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Thick film Hybridization of Indigenously Developed “10W DC-DC
Converter” for Satellite Power Distribution Systems
Anju Singh*, A. Raghuraman, PushpaNaresh Kumar, V. Venkatesh, and K. M. Bharadwaj Hybrid Micro Circuits Division,
ISRO Satellite Centre, Bangalore-560 015, India
E-mail*: singh@isac.gov.in
ABSTRACT
Thick film technology based Hybrid Microcircuits (HMCs) have become the assembly of choice over Surface
Mount Technology (SMT) based electronics for several of its prominent advantages for miniaturization.
Hybrid DC-DC converters are the miniaturized power supplies fabricated using Thick film technology with
active dice, passive chip components & magnetics attached on a ceramic substrates then housed into a
hermetic package. Thick film technology with chip & wire and surface mount components housed in hermetic
hybrid package allows ultimate power density & superior thermal performance. Thus hybrid DC-DC
converters have always been preferred over SMT based converters due to smaller size, weight, good thermal
& mechanical performance and high reliability for space applications. In-house developed 10W DC-DC
converter has multiple outputs (+15V, -15V & +5V) for wide-input voltage (42V to 70V) operations for power
distribution systems. The challenge was to develop the converter hybrid that meets required electrical
performance with High Reliability requirements for space applications with minimum size and weight.
This paper highlights the major challenges faced during the development of 10W Hybrid DC-DC converter.
Thermal analysis for critical components and unique layout design for meeting high reliability requirements
for space applications are discussed in this paper. Innovative Thick film process technology, involving
fabrication of special subassemblies & newly developed processes has also been highlighted. In addition to
this, process qualification details with high reliability requirements have been addressed in this paper.
KEYWORDS: Hybrid Microcircuit (HMC), DC-DC Converter, Thick film Processes, Sub-assembly, Process
Qualification, Proto Module Fabrication.
1. INTRODUCTION
Electrical energy is one of the vital elements in keeping a satellite operational in orbit. Solar panels convert
solar energy to electrical energy. The electrical energy is then processed, stored, regulated, and distributed to
electronic loads. Power conditioning and distribution in Satellite Power Systems include DC-DC Converters
which receive power from spacecraft power bus, which is mission and satellite design dependent, and convert
variable input voltage to multiple regulated DC voltages as per subsystem requirements. In-house developed
10W DC-DC converter design is based on Flyback Topology switching at 200 KHz with multiple outputs
(+15V, -15V & +5V) for wide-input voltage (42V to 70V) operations for power distribution systems. The
converter has features like built-in EMI Filter, indigenously developed LDO for +5V line, Inrush current
limit, Over Voltage, Under Voltage & current limit protections. This converter has been hybridized using
Thick film technology to achieve smaller size, lighter weight, improved electrical & thermal performance
and high reliability for space applications. Newly developed HMC converter emerged with unique
methodology of fabrication & assembly with development of special sub-assemblies and New Thick Film
technology. The details of fabrication, qualification of processes involved and the challenges faced during the
realization of 10W HMC DC-DC converter are discussed in the paper.
01
2. THICK FILM TECHNOLOGY FOR POWER HYBRIDS
Design & Development cycle of any Power Hybrid for space application consists of the following:
(i) Package/Substrate Selection
(ii) Component Selection (Bare Dice, Chip Resistors/Capacitors, Magnetic Coils)
(iii) Thick Film Compositions for Deposition of Various Functional Layers
(iv) Thick Film Resistors & Interconnections Design
(v) Thermal Analysis
(vi) Layout Design
(vii) Masks Generation & Fabrication
(viii) Process Technology Qualification
(ix) Proto Module Fabrication Followed by Circuit Type Qualification
2.1 DESIGN OF 10W HYBRID DC-DC CONVERTER
(i) Package & Substrate Selection :
The reliability of a hybrid circuit starts with proper selection of package & substrate. Package & substrate
selection is very critical to achieve required thermal & electrical performance and mechanical specifications
including vibration & shock requirements. To achieve this, package material should have mechanical
properties to withstand harsh environments, high thermal conductivity and closely matching coefficient of
thermal expansion (CTE 5.5ppm/°C) with substrate, Si chips & various adhesives/solders used for fabrication.
