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Extremely High Temperature and High Pressure (x-HTHP) Endurable SOI Device and Sensor Packaging for Harsh Environment Applications

K.Y.Au*, F.X. Che, Eva Wai Leong Ching v/ Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-02

Innovis Tower, Singapore 138634 *E-mail: auky@ime.a-star.edu.sg Tel: +65-67705463 Fax: +65-67745747

Abstract—There are increasing interest in high temperature endurable sensor or device packaging solution for a set of wide ranging applications that include oil and gas production or deep sea exploration, advanced automotive application, aerospace, more electric aircraft initiative (MEA) or engine health management (EHM) systems, geothermal energy harvesting as well as other renewable energy industries. Besides high temperature endurable capability of the electronic device, heavy industries such as oil and gas down-hole drilling or exploration, deep sea or geological survey applications which require additional capability to endure high ambient pressure with target reliability requirement of up to 2501C and 30Kpsi. A unique packaging solution is therefore required to address all the challenges associated with extreme high temperature and high pressure (x-HTHP) endurable electronics continuous operation in harsh x-HTHP ambient environment. In this paper, we demonstrated a packaging solution for sensor device with high reliability under harsh condition with extreme high temperature and high pressure through reliability test and finite element analysis.

Keywords: High temperature and high temperature, packaging, reliability, finite element analysis

I. INTRODUCTION

Harsh environment endurable electronics have vast application ranging from renewable energy, aerospace industry to sensor electronics in extremely high pressure down-hole oil drilling and exploration. For electronic devices to work under a combine load of 250"C and 30Kpsi requires a completely different approach to electronic packaging. In this work, a set of bill of material (BOM) and metallization design are benchmarked and proposed to specifically address high temperature induced interconnect failures. This includes failure caused by excessive interdiffusion between bulk interconnect materials and substrate metallization which ultimately causes Kirkendall voids formation as the HTHP reliability test progresses and results in open failures or coefficient of thermal expansion (CTE) mismatch induced thermal mechanical stress that causes cracks in passive component or early device failure. Due to the additional 30Kpsi extreme pressure durability requirement on the electronic package, the target TV (test vehicle) designed and developed in this work seeks to isolate and solve the extreme pressure reliability requirement from the 250°C high temperature specification separately. This will significantly simplify the BOM selection process to address both 250°C and 30Kpsi HTHP reliability requirements simultaneously. The TV used in this work is a multichip module (MCM) package design using commercially available SOI (silicon on insulator) based high temperature endurable sensor device and a voltage regulator chip. The devices will be bonded on HTHP endurable ceramic substrate with a unique metallization that is

designed to be compatible with Aluminum wire bond at extreme high temperature. High melting point Ag sintering material (Tin>900°C) and Au based high temperature solder [1,2] are evaluated in this work with respect to the reference material and commonly used high lead solder. Aluminum wedge wire bonding is used to form monolithic interconnects between the SOI chip and the substrate. The assembled high temperature endurable sensor device is then mounted on to a specially designed rugged casing that once is hermetically sealed and has the capability to completely isolate ambient 30Kpsi pressure from the sensor device that is housed internally. The fully assembled x-HTHP endurable sensor package is then subjected to thermal cycling test (-55'C —250dC/ 500 cycles), high temperature storage test (2500C/ 500cycles) and finally pressure cycling test (10 cycles, atmospheric pressure to 30Kpsi). Post reliability electrical test shows all sensor packages remain fully functional with temperature sensing accuracy and tolerance within f5%.

Il. HTHP ENDURABLE SENSOR PACKAGE

Fig. 1 illustrates a cross sectional schematic of the HTHP MCM sensor device package. The package consists of 2 SOI devices, i.e. a voltage regulator used to power the temperature sensor device in conjunction with 2 passive components. A unique and specially designed metal hermetic casing design, 60mm x 25mm in size, houses the high temperature endurable electronic circuitry. Aluminum wedge wire bonding establishes the interconnects between the SOI chip to the ceramic substrates. Glass / ceramic to metal seals preserve the hermeticity of the metal housing to the external leads at HTHP. The external leads establish electrical connection between the internal sensor devices to the outside world. In a general sense, the HTHP sensor package is designed in such a way that pressure and temperature endurance problems are being tackled and solved separately. All the selected and established BOM, especially the right substrate metallization to avoid excessive interdiffusion with solder and wire bond materials, are carefully chosen to ensure continuous functionality of the device at high temperature of 250°C and high pressure of 30Kpsi.

Figure 1. HTHP endurable sensor package.

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The electrical schematic diagram as well as the fully (a) assembled voltage regulator / temperature sensor system on the ceramic substrate is shown in Fig. 2. A 15 volts external source applied to the external leads powers the voltage regulator which output the correct voltage to enable the

functionality of the temperature sensor. The entire circuitry must remain operational at 250°C.

v

(b)

Figure 2. (a) SOI devices assembled on ceramic substrate (b) Electrical schematic diagram of the sensor package.

III. THERMAL MECHANICAL SIMULATION OF HTHP SENSOR

PACKAGE USING FEA

In the initial design stage, finite element analysis (FEA) is employed to provide a general understanding of the HTHP sensor package design requirements and subsequently used to finalize critical boundary conditions and dimensional optimization for HTHP endurance. Fig. 3 (a) shows the FEA model of the sensor package used for mechanical and thermal mechanical (TM) simulation. Different casing materials such as Kovar, Ti, and Inconel, and different dia attach materials such as sintered Ag and high Pb are modeled for comparison. Table I lists material properties used in FEA simulation. Simulation results as illustrated in Fig. 3(b) & (c) show CTE matching is crucial for sensor package thermal mechanical performance up to 250°C.

TABLE I. MATERIAL PROPERTIES USED IN FEA SIMULATION

Materials

Young's

modulus (GPa)

Poisson's

ratio

CTE

(ppm/K)

Yield stress

(MPa)

SO] die 130 0.3 2.8

Substrate-A1203 300 0.21 8.2 257

Substrate-AIN 330 0.24 4.5 320

Sintered Ag 40 0.37 20 5

AuSn 68 0.4 16 200

Soft solder (high Pb) 19 0.4 29.5 10.5

Cu 121 0.34 17 121

Kovar(NiCoFe) 159 0.317 5.5 270

Inconel 198 0.29 13.55 896

Ti Alloy (Grade 5) 114 0.34 9.5 532

Figure 3. (a) FEA model of the sensor package for TM simulation, (b) die and (c) ceramic substrate bending stress under a combine loading of 250°C and

30Kpsi.

0 2000 4000 6000 8000 10000 12000

time (s)

Figure 4. Strain energy density accumulation when sensor is subjected to thermal cycling under different casing materials.

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Although results shows lower CTE casing material helps in thermal mechanical stress management, high CTE alloy such as high grade stainless steel or Inconel on the other hand, possess high bending yield stress and superb corrosion resistant which are key characteristics for high pressure corrosive harsh environment endurance. Hence it can be deduced that using a lower CTE alloy such as Kovar or Ti as casing material will yield a lower strain energy density built up during thermal cycling and potentially increasing the lifespan of the package as shown in Fig. 4.

The simulation result suggests the need for a unique casing design that consists of a high pressure harsh corrosive environment endurable exterior alloy with a low CTE alloy interior. This unique design ensures the internal delicate circuitry is protected from corrosion and physical destruction due to high pressure induced mechanical bending in the metal casing. Fig. 5 (a) and (b) shows FEA analysis results of the unique design under thermal loading of -55°C to 250°C and a special rugged metal design clearly demonstrates a lower strain energy density compares to other combination. This unique design ensures low package deformation and mitigates a portion of the thermal mechanical stress of the ceramic substrate electronics at 250°C / 30Kpsi.

(a)

(b) 14

Incon a I+5 mmKovar+A1203+Pb

Inconel+5mmKovar+AI203+AuSn ~~-

--- Inconel+A1203+Pb / /

/ /

0 2000 4000 6000 8000 10000 12000

time (s)

Figure 5. (a) Speical casing design shows reduced stain energy density, (b) Magnitude of built up of solder joint strain energy density for normal

metal versus special casing design.

IV. PACKAGING TECHNOLOGY OF HT-HP ENDURABLE

ELECTRONIC PACKAGES

As describe previously, the external special alloy casing design isolates the high pressure high corrosion environment while reducing thermal mechanical stress experienced by the internal electronic components, however, to ensure robust thermal mechanical performance and reliability of electronics components at 250°C, careful BOM and metallization selection is critical. Firstly, high melting point solder is used as passive component and die attach material. Fig. 6 shows the list of all benchmarked HT (high temperature) endurable material and their unique characteristic. HTS (high temperature storage) test at 250°C is perform to investigate the die attach materials interaction with custom design barrier metallization fabricated on both silicon and ceramic substrate surface. As shown in Fig. 6, all materials evaluated show no substantial Kirkendall voids formation and crack after 500 hours tested at 250°C. This evaluation suggests that the selected high temperature die attach material remains robust after HTS test.

Aluminum wire bond has good vibration endurance for harsh environment electronics, but conventional substrate metallization such as Ni/Au or NiPdAu used in conjunction with gold base solder and Ag sintering paste is not compatible with Aluminum (Al) wire because of the high interdiffusion rate between Au and Al.

Figure 6. AuGe, AuSn, Ag Sintering and High lead solder FIB / SEM inspection after 500 hours HTS at 250°C.

TABLE II. WIRE PULL STRENGTH OF AL WIRE AFTER HTS RELIABILITY TEST AT 250°C FOR 500 HOURS

HTS 25000 time

(hours)

Al wire on Ni/Au (gf)

At wire on special

metallization surface (gf)

time 0 5.89 5.89

250 hours 0.7 3.3

500 hours 0.6 4.8

This excessive interdiffusion at high temperature between Au and Al is observed experimentally as shown in Fig. 7. The experiment comprise of Aluminum wire bonded on two separate ceramic substrate samples with Ni/Au metallization and a special metallization design for high temperature endurable environment. Both samples with Al wire is then subjected to 500 hours storage test at 250°C. Wire pull and FEB (Focus Ion Beam) / SEM is use to characterize the output response of post reliability. Table II shows Al wire maintain a pull strength of 4.8gf (higher than the 2.Ogf military standard requirement) after 250°C HTS for the test sample bonded on the special metallization while the strength on conventional NiAu surface degrades tremendously below specification. FIB/ SEM of the 1 mil Al wire on both surfaces are shown in Figure 7. However, Al wire bonded and aged on the special metallization exhibit no significant increase in kirkendall voids and observed no interfacial cracks at the intermetallic interface as shown in Fig. 8. The same experiment is also conducted internally on Au wire with the same special metallization. No degradation of wire pull / ball shear strength and no built up of kirkendall voids are observed.

These preliminary evaluation shows that the special metallization is compatible to Au solders / Ag sintering material and Aluminum wire. Hence a complete BOM / metallization solution has been formulated for 250°C HT endurable electronic packaging.

Figure 7. AI wire on NiAu surface after HTS 250C for 500 hours. Large kirkendall voids appears at the AI / metallization interface.

Figure 8. Al wire on special metallization surface after HTS 250C for 500 hours. No significant kirkendall voids appears at the Al / metallization

interface.

V. HTHP SENSOR ASSEMBLY PACKAGING SOLUTION

The assembly process flow is show in Fig. 9. The substrate and the special metal casing are manufactured separately and are bonded together during the assembly process. Al wire bond forms the interconnect between the SOI device and the special metallization on the substrate. Solder and sintering material described in the previous section are used as passive component and die attach material. Fig. 10 shows the post assembly image of all interconnects. The fully assembled HTHP sensor package prior to hermetic sealing is shown in Fig. 11. A thick unique metallic casing is custom design to be both rugged and corrosion resistance to ambient high pressure harsh environment but with reduced CTE mismatch for improving thermal mechanical performance. The fully assembled substrate is subsequently bonded onto the special metallic casing and Aluminum wire form interconnects between the substrate and the casing lead. All aspect of the sensor package design, from BOM material properties selection, special casing design device to hermetic seal and electrical isolation materials are carefully chosen to achieve 250°C and 30Kpsi reliability performance.

The top lid is then hermetically welded onto the sensor package to form the final HTHP endurable product as shown in Fig. 12.

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0 50 100 150 200 250 300

Temperature (°C)

(a)

(U)

Figure 11. Fully assembled HTHP endurable rugged sensor package.

Figure 12. Top lid welded HTHP sensor package.

Passive ComponentSMT Attach using

solder or Ag sintering

FI

i F

Alu Wire bond

• Substrate to Lead Interconnect

Hermetic Welding

Figure 9. HTHP sensor package assembly process flow.

(a)

VL HTHP SENSOR PACKAGE ELECTRICAL

CHARACTERIZATION

The fully assembled and hermetically welded sensor package as shown in Fig. 12 is subjected to electrical testing to check the electrical performance of the device after packaging with an ambient temperature ranging from room temperature up to 300°C. The sensor package temperature (in voltage) output is then compared to the SOI temperature sensor die specification from the vendor. The voltage regulator output versus temperature is also measured. The result is shown in Fig. 13. The result shows no significant variation observed between the HTHP rugged sensor package output measurements versus the S01 device manufacturer specification. The sensor accuracy is within +/- 3°C ranging from room temperature to 300°C. Similarly, the voltage regulator output voltage range from 4.9volts to 5.1volts and is within the 2% voltage deviation range. The result confirms that the HTHP rugged sensor package is capable to operate up to 300°C with no degradation of signal integrity and reliability failure.

Figure 10. (a) Al wire bond on SOI device with Al bond pad, (b) MLCC passive component attach using SMT method.

1 Au-G e P- Ag Sinter

Reliability Test

Post Reliability

Result Electrical Test

High Temperature 4 pcs 4 pcs 4 pcs Storage. 500 hours

at 250'C.

Passed: 12/12

4 pcs 4pcs 4 pcs

Thermal Cycling 500 cycles

-55'C to +2501C

Passed: 12/12

Non Sealed Case

Vibration Mil-Std-883 Method 2007. Level A 20G

Vibration

Passed : 6/6

2 pcs 2 pcs 2 pcs

Shock Mil-Std-883Shock Method 2002 Level B Passed : 3/3

1506G

Pressure cycling test.

30 kpsi 112 Cycle Passed 5/5

Post HTS + TC units 5 pcs

Sand & Dust Test

MIL-STD-810G Passed 4/4 Method 510.5

Case Lid Sealed

1pcs Ipcs 1pcs Salt spray Test MIL-STD-81OG

Passed 3/3 Method 509.4

(b)

5.5

5.3

5.1

4.9

4.7

4.5 -50 0 50 100 150 200 250 300

Temperature ("C) Figure 13. (a) Actual Sensor output voltage compare to sensor device vendor

specification. Both theoretical and measure output matches perfectly, (b) Voltage regulator output voltage variation versus temperature.

TABLE III.: FULL RELIABILITY ASSESSMENT OF THE HTHP RUGGED SENSOR PACKAGE OF 250°C AT 30KPSI.

VII. RELIABILITY ASSESSMENT OF THE RUGGED SENSOR

PACKAGE IN HTHP HARSH OPERATING ENVIRONMENT

Table Ill illustrates all reliability tests that were conducted on the HTHP rugged sensor package, taking into account deep sea exploration, oil and gas drilling and aerospace application. The reliability test focuses on non-lead rugged BOM and package design. The test includes high temperature storage at 250°C for 500 hours, -55'C to 250°C TC for 500 cycles, sand

and dust test, shock and vibration test, salt spray test as well as 30Kpsi pressure cycling test. All HTHP sensor package passed all the tests as shown in Table III. This clearly shows the combination of unique metal casing, capable of ensuring 30Kpsi and corrosive ambient environment while is still able to manage thermal mechanical stress as well as the special substrate metallization in which a single surface finish is compatible with Au base solder, Ag sintering and Aluminum wire at prolong high temperature exposure.

VIII. CONCLUSIONS

This study demonstrated a packaging solution for sensor device with high reliability under harsh condition with extreme high temperature and high pressure through reliability tests and finite element analysis. unique casing design and optimized BOM for die attach material, substrate material are presented. Some important results are summarized in the following:

1. The sensor package developed in this work is able to withstand thermal cycling condition of -55°C to 250°C for 500 cycles and high temperature storage test of 250°C for 500 hours with no failure.

2. The sensor package also passed 30Kpsi pressure cycling test for 10 cycles and other harsh environemental test such as vibration test, salt spray test and sand and dust test.

3. The reliability results demonstrate successful x-HTHP reliablity achievement necessary for future electronic packages to maintain full device operation in an extrmemely harsh environmental condition.

ACKNOWLEDGMENT

This work is the result of a project initiated by Rugged E consortium. The authors greatly appreciate the members' participation in discussions and encouragement throughout the course of the project which makes this research possible.

REFERENCES

[1] L. C. Wai, W. W. Seit, E. Phua, M. Z. Ding, V. S. Rao, and D. Rhee, "Study on Silver Sintered Die Attach Material with Different Metal Surfaces for High Temperature and High Pressure (300°C/30kpsi) Applications." Proc. 15th IEEE Electronics Packaging Technology Conference, 2013, DOI: 10.1109/EPTC.2013.6745738. [2] L. C. Wai, M. Z. Ding, D. Rhee, and V. S. Rao, "Process Characterization of Highly Conductive Silver Paste Die Attach Materials for Thin Die on QFN." Proc. 14th IEEE Electronics Packaging Technology Conference, 2012, pp.377-383.

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