Part 2: Support Electronics and Materials Used in Making High-Temperature-Tolerant Circuits
Sensors That Can Take the Heat
Jay Goetz,
Honeywell SSEC
The first part in this series of articles examined the bleak state of high-temperature-tolerant electronics available to designers. Conventional solutions were discussed, including standard component limitations with design work-arounds; new options were presented, such as high-temperature extensions of silicon devices; and the hope for future development of wide bandgap semiconductors was considered.
This part of the series provides an introduction to the rest of the components required to make high-temperature-tolerant systems, as well as the interconnection and packaging materials typically used.
Passive and Discrete Devices
For passive components, packaging durability at high temperatures is of prime importance. Very often, joining, sealing, and package ruggedness are the main issues affecting the reliability of the components because the basic property materials are robust. To help guide you in choosing devices that can stand up to high temperatures, this article summarizes results of many thermal and aging tests of passive high-temperature-tolerant electronic components.
Resistors. Although less complex than other passive devices, resistors are the most commonly used component in electronic design. Thus, failure modes, shifts, and thermal drifts all can have adverse effects on a circuit designed for high-temperature operation. In addition, mismatched material thermal coefficients of expansion (TCE) cause thermal stress at the resistor-substrate interface.
Thin film resistors are ideal miniature high-temperature-tolerant devices because of their good resolution, stability, high-frequency performance, and low thermal coefficient of resistance (TCR). Honeywell has a process for making thin film chrome silicon (CrSi) on SOI ICs. These films are deposited after underlayer processing and before the back-end conductor films are deposited and patterned. Resistor values have trimmed accuracies of ±2% and temperature coefficients of <300 ppm/ºC, with excellent matching to ~50 ppm/ºC. Other thin film materials used include tantalum, tantalum nitride, and nickel-chromium.
Carbon-composition resistors, marginal performers for noise and tolerance at low temperatures, are even more unsuitable at temperatures above 130ºC. Two factors are to blame:
• Differences in the TCE of their constituent conducting and nonconducting elements
• Low melting temperature of their binder (usually a phenol-formaldehyde resin)
Metal-film resistors are more stable at lower temperatures than the carbon-composition types, but they begin to degrade by sublimation of their resistive elements above 165ºC. Integrity of their epoxy coatings is also an issue at high temperatures.
Wirewound resistors are designed for power applications and are usually well suited for high-temperature environments because of their more stable TCR. Manufacturers have coated this type of resistor with vitreous enamel, enabling the part to perform well to 300ºC. Without specific coatings for high temperatures, though, these parts must be kept from exceeding their potting material temperature limits, and they should be derated significantly. Manufacturers recommend derating a resistor to 20% of the room temperature power rating for operation at 200ºC. Tests have shown, however, that the part can be used at this derating factor for thousands of hours successfully. Dale, Caddock, and Huntington Electric make wirewound resistors for which there are temperature test data available.
Thick film resistors are low cost in volume applications, have high resistivity, and can be designed for high temperatures. Heraeus-Cermalloy makes a special high-temperature-tolerant resistor ink that—when printed on ceramic substrate with Pd/Ag termination suitable—is for temperatures up to 500ºC. Good to 1000ºC as a thick film material, ruthenium has good stability, low TCR, and low noise.
A summary of published test results for different types of resistors follows (for more detail, see the suggestions for further reading at the end of the article):
• The nominal value of 0.1 to 5 W wirewound resistors typically drifted within ±4% over several hundred hours.
• Shifting of a couple precent can occur if the wirewound resistor is thermally cycled. The shift usually—but not always—returns to the original value from prior to the shift.
• Some wirewound resistors have survived tests of 10,000 hr. with 1000 thermal cycles from –55ºC to 225ºC, making them a good solution for medium-power resistive needs.
• Temperature effects of 1% from 0ºC to 300ºC are seen with the thick film printed resistors, and despite aging effects showing drifts of 3% in a couple hundred hours, they are good candidates for low-power, high-temperature use.
• Drifts of the printed resistors were different for different shapes, with a square geometry having the least drift.
• CrSi thin film resistors drifted <0.7% over 1700 hr. at 200ºC. These resistors have a TCR that is typically –226 ppm/ºC, with a standard deviation of 35 ppm/ºC. Power levels must be kept small, however, to avoid greater drift caused by self-heating.
Beware: Manufacturers will sometimes adjust the value of a wirewound resistor by notching the wire. The notch serves as a nucleation site for wire fatigue damage to collect and is the usual cause of failure for the device.
From a performance standpoint, Johnson noise is also a concern, especially in the case of signal conditioning circuits for low-level signals, because signals are usually amplified, which enhances the noise as well as the signal.
The Johnson noise voltage varies as:
Erms = 
(4kRT
f) (4)
where:
R = resistance value
T = temperature
f = bandwidth of interest
k = Boltzmann constant
As the equation demonstrates, you can reduce the noise by reducing resistance or by reducing the bandwidth of the signal.
Capacitors. Increasing temperature often significantly changes capacitance because of strong temperature dependence of the dielectric constant. Higher temperatures also affect equivalent series resistance (ESR) and dissipation factors. High-temperature capacitors usually have low capacitance values because of these dielectric effects and because packages are kept small to prevent mechanical breakage caused by thermal packaging stresses. Electrolytic capacitors don’t operate above 150ºC because of die lectric breakdown.
High-temperature applications requiring values of several tens of microfarads typically use wet-slug tantalum devices, but switching applications suffer because of higher values of ESR. In this case, the capacitors should be parallel to get high values with low ESR. Solid tantalum capacitors are available in values as high as several microfarads.
X7R and NPO dielectric ceramic capacitors have successfully operated at high temperatures. X7R dielectric has a large temperature drift, but NPO dielectric tends to be stable for long periods of time, even at temperatures as high as 500ºC and exceeding 5000 hr. in some tests. Their values, however, are usually <<1 µF.
Various types of polymer-film capacitors have good high-temperature characteristics up to 200ºC. Custom Electronics is developing Teflon electrostatic capacitors that have low thermal coefficients of capacitance from room temperature to 200ºC, though the parts are usually small valued. Other dielectric materials have promising test results, including Teflon perflouroalkoxy and polybenzemidazole-PBI.
Glass dielectric capacitors also have excellent stability and strong reliability and exhibit high quality at high temperatures—even above 300ºC—but they tend to be large for their capacitance values.
Published temperature test results demonstrate that:
• The loss of hermetic seal is the primary failure mode of wet-slug tantalum capacitors. Tests have shown that at 200ºC there is a gradual loss of functionality above 2500 hr.
• At 200ºC, solid tantalum capacitors typically exhibit an initial aging shift in capacitance of a couple percent, but then stabilize for several thousand more hours. The TC of capacitance for these capacitors is typically about 350 ppm/ºC.
• X7R dielectric ceramic capacitors typically show some initial aging at 200ºC, but then stabilize for several thousand more hours. Cycling temperature, however, produces a 2.5% temporary shift in value.
• An improved high-temperature-tolerant dielectric material from KD Components makes the company’s ceramic capacitors capable of reliable operation to 300ºC. Tests run for 1000 hr. at 300ºC indicate no failure or insulation resistance degradation.
Crystals. The results of 5500-hr. drift tests of crystals from CINOX, Anderson, and Q-Tech show that current crystal fabrication technology can make an oscillator stable to ±800 ppm from –55ºC to 225ºC. The packaging techniques used in high-temperature crystals are a critical part of the overall component design. Mismatches in TCE between the internal structures can cause cracking and immediate failure or gradual degradation of performance. You can’t just look at the thermal coefficient of frequency without considering long-term aging effects. MF Electronics also manufactures crystal oscillators that operate up to 200ºC.
Several analog and digital functions can be integrated on a single chip, resulting in gate arrays and ASICs. This approach provides higher levels of integration and correspondingly increases the reliability of the system because of reductions of interconnections.
Multichip Modules
High-temperature-tolerant multichip modules have been fabricated for two aircraft turbine engine applications (see the photos). The improvements possible with a multichip module approach are shown in Table 4. The lower complexity modules, Case 2, have passed tests for wire resonance (vibration), cover resonance, high voltage insulation resistance, residual gas analysis, impact shock (20 g’s, 11 mS both directions, all axes), thermal shock (–65ºC to 150ºC), moisture resistance, salt atmosphere, wire bond strength, and die and capacitor bond strength. This multichip module is being qualified at a higher assembly level for application on a military aircraft engine program.
TABLE 4 Color
x
Turbine Engine Case 1
SCP
MCM
Turbine Engine Case 2
SCP
MCM
Number of components
8
1
4
1
Number of pins
469
64
39
9
Total chip MTBF (hours) 1
97,000
97,000
1,500,000
1,500,000
Connections MTBF (hours) 2
37,000
273,000
448,000
1,940,000
Board/MCM combined MTBF
27,000
71,600
345,0
848,000
MCM improvement in MTBF
1. Based on Reliability Database Projected to 150°C
2. Based on MIL-Hdbk-217
TABLE 5
Material (resin)
Tg (°C)
CTE (ppm/°C)
Thermal
Conductivity (W/m°C) Dielectric
Constant (MHz)Recommended
Maximum Board
Epoxy (FR4)
120-140
69
0.19
3.6
135°C
Polyimide
240-300
50
0.18
3.2
260°C
Cyanate ester
260
55
0.2
3.1
260°C
Polyimide-kevlar
x
x
x
x
260°C
BT
250
xx
0.2
3.1
190°C
Circuit Construction Materials
Packaging and Interconnections for Components and Multichip Modules (MCMs). If the high-temperature-tolerant electronics you’re designing are going to be exposed to elevated temperatures for extended periods of time, choose the materials used for substrates, die attach, wire bonding, and metal interconnects carefully, and make sure they have appropriate properties. Substrate materials can warp and de-laminate, and mismatched TCEs can cause die or substrates to crack. You should also be aware of more subtle life or performance conditions, such as gradual thermally in duced wire-bond resistance increase caused by formation of intermetallics, or increased leakage of packaging substrate materials because of dielectric strength degradation.
Some of the important properties for high-temperature-tolerant electronics packaging and interconnections include:
• Materials should have matching TCEs
• Interfacial materials should be chemically inert
• Interconnect metals should not be mixed if possible
• Substrates should have high dielectric strength at high temperatures
In general, ceramic substrates and gold eutectic epoxy attach materials work better above 250ºC. Cyanate ester and polyimide PCBs and polyimide or silicone attach materials work well only below 250ºC. If the materials have large TCE differences and significant thermal cycling is expected, silver-filled silicone works well as a conductive attach adhesive. But limit the upper temperature range to 250ºC. An example of TCE mismatch is the use of cyanate ester PCBs with surface-mount (SMT) capacitors and resistors at or above 250ºC. For these temperatures, ceramic substrates are recommended, especially if thermal cycling is expected.
Traditionally, high-temperature-tolerant applications use throughhole components because of their availability. These components can withstand greater TCE differences between components and circuit boards than can leadless packages because of the strain relief provided by component leads.
Lately, however, many manufacturers use SMT components because of the space savings they provide. More passive SMT components, for example, are available with high-temperature-tolerant terminations, making the decision to go to SMT easier. Leaded gull-winged or J-leaded SMT ICs can also provide sufficient strain relief. In general, larger leadless chip carrier SMT components should be avoided because of the potential of cracking caused by TCE differences in board and component materials.
Board Materials. Choose circuit-board materials carefully to achieve a stable platform for electronic components or circuit die that will operate in temperature ex tremes (see the sidebar, “Multichip Mod ules,”). Some of the desirable properties of good packaging material include:
• Good thermal conductivity
• Thermal shock resistance
• High electrical resistivity
• High mechanical and chemical inertness
• Thermal expansion that matches the components and adhesives
Table 5 shows the temperature ranges recommended for the board materials, depending on the type of application:
Manufacturers are testing new techniques that use conductive inks with polymer insulative dielectric layers on metallic substrates to produce cost-effective, high-temperature-tolerant circuit boards. Ormet is one of the suppliers of these inks.
Case Materials. Useful up to 1000ºC, ceramic materials are recommended for high-temperature applications that don’t require a lot of heat dissipation. Power de vices with dissipation of more than 0.75 W/ cm2 should use metal cases for better conductivity. Metal also protects against high vibration and shock.
Manufacturers primarily use alumina, beryllia, and aluminum nitride when fabricating these cases. Aluminum nitride is a new material that offers superior thermal conductivity and a TCE better matched to silicon than the other two. The drawback is that it is more expensive.
Producers also use Kovar for cases because of its low TCE. Copper-tungsten has better thermal conductivity and a low TCE and is a good choice for power devices. Lids are made of metal or ceramic, typically alumina or Kovar.
Some vendors produce liquid crystal polymer IC packages. These hold promise for a low-cost, high-temperature-tolerant packaging solution.
Wiring/Wirebonds, Interconnections, and Solder/Electrically Conductive Adhesives. Multichip modules typically use wirebonds for die connection to substrates. Wirebond compatibility and wire integrity when stressed or fatigued are issues to be dealt with (see Figure 9).
In general, dissimilar material bond interfaces cause problems with interdiffusion (void formation) and intermetallic formation (brittle phases). The following list indicates the survivability of certain wirebond interfaces (wire-pad order):
• Al-Au: 175ºC (Note, however, that Al wire bon ded to good-qual ity gold pack age leads works well to 300ºC.)
•Al-Ag: 175ºC
•Cu-Al: 200ºC
• Cu-Au: 300ºC
• Al-Ni: 300ºC
• Al-Al: 660ºC
• Au-Au: 1064ºC
Lead frames provide power, ground, and signal I/O connections. Their important properties include electrical and thermal conductivity, TCE, and strength. Lead frames with high TCEs can cause stress problems on IC die and packages exposed to high thermal swings. Kovar and Alloy 42 are good to 500ºC.
For wiring interconnection, TFE Teflon-coated, silver-plated hookup wire can be used for temperatures up to 250ºC. Above that, nickel-coated copper conductors with mica and glass braid cladding has been used successfully.
Connectors used above 200ºC should have contacts with thick, soft gold (100 µin.) over nickel plating (100 µin.). Nickel-plated solderless terminals can be used at lower temperatures.
Several solders can be used in making high-temperature-tolerant circuits:
• Commercial temperature range solder is 60 Sn–40 Pb, with a melting point of 183ºC.*
• For temperatures below 280ºC, you can use a 97.5 Pb 1.5 Ag 1.0 Sn* with a melting point of 309ºC, or a 95 Pb–5 Sn* with a melting point of 300ºC.
*This is environmentally unsafe because of public health concern over lead poisoning of children. A better choice for environmental reasons is the 95 Sn–5 Sb alloy, with a melting temperature of 234ºC, or the 96.5 Sn–3.5 Sb, with a melting temperature of 221ºC.
• For temperatures above 300ºC, Indalloy IND. 183 (88 Au-12 Ge) can be used, with a melting point of 356ºC.
TABLE 6
x
Stiffer Materials
More Flexible Materials
Electrically/thermally
conductiveGold eutectics
Silver-filled glassSilver-filled polyimides/epoxies Tin-lead-based solders Indium-based solders
Thermally conductive
Tin-oxide/indium-oxide/berylia-filled glass
AIN-filled/alumina-filled epoxy
Nonconductive
Unfilled glass
Unfilled epoxies Epoxy silicone polyimides
TABLE 7
Product Name
Material Type
Maximum Operating Temperature
EPO-TEK E3081
AG-filled epoxy
250°C
Ablestik 71-1
AG-filled polyimide
240°C
Indalloy IND. 183
88 Au/12 Ge braze
356°C
Grace specialty polymers
AG-filled silicone
230°C
Electrically conductive adhesives provide a simpler process for assembly than solders, but at higher expense. They’re usable up to 250ºC and don’t require processing at the high temperatures of solders. Although these are new materials in need of further study, current results indicate that the electrical performance is satisfactory.
Die Adhesives, Epoxies, and Circuit Encapsulants. Selection of a die attach material for MCMs or IC packages depends on melting temperature, elasticity, thermal conductivity, and electrical conductivity. Material types can be classified as shown in Table 6. Some examples of good high-temperature-tolerant conductive epoxies and attach adhesives are listed in Table 7.
High-temperature-tolerant die-attach materials—including gold eutectics or silver-filled glasses—tend to be stiffer. If the high electrical conductivity (and cost) of the gold eutectic is not needed, silver-filled glass can be used and is somewhat more ductile. Aluminum nitride and beryllia-filled glass have less thermal conductivity and are electrically insulating. Stress-sensitive large die can use cyanate ester die attach.
The most widely used molding compounds are thermoset epoxies, which require high temperatures and pressures for molding and have low purity. Some polyimides are also recommended for thermal and moisture stability, with low TCE and high purity as well. Plastics are limited in temperature use by depolymerization, which occurs between 190ºC and 230ºC for epoxies and between 260ºC and 280ºC for silicones. Operation above the glass transition temperature—150ºC–180ºC for epoxies—increases the thermal strain between the molding compound, die, and lead frame because of increased TCE. Silicones are often used to absorb thermal stress on the die, thanks to their improved compliance. Their poor adhesion to the lead frame, however, can provide a path for moisture ingress.
Summary
This part of the series provides an overview of the support electronics, interconnections, and packaging materials that go into high-temperature-tolerant systems. These elements provide the critical support for circuitry and the physical foundation and glue that hold these systems together. During the selection and design of these components, special consideration must be given to the ruggedness, temperature expansion, and electrical performance effects of the passives, interconnections, and materials. Research information is available from the HiTeC and HITEN consortiums regarding the latest advances in these areas.
The next and final part of this series will present application information, including architectures and design techniques for high-temperature sensor interfacing and data acquisition.
For Further Reading
Brusius, P. 1998. “Some Reliability Aspects of High-Temperature IC’s,” Proc 4th Annual High Temperature Electronics Conference, Albuquerque NM.
Caruso, M. May/October 1998. “A New Perspective on Magnetic Field Sensing,” Proc Sensors Expo.
Day, J., and M. Roach, 1998. “Ceramic Dielectric Performance under High-Temperature Life Test,” Proc 4th Annual High-Temperature Electronics Confer ence, Albuquerque NM.
Goetz, J., and H. Middleton. May 1999. “Designing Sensor-Based Systems for High-Temperature Environments,” Proc Sensors Expo.
Grzybowski, R. 1998. “Long-Term Behavior of Passive Components for High-Temperature Applications—An Update,” Proc 4th Annual High Temperature Electronics Conference, Albuquerque NM 1998.
Grzbowski, R., and B. Gingerich. June 1998. “High-Temperature Integrated Circuits and Passive Components for Commercial and Mil itary Applications,” Proc ASME Turbo Expo, Stockholm Sweden.
Hattori, M. “Needs and Applications of High-Tem p erature LSIs for Automotive Electronic Systems,” Toyota Motor Corp.
High-Temperature Electronics, Ed. by F. McCluskey, R. Grzybowski, and T. Podlesak, CRC Press, 1997.
HITEN Regional Seminar—Europe. High-Temperature Electronics. December 16, 1998.
HITEN Report-1997, www.hiten.com, London England.
Johnson, R. July 1999. “High-Temperature Silicon-On-Insulator Pressure Sensor Technology,” Proc HITEN Conference, Berlin, Germany.
Lewis, T. 1998. “Military Aircraft Turbine Engine Electronics and Requirements,” Proc 4th Annual High Temperature Electronics Conference, Albuquerque NM.
Miller, J., P. Nicastri, and S. Zarei. February 1999. “Future Power System Architectures Face Formidable Hurdles,” PCIM Magazine:44-45.
Naefe, J., W. Johnson, and R. Grzybowski. 1998. “High-Temperature Storage and Thermal Cycling Studies of Heraeus-Cermalloy Thick Film and Dale Power Wirewound Resistors,” Proc 4th Annual High Temperature Electronics Confer ence, Albuquerque NM.
Naefe, J., W. Johnson, and R. Grzybowski. 1998. “High-Temperature Storage and Thermal Shock Studies of Passive Component Attach Materials,” Proc 4th Annual High-Temperature Electronics Conference, Albuquerque NM.
Normann, R., and B. Livesay. 1998. “Geothermal High Temperature Instru mentation Applications,” Proc 4th Annual High Temperature Electronics Confer ence, Albuquerque NM.
Travis, B. October 1998. “Keeping HAL Cool in 2003,” EDN Magazine.
Welch, Jr., R., C. Olsen, and G. Patyk. August, 1998. “Integrated Motor-Drive,” PCIM Maga zine: 22-27.
Editor’s Note
The figures, equations, and tables in Part 2 are numbered consecutively from those in Part 1. The information in this series of articles will be presented at Sensors Expo Detroit, September 19–21, 2000.
Jay Goetz is an Applications Engineer, Honeywell SSEC, 12001 Hwy. 55, Plymouth, MN 55441; sj.goetz@worldnet.att.net. For inquiries on information or products mentioned in this article, contact Honeywell at 800-323-8295 or visit its Web site at www.ssec.honeywell.com.