To meet these requirements metal package with Molybdenum (Mo) base and KOVAR ring has been chosen
for the converter HMC.
Substrates for hybrid circuits serve key functions like (a) Mechanical support for the assembly of the devices,
(b) base for the electrical interconnect pattern & batch fabricated film resistors, (c) medium for the dissipation
of heat from devices.Components like MOSFETs, schottky diodes, high power transistors and transformer
dissipate significant power. In order to handle this power, 96% Alumina substrate is selected. And based on
preliminary feasibility study, appropriate size of ceramic substrate is chosen to house the fabricated substrate
into existing metal package.
(ii) Component Selection :
All active components (ICs, MOSFETS, Diodes, etc.) are used in bare form which confirm to MIL-PRF-
38534 with Class-K element evaluation. Other passive components like chip resistors and chip capacitors are
of space grade with failure rate of Level-S. All components are derated as per requirements of space
application. All these parts used are processable & mountable using conductive/non-conductive
adhesives/solders, as applicable based on thermal analysis carried out as shown in Table 1.
(iii) Thick film Composition Selection:
Thick-film pastes, also referred to as inks, are thixotropic screenable compositions used in forming the
conductor, resistor, and dielectric patterns on a ceramic substrate. Pastes are chosen based on their functional
requirements and compatibility with Alumina substrates for high reliability and compliant to REACH & RoHS
guidelines.
(iv) Thick film Resistor & Interconnection Design:
Design of Thick film resistors is an important activity. Thick film resistors, as required by the circuit to handle
more power, have been designed with 10Ω/, 100Ω/ and 1KΩ/ sheet resistivity pastes. Resistor values
are adjusted to the required value by Laser trimming, wherever required. Wire bonds (Au/Al) and track widths
of conductor patterns are designed to carry high currents as per design requirements with appropriate derating
guidelines.
02
Thermal management of Power Hybrids consists of materials and processes that must be utilized to provide
effective heat removal from power dissipating components. Its primary objective is to ensure predictable and
reliable performance of all components within their safe operating limits. Proper selection of interface
materials and attachment mechanisms are required to achieve the best thermal resistance (θjc) from case to
junction so that critical heat dissipating elements perform within safe operating junction temperatures.
From Fourier’s law of heat conduction it is known that,
𝜽 = 𝒍
𝑲. 𝑨… … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … . … … … … . … … . (𝟏)
Where,= Thermal Resistance in ºC/W
l = Length of heat flow in Inch
K= Thermal conductivity in W/in-ºC
A= Area of cross section of heat source in sq. inch
Fig-1 shows typical heat flow for a die mounted inside a HMC assuming heat spreading to be at 45.
𝜽𝒋𝒄 = 𝟏
𝟐𝑲(𝒃 − 𝟏)𝒍𝒏 (
𝒃
𝒂) (
𝒂 + 𝟐𝒍
𝒃 + 𝟐𝒍) … … … … … … … … … … … … . … … (𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 𝟐 𝑓𝑜𝑟 𝑟𝑒𝑐𝑡𝑎𝑛𝑔𝑢𝑙𝑎𝑟 𝑑𝑖𝑐𝑒)
𝜽𝒋𝒄 = 𝒍
𝑲. 𝒂(𝒂 + 𝟐𝒍)… … … … … … … … … … … … … … … … … … … … … … … … (𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 𝟑 𝑓𝑜𝑟 𝑠𝑞𝑢𝑎𝑟𝑒 𝑑𝑖𝑐𝑒)
Fig-1: Thermal Resistance of a Die Mounted Inside HMC Package
From Fig.1, thermal resistance (θjc) for a die mounted inside HMC package can be calculated as
θjc= 1 + 2 + 3 + 4 + 5 (°C/W) ………………………………………………………...……..(4)
Typical calculation of thermal resistance (θjc) for all high power dissipating components has been done and
shown in table below:
Table 1: Thermal Calculations for 10W HMC Converter
Material/
Interface
Thermal Resistance (°𝐂/𝐖) as per equation 2 & 3
MOSFET-1 MOSFET-2 Power
Transistor
Power
Diode
PWM IC
Size/unit 0.257”X0.257” 0.182”X0.116” 0.142”X0.142” 0.125”X105” 0.142”X0.185”
Silicon Die (1) 0.054 0.156 0.163 0.241 0.128
Epoxy (2) 0.192 0.511 0.534 0.754 0.428
Alumina(3) 0.486 1.17 1.221 1.650 1.001
Epoxy (4) 0.105 0.231 0.240 0.311 0.202
Package (5) 0.092 0.181 0.188 0.233 0.162
Total Thermal Resistance θjc = 1 + 2 + 3 + 4 + 5
jc 0.931 2.250 2.349 3.190 1.923
03
(v) Thermal Analysis:
Dissipation for Devices
Dissipation (W) 0.7 1.7 0.2 0.8 0.5
Temperature Gradient at Devices
Temp. Rise (°𝐂) 0.652 3.825 0.469 2.552 0.962
Based on thermal calculations with appropriate interface materials, calculated maximum temperature rise
shows that the junction temperatures of critical power dissipating elements are within the limits.
(vi) Layout Design:
Detailed circuit schematic for hybridization is provided by Power Systems Group to HMC Division. Having
selected the technology, package/substrate & components, appropriate design guidelines (ISRO-PAS-206)
have been followed to generate the layout for HMC fabrication. Innovative design methodology was used to
design the layout for 10W HMC DC-DC converter. Based on circuit partitioning, components power
dissipation & attachment processes, entire layout was designed into two separate sections namely ‘Power
Section’ & ‘Control Section’. This methodology facilitates circuit isolation, better thermal management and
ease of fabrication. Partitioning of layout also facilitates electrical testing/tuning of fabricated section
separately at substrate level, hence building more confidence on each & every section of the circuit. And prior
to substrate assembly into package, partitioning aids rework at substrate level as package level rework is
extremely difficult. Special attention was given for designing conductor tracks and conductor pads of critical
components as per their electrical & power dissipation requirements.
2.2 DEVELOPMENT OF 10W DC-DC CONVERTER
The converter is configured with a fixed frequency flyback topology incorporating magnetic
feedback and typical three user output voltages of +15V,-15V and +5V. Figure 2 shows the functional
block diagram of the converter and electrical specifications are given in Table 3.
2.2.1 CHALLENGES & DEVELOPMENTAL APPROACH
Thick film facility of HMC Division has already developed 4.5W DC-DC Hybrid Converter in past and
have flown successfully in various spacecrafts. Development of 10W DC-DC Hybrid Converter uses
Thick Film Technology along with recently qualified Innovative Processes Technology used for
realization of High Power Hybrids. The major challenge in development of this converter was to
accommodate the converter in available metal package (size 2.40” X 2.88” X 0.60”) which already had
successful flight history.
Fig-2: Functional Block Diagram of 10W DC- DC Converter
04
Development of 10W DC-DC Converter involves the following major phases:
(I) Innovative Thick Film Processes
(II) Design of special Sub-assemblies & their attachment technique
(III) Unique assembly sequence of fabrication
(I) Innovative Thick film Processes:
The First Challenge in developing the Converter is Large Power Dissipation & Current Handling
Capabilities with stringent Line and Load regulation specifications. This calls for keeping Thermal and
Electrical Resistances to minimum for achieving good Thermal & Electrical performance. As a result of
thermal analysis carried out for high power dissipating components including MOSFETs & Schottky
diodes, all bare dice have been assembled using conductive epoxy along with Chip & Wire assembly
techniques. Attachment of fabricated substrates to Molybdenum (Mo) base package has been carried out
with Conductive Film Adhesive. This has led to efficient Thermal Performance of the Converter. Large
diameter Gold/Aluminum wire and Gold Ribbons have been used for interconnections to handle large
currents. Electrical Resistances are kept to minimum by soldering enameled copper wire avoiding Thick
Film Tracks to a large extent in Power Section and soldering enameled copper wire on to Package leads.
To meet all these challenges following New Process techniques have been employed:
a) Realization of conductor tracks/pads with New Thick film Gold Composition suitable for soldering
and chip & wire assembly.
b) Design of Thick film Resistors with 10Ω/ Sheet Resistivity.
c) Stacking & attachment of Multilayer Ceramic Chip Capacitors using solder (Refer Fig.3).
d) Vertical mounting of Ceramic Chip Capacitors using Epoxy/solder (Refer Fig. 4).
e) Attachment of CWR29 Tantalum Chip Capacitors and RM0505 Chip Resistors using Epoxy (Refer
Fig.4)
f) Assembly of magnetic coils including vertical mounting of their attachment (Refer fig. 5).
g) Attachment of Special sub-assembly of Stacked Multilayer Ceramic Chip Capacitor (MLCC) on child
substrate using solder and attachment of this child substrate using non-conductive epoxy on Magnetic
coil which is eventually mounted on mother substrate (Refer Fig. 6).
h) Soldering of enameled Copper wire on Thick film Gold metallization& directly onto the capacitor
termination (refer Fig.3).
i) Attachment of multiple Substrates to Molybdenum (Mo) base with adhesive temporarily and their
interconnections for Substrate level electrical testing.
j) Attachment of multiple Substrates to Package with Molybdenum base using Conductive Film adhesive
and their interconnections (refer fig. 7).
k) Gold Ribbon bonding on New Thick film metallization & Package leads.
l) Soldering of enameled Copper wire on to Package Leads for I/O Connections (refer fig.7).
Fig. 5: Vertical mounting of magnetic coils
using non-conductive epoxy. This saves enormous substrated for other components
assembly to be fabricated.
Fig. 3: Stacking of MLC capacitor using solder.
Soldering of enameled coil wire is also indicated.
Fig. 6: Sub-assembly of two stacked
capacitor on child substrate and
mounted on magnetic coil and
attached to the mother substrate later.
Fig. 4: Vertical mounting of MLC capacitor,
mounting of Tantalum Capacitor &
RM0505 resistor is indicated.
05
(II) Design of Special Subassemblies & Attachment Techniques
The Second Challenge is to realize the Converter HMC with minimum size & weight and realize the
converter in an existing package which had flight history. To achieve this, new type of Special
Subassemblies (Fig. 3 & 6) are designed and fabricated. Subassembly in Fig. 3 consists of stacking of
four nos. of MLC Capacitors using solder attachment. This subassembly resulted in high capacitance
value & high volumetric efficiency with minimum assembly area on substrate. Design of sub-assembly
shown in Fig. 6, includes assembly of two stacked MLC Chip Capacitors on child substrate which is
mounted over horizontally attached magnetic coil on mother substrate. This subassembly is unique in
nature and has not only facilitated miniaturization of HMC Converter significantly but has also aided in
excellent electrical performance of the Converter by keeping electrical resistances to bare minimum.
(III) Unique Fabrication & Assembly Sequence
The Third Challenge was to realize the Converter in such a way that descending hierarchy to be
maintained with respect to high temperature processes. Sequence of fabrication is chosen such that the
one level of assembly process does not become an obstacle to next level of assembly. Secondly, apart
from special layout design, fabrication/assembly is done in such a way that detailed functional testing is
carried out at substrate level and then transfer of Substrates to Package is carried out carefully so that the
Converter performs at Package level meeting 100% electrical specifications. This has been achieved by
partitioning the circuit into two sections i.e., two substrates (power section & control section) and then
assembling them on to a temporary test jig with interconnections so that electrical testing is carried out
and any rework required due to unsatisfactory electrical performance, can be done at substrate level
itself. Eventually, the Substrates are disassembled from the temporary test jig and mounted into the
Package and interconnections are carried out from substrate to Package I/Os. This is a very critical &
challenging stage of assembly as there is little scope for rework once substrate is assembled into the
Package. Later, 100% Electrical testing is carried out once again to confirm electrical performance at
Package level.
The fabrication of converter HMC right from substrate fabrication to assembly of substrates with special
subassemblies and other unique assembly operations & their integration into package with
interconnections, is totally unique and challenging for realization of 10W HMC DC-DC Converter in an
existing Power Package. New Thick Film techniques along with special subassemblies developed and
unique fabrication & assembly sequence is the Innovative Thick Film Process Technology developed for
fabrication of 10W HMC DC-DC Converter.
2.2.2 PROCESS TECHNOLOGY QUALIFICATION
The Innovative Thick film Process Technology developed has been qualified through a rigorous
qualification plan as per guidelines of ISRO-PAS-206 & MIL-Std-883. The samples required for
qualification were fabricated by Thick film Facility of HMC Division and the evaluations were conducted
by Systems Reliability Group (SRG). The following table provides the typical qualification plan followed
for various new processes techniques employed and special subassemblies developed:
Table 2: Process Qualification Plan for 10W HMC Converter
Process Qualification Plan
Group Tests & Test Conditions Samples
Pre-
Conditioning
Pre-cap visual inspection All
Samples
(6 nos.) Initial electrical testing
Bake +125°C for 72 hours
Temperature Cycling -55°C to +125°C, 15 cycles
Mechanical Shock (1000g, SRS)
Seal leak (5 X10-8atm cc/sec)
Sub-Group 1 Temperature Cycling -55°C to +125°C, 100 cycles 2 nos.
06
Electrical testing
Mechanical shock (1500g SRS)
Electrical testing
De-lid & visual inspection
Die shear
Sub-Group 2 High temp. storage 125°C 1000 hours 2 nos.
Electrical Testing
Mechanical shock (1500 g, SRS)
Electrical Testing
De-lid & visual inspection
Die shear
Sub-Group 3 Vibration (Sine 20-100Hz, 20g), (Random 21.4g rms) 2 nos.
Electrical testing
De-lid & visual inspection
Die shear
All new process techniques were evaluated & qualified by SRG, ISAC successfully, thereby qualifying the
Innovative Thick film Process Technology for Flight fabrication of 10W HMC DC-DC Converter.
2.2.3 PROTO MODULE FABRICATION
Fig-7 shows 10W HMC DC-DC Converter housed in Molybdenum Base Metal Package. After
satisfactory electrical performance of the converter, thermo vacuum testing has been conducted and
junction temperature measurement was carried out for the most critical heat dissipating elements in the
converter Hybrid Module. Thermo Vacuum test confirmed thermal reliability aspects for space application
and assured that converter operates within the safe device junction temperature limits. In addition to this,
proto module was also subjected to EMI/EMC tests. Proto module complies with all EMI/EMC
requirements as per MIL-STD-461C for space application. The converter HMC meets all the electrical
specifications given in Table 3.
Proto modules of 10W HMC DC-DC Converter
was fabricated adopting newly qualified
Innovative Process Technology along with
existing qualified thick film technology for power
Hybrids. After assembly of both the Substrates
(Power & Control Sections), the individual
Substrates are tested electrically. Then the
Substrates are assembled on to temporary test jig
and interconnections are carried between them.
Electrical testing has been carried out using a
special test jig designed to test the converter at
substrate level. Converter Hybrid was tested for
all electrical parameters at substrate level before
its housing into package. After successful
electrical testing the individual substrates are
disassembled from temporary jig and housed into
a Power Package followed by carrying out
Substrates to Package I/O interconnections. The
Converter has been tested in Package for 100%
Electrical performance.
