37 th International Symposium
on Microelectronics, Nov. 14-18, 2004, Long Beach, CA.
[4] Ruben Perrone et al.: Development and Evaluation of
Photodefined Elements for Microwave Modules in LTCC
for Space Applications, 14 th European Microelectronics and
Packaging Conference, June 12-15, 2005, Brugge/Belgium.
[5] Günter Reppe et al.: Development and Evaluation of
Fine Line Structuring Methods for Microwave Packages in
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Packaging Conference, June 12-15, 2005, Brugge/Belgium.