- Email: [email protected]
Lead free packaging of Pt micro-heater for high temperature gas sensors
Fuel 112 (2013) 550–556
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Lead free packaging of Pt micro-heater for high temperature gas sensors R.P. Sharma a,⇑, P.K. Khanna b,1 a b
Department of Electronics Fiber Optics, Birla Technical Training Institute, Pilani 333 031, Rajasthan, India Hybrid Microcircuits Group, Central Electronics Engineering Research Institute (CEERI)/Council of Scientific and Industrial Research (CSIR), Pilani 333 031, Rajasthan, India
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 28 February 2011 Received in revised form 23 February 2012 Accepted 28 February 2012 Available online 20 March 2012
There is a continuing demand for the development of fast, sensitive and reliable sensors for applications in harsh industrial environments. Gas sensors are finding great importance in fuel and energy sectors. Monitoring and control of combustion related emissions result in efficient use of fuels and subsequent energy savings. Microelectronic gas sensors have advantages like small size, low power combustion, and possibility of in situ control and monitoring. These gas sensors are commonly employed in fire detectors, emission control system, CO2 operated enhanced geothermal system (EGS), supercritical water systems, organic rankine cycles used in binary power plants, high temperature gas-reactors in next-generation nuclear power plants and in gas processing. Packaging of microelectronic devices is equally important to the development practice. Input/Output connections are fabricated on these systems during this process. Thermo-mechanically strong lead attachments on sensor devices are necessary for high temperature applications. The conventional joining techniques like soldering, wire bonding, flip chip bonding and lead welding show failure in high temperature operative microelectronic systems. Present work is focused on the development and characterization of Pb free packaging of Pt micro-heaters, a main functionary of the gas sensors. Thick film Pt heater printed on ceramics substrate has been interconnected to Cu metal using Indium interlayer. The thermal and mechanical strengths of the interconnect specimen are determined and analyzed. The ultimate tensile strength (UTS) and ultimate shear strength (USS) values are plotted for different reaction parameters. The isothermal solidification morphology and crystallography are examined and discussed. The tests of thermal shock, pressure, operating life and vibration have been performed in an environmental lab to predict the compatibility of the packaging for deployment in harsh environments. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Gas sensors Pb free Pt heater Isothermal solidification UTS
1. Introduction There is a continuing demand from industries for the development of fast, reliable and low cost sensors for application in harsh environments. Gas sensors based intelligent systems and controls are useful in air quality monitoring, detection of toxic and explosive gases, energy efficiency and in controlling the emissions during combustion processes. The impact of limited fossil fuel supplies has revived interest in efficient use of fuel. Continuous monitoring of combustion processes and predictive emission modeling tools in power plants provide better control on combustion. This practice leads to reduction of toxic emissions and subsequent energy savings [1]. In close proximity to engines very high temperatures are generated. Monitoring the levels of CO2 and CO is useful in early fire alarms and detection of fuel leaks in jet engines [2]. Concurrent monitoring of chemical signature of a fire is preferred over the ⇑ Corresponding author. Tel.: +91 9461508599. E-mail addresses: [email protected] (P.K. Khanna). 1 Tel.: +91 1596 252263.
(R.P.
Sharma),
[email protected]
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.02.070
standard smoke detection system. CO/CO2 ratio is a chemical signature for determining fire condition [3]. Gas sensors are commonly used in monitoring of geothermal reservoirs during simulation and operation. CO2 based enhanced geothermal systems (EGSs) have been examined in a number of works over the past years [4–6]. Loloee et al. 2008 developed a high temperature H2 monitoring system for power plants [7]. Afridi et al. 2004, Belmonte et al. 2006 and Graf et al. 2006 developed micro-hot plate based gas sensors for high temperature applications [8–10]. Micro-hot plate in a gas sensor is the basis of gas sensitive metal oxide layers. Pt micro-heater is a main functionary of the microhot plates. A wide range of temperatures up to 900 °C is generated in Pt micro-heater. Different electrical response in the same sensitive layer is observed at different temperatures for a variety of gases or a gaseous mixture. The application of a voltage to the Pt heaters causes the temperature of the micro-hot plate to increase, which in turn enhances the sensitivity of the sensor [11]. The high temperature applications emphasize on reliable interconnections on the Pt micro-heater in the gas sensors. In high temperature operative microelectronic systems the conventional joining techniques show failure [12]. The conventional solder alloys
R.P. Sharma, P.K. Khanna / Fuel 112 (2013) 550–556
contain Pb, which causes environmental problems. Lead bearing solder are ‘‘the silent hazard’’ for the environment [13–15]. Interconnections based on isothermal solidification [16–18] offers solution to these challenges. Intermetallic phases (IPs) of high mechanical and thermal stability are formed during the isothermal solidification process. The interconnect fabrication temperature is lower to its service temperature which is above 600 °C. This is low power consuming, Pb-free joining technique and therefore found suitability with the environment concerns. With these properties the reported interconnection technique is advantageous over the conventional soldering, wire bonding, flip chip bonding, tape automated bonding and lead welding techniques. Present course of work is focused on the development and characterization of Pb free interconnections of high thermo-mechanical strengths on Pt micro-heater. Thick film Pt heater, printed on ceramics substrate, has been interconnected to Cu metal using Indium interlayer. The thermal and mechanical strengths of the interconnect specimen are determined and reported in this paper. The ultimate tensile strength (UTS) and ultimate shear strength (USS) values are plotted for different reaction time, temperature, pressure and interlayer thickness. The isothermal solidification morphology and crystallography are examined and discussed. The environment test results reveal the reliability of isothermal solidification based packaging for deployment of the sensor devices in stringent environments. 2. Experimental Binary phase diagrams of Pt–In and Cu–In combinations have been studied and analyzed to conclude on their use for fabricating high temperature stable Pt–Cu joints at low temperatures. Both the metals Pt and Cu form high temperature stable intermetallic phases (IPs) with Indium metal [19–22]. Figs. 1 and 2 reveal the possible IP compositions. High purity (99.999%) Indium has been used as interlayer metal. The Indium layer melts at 156 °C and form intermetallic phases with substrate metals. These phases are stable in a wide temperature range up to 1455 °C. The specially designed heater contains a
551
hybrid of conventional Meander design and S-shaped design with square contact pads (Fig. 3, Table 1). The design is printed with DuPont Pt thick film paste No. 9141 on 100 100 ceramic substrates using DEK printer. A steel mesh of 325 counts is used for screen preparation to produce fine grain size. After a 15 min drying at the room temperature these prints are fired up to 900 °C in a Muffle furnace. During firing process the organic materials present in the Pt thick film paste are evaporated at 350 °C. The binders melt at the temperatures above 600 °C and produce strong bonding between the metal-rich thick film layer and the ceramic substrate. The post firing thickness of conductor print is obtained in the range of 17– 19 lm. To prepare the Cu substrate sheet for the interconnection, a Cu bar of high purity is pressure rolled in rolling machine and grinded to smoothness by using different paper numbered from 400 to 4000. This Cu-sheet of constant thickness is then cut in 4 4 mm2 pieces. High purity Indium (99.999%) foils with prescribed dimensions and 2–20 lm thickness values are prepared by thermal deposition techniques and pressure rolling techniques. The foils and the sheets are then cleaned ultrasonically for 60 min in acetone. The Indium foil is sandwiched between the Pt heater contact pads and Cu-metal sheet with a ceramic substrate cover over the top Cu layer. The assembly is heated up rapidly to process temperature values and remains at hold for the specified reaction time under a mechanical pressure to accomplish isothermal solidification reaction of Pt–In and Cu–In. The electric input to heating units of specially designed load press device is connected via temperature controller. The temperature of the bonding environment in load press device has a continuous measurement by a closely placed thermocouple. These interconnects developed are gone through joint strength tests. The maximum temperature up to which the interconnection remains stable defines the thermal stability of the interconnection. The sample fabricated is clamped on a static base with a heating element arrangement. The temperature applied through this element is controlled by a temperature controller. A constant force of 10 N is applied vertically upwards, with a drawing speed of 1 mm/min, on the Cu substrate sheet. The temperature is increased
Fig. 1. The Pt–In binary phase diagram.
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Fig. 2. The Cu–In binary phase diagram.
Fig. 3. The Pt heater design for the experiment.
Table 1 The Pt heater design specifications. Conductor metal
DuPont thick film paste No. 9141
Area of the heater (with contact pads area) Line width of the heater Area of the square contact pads Peripheral of the design
72.1 mm1 1 mm 32 mm2 191.3 mm
characterization of the interconnection. USS is determined for a set of 10 samples, which have been fabricated under the same process parameters. The test results for the thermal and mechanical stabilities of these samples have been analyzed and reported in tables and graphs. SEM imaging techniques have been employed for the surface growth characterization. The continuity of the phase formation on the joining surfaces has been reported in SEM images. The tests for sustainability of the packaging in harsh environment have been carried out in an environment lab and the results are reported in tabular form. The fabricated samples are tested for 4/ +50 °C thermal shock to predict the efficiency of joint in changing environments. Vibration test is conducted on a vibration platform in the environmental lab for 60 min duration. To check the sustainability of the interconnection in pH varying environment, samples fabricated are dipped in solutions of different pH ranging 5 to +9 for 24 h. Under the shelf life test the interconnected specimens are stored in different humidity conditions for 240 h. 3. Results and discussion 3.1. The thermal stability of the Pt heater interconnection
slowly under the constant pull off condition. The temperature at which the fabricated bond delaminates is noted down. This experiment has been carried out in an inert environment. This gives the thermal stability of the sample under test. This process is repeated for a set of 10 samples, which have been fabricated under the same process parameters. UTS and USS are determined against the applied tensile and shear at constant temperatures. The shearing stress is generated in the fabricated sample by applying a tangential force on the top metal layer while keeping the ceramic side static. Clamps of the static base has been used for this purpose and pull tester is used for the measurement of the tangential force applied. The interconnection breaking force gives the total shear strength of the sample. The ultimate shear strength (USS) is calculated from this value and reported for the mechanical