Fig. 7: Internal construction of a full-fledged 10W
hybrid DC-DC converter. Converter is packaged into
a metal Mo base package
07
Output Voltage (V) +15 +5 -15
Nominal Current (mA) 250 500 250
Input Current (mA) @ No Load 14mA
Nominal Power (mW) 3750 2500 3750
Maximum Output Current (mA) 500 1200 500
Total Nominal Output Power 10W
Maximum Output Power 12W
Output Ripple Voltage (Max) (mV) 30 – 50
Line Regulation (%) ±2 ±0.5 ±2
Load Regulation (%) ±3 ±0.5 ±3
Cross Regulation (%) ±3 ±0.5 ±3
Over current limit (% of full load rated current) 130% – 150%
Turn ON response (ms) 5
Efficiency ≥69% @ 100% Load
Switching Frequency 200 KHz ±10%
Over Current, Over Voltage Protection Pulse by Pulse and Hick-Up Mode
Isolation I/P to O/P – 100MΩ
TM Monitoring 5V
EMI/EMC Compliance As per MIL-STD-461C
(meets CE03, CS01, CS02& CS06)
Operating Temperature range -55ºC to +125 ºC
Mechanical Specifications
HMC SMT
Size 2.880” x 2.400” x 0.600” 3.070” x 3.220” x 1.18”
Weight 154 grams 220gms
3. CONCLUSION
Thick film facility of HMC Division has fabricated 10W HMC DC-DC Converter Proto module adopting
Innovative Thick film Process Technology which is Qualified for Space application. The performance of the
converter HMC is excellent and meets all the requirements of subsystem. Development of 10W HMC DC-
DC Converter was a difficult task consisting of many challenges. Major challenge was to accommodate the
converter in available metal package (size: 2.40” X 2.88” X 0.600”). Next challenge was to develop the
converter HMC handling Large Power dissipation and meeting stringent Line and Load regulation
specifications with good efficiency. For this, thermal analysis was carried out to decide appropriate & efficient
attachment processes ensuring safe junction to case temperature for high power dissipating elements. Design
& Development of the converter HMC was carried out successfully adopting Innovative Thick Film Process
Technology and developing special subassemblies which enabled the realization of 10W HMC DC-DC
Converter with minimum size & weight meeting the required electrical & thermal performance. Unique
method of layout design based on partitioning, fabrication & assembly sequence of individual substrates and
their testing individually at substrate level prior to package assembly, enabled realization of converter without
any rework/repair at package level which was a real challenging task. Based on the mechanical specifications
as given in Table 3, it is evident that low size & weight achieved for Hybrid Converter developed using
Thick Film Hybrid Technology makes it a winning candidate over SMT converter for space applications.
08
Table 3: 10W HMC DC-DC Converter Specifications
Electrical Specifications
Input Voltage (V) 42V to 70V
Qualification Samples & Proto Module of 10W DC-DC Converter and HMC Evaluation Section of SRG for
conducting process Qualification tests.
6.
ABOUT THE AUTHORS:
1. ‘Design and processing of Power Hybrids’ – Al Krum, Hybrid Circuit Technology, Jan. 1987.’
2. ‘Thermal Management Handbook for Electronic Assembly’ – Jerry E. Sergent and Al Krum.
3. Manufacturing Power Hybrid Circuits Dr.R.F.David, Electronics Packaging and Production, March 1996.
4. Power Hybrids for Space Application, Pushpa Naresh Kumar, C.S.Madhusudhana, A.Raghuraman, G.Yadagiri,
K.K.Goswami and R.Madhavan, EMIT ‘98.
5. Thermal Resistance of power devices in Hybrid Microcircuits, A.Raghuraman, D.Ramakrishnan, Pushpa Naresh
Kumar, T. Kanthimathinathan & Arun Batra, IINC 2004.
A. Raghuraman received B.Sc. degree in Physics from Madras University in 1979 and M.Sc. degree in
1989 from Annamalai University. He joined ISRO in 1983. Currently he is heading Thick film Section of
HMC Division. At present he is involved in design and development of various Thick film Hybrids for
Space applications including DC-DC Converter Hybrids. He is a Life Member of IMAPS (India chapter)
and has a few papers published to his credit.
Anju Singh has received B.E. degree in Electronics & Communication Engineering from Govind Ballabh
Pant Engineering College (GBPEC), Pauri Garhwal in 2004. She joined Space Applications Centre, ISRO
in 2006 and at later years of her service she joined ISRO Satellite Centre, ISRO in 2010. Since, then she
is involved in Design & Development of various Thick film Hybrids for Space Applications including
DC-DC Converter Hybrids & Fine Line Technology based Multichip Modules.
09
REFERENCES
4. FUTURE SCOPE
Development of 10W HMC DC-DC Converter has been a major challenge, the next challenge will be the
Product Qualification. Attempts are being made for further miniaturization of the converter by using Package
with all KOVAR construction.
5. ACKNOWLEDGMENT
Authors wish to thank Director, ISAC for entrusting Thick film Section of HMC Division the task of
miniaturization of 10W HMC DC-DC Converter & providing constant guidance and support. Special thanks
to PSG for providing excellent Circuit Design and Testing support, Thick Film Team for fabricating Process
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