04-0507-Electronics Packaging Considerations for Space Applications

8/2/2019 04-0507-Electronics Packaging Considerations for Space Applications

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Electronics Packaging Con siderations for Space Ap plicationsP.J. ZuluetaJet Propulsion Laboratory, California Institute of Technology4800 Oak Grove Drive, Pasadena, CA 91109

AbstractThe functionality of spacecraft electronics must be maintained in the harsh environments found in space. Theradiation environments can consist of either low-energy x-rays at the surface of the spacecraft or high-energyelectrons, high-energy protons and high-energy photons within the spacecraft. Space radiation varies significantlyand induces various types of effects (e.g.. cumulative ionization, low dose rate effects, single event effects anddisplacement damage) in common device technologies. Harsh radiation environments can also induce electricalcharging in spacecraft materials. Within the solar wind, direct irradiation by the Suns emissions creates relativelylow-voltage charging. Large particle currents, carried by the solar wind, interact with planetary magnetospherescreating trapped high-energy particle belts around certain planets (e.g., Earth, Saturn and Jupiter). Spacecraftpassing through these belts can charge spacecraft materials with internal fields that can cause damagingelectrostatic discharges. Another harsh environment found in space is extreme low temperature. This can affect thefunctionality of spacecraft electronics, particularly for interplanetary or deep space applications. Devices andassemblies can experience performance and reliability issues either at extreme low temperatures or as a result ofcycling to extreme low temperatures. Mitigation of these dam aging effects leads to understanding physics of failureand m aking changes in the device design, electronic assembly or the electronic packaging materials.1. IntroductionSpacecraft encounter harsh extremes of radiationexposure and temperature variation to the extent thatspecial considerations must be made in the packagingof the electronics to mitigate their harmful effects. Thisbecomes particularly important as there is increasinginterest in the use of unhardened commercialelectronics technologies in space systems, driven bytheir higher performance, lower semiconductormanufacturing costs and advanced levels ofintegration. These advantages are partially offset byincreased radiation testing, environmental testing costsand the risk that process changes in commercialtechnologies may inadvertently compromise theirreliability. This paper will focus on how both spaceradiation and extreme cold environments can affectspacecraft electronics and will address considerationsin packaging to mitigate their damaging effects.2. Radiation Sources [ I ]There are three primary components of the naturalspace environment; 1) high-energy electrons andprotons trapped by planetary magnetic fields, 2)galactic cosmic rays (GCRs) originating from outsidethe solar system, 3) the anomalous component and 4)

solar event particles. There m ay also be, rapped heavyions, but their energies are low enough to be stoppedby typical spacecraft shielding.2.1 High-enerev electrons and protons can penetratespacecraft shielding and are usually the only trappedparticles that are a concern for Earth orbits. Theregions of trapped electrons and protons, sometimescalled Van Allen radiation belts, are most significantbetween the altitudes of approximately 1,000 km to32,000 km. Their extent is greater than this, but theparticle flux is much smaller outside this altituderange. The distribution of electrons and protons varywith altitude. Electrons, whose energies reach amaximum of about 7 MeV, show tw o altitude peaks atabout 4,000 km and 24,000 km. The location ofprotons is restricted primarily to one belt, but theirenergies can be in excess of several hundred MeV.The dipole that represents Earths magnetic field istilted and offset from its center. The result is a specialgeographic location off the coast of Brazil called theSouth Atlantic Anomaly, in which an intense radiationbelt exists at an altitude lower than without the offset.This region reaches down to the upper part of theatmosphere and is not an issue for orbits that would be

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in the intense radiation belts anyway, but is an issue forspacecraft attempting to avoid harsh environments.2.2 Galactic cosmic rays originate from outside thesolar system and occur everywhere in space. Thesehighly energetic particles with a w ide range of atomicnumbers, reach energies in excess of 10 GeVhucleon.The flux rate is very low compared to the particles inthe radiation belts. However, a single GCR can depositsufficient charge in an integrated circuit transistor (orchange the state of intemal storage elements) and causemore complex behavior, such as latchup.2.3 The anomalous component is another source ofparticles composed of helium and some heavier ionswith energies less than -50 MeVlnucleon. They werenamed this because their presence is random. Theanomalous component is not a very severeenvironment compared to GCRs in interplanetaryspace behind moderate or heavy spacecraft shielding,because shielding is effective against this relativelylow-energy environment. However, the anomalouscomponent can be significant compared to GCRs forspacecraft heavily protected by magnetic shielding, butonly lightly protected by mass shielding. The reason isthat the anomalous component is only singly ionizedand this tends to make it able to penetrate magneticshielding. If mass shielding does not remove it, whilemagnetic shielding removes most of the GCRenvironment, the anomalous component can be asignificant part of the surviving heavy ions.2.4 Solar events produce varying quantities ofelectrons, protons and lower energy charged particles.Solar activity varies widely where very high fluxes ofparticles may occur over a period of hours or days.Solar event protons are the primary contributions toionizing dose and displacement damage for spacecraftin interplanetary space. Also, the relative abundance o fprotons can be an important contribution to singleevent effects in proton-susceptible devices.3. Radiation Effects [2]3.1 Lone-Term Ionization Effects. Damage toelectronics and materials can arise from the long-termeffects of ionizing radiation. When a photon or chargedparticle travels through a material, it interacts withelectrons in the material and causes som e of the atomsto become ionized, creating electron-hole (e-h) pairs.The damage can occur in both semiconductors and

insulators. One distinction between semiconductor andinsulator is whether this effect accumulates ordissipates. Device insulators e.g., gate or field oxidesin a CMOS device, exhibit a cumulative effect. Someof this charge will be trapped at thesemiconductor/insulator surface. In MOS structures,the trapped charge will cause a shift in the gatethreshold voltage and can result in extremely largeleakage currents. Mobility (which affects switchingspeed and drive current) is also degraded. The trappedholes will eventually become neutralized, as the oxideanneals, but the anneal time can take years at roomtemperature.Long-term ionization effects in optical materials areevident as an increase in optical absorption. Theabsorption rate is a strong function of the type ofmaterial. For example, fused quartz generally colorsless than alkali glasses from a given ionizing dose.Quartz crystal used for precision oscillators or filterscan show significant resonant frequency shifts. Naturalquartz shows the largest frequency shift; syntheticquartz shows less and swept synthetic quartz even less.Accumulated trapped charge is measured by theaccumulated ionization, which in tum is measured bythe sum of the energy lost by the particles to thematerial via interactions with the electrons. The totalenergy, per unit mass of material, transferred to thematerial via ionization from all ionizing particles, iscalled the total ionizing dose (TID ) or total dose. Thisdose is typically measured in rads (material specified),with one rad defined as 100 ergs of deposited energyper gram of material. Total dose levels encountered bysatellites and space probes are typically between 10and 100 h a d (Si), although systems exist withrequirements above and below this range.The devices and materials of concern and the mostserious radiation induced effects are:1. MOS devices (threshold voltage shift, decrease indrive current and switching speed, enhanced leakage).2. Bipolar tran sistors (h FE degradation, espec ially atlow collector current, IC; leakage current), and junctionfield effect transistors (JFETs) (enhanced source-drainleakage current).3. Analog microcircuits (offset voltage, offset currentand bias-current changes, gain degradation).4.Digital microcircuits (enhanced transistor leakage orlogic failure due to ionizing dose induced hFE& VTchanges).

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5 . Quartz resonant crystals (frequency shifts).6. Optical materials (increased absorption).7. External polymeric surfaces (mechanicaldegradation).3.2 Transient Ionization Effects (Interference).Interference is defined as transient ionization effectsthat persist only while the electronics are beingirradiated. Interference effects depend primarily on therate of ionization energy deposition, Le., the dose ratemeasured in rads (material)/sec.There are four types of interference in electronicdevices and optical materials:1. Primary photocurrents in low current input stages tothe electronics.2. Electron emission from cathodes of electronmultiplier-type detectors.3. Ionization-induced conductivity in photo-sensitivematerials, such as those in detector surfaces.4. Ionization-induced fluorescence in optical materialssuch as detector windows and lenses.3.3 Displacement Effects. Displacement of atoms incrystal lattices cause permanent changes to materialproperties. Only the most sensitive devices are affectedsignificantly by displacement effects.Displacement effects can affect the following devicesand properties:( I ) Bipolar tran sistor s with lo w fT (hFE, VCE AT, VBESAT).(2 ) PN junction diodes (VF,VB).(3 ) Light emitting diodes (VF, VB, light emittingefficiency).(4) Semiconductor photodetectors (quantumefficiency).(5)Devices incorporating lateral p-n-p transistors (hFE,VC E AT , VBE AT).(6) MOSFE Ts (resistance, leakage current).4. Mitigation of Radiation Effects [3]The energies of the trapped particles are low enough sothat mass shielding thicknesses practical for spacecraftuse can provide significant protection (particularly forthe electrons). Shielding is very effective against theheavy-ion component of solar events and is lesseffective against the more penetrating protons.Localized or spot shielding may be needed to obtainadequate protection without adding excessive weight

to the spacecraft. The amount of shielding isdetermined with the use of a radiation design margin(RDM). The definition of RDM is the ratio ofradiation capability of the part or component to theexpected radiation environment. The padcomponentradiation capability is defined as the fluence (or dose),flux (or dose rate) of charged particles or nuclearradiation which will produce enough degradation orradiation-induced interference in the partcharacteristics to cause the part to operate outside of itsspecification for the particular circuit application.The general use of an RDM acknowledgesuncertainties in environmental calculations and partradiation hardness determinations. The selection of anRDM may be somewhat arbitrary and will tend to bedriven by mass limitations, acceptable risk versus costand the overall radiation hardness program. RDM,which implies a margin, is really a misnomer, becauseis assumes that a conservative margin actually exists. Itma y be more appropriate to refer to a radiation designfactor.Typically, once the mission design is confirmed, theTID as a function of shielding thickness (dose-depthdata) are generated for a simplified geometric massmodel, such as the spherical shell model. Figure 1below is an example of a flight mission at 1Astronomical Unit (AU) from the sun during the solarmax period. Standard practice is to apply the dose-depth curve at 95% confidence level for the flightassembly (unit) design. This radiation dose curve canbe used to obtain conservative first-look shieldeddose values without hardware configuration modeling.As electronics parts scaling continues, it is prudent tocarefully re-examine RDMs of high magnitude or torefine the part radiation hardness determinationtechnique if lower RDMs is demanded. The partradiation hardness test is generally a cost driver. Thisis primarily due to the fact that a more accurate testrequires more samples, more realistic flight simulatingradiation sources and conditions, and longer test time.More extensive radiatiodshielding calculations alsotend to be cost drivers, but it relieves the shieldingrequirement and therefore saves more m ass.

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i%na** r*Figure 1. Radiation Dose from Solar Flare Protons for1year at 1 AU5. Internal Charging Effects5.1 NASA-HDBK-4002 [4], Avoiding ProblemsCaused by Spacecraft On-Orbit Internal ChargingEffects describes internal charging, why it is ofconcern to spacecraft designers, general designguidelines, quantitative design guidelines an d a typicalmaterials characteristics list. Selected packaging-related excerpts from the handbook follow:The handbook applies to any spacecraft whose circularEarth orbit is described by the labeled regions ofFigure 2, as well as other energetic plasmaenvironments.

liM tm fB * 1300%C%cu2a?&n Orsr Aim& kmFigure 2 . Earth Regimes of Concern for On-OrbitInternal Charging Hazards for Spacecraft with CircularOrbitsThe distinction between surface charging andinternal charging is that internal charging is causedby energetic penetrating particles that can penetrateand deposit charge very close to a potentiallydamaging site. Surface charging is on areas that can beseen and touched on the outside of a spacecraft.

Surface discharges occur on or near the outer surfaceof a spacecraft and discharges must be coupled to aninterior site. Discharge energy from surface arcs isattenuated by the coupling factors and therefore is lessthreat to internal electronics.External wiring and antenna feeds are susceptible tothis threat. Internal charging, by contrast, may cause adischarge directly to a circuit pin or wire with verylittle attenuation. Geosynchronoudgeostationary(GEO ) orbit (a circular orbit in the equatorial plane ofthe Earth at -35,063 km altitude) is perhaps the mostcommon example of a region where spacecraft areaffected by internal electrostatic discharges, but thesame problem can occur at lower Earth altitudes, polarorbits and at Jupiter.Electrons and ions will penetrate matter. The depth ofpenetration o f a given species (electron, proton or otherion) depends on its energy, its atomic mass and thecomposition of the target material. Figure 3 shows thedepth of penetration versus energy of electrons andprotons into aluminum. Only particles with an energythat corresponds to a range greater than the spacecraftshield thickness can penetrate into the spacecraftinterior. If the material is not aluminum, an equivalentpenetration depth may be approximated by substitutingan equivalent material density.

I *3 2 >. *.I# *I*Figure 3. Electroflroton Penetration Depths inAluminum

Figure 4 illustrates the concept that energetic electrons(100 keV to 10 MeV) will penetrate into interiorportions of a spacecraft. Having penetrated, theelectrons may be stopped in dielectrics or on

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ungrounded conductors. If too many electronsaccumulate, the resultant high electric fields inside thespacecraft may cause an electrostatic discharge to anearby circuit.

Saturn

acts as a greenhouse and further heats the surface.Mercury rotates slowly and has a thin atmosphere.Consequently, night temperatures can be more than500C lower than the day temperature shown.

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Ungrounded Metal EnergeticAccumuIaIes Electrons. ElectronsMavCruse Dwc)Iar(le to Trace Pcnetrale She11I

Trace \

/ May Gauss DischargeAlummumShell $6 race

Figure 4. Illustration of the Internal Charging Process5.2 General Packaging Design Guidelines forMitigation of Charging Effects

Shield all electronic elements with sufficientaluminum equivalent thickness so that theinternal charging rate is benign.Ground all structural elements.Have a conductive path to the structure for allcircuitry.Limit usage of excellent dielectrics.Make all interior dielectrics electrically leaky.Ground radiation spot shields.Use low pass filters on interface circuits.Isolate the primary and secondary windings ofall transformers.Provide a conductive bleed path forsometimes forgotten conductors (includingstructural elements)Route cable harness away from apertures.Provide additional protection for externalcabling.Carry grounds across all articulated androtating joints.

6. Extreme Cold TemperatureIn general, the surface temperature of the planetsdecreases with increasing distance from the Sun (Table1) . Venus is an exception because its dense atmosphere

Mercu -173 to 427Venus -89 to 58-87 to -5

Mercu -173 to 427Venus -89 to 58-87 to -5

I Uranus I -216 ~ 1Neptune I -2 14Pluto I -233 to -223Table 1. Planet Surface Temperatures, "C

Thermal control systems on spacecraft can sometimesbe impractical, depending on the mission requirements.As the scaling of mechanical and electronic devicesdecreases, the ability to build smaller spacecraft andinstruments increases. This enables missions thatwould not otherwise be possible and also increases thepossibility for multiple spacecraft to increase coverageand mission reliability. Smaller spacecraft andinstruments imply smaller volume, surface area andthermal mass. This, in turn, can result in larger thermalexcursions for the spacecraft, but can also result in lesssolar radiation and hence, less energy available fromsolar panels and for heaters. For packaging, methodsfor minimizing heat loss between electronics andactuators or sensors should be developed. Thishighlights the need for electronics to operate and cyclein extreme colder environments.The selection of devices for extreme coldenvironments becomes particularly important.Resistors appear to operate well, but multilayercapacitors and inductors show variations withdecreasing temperatures. The performance of activedevices varies and supports the need for testing toefficiently characterize them. MOSFET s, for example,exhibit hot carrier injection degradation thatsignificantly reduces device lifetime, even as electricalparameters improve (channel resistance, propagationdelay decreases and transistor gain, switching speedincreases). Decreasing propagation delay increases thepossibility of hold time errors. Thus, appropriateadjustments in design or timing margins need to bemade for proper system performance.

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Some temperature inducedpackaging include:Cyclic temperatureeffects on electronics

variations induces lowcicle fatigue of circuit interconnects, boardvias and solder joints.Stresses due to coefficient of thermalexpansion (CTE) mismatches in circuitmaterials can result in cracking and failure ofcomponents and/or their attachments.Dimensional changes in optical systems dueto thermal material contraction and expansionof can affect the optical alignment anddegrade efficiencies.Cold temperature cycling can enhancewhisker growth in device terminations andpackage leads.Thermal Design Requirements are used to ensure thatthe packaged assembly will operate as intended overthe range of m ission environments seen during its life.

Design requirements usually include margin beyondthe intended use environment that account forvariations in the intended application and uncertaintiesin the predicted mission temperatures. Temperatureaffects most mechanical and electrical designs due totemperature-dependent material properties. Electronicsand their packaging must be designed to accommodatethese thermal effects to ensure their intended functionover the anticipated thermal excursions.7. ConclusionsAt the present time, commercial devices can be appliedsuccessfully in many space applications even thoughthey are susceptible to radiation damage. The key tosuccess is thorough testing and characterization ofdevices, as well as an understanding of the variouseffects. In addition, it is possible to implement systemarchitectures that can correct for damaging effects withthe use of shielding. Similarly, an understanding o f theeffects of spacecraft charging and extreme coldtemperature on spacecraft electronics and theirpackaging aids in developing procedures to mitigatethese effects.8. Bibliography

w-[3] JPL IOM1988.[4] NASA-HDBK-4002, Avoiding Problems Causedby Spacecraft On-Orbit Intemal Charging Effects,February 1999

[l] L.D. Edmonds, C.E. Barnes, L.Z. Scheick, AnIntroduction to Space Radiation Effects onMicroelectronics, May 200 0[2] Practice No. PD-ED-1260, Radiation DesignMargin Requirement, May 1996

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04-0507-Electronics Packaging Considerations for Space Applications