Thermal stabilities of the interconnection specimen fabricated for different process parameter settings are measured during this course of study. The statistical data in Table 2 are based up on the measurements of samples fabricated at 300 °C temperature for 25 min Table 2 Un-bonding temperatures of Pt–In–Cu interconnections fabricated at 300 °C. Temperature interval (°C)
Number of specimen un-bonded per interval
<580 581–590 591–600 601–610 611–620
Nil 2 1 2 5
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under constant pressure of 0.5 MPa. A 20 lm thick Indium layer has been used for the preparation of these interconnections. The thermal stabilities are ranging from 580 to 620 °C for the samples under test. The thermal stability measurements for Pt–In–Cu specimens provide basis for its use in fabricating high temperature stable Input/Output (I/O) connections on heating elements during the development of gas sensors. The micro-heater assembly is a main integrand of the gas sensors and conventional bonding techniques show failure at the high temperatures generated in these heaters. This technique provides solution to the challenges arising from interconnection failure in microelectronic assembly [23–25] by establishing itself as an alternative technique. 3.2. The mechanical strengths of the Pt heater interconnections The tensile behaviors of the bonded joints are measured in terms of the UTS. The effects of process parameters on the UTS of Pt heater interconnections have been observed and reported in Figs. 4–7. The tensile strengths of the fabricated specimens are measured between 15 and 47 MPa for different reaction times ranging from 1 min to 40 min. The effect of temperature on the interconnect strength is shown in Fig. 5. For a 20 lm thick interlayer it is obtained between 36 and 44 MPa. With Increment in the temperature the diffusion of the solidus Pt and Cu in Indium liquidus increases and the UTS of the interconnection increases rapidly. The interlayer thickness deals with the width of the intermetallic zone of an interconnection, which in turn contribute in the strength of the bonded joint. For the Pt–In–Cu interconnection fabricated for different interlayer thickness ranging from 2 to 20 lm the maximum UTS of 44 MPa has been reported for an interconnect fabricate at 300 °C, for 15 min reaction time and under constant pressure of 0.5 MPa (Fig. 6). The effect of pressure on the UTS of the bonded joint has been analyzed in Fig. 7. The change in the interconnect UTS is linear with the pressure. Akselsen [26] reviewed the Al2O3–Pt diffusion bonding and summarized that the pressure applied during the bonding process reduces the surface asperities and the strength of the interconnect increases. The shear strength measurements for Pt/In, Pd/ In and Zr/Sn systems have been reported by Studnitzky and Schmid-fetzer [19]. These studies are focused on the phase formation [27–30] during the bonding. Square shaped interconnections are fabricated by these researchers.
Fig. 5. Effect of temperature on UTS of Pt–In–Cu interconnection.
Fig. 6. Effect of the interlayer thickness on UTS of Pt–In–Cu interconnection.
Fig. 7. Effect of pressure on UTS of Pt–In–Cu interconnection.
Fig. 4. Effect of reaction time on UTS of Pt–In–Cu interconnection.
The measured shear strengths of the samples fabricated in this course of study are analyzed. 45 MPa is the highest shear strength
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measured during the experiments. The effects of process parameters on the USS of interconnection fabricated are reported in Figs. 8–11. The resistivity of the Pt heater interconnections against the applied torque is proving the compatibility of this bonding technique with portability. The highest measured Ultimate torsion strength is 44 MPa and this show the significance of the innovative technique. 3.3. The surface growth during the Pt heater interconnection process The growth of the intermetallic phases during the isothermal solidification process has been analyzed with SEM imaging. An optical view of the IP growth in the reaction zones is presented in Fig. 12. Change in color show different intermetallic phases growth on the interconnecting surfaces. Brightness of Pt composite IP on Cu surface is visible in Fig. 12. Un-reacted Indium out flown during the experiment gets stick on the peripheral of the contact pad. A low magnification SEM image of the delaminated Cu surface of a strong Pt–In–Cu interconnection fabricated at 300 °C is shown in Fig. 13. This depicts the interconnection zones spread all over the area of the contact with the Pt square contact pad. The bright texture of surface show firmly bonded intermetallic of Pt–In–Cu.
Fig. 10. Effect of the interlayer thickness on the USS of Pt–In–Cu interconnection.
Fig. 11. Effect of pressure on the USS of Pt–In–Cu interconnection.
Fig. 8. Effect of reaction time on the USS of Pt–In–Cu interconnection.
Fig. 12. Optical view of reaction zones on Cu and Pt surfaces.
Fig. 9. Effect of temperature on the USS of Pt–In–Cu interconnection.
Kim and Kim [31] investigated the interfacial reaction of lead free solders with Pt. The SEM images of the interconnection cross section have been reported by these researchers. The effects of Pt–Sn intermetallic layer thickness and the reaction time on the diffusion controlled growth at the interface has been reported. The good wetting properties of Pt [32–35] are highlighted along with the wider use of Pt based conductors in the hybrid integrated circuits [36]. Wang and Liu [37] investigated the coupling effects in
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R.P. Sharma, P.K. Khanna / Fuel 112 (2013) 550–556 Table 3 The environmental test results of Pt heater interconnections. Test performed
Number of samples tested
Number of the test qualifying samples
Thermal shock Vibration pH dip Shelf life
5 5 5 5
5 4 5 5
Fig. 13. A low magnification SEM of the reaction zone on Cu surface.
Fig. 15. Voids on the surface of sample failed during vibration test.
3.4. The environmental test results for Pt heater interconnections A set of five specimen interconnections fabricated for the same process parameters as discussed in Section 3.1 has undergone through a test among the selected four environmental tests. Total 20 specimens have been used to conduct all the four environmental tests. These tests have been conducted in a specially designed environment lab. The results of the environmental tests are summarized in Table 3. One test sample out of the five samples under vibration test shows failure. The delaminated surfaces of this failed sample examined with high resolution SEM reveal voids in the interconnection zone (Fig. 15). The environment test results show the efficient workability and long time sustainability of isothermal solidification based interconnections of the Pt heater in harsh and stringent environments. 4. Conclusions
Fig. 14. The continuity of crystalline structures on the surface results an interconnection of high thermo-mechanical strength.
sandwiched solder joints of Pt–Sn–Cu compositions and reported the sluggish intermetallic growth at the Pt–Sn interface. The metallographic of the delaminated Pt and Cu surfaces are reported in Fig. 14. The continuity of surface structures is visible on both the surfaces of a strong interconnection. Surface continuity depicts the homogeneity of IP growth which in turn reflects a strong interconnection. The surfaces of Cu and Pt sides visible in Fig. 14 are free from voids and cracks. Voids and surface cracks cause weak interconnection.
The isothermal solidification based interconnection technique is a Pb free packaging technique. Thermally and mechanically stable interconnections are fabricated in flux free and environmental friendly manner. The outcome of the thermal strength measurements establishes the isothermal solidification based interconnections for gas sensor applications where high temperatures stable interconnections are required. All the samples fabricated in this course of study are stable up to 580 °C. The UTS can be increased up to 47 MPa with this interconnection technique. 45 MPa USS and 44 MPa of the ultimate torsion strength establish this technique for strong lead attachment and reliable packaging on micro-systems. The obtained thermo-mechanical stabilities are best suited for the sensor systems operative at elevated temperatures and under harsh environment conditions. Samples passed the environmental resistance tests in 95% of cases. In the present scenario when researchers are driving their full efforts to enlarge and diversify the portfolio of sensor based automation in harsh industrial processes, the gas sensors are useful in safety and product quality.
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Gas sensors are finding importance in supercritical water systems, organic rankine cycles used in binary power plants, high temperature gas-reactors in next-generation nuclear power plants and in gas processing. In addition to the safety issue, the monitoring of combustion exhaust gases has environmental appeals. Under the environmental conditions of above mentioned applications, reliable packaging on sensor parts is needed. The isothermal solidification based interconnections on gas sensor Pt heater show potential to fulfill this demand. The X-ray diffraction analysis of the interconnect cross section is proposed for the future work to study the intermetallic phase formation. Acknowledgements The authors wish to thank Dr. P.S. Bhatnagar, Director BTTI; Prof. D. Kumar of Kurukshetra University, Kurukshetra, Dr. N. Suri, Mr. S. Kumar and Mr. I.C. Sharma of HMC group for their support. Authors are equally thankful to the Director CEERI for his encouragement and permission to publish the results. References [1] [2] [3] [4] [5] [6] [7] [8]
Akbar S, Dutta P, Lee C. Int J Appl Ceram Technol 2006;3(4):302. Thiele JA, Cunha MP. Sens Actuat B 2006;113:816. Kohl D, Kelleter J, Petig H. Sens Update 2001;9:161. Brown DW. In: Proceedings 25th workshop on geothermal reservoir engineering. Stanford Univ.; 2000. p. 233. Atrens AD, Gurgenci H, Rudolph V. Geothermics 2010;39(2):161. Pritchett JW. Geotherm Resour Coun Trans 2010;33:235. Loloee R, Chorpenning B, Beer S, Ghosh R. Sens Actuat B 2008;129:200. Afridi M, Hefner A, Berning D, Ellenwood C, Varma A, Jacob B, et al. Solid State Electr 2004;48:1777.
[9] Belmonte J, Puigcorbe J, Arbiol J, Vila A, Morante JR, Sabate N, et al. Sens Actuat B 2006;114:826. [10] Graf M, Barrettino D, Kirstein K, Hierlemann A. Sens Actuat B 2006;117: 346. [11] Lee CY, Chiang CM, Wang YH, Ma RH. Sens Actuat B 2007;122(2):503. [12] Khanna PK, Bhatnagar SK, Chang LS, Gust W. Z Metallkd 1999;90(7):470. [13] Lustberg M, Silbergeld E. Arch Intern Med 2002;162(21):2443. [14] Ciocci R, Pecht M. Microelectr Int 2006;23(2):45. [15] Kamal M, Gouda ES. Mater Manuf Process 2006;21(8):736. [16] Khanna PK, Bhatnagar SK, Dalke G, Brunner D, Gust W. Mater Sci Engg B 1995;33:L6. [17] Sharma RP, Khanna PK, Kumar D. Thin Solid Films 2010;519:987. [18] Sharma RP, Khanna PK, Kumar D, Suri N, Pandey HC. Integr Ferroelectr 2010;120(1):49. [19] Studnitzky T, Fetzer RS. J Electron Mater 2003;32(2):70. [20]. [21] Desk handbook: phase diagrams for binary alloys. ASM Int.; 2000. [22] Plumbridge WJ. J Electron Mater 2008;37(2):218. [23] Longford A. Microelectr Int 2005;22(2):17. [24] Huang CY, Li MS, Ku CL, Hsieh HC, Li KC. Microelectr Int 2009;26(3):41. [25] Smith CA. Circ World 2007;33(1):15. [26] Akselsen OM. J Mater Sci 1992;27:569. [27] Kuper C, Peng W, Pisch A, Goesmann F, Fetzer RS. Z Metallkd 1998;89:855. [28] Studnitzky T, Onderka B, Fetzer RS. Z Metallkd 2002;93:48. [29] Studnitzky T, Fetzer RS. Z Metallkd 2002;93:895. [30] Massalski TB. Binary alloy phase diagrams. 2nd ed., ASM Int.; 1990. [31] Kim TH, Kim YH. J Miner Met Mater 2004;56(6):45. [32] Meagher B, Schwarcz D, Ohring M. J Mater Sci 1996;31:5479. [33] Klein M, Wiens B, Hutter M, Oppermann H, Aschenbrenner R, Reichl H. In: Proceedings 50th electronic components and technology conference; 2000. p. 45. [34] Kuhman JF, Chinang CH, Harde P, Reier F, Oesterle E. Mater Sci Engg A 1998;242:22. [35] Yang SC, Chang WC, Wang YW, Kao CR. J Electron Mater 2009;38(1):25. [36] Guttensohn AE, Hanan JD, Holderfield D. Hybrid Circ Technol 1987;4:21. [37] Wang SJ, Liu C. Acta Mater 2007;55(10):3327.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Lead free packaging of Pt micro-heater for high temperature gas sensors R.P. Sharma a,⇑, P.K. Khanna b,1 a b
Department of Electronics Fiber Optics, Birla Technical Training Institute, Pilani 333 031, Rajasthan, India Hybrid Microcircuits Group, Central Electronics Engineering Research Institute (CEERI)/Council of Scientific and Industrial Research (CSIR), Pilani 333 031, Rajasthan, India
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 28 February 2011 Received in revised form 23 February 2012 Accepted 28 February 2012 Available online 20 March 2012
There is a continuing demand for the development of fast, sensitive and reliable sensors for applications in harsh industrial environments. Gas sensors are finding great importance in fuel and energy sectors. Monitoring and control of combustion related emissions result in efficient use of fuels and subsequent energy savings. Microelectronic gas sensors have advantages like small size, low power combustion, and possibility of in situ control and monitoring. These gas sensors are commonly employed in fire detectors, emission control system, CO2 operated enhanced geothermal system (EGS), supercritical water systems, organic rankine cycles used in binary power plants, high temperature gas-reactors in next-generation nuclear power plants and in gas processing. Packaging of microelectronic devices is equally important to the development practice. Input/Output connections are fabricated on these systems during this process. Thermo-mechanically strong lead attachments on sensor devices are necessary for high temperature applications. The conventional joining techniques like soldering, wire bonding, flip chip bonding and lead welding show failure in high temperature operative microelectronic systems. Present work is focused on the development and characterization of Pb free packaging of Pt micro-heaters, a main functionary of the gas sensors. Thick film Pt heater printed on ceramics substrate has been interconnected to Cu metal using Indium interlayer. The thermal and mechanical strengths of the interconnect specimen are determined and analyzed. The ultimate tensile strength (UTS) and ultimate shear strength (USS) values are plotted for different reaction parameters. The isothermal solidification morphology and crystallography are examined and discussed. The tests of thermal shock, pressure, operating life and vibration have been performed in an environmental lab to predict the compatibility of the packaging for deployment in harsh environments. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Gas sensors Pb free Pt heater Isothermal solidification UTS
1. Introduction There is a continuing demand from industries for the development of fast, reliable and low cost sensors for application in harsh environments. Gas sensors based intelligent systems and controls are useful in air quality monitoring, detection of toxic and explosive gases, energy efficiency and in controlling the emissions during combustion processes. The impact of limited fossil fuel supplies has revived interest in efficient use of fuel. Continuous monitoring of combustion processes and predictive emission modeling tools in power plants provide better control on combustion. This practice leads to reduction of toxic emissions and subsequent energy savings [1]. In close proximity to engines very high temperatures are generated. Monitoring the levels of CO2 and CO is useful in early fire alarms and detection of fuel leaks in jet engines [2]. Concurrent monitoring of chemical signature of a fire is preferred over the ⇑ Corresponding author. Tel.: +91 9461508599. E-mail addresses: [email protected] (P.K. Khanna). 1 Tel.: +91 1596 252263.
(R.P.
Sharma),
[email protected]
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.02.070
standard smoke detection system. CO/CO2 ratio is a chemical signature for determining fire condition [3]. Gas sensors are commonly used in monitoring of geothermal reservoirs during simulation and operation. CO2 based enhanced geothermal systems (EGSs) have been examined in a number of works over the past years [4–6]. Loloee et al. 2008 developed a high temperature H2 monitoring system for power plants [7]. Afridi et al. 2004, Belmonte et al. 2006 and Graf et al. 2006 developed micro-hot plate based gas sensors for high temperature applications [8–10]. Micro-hot plate in a gas sensor is the basis of gas sensitive metal oxide layers. Pt micro-heater is a main functionary of the microhot plates. A wide range of temperatures up to 900 °C is generated in Pt micro-heater. Different electrical response in the same sensitive layer is observed at different temperatures for a variety of gases or a gaseous mixture. The application of a voltage to the Pt heaters causes the temperature of the micro-hot plate to increase, which in turn enhances the sensitivity of the sensor [11]. The high temperature applications emphasize on reliable interconnections on the Pt micro-heater in the gas sensors. In high temperature operative microelectronic systems the conventional joining techniques show failure [12]. The conventional solder alloys
R.P. Sharma, P.K. Khanna / Fuel 112 (2013) 550–556
contain Pb, which causes environmental problems. Lead bearing solder are ‘‘the silent hazard’’ for the environment [13–15]. Interconnections based on isothermal solidification [16–18] offers solution to these challenges. Intermetallic phases (IPs) of high mechanical and thermal stability are formed during the isothermal solidification process. The interconnect fabrication temperature is lower to its service temperature which is above 600 °C. This is low power consuming, Pb-free joining technique and therefore found suitability with the environment concerns. With these properties the reported interconnection technique is advantageous over the conventional soldering, wire bonding, flip chip bonding, tape automated bonding and lead welding techniques. Present course of work is focused on the development and characterization of Pb free interconnections of high thermo-mechanical strengths on Pt micro-heater. Thick film Pt heater, printed on ceramics substrate, has been interconnected to Cu metal using Indium interlayer. The thermal and mechanical strengths of the interconnect specimen are determined and reported in this paper. The ultimate tensile strength (UTS) and ultimate shear strength (USS) values are plotted for different reaction time, temperature, pressure and interlayer thickness. The isothermal solidification morphology and crystallography are examined and discussed. The environment test results reveal the reliability of isothermal solidification based packaging for deployment of the sensor devices in stringent environments. 2. Experimental Binary phase diagrams of Pt–In and Cu–In combinations have been studied and analyzed to conclude on their use for fabricating high temperature stable Pt–Cu joints at low temperatures. Both the metals Pt and Cu form high temperature stable intermetallic phases (IPs) with Indium metal [19–22]. Figs. 1 and 2 reveal the possible IP compositions. High purity (99.999%) Indium has been used as interlayer metal. The Indium layer melts at 156 °C and form intermetallic phases with substrate metals. These phases are stable in a wide temperature range up to 1455 °C. The specially designed heater contains a
551
hybrid of conventional Meander design and S-shaped design with square contact pads (Fig. 3, Table 1). The design is printed with DuPont Pt thick film paste No. 9141 on 100 100 ceramic substrates using DEK printer. A steel mesh of 325 counts is used for screen preparation to produce fine grain size. After a 15 min drying at the room temperature these prints are fired up to 900 °C in a Muffle furnace. During firing process the organic materials present in the Pt thick film paste are evaporated at 350 °C. The binders melt at the temperatures above 600 °C and produce strong bonding between the metal-rich thick film layer and the ceramic substrate. The post firing thickness of conductor print is obtained in the range of 17– 19 lm. To prepare the Cu substrate sheet for the interconnection, a Cu bar of high purity is pressure rolled in rolling machine and grinded to smoothness by using different paper numbered from 400 to 4000. This Cu-sheet of constant thickness is then cut in 4 4 mm2 pieces. High purity Indium (99.999%) foils with prescribed dimensions and 2–20 lm thickness values are prepared by thermal deposition techniques and pressure rolling techniques. The foils and the sheets are then cleaned ultrasonically for 60 min in acetone. The Indium foil is sandwiched between the Pt heater contact pads and Cu-metal sheet with a ceramic substrate cover over the top Cu layer. The assembly is heated up rapidly to process temperature values and remains at hold for the specified reaction time under a mechanical pressure to accomplish isothermal solidification reaction of Pt–In and Cu–In. The electric input to heating units of specially designed load press device is connected via temperature controller. The temperature of the bonding environment in load press device has a continuous measurement by a closely placed thermocouple. These interconnects developed are gone through joint strength tests. The maximum temperature up to which the interconnection remains stable defines the thermal stability of the interconnection. The sample fabricated is clamped on a static base with a heating element arrangement. The temperature applied through this element is controlled by a temperature controller. A constant force of 10 N is applied vertically upwards, with a drawing speed of 1 mm/min, on the Cu substrate sheet. The temperature is increased
Fig. 1. The Pt–In binary phase diagram.
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Fig. 2. The Cu–In binary phase diagram.
Fig. 3. The Pt heater design for the experiment.
Table 1 The Pt heater design specifications. Conductor metal
DuPont thick film paste No. 9141
Area of the heater (with contact pads area) Line width of the heater Area of the square contact pads Peripheral of the design
72.1 mm1 1 mm 32 mm2 191.3 mm
characterization of the interconnection. USS is determined for a set of 10 samples, which have been fabricated under the same process parameters. The test results for the thermal and mechanical stabilities of these samples have been analyzed and reported in tables and graphs. SEM imaging techniques have been employed for the surface growth characterization. The continuity of the phase formation on the joining surfaces has been reported in SEM images. The tests for sustainability of the packaging in harsh environment have been carried out in an environment lab and the results are reported in tabular form. The fabricated samples are tested for 4/ +50 °C thermal shock to predict the efficiency of joint in changing environments. Vibration test is conducted on a vibration platform in the environmental lab for 60 min duration. To check the sustainability of the interconnection in pH varying environment, samples fabricated are dipped in solutions of different pH ranging 5 to +9 for 24 h. Under the shelf life test the interconnected specimens are stored in different humidity conditions for 240 h. 3. Results and discussion 3.1. The thermal stability of the Pt heater interconnection
slowly under the constant pull off condition. The temperature at which the fabricated bond delaminates is noted down. This experiment has been carried out in an inert environment. This gives the thermal stability of the sample under test. This process is repeated for a set of 10 samples, which have been fabricated under the same process parameters. UTS and USS are determined against the applied tensile and shear at constant temperatures. The shearing stress is generated in the fabricated sample by applying a tangential force on the top metal layer while keeping the ceramic side static. Clamps of the static base has been used for this purpose and pull tester is used for the measurement of the tangential force applied. The interconnection breaking force gives the total shear strength of the sample. The ultimate shear strength (USS) is calculated from this value and reported for the mechanical
Thermal stabilities of the interconnection specimen fabricated for different process parameter settings are measured during this course of study. The statistical data in Table 2 are based up on the measurements of samples fabricated at 300 °C temperature for 25 min Table 2 Un-bonding temperatures of Pt–In–Cu interconnections fabricated at 300 °C. Temperature interval (°C)
Number of specimen un-bonded per interval
<580 581–590 591–600 601–610 611–620
Nil 2 1 2 5
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under constant pressure of 0.5 MPa. A 20 lm thick Indium layer has been used for the preparation of these interconnections. The thermal stabilities are ranging from 580 to 620 °C for the samples under test. The thermal stability measurements for Pt–In–Cu specimens provide basis for its use in fabricating high temperature stable Input/Output (I/O) connections on heating elements during the development of gas sensors. The micro-heater assembly is a main integrand of the gas sensors and conventional bonding techniques show failure at the high temperatures generated in these heaters. This technique provides solution to the challenges arising from interconnection failure in microelectronic assembly [23–25] by establishing itself as an alternative technique. 3.2. The mechanical strengths of the Pt heater interconnections The tensile behaviors of the bonded joints are measured in terms of the UTS. The effects of process parameters on the UTS of Pt heater interconnections have been observed and reported in Figs. 4–7. The tensile strengths of the fabricated specimens are measured between 15 and 47 MPa for different reaction times ranging from 1 min to 40 min. The effect of temperature on the interconnect strength is shown in Fig. 5. For a 20 lm thick interlayer it is obtained between 36 and 44 MPa. With Increment in the temperature the diffusion of the solidus Pt and Cu in Indium liquidus increases and the UTS of the interconnection increases rapidly. The interlayer thickness deals with the width of the intermetallic zone of an interconnection, which in turn contribute in the strength of the bonded joint. For the Pt–In–Cu interconnection fabricated for different interlayer thickness ranging from 2 to 20 lm the maximum UTS of 44 MPa has been reported for an interconnect fabricate at 300 °C, for 15 min reaction time and under constant pressure of 0.5 MPa (Fig. 6). The effect of pressure on the UTS of the bonded joint has been analyzed in Fig. 7. The change in the interconnect UTS is linear with the pressure. Akselsen [26] reviewed the Al2O3–Pt diffusion bonding and summarized that the pressure applied during the bonding process reduces the surface asperities and the strength of the interconnect increases. The shear strength measurements for Pt/In, Pd/ In and Zr/Sn systems have been reported by Studnitzky and Schmid-fetzer [19]. These studies are focused on the phase formation [27–30] during the bonding. Square shaped interconnections are fabricated by these researchers.
Fig. 5. Effect of temperature on UTS of Pt–In–Cu interconnection.
Fig. 6. Effect of the interlayer thickness on UTS of Pt–In–Cu interconnection.
Fig. 7. Effect of pressure on UTS of Pt–In–Cu interconnection.
Fig. 4. Effect of reaction time on UTS of Pt–In–Cu interconnection.
The measured shear strengths of the samples fabricated in this course of study are analyzed. 45 MPa is the highest shear strength
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measured during the experiments. The effects of process parameters on the USS of interconnection fabricated are reported in Figs. 8–11. The resistivity of the Pt heater interconnections against the applied torque is proving the compatibility of this bonding technique with portability. The highest measured Ultimate torsion strength is 44 MPa and this show the significance of the innovative technique. 3.3. The surface growth during the Pt heater interconnection process The growth of the intermetallic phases during the isothermal solidification process has been analyzed with SEM imaging. An optical view of the IP growth in the reaction zones is presented in Fig. 12. Change in color show different intermetallic phases growth on the interconnecting surfaces. Brightness of Pt composite IP on Cu surface is visible in Fig. 12. Un-reacted Indium out flown during the experiment gets stick on the peripheral of the contact pad. A low magnification SEM image of the delaminated Cu surface of a strong Pt–In–Cu interconnection fabricated at 300 °C is shown in Fig. 13. This depicts the interconnection zones spread all over the area of the contact with the Pt square contact pad. The bright texture of surface show firmly bonded intermetallic of Pt–In–Cu.
Fig. 10. Effect of the interlayer thickness on the USS of Pt–In–Cu interconnection.
Fig. 11. Effect of pressure on the USS of Pt–In–Cu interconnection.
Fig. 8. Effect of reaction time on the USS of Pt–In–Cu interconnection.
Fig. 12. Optical view of reaction zones on Cu and Pt surfaces.
Fig. 9. Effect of temperature on the USS of Pt–In–Cu interconnection.
Kim and Kim [31] investigated the interfacial reaction of lead free solders with Pt. The SEM images of the interconnection cross section have been reported by these researchers. The effects of Pt–Sn intermetallic layer thickness and the reaction time on the diffusion controlled growth at the interface has been reported. The good wetting properties of Pt [32–35] are highlighted along with the wider use of Pt based conductors in the hybrid integrated circuits [36]. Wang and Liu [37] investigated the coupling effects in
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R.P. Sharma, P.K. Khanna / Fuel 112 (2013) 550–556 Table 3 The environmental test results of Pt heater interconnections. Test performed
Number of samples tested
Number of the test qualifying samples
Thermal shock Vibration pH dip Shelf life
5 5 5 5
5 4 5 5
Fig. 13. A low magnification SEM of the reaction zone on Cu surface.
Fig. 15. Voids on the surface of sample failed during vibration test.
3.4. The environmental test results for Pt heater interconnections A set of five specimen interconnections fabricated for the same process parameters as discussed in Section 3.1 has undergone through a test among the selected four environmental tests. Total 20 specimens have been used to conduct all the four environmental tests. These tests have been conducted in a specially designed environment lab. The results of the environmental tests are summarized in Table 3. One test sample out of the five samples under vibration test shows failure. The delaminated surfaces of this failed sample examined with high resolution SEM reveal voids in the interconnection zone (Fig. 15). The environment test results show the efficient workability and long time sustainability of isothermal solidification based interconnections of the Pt heater in harsh and stringent environments. 4. Conclusions
Fig. 14. The continuity of crystalline structures on the surface results an interconnection of high thermo-mechanical strength.
sandwiched solder joints of Pt–Sn–Cu compositions and reported the sluggish intermetallic growth at the Pt–Sn interface. The metallographic of the delaminated Pt and Cu surfaces are reported in Fig. 14. The continuity of surface structures is visible on both the surfaces of a strong interconnection. Surface continuity depicts the homogeneity of IP growth which in turn reflects a strong interconnection. The surfaces of Cu and Pt sides visible in Fig. 14 are free from voids and cracks. Voids and surface cracks cause weak interconnection.
The isothermal solidification based interconnection technique is a Pb free packaging technique. Thermally and mechanically stable interconnections are fabricated in flux free and environmental friendly manner. The outcome of the thermal strength measurements establishes the isothermal solidification based interconnections for gas sensor applications where high temperatures stable interconnections are required. All the samples fabricated in this course of study are stable up to 580 °C. The UTS can be increased up to 47 MPa with this interconnection technique. 45 MPa USS and 44 MPa of the ultimate torsion strength establish this technique for strong lead attachment and reliable packaging on micro-systems. The obtained thermo-mechanical stabilities are best suited for the sensor systems operative at elevated temperatures and under harsh environment conditions. Samples passed the environmental resistance tests in 95% of cases. In the present scenario when researchers are driving their full efforts to enlarge and diversify the portfolio of sensor based automation in harsh industrial processes, the gas sensors are useful in safety and product quality.
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Gas sensors are finding importance in supercritical water systems, organic rankine cycles used in binary power plants, high temperature gas-reactors in next-generation nuclear power plants and in gas processing. In addition to the safety issue, the monitoring of combustion exhaust gases has environmental appeals. Under the environmental conditions of above mentioned applications, reliable packaging on sensor parts is needed. The isothermal solidification based interconnections on gas sensor Pt heater show potential to fulfill this demand. The X-ray diffraction analysis of the interconnect cross section is proposed for the future work to study the intermetallic phase formation. Acknowledgements The authors wish to thank Dr. P.S. Bhatnagar, Director BTTI; Prof. D. Kumar of Kurukshetra University, Kurukshetra, Dr. N. Suri, Mr. S. Kumar and Mr. I.C. Sharma of HMC group for their support. Authors are equally thankful to the Director CEERI for his encouragement and permission to publish the results. References [1] [2] [3] [4] [5] [6] [7] [8]
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