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.

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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

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(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.

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(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.

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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

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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