- Email: [email protected]
Quasi hermetic packaging for new generation of spaceborn microwave equipment
Microelectronics Reliability 49 (2009) 1326–1329
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Invited Paper
Quasi hermetic packaging for new generation of spaceborn microwave equipment Philippe Monfraix *, Regis Barbaste, Jean Luc Muraro, Claude Drevon, Jean Louis Cazaux Thales Alenia Space, 26 Avenue J.F. Champollion, BP 33787, 31037 Toulouse Cedex 1, France
a r t i c l e
i n f o
Article history: Received 17 July 2009
a b s t r a c t This paper presents the introduction of the quasi hermetic encapsulation of microwave hybrids for space application through different approaches evaluated at Thales Alenia Space – France. Thanks to the improvement for many years of microwave organic materials, it is now realistic to propose advanced packaging solutions like the chip on board approach with glob top encapsulation of active devices directly bonded on printed circuit boards for space applications. To validate this packaging approach, very significant reliability test-plans have been proposed and performed on the different technological processes and materials in agreement with standard space quality requirements. Results will be presented and a discussion on the nature of the stresses applied during the tests will be proposed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Future satellite payload will exhibit higher functionalities (such as multi-spot beam coverage, re-configurability, on-board processing) in higher frequencies (Ka, Q/V bands) compared to current telecom applications, mainly in Ku-band. For the packaging of the electronic equipment, from low to microwave frequencies, this drastic evolution is also in parallel with miniaturisation and costreduction. Standard packaging techniques for space application involve traditionally fully hermetic hybrid modules (like laser/seam welding or static oven sealing processes) based on metallic (KovarÒ, Cu/W, etc.) or ceramic housing. To face these new challenges, a possible route is to introduce a quasi hermetic packaging approach with an optimised partitioning of technologies. 2. Microwave amplifier modules for multimedia application at 30 GHz For the design of a Focal Array Fed Reflector (FAFR) antenna for multimedia application in Ka-band [1], where this approach can finally replace four passive antennas (one single feed per beam), a major challenge was the design of a very large number of low noise amplifier (LNA) modules with both nominal and redundant paths. The LNA modules are parts of a dual polarised focal array feed reflector antenna including the following elements (cf. Fig. 1): radiating element of horn type, ortho-mode transducer (OMT), band-pass filter (BPF) which provides wideband out-of-band rejection in order to eliminate extra system uplink interferences, * Corresponding author. Tel.: +33 5 34 35 60 70. E-mail address: [email protected] (P. Monfraix). 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.07.051
low noise amplifiers with including a redundancy switch, located just behind the radiating elements for G/T maximisation, beam forming network (BFN) connected to LNA modules and generating illumination laws required for multiple spots coverage. The choice of the technology for the LNA modules has a direct impact on focal array architecture and layout. The complete module design has to be very innovative to comply with severe volume constraints (small pitch from module to module), tight RF performances and low recurring cost. That is why we developed a solution using MMIC devices directly bonded on an organic substrate (printed circuit board). Transition between waveguide access and the microstrip line is achieved by Vivaldi slots designed on the substrate. This substrate is then directly embedded in the waveguide access. It finally offers a technical solution well adapted to production of a significant number of items (more than 300 required for a fully equipped antenna) at low cost. The LNA modules are based on the following elements (cf. Fig. 2): a waveguide/microstrip transition, a PIN diode switch to divide nominal and redundant accesses, two MMICs (with a constant current biasing network) followed by a gain and phase corrector in each access, a Wilkinson combiner to combine the nominal and redundant accesses, a phase corrector, a microstrip/waveguide transition. These modules have to be implemented within the focal array mesh (about 1.2k = 12 mm at 30 GHz). This was reached with the introduction of new materials and technologies such as well-suited
P. Monfraix et al. / Microelectronics Reliability 49 (2009) 1326–1329
1327
Fig. 1. Design and layout of the focal array.
Fig. 2. Picture of the LNA modules at EQM level.
printed circuit board substrate, chip on board assembly and glob top encapsulation. Thales Alenia Space – France (TAS-F) is involved on those new processes both for microwave and low frequency applications [2]. Large investigations have resulted in the choice of epoxy based resins with silica fillers designed with very low ionic contents and out-gassing values, in agreement with MIL standard. With a ‘‘dam & fill” process, an accurate positioning of the encapsulation resin can then be assumed. The technological construction is described below: a RO4003TM board (substrate designed for microwave applications) is assembled on an aluminium structure, the 100 lm thick GaAs MMIC devices, 0805 SMT passives and alumina substrates are then directly bonded on the board using a flexible adhesive (to compensate the thermal coefficient mismatch between these components and the soft board), all the actives (GaAs MMIC, Si transistor) devices are encapsulated by glob top process based on a dam & fill technique. Thanks to this innovative packaging approach for space application, the integration achieved (72.4 mm 30.1 mm, 31.6 g for each LNA module) is four times lighter and half in volume compared to a standard fully hermetic solution. 3. Very integrated TM/TC section for Linearized Driver Limiter Amplifier (LDLA) The LDLA equipment is a key component of a satellite payload as it is localised before and then driving the electronic tubes. As depicted in Fig. 3, it is typically composed of two RF sections (a 60– 100 dB gain block and a linearized block to compensate tubes distortion) and a mixed digital/DC section called TM/TC which is in charge to monitor all the tele-metry (TM) and tele-command
Fig. 3. Picture of the current Linearized Driver Limiter Amplifier equipment.
(TC) signals. Concerning this TM/TC section, the current packaging is a thick film alumina substrate encapsulated in a fully hermetically sealed MCM-C module (MultiChip Module, High Temperature Cofired Ceramic technology) with a total size of 74 mm 51 mm. Replacing the thick film substrate by a eight layers printed circuit board, introducing smaller surface mount components (down to 0402 dimension) and thanks to a glob top encapsulation process of active devices directly bonded on the board, we can achieve a size integration by a factor 2 with a weight reduction over 50% (cf. Fig. 4). 4. Reliability tests over advanced materials and processes for space applications In the frame of the FAFR amplifier modules, many reliability tests have been achieved according to MIL or ECSS specifications.
1328
P. Monfraix et al. / Microelectronics Reliability 49 (2009) 1326–1329
Up face of the board : active devices – glue and wire processes (before glob top encapsulation)
Bottom face of the board : passive devices down to 0402 format – surface mount process
Fig. 4. Picture of the enhanced TM/TC section (up face after glob top encapsulation). Only half of the footprint is used compared to the current solution.
4.1. Printed circuit board RO4003CTM substrate used for the amplifier modules is a double sided printed circuit board based on a glass reinforced hydrocarbon/ceramic thermoset laminate designed for high performance sensitive applications. It is a low loss material which can be fabricated using standard epoxy/glass (FR4) processes. As a non Teflon based material, this material does not require specialised via preparation process such as sodium etch. Designed for application at 30 GHz, this substrate has very tight mechanical constraints and etching tolerance on line/space width of ±15 lm. The size of the RO4003TM board is 72 mm 30 mm with a thickness of 0.3 mm. The reliability tests done for the process qualification of this substrate are following the ECSS-Q 70-10A standard and are divided in three test-files: life-test for 1000 h at 125 °C, 500 thermal cycles in the [ 55 °C, +125 °C] temperature range according to MIL Std 883 Method 1010 condition B with an intermediate status after 200 cycles, humidity test for 10 days at 40 °C and 93% HR.
After all these tests, we can notice: no degradation after visual inspection on the surface metallization and the via holes (cross-section done on test-samples coming from the three test-files), no degradation on the metallization adherence (with values above 8 N/cm after these tests), no electrical deviation about the isolation resistance, the breakdown voltage and the interconnection resistance.
4.2. Assembly of active and passive devices on board Traditionally, microwave hybrid modules manufactured for space application involve GaAs active devices and alumina substrates matched with substrates in term of coefficient of thermal
expansion (CTE). For the chip on board approach, the large difference of CTE is an issue to be solved by using other interface. For the assembly of the components on the board, the objective was then to work on a flexible adhesive for mismatch CTE that can be used on automatic dispensing system for production, with extractable ionic content and out-gassing characteristics in agreement with MIL Std 833 Method 5011. Many commercial products were selected and evaluated on preliminary technological tests and the choice has been done on an epoxy reference from Emerson and Cuming. The ionic content analysis exhibit excellent results with for instance 32 ppm chloride ion and 33 ppm ammonium ion (all other values are below 10 ppm for sodium, fluoride nitrate ions). The reliability test for the process qualification of these packaging assemblies is up to 500 thermal cycles in the [ 55 °C, +125 °C] temperature range according to MIL Std 883 Method 1010 condition B with an intermediate status after 200 cycles. The components assembled for this process are GaAs MMIC (3 mm 2 mm), alumina substrates (8.3 mm 5.6 mm), 0805 SMD capacitors. After all these tests, we can notice: no cracks or delaminating effect after visual inspection and on cross-sections done on the joint of the epoxy glue, before and after the reliability test, value of different components strength on the board substrate with the new adhesive after shear test is above 2–4 for times above the according to MIL Std 883 Method 2019.5.
4.3. Wire-bonding on soft substrate The needs for wire-bonding for the amplifier modules concern the connection of GaAs MMIC devices and thin film alumina substrates on the RO4003TM soft board. The process developed for the MMIC and alumina on the board is 25 lm gold wire thermo-sonic bonding on both automatic and manual die-bonder. A key-point of this process is to closely manage the loop of the wire: for electrical reason, the inductance of the wire has to be minimised (Ka-band). The reliability test done for the process qualification is a storage of wires bonded on the RO4003CÒ board for 31 days at 175 °C (with
P. Monfraix et al. / Microelectronics Reliability 49 (2009) 1326–1329
1329
an intermediate status after 15 days, mid-term) with 50 shear tests done at each step. This is equivalent to a storage for 24 h at 250 °C, standard reliability test done on alumina substrates for wire-bonding qualification. On a manual die-bonder, we obtain the following results:
Life test: This test evaluates the functionality of the component and its electrical characteristics during all the mission. The test is applied on eight samples and consists to a storage at 125 °C for 2000 h under nominal biasing conditions for half of the samples and reverse biasing conditions for half of the samples.
before ageing: the mean shear test value is 8.3 g, at mid-term, the mean shear test value is 7.2 g, after ageing, mean shear test value is 7.2 g.
Concerning the humidity test, we selected a reverse biasing condition with VDS = 0 V and VGS < 0 in order to exhibit surface effects and not remove humidity on active area due to thermal dissipation. Prior to these test-files, all the modules were subjected to screening tests, including 10 thermal cycles, five mechanical shocks and a burning operation at 125 °C for 240 h, a standard procedure to hybrid production to detect any failure of components or manufacturing operations. These modules were also submitted to pre-conditioning tests, as defined in the JEDEC standard [4]. Electrical characteristics such as [S] parameters and static parameters, output characteristics (IDS = f(VDS, VGS)) and Schottky characteristics (direct and reverse); have been monitored before and along all these tests. All the modules designed and manufactured successfully succeeded the five environmental test-files with no deviation on the electrical characteristics between initial and final steps.
On an automatic die-bonder, we obtain the following results: before ageing: the mean shear test value is 6.94 g, at mid-term, the mean shear test value is 6.71 g, after ageing, mean shear test value is 7.07 g. The standard deviation is smaller with an automatic die-bonder (around 0.6 g) compared to a manual operation (around 1 g). For both manual and automatic processes, the mean value of bond pull at four times the standard deviation is better than 3 g before ageing and better than 2.5 g after ageing. All these results on thermo-sonic bonding are finally in agreement (and even better) with the MIL Std 833 Method 2011.7 condition C.
5. Conclusions and prospects 4.4. Glob top encapsulation Glob top technology has been developed in the 1960s and is a very familiar process for consumer applications such as mobile, avionics or automotive. It consists to encapsulate bare die devices, including wire-bondings, with a epoxy, silicon, urethane, etc. based resin. The first standard epoxy based materials showed very high ionic contents values (K+, Na+, F , Cl ), above the MIL [3] specification (200 ppm for Cl , 50 ppm for F , K+, Na+). The new epoxy based generation, with data ranging from some traces to 30 ppm, is now strongly below this limit. A related process called dam & fill technology (using a high viscosity resin as a dam around the die and a low viscosity one to encapsulate the die and wire-bondings) shows promising results for microwave applications because the pattern of the encapsulated area can be controlled with a high mechanical accuracy using automatic dispensing equipment. A test vehicle has been designed, taking into account all the materials and processes qualified in the above section. To evaluate the reliability of this encapsulation towards space applications requirements, five test-files are defined: Radiation test: With maximum values established to 300 krad for GaAs MMIC and 70 krad for silicon bipolar transistor, this test is a full 3D finite elements simulation that give the cumulated dose effect at the equipment/component level, taking into account the environment conditions (geostationary orbit, location of the equipment on the satellite, etc.). Mechanical tests: A LNA module will be stressed to the combination of random vibrations and up to 500 thermal cycles (according to MIL Std 883 Method 1010 condition B). Thermal cycles: Based on materials with different CTEs, this test gives confidence on the mechanical assembly of the encapsulation process. The definition is 500 cycles in the [ 55 °C, +125 °C] temperature range with intermediate steps at 200 and 350 cycles, test applied on six modules. Humidity test: Humidity is one of the main responsible for corrosion in semi-conductor devices and can cause phenomenon such as electro-oxidation or metallic migration. The defined test is a storage at 85 °C and 85% relative humidity for 1000 h under reverse biasing conditions, test applied on six modules.
Thanks to strong improvement of organics such as printed circuit boards, adhesive and glob top materials, it is now realistic to introduce a non hermetic packaging of active devices for space application. Based on the design of LNA modules for a focal array fed reflector antenna, advanced materials and processes were introduced and passed with success all environmental tests in agreement with ESA and CNES requirement. This concerns a thermoset microwave printed circuit board RO4003CTM, assembly of components on board with mismatched CTE, 25 lm thermosonic wire bonding process on soft substrate and glob top encapsulation. For the needs of the focal array antenna and the needs of the glob top encapsulation reliability tests, more than seventy LNA modules or equivalent have been manufactured and tested. This shows the maturity at industrial level of this technology. The introduction of this packaging concept, already existing for consumer application, offers finally a great opportunity for the size and weight reduction of space equipment, as shown also for the mixed digital/DC section of a Linearized Driver Limiter Amplifier. This impact has to be analysed now on other standard microwave products like receiver, down/up converter, etc. and on other semi-conductor (GaAs, SiC, GaN, etc.) process foundries. Acknowledgements Activities on FAFR modules were supported by the European Space Agency (ESA) under DOMINO 2, Artes 3 program and the French Space Agency (CNES) under TCS21 program. The authors wish to thank also all technical people within ESA, CNES and Thales Alenia Space, France who contributed to the success of these different programs. References [1] Chane H, et al. Recent developments in feed array for Ka-band FAFR antenna. In: European conference on antennas and propagation (EUCAP 2006), Nice. [2] Monfraix P, et al. Non hermetic space microwave packaging: a reality for space application. In: IEEE MTT-Symposium 2006, San Francisco. [3] MIL Standard 883E. [4] JESD22–A113C. Pre-conditioning of non hermetic surface mount devices prior to reliability testing.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Invited Paper
Quasi hermetic packaging for new generation of spaceborn microwave equipment Philippe Monfraix *, Regis Barbaste, Jean Luc Muraro, Claude Drevon, Jean Louis Cazaux Thales Alenia Space, 26 Avenue J.F. Champollion, BP 33787, 31037 Toulouse Cedex 1, France
a r t i c l e
i n f o
Article history: Received 17 July 2009
a b s t r a c t This paper presents the introduction of the quasi hermetic encapsulation of microwave hybrids for space application through different approaches evaluated at Thales Alenia Space – France. Thanks to the improvement for many years of microwave organic materials, it is now realistic to propose advanced packaging solutions like the chip on board approach with glob top encapsulation of active devices directly bonded on printed circuit boards for space applications. To validate this packaging approach, very significant reliability test-plans have been proposed and performed on the different technological processes and materials in agreement with standard space quality requirements. Results will be presented and a discussion on the nature of the stresses applied during the tests will be proposed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Future satellite payload will exhibit higher functionalities (such as multi-spot beam coverage, re-configurability, on-board processing) in higher frequencies (Ka, Q/V bands) compared to current telecom applications, mainly in Ku-band. For the packaging of the electronic equipment, from low to microwave frequencies, this drastic evolution is also in parallel with miniaturisation and costreduction. Standard packaging techniques for space application involve traditionally fully hermetic hybrid modules (like laser/seam welding or static oven sealing processes) based on metallic (KovarÒ, Cu/W, etc.) or ceramic housing. To face these new challenges, a possible route is to introduce a quasi hermetic packaging approach with an optimised partitioning of technologies. 2. Microwave amplifier modules for multimedia application at 30 GHz For the design of a Focal Array Fed Reflector (FAFR) antenna for multimedia application in Ka-band [1], where this approach can finally replace four passive antennas (one single feed per beam), a major challenge was the design of a very large number of low noise amplifier (LNA) modules with both nominal and redundant paths. The LNA modules are parts of a dual polarised focal array feed reflector antenna including the following elements (cf. Fig. 1): radiating element of horn type, ortho-mode transducer (OMT), band-pass filter (BPF) which provides wideband out-of-band rejection in order to eliminate extra system uplink interferences, * Corresponding author. Tel.: +33 5 34 35 60 70. E-mail address: [email protected] (P. Monfraix). 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.07.051
low noise amplifiers with including a redundancy switch, located just behind the radiating elements for G/T maximisation, beam forming network (BFN) connected to LNA modules and generating illumination laws required for multiple spots coverage. The choice of the technology for the LNA modules has a direct impact on focal array architecture and layout. The complete module design has to be very innovative to comply with severe volume constraints (small pitch from module to module), tight RF performances and low recurring cost. That is why we developed a solution using MMIC devices directly bonded on an organic substrate (printed circuit board). Transition between waveguide access and the microstrip line is achieved by Vivaldi slots designed on the substrate. This substrate is then directly embedded in the waveguide access. It finally offers a technical solution well adapted to production of a significant number of items (more than 300 required for a fully equipped antenna) at low cost. The LNA modules are based on the following elements (cf. Fig. 2): a waveguide/microstrip transition, a PIN diode switch to divide nominal and redundant accesses, two MMICs (with a constant current biasing network) followed by a gain and phase corrector in each access, a Wilkinson combiner to combine the nominal and redundant accesses, a phase corrector, a microstrip/waveguide transition. These modules have to be implemented within the focal array mesh (about 1.2k = 12 mm at 30 GHz). This was reached with the introduction of new materials and technologies such as well-suited
P. Monfraix et al. / Microelectronics Reliability 49 (2009) 1326–1329
1327
Fig. 1. Design and layout of the focal array.
Fig. 2. Picture of the LNA modules at EQM level.
printed circuit board substrate, chip on board assembly and glob top encapsulation. Thales Alenia Space – France (TAS-F) is involved on those new processes both for microwave and low frequency applications [2]. Large investigations have resulted in the choice of epoxy based resins with silica fillers designed with very low ionic contents and out-gassing values, in agreement with MIL standard. With a ‘‘dam & fill” process, an accurate positioning of the encapsulation resin can then be assumed. The technological construction is described below: a RO4003TM board (substrate designed for microwave applications) is assembled on an aluminium structure, the 100 lm thick GaAs MMIC devices, 0805 SMT passives and alumina substrates are then directly bonded on the board using a flexible adhesive (to compensate the thermal coefficient mismatch between these components and the soft board), all the actives (GaAs MMIC, Si transistor) devices are encapsulated by glob top process based on a dam & fill technique. Thanks to this innovative packaging approach for space application, the integration achieved (72.4 mm 30.1 mm, 31.6 g for each LNA module) is four times lighter and half in volume compared to a standard fully hermetic solution. 3. Very integrated TM/TC section for Linearized Driver Limiter Amplifier (LDLA) The LDLA equipment is a key component of a satellite payload as it is localised before and then driving the electronic tubes. As depicted in Fig. 3, it is typically composed of two RF sections (a 60– 100 dB gain block and a linearized block to compensate tubes distortion) and a mixed digital/DC section called TM/TC which is in charge to monitor all the tele-metry (TM) and tele-command
Fig. 3. Picture of the current Linearized Driver Limiter Amplifier equipment.
(TC) signals. Concerning this TM/TC section, the current packaging is a thick film alumina substrate encapsulated in a fully hermetically sealed MCM-C module (MultiChip Module, High Temperature Cofired Ceramic technology) with a total size of 74 mm 51 mm. Replacing the thick film substrate by a eight layers printed circuit board, introducing smaller surface mount components (down to 0402 dimension) and thanks to a glob top encapsulation process of active devices directly bonded on the board, we can achieve a size integration by a factor 2 with a weight reduction over 50% (cf. Fig. 4). 4. Reliability tests over advanced materials and processes for space applications In the frame of the FAFR amplifier modules, many reliability tests have been achieved according to MIL or ECSS specifications.
1328
P. Monfraix et al. / Microelectronics Reliability 49 (2009) 1326–1329
Up face of the board : active devices – glue and wire processes (before glob top encapsulation)
Bottom face of the board : passive devices down to 0402 format – surface mount process
Fig. 4. Picture of the enhanced TM/TC section (up face after glob top encapsulation). Only half of the footprint is used compared to the current solution.
4.1. Printed circuit board RO4003CTM substrate used for the amplifier modules is a double sided printed circuit board based on a glass reinforced hydrocarbon/ceramic thermoset laminate designed for high performance sensitive applications. It is a low loss material which can be fabricated using standard epoxy/glass (FR4) processes. As a non Teflon based material, this material does not require specialised via preparation process such as sodium etch. Designed for application at 30 GHz, this substrate has very tight mechanical constraints and etching tolerance on line/space width of ±15 lm. The size of the RO4003TM board is 72 mm 30 mm with a thickness of 0.3 mm. The reliability tests done for the process qualification of this substrate are following the ECSS-Q 70-10A standard and are divided in three test-files: life-test for 1000 h at 125 °C, 500 thermal cycles in the [ 55 °C, +125 °C] temperature range according to MIL Std 883 Method 1010 condition B with an intermediate status after 200 cycles, humidity test for 10 days at 40 °C and 93% HR.
After all these tests, we can notice: no degradation after visual inspection on the surface metallization and the via holes (cross-section done on test-samples coming from the three test-files), no degradation on the metallization adherence (with values above 8 N/cm after these tests), no electrical deviation about the isolation resistance, the breakdown voltage and the interconnection resistance.
4.2. Assembly of active and passive devices on board Traditionally, microwave hybrid modules manufactured for space application involve GaAs active devices and alumina substrates matched with substrates in term of coefficient of thermal
expansion (CTE). For the chip on board approach, the large difference of CTE is an issue to be solved by using other interface. For the assembly of the components on the board, the objective was then to work on a flexible adhesive for mismatch CTE that can be used on automatic dispensing system for production, with extractable ionic content and out-gassing characteristics in agreement with MIL Std 833 Method 5011. Many commercial products were selected and evaluated on preliminary technological tests and the choice has been done on an epoxy reference from Emerson and Cuming. The ionic content analysis exhibit excellent results with for instance 32 ppm chloride ion and 33 ppm ammonium ion (all other values are below 10 ppm for sodium, fluoride nitrate ions). The reliability test for the process qualification of these packaging assemblies is up to 500 thermal cycles in the [ 55 °C, +125 °C] temperature range according to MIL Std 883 Method 1010 condition B with an intermediate status after 200 cycles. The components assembled for this process are GaAs MMIC (3 mm 2 mm), alumina substrates (8.3 mm 5.6 mm), 0805 SMD capacitors. After all these tests, we can notice: no cracks or delaminating effect after visual inspection and on cross-sections done on the joint of the epoxy glue, before and after the reliability test, value of different components strength on the board substrate with the new adhesive after shear test is above 2–4 for times above the according to MIL Std 883 Method 2019.5.
4.3. Wire-bonding on soft substrate The needs for wire-bonding for the amplifier modules concern the connection of GaAs MMIC devices and thin film alumina substrates on the RO4003TM soft board. The process developed for the MMIC and alumina on the board is 25 lm gold wire thermo-sonic bonding on both automatic and manual die-bonder. A key-point of this process is to closely manage the loop of the wire: for electrical reason, the inductance of the wire has to be minimised (Ka-band). The reliability test done for the process qualification is a storage of wires bonded on the RO4003CÒ board for 31 days at 175 °C (with
P. Monfraix et al. / Microelectronics Reliability 49 (2009) 1326–1329
1329
an intermediate status after 15 days, mid-term) with 50 shear tests done at each step. This is equivalent to a storage for 24 h at 250 °C, standard reliability test done on alumina substrates for wire-bonding qualification. On a manual die-bonder, we obtain the following results:
Life test: This test evaluates the functionality of the component and its electrical characteristics during all the mission. The test is applied on eight samples and consists to a storage at 125 °C for 2000 h under nominal biasing conditions for half of the samples and reverse biasing conditions for half of the samples.
before ageing: the mean shear test value is 8.3 g, at mid-term, the mean shear test value is 7.2 g, after ageing, mean shear test value is 7.2 g.
Concerning the humidity test, we selected a reverse biasing condition with VDS = 0 V and VGS < 0 in order to exhibit surface effects and not remove humidity on active area due to thermal dissipation. Prior to these test-files, all the modules were subjected to screening tests, including 10 thermal cycles, five mechanical shocks and a burning operation at 125 °C for 240 h, a standard procedure to hybrid production to detect any failure of components or manufacturing operations. These modules were also submitted to pre-conditioning tests, as defined in the JEDEC standard [4]. Electrical characteristics such as [S] parameters and static parameters, output characteristics (IDS = f(VDS, VGS)) and Schottky characteristics (direct and reverse); have been monitored before and along all these tests. All the modules designed and manufactured successfully succeeded the five environmental test-files with no deviation on the electrical characteristics between initial and final steps.
On an automatic die-bonder, we obtain the following results: before ageing: the mean shear test value is 6.94 g, at mid-term, the mean shear test value is 6.71 g, after ageing, mean shear test value is 7.07 g. The standard deviation is smaller with an automatic die-bonder (around 0.6 g) compared to a manual operation (around 1 g). For both manual and automatic processes, the mean value of bond pull at four times the standard deviation is better than 3 g before ageing and better than 2.5 g after ageing. All these results on thermo-sonic bonding are finally in agreement (and even better) with the MIL Std 833 Method 2011.7 condition C.
5. Conclusions and prospects 4.4. Glob top encapsulation Glob top technology has been developed in the 1960s and is a very familiar process for consumer applications such as mobile, avionics or automotive. It consists to encapsulate bare die devices, including wire-bondings, with a epoxy, silicon, urethane, etc. based resin. The first standard epoxy based materials showed very high ionic contents values (K+, Na+, F , Cl ), above the MIL [3] specification (200 ppm for Cl , 50 ppm for F , K+, Na+). The new epoxy based generation, with data ranging from some traces to 30 ppm, is now strongly below this limit. A related process called dam & fill technology (using a high viscosity resin as a dam around the die and a low viscosity one to encapsulate the die and wire-bondings) shows promising results for microwave applications because the pattern of the encapsulated area can be controlled with a high mechanical accuracy using automatic dispensing equipment. A test vehicle has been designed, taking into account all the materials and processes qualified in the above section. To evaluate the reliability of this encapsulation towards space applications requirements, five test-files are defined: Radiation test: With maximum values established to 300 krad for GaAs MMIC and 70 krad for silicon bipolar transistor, this test is a full 3D finite elements simulation that give the cumulated dose effect at the equipment/component level, taking into account the environment conditions (geostationary orbit, location of the equipment on the satellite, etc.). Mechanical tests: A LNA module will be stressed to the combination of random vibrations and up to 500 thermal cycles (according to MIL Std 883 Method 1010 condition B). Thermal cycles: Based on materials with different CTEs, this test gives confidence on the mechanical assembly of the encapsulation process. The definition is 500 cycles in the [ 55 °C, +125 °C] temperature range with intermediate steps at 200 and 350 cycles, test applied on six modules. Humidity test: Humidity is one of the main responsible for corrosion in semi-conductor devices and can cause phenomenon such as electro-oxidation or metallic migration. The defined test is a storage at 85 °C and 85% relative humidity for 1000 h under reverse biasing conditions, test applied on six modules.
Thanks to strong improvement of organics such as printed circuit boards, adhesive and glob top materials, it is now realistic to introduce a non hermetic packaging of active devices for space application. Based on the design of LNA modules for a focal array fed reflector antenna, advanced materials and processes were introduced and passed with success all environmental tests in agreement with ESA and CNES requirement. This concerns a thermoset microwave printed circuit board RO4003CTM, assembly of components on board with mismatched CTE, 25 lm thermosonic wire bonding process on soft substrate and glob top encapsulation. For the needs of the focal array antenna and the needs of the glob top encapsulation reliability tests, more than seventy LNA modules or equivalent have been manufactured and tested. This shows the maturity at industrial level of this technology. The introduction of this packaging concept, already existing for consumer application, offers finally a great opportunity for the size and weight reduction of space equipment, as shown also for the mixed digital/DC section of a Linearized Driver Limiter Amplifier. This impact has to be analysed now on other standard microwave products like receiver, down/up converter, etc. and on other semi-conductor (GaAs, SiC, GaN, etc.) process foundries. Acknowledgements Activities on FAFR modules were supported by the European Space Agency (ESA) under DOMINO 2, Artes 3 program and the French Space Agency (CNES) under TCS21 program. The authors wish to thank also all technical people within ESA, CNES and Thales Alenia Space, France who contributed to the success of these different programs. References [1] Chane H, et al. Recent developments in feed array for Ka-band FAFR antenna. In: European conference on antennas and propagation (EUCAP 2006), Nice. [2] Monfraix P, et al. Non hermetic space microwave packaging: a reality for space application. In: IEEE MTT-Symposium 2006, San Francisco. [3] MIL Standard 883E. [4] JESD22–A113C. Pre-conditioning of non hermetic surface mount devices prior to reliability testing.