Failure Mechanisms in Electronic Products at High Altitudes

Transcription

1 Failure Mechaniss in Electronic Products at High Altitudes N. Blattau, and C. Hillan CALCE Electronic Products and Systes Center Building 89, Roo 1103 University of Maryland College Park, MD Tel.: (301) Fax: (301) Introduction Understanding the risks of using coercial off-theshelf (COTS) electronic coponents, specifically laptop coputers, in a non-coercial environent is critical to designers of ilitary systes. The environent studied in this paper is that of an unpressurized aircraft cockpit operating at 35,000 feet. During operation in this environent, COTS ay be exposed to low atospheric pressure and ay experience low teperatures and huidity. The Departent of Defense Design Criteria Standard for Huan Engineering, MIL-STD-1472F, provides guidance as to the required environent in unpressurized cockpit. At 35,000 feet, the atospheric pressure in the cockpit is expected to decrease to around 0.28 atospheres. In addition, the teperature will tend to range between 5 C and 20 C 1 and the relative huidity will be aintained to at least 45% 2. The assued environental paraeters used for this study are shown in Table 1. The typical laptop coputer will contain parts that ay be sensitive to high altitude failures, including: 1. Hard Drive 2. Batteries 3. Capacitors 4. Liquid Crystal Display 5. Meory 6. CPU Solid State Electronics (Meory and Microprocessors) The CPU eory and chipset ake up ost of the solid state electronics of the laptop. The chipset fors the bridge between the CPU and the various I/O devices of the coputer. Modern laptops have alost all functions such as video, sound, and interfaces integrated into the chipset. At the environent specified two conditions can adversely effect the operation of the solid state electronics. These are reduced air density and cosic rays. The reduction in air density can degrade the theral anageent syste used to cool the devices. Cosic rays can cause soft failures to occur in the solid state electronics. Teperature Altitude Pressure Huidity Absolute iniu: 5 C Noinal: 20 C feet axiu 0.25 atospheres 45 % RH Table 1: Assued environental conditions Soft fails 3 of electronics have been known to occur fro nuclear particles. The two priary sources for these particles are 1) decay fro radioactive atos and 2) extraterrestrial cosic rays. However, the density of cosic rays changes greatly with altitude and is therefore of great concern to electronics used at high altitudes. Once root-cause is positively identified, flex cracking, and a quantitative description of the influence of stress drivers has been presented, it is possible to ake an infored decision on possible corrective actions. Eventual selection of successful 1 Dependent if the aircraft is used in noinal or Arctic conditions. 2 To ensure huan cofort. This is surprising, considering that cities in the Aerican Southwest typically experience relative huidities in the single digits. 3 Failure is not peranent. Typical failure ode is data corruption or reboot of the operating syste

2 corrective actions will take into consideration effectiveness, tie, cost and outcoe on functionality. Cosic Rays IBM has conducted extensive studies detailing cosic ray intensities and soft error rates (SER) for digital electronics. The diagra in Figure 1 details how cosic rays cascade through the earth s atosphere. High energy particles hit the earth s atosphere shattering atospheric atos causing a cascade of atoic particles. The initial particle flux striking the earth s atosphere contains about 1000 particles/ 2 s 1. This increases to 1,000,000 particles/ 2 s -1 at airplane altitudes (40,000 ft). At lower altitudes ore particles are absorbed by the denser air until the particle flux is reduced to 1 particles/c 2 s -1 at sea level [1]. Experients conducted by IBM and others have shown that the failure rate due to cosic rays is about 100 ties greater for electronics at airplane altitudes than those at terrestrial altitudes [1]. At space altitudes soft failures have also been docuented. For exaple, IBM ThinkPad coputers on the MIR Space station (1,200,000 ft) have shown upsets every nine hours while other laptop coputers on the Space Shuttle (600,000 ft), have shown upset rates of one per hour [2]. Using the graph in Figure 2, and taking 0 g/c2 pressure as space it is easy to see that the neutron flux is higher at 35,000 feet; which eans that the likelihood of a soft failure could be higher at airplane altitudes. Figure 2: Neutron flux vs. atospheric pressure [1] Figure 1: Cosic rays cascading through the atosphere, n neutrons, p protons, e electrons (positron pair), µ uons, γ- gaa ray, π pion, n neutrino [1] Of all the particles present in the cascade, only neutrons and pions can cause significant soft failures in digital devices [1]. However, the ean lifetie of a pion is 26ns and therefore does not exist long enough to ipact the electronic device and cause failure. The ean lifetie of a neutron is stable and neutron ipacts are known to cause soft failure of electronics. The graph in Figure 2 shows the energy and neutron flux as a function of atospheric pressure. Atospheric pressure can be converted to altitude by using the forula in Equation 1. An altitude of 35,000 ft is therefore roughly equivalent to a pressure of 278 g/c ( H ) + ( ) A = 1033 H Equation 1: Altitude (feet) to pressure (g/c 2 ) [1] Failure Behavior CPU s and eory chips store inforation as sall packets of electrical charge. A soft failure or single event upset (SEU) occurs when electronic noise causes the electrical charge storing the inforation to change. This electronic noise coes fro various sources such as lightning and noisy power supplies. Manufacturers have tried to design their electronics to be iune fro ost sources. However, not all sources of electronic noise can econoically shielded against. Therefore, cosic rays are still a significant source of electronic noise at high altitudes. Most copanies that design high altitude electronic systes use redundant designs or radiation hardened

3 coponents to reduce the occurrences of soft failures or single event upsets (SEU s). Copanies then test the devices under neutron bobardent to ensure that occurrences of SEU s are liited to acceptable levels. The use of COT s devices precludes the use of radiation hardened coponents and custo redundant designs. There are steps that can itigate soe of the SEU s that will occur at high altitude. The use of errorcorrection code (ECC) eory in the laptop can identify and repair eory that has been corrupted by a SEU. The ability of the laptop to support the use of ECC eory is dependent on, it s support by the chipset and if the laptop anufacturer has configured the ECC eory support option. The ain eory chips are not the only eory that is susceptible to soft failures. There is eory built into the CPU, such as cache that can also be affected. There are no viable itigation techniques for reducing the chance of soft failures occurring in these eories. Theral Manageent Theral anageent in laptops is priarily through a syste of heat-pipes with a fan for forced air cooling as shown in Figure 3. power that can be dissipated. As shown in the following equation, the ass flow rate of air over the heatsink is directly proportional to the density of the air. 3 = K ρnd K : constant for geoetric and dynaic siiliar operation : ass flow rate (kg/ or lb /) N : fan speed (RPM) D : fan diaeter ( or ft) 3 ρ : air density (kg/ or lb /ft 3 The following exaple illustrates the effects of reduced air density on convection cooling. Siple CPU teperature rise exaple 4 : Altitude 0 feet feet Air density (ρ) kg/ kg/ 3 Air teperature (T) 30 C 20 C Theral conductivity (k) W/ K W/ K Absolute viscosity (µ) x x 10-6 N s/ 2 N s/ 2 Table 2: Properties of air at altitude ) Fan CPU Assuptions for saple calculations: Power dissipation for a typical obile CPU: 5 Watts Size: Length (L) = 51, Height = 3 Surface area = x Air velocity (U) = 2.5 /s The Reynolds nuber equation: Heat Pipes Figure 3: Laptop cooling syste The priary effect of altitude on theral anageent is the reduced efficiency of convection based heatsinking schees. Because of this reduced cooling efficiency, the operating teperature of the CPU will increase for the sae abient air teperature. The decrease in air density as the altitude increases is responsible for this reduced cooling efficiency and ust be accounted for when doing theral calculations. The lower air density reduces the ass flow of air over heat sinks, reducing the aount of ρul Re L = µ Nusselt nuber deterination: Using the Jacob correlation for a square shape we have B=0.160 and n=0.699 Nu D = BRe n D The heat transfer coefficients fro the average Nusselt nuber are: h c Nu k = L 4 Resburg, Ralph. Advanced Theral Design of Electronic Equipent. Chapan and Hall 1998

4 The teperature rise is found by: q T = h A c Altitude 0 feet feet Re L Nu D h c W/ 2 K W/ 2 K T 35.5 K 84.6 K Table 3: Theral calculation results The forulas and the assued environents yield the teperature changes shown in Table 3. This siple exaple illustrates that the CPU teperature will be 35.5 C above abient at sea level and 71.5 C above abient at feet. The teperature of the abient air in the aircraft at feet surrounding the laptop coputer will ost likely be 20 C or less so the CPU teperature will be around C. A noinal outside ground level teperature of 30 C (86 F) yields a CPU teperature of 65.5 C. The feet teperature value is outside the teperature specifications of 100 C for ost obile CPUs (Intel) as shown in Table 4. While this siple exaple doesn t accurately reflect the actual cooling syste of a laptop, it does indicate that a potential for inadequate cooling of the CPU could exist. Failure Behavior Most obile CPU s include a theral diode that onitors the teperature of the die. When the teperature reaches a preset liit, the CPU ay invoke schees to reduce its power dissipation in an attept to lower the teperature. This could include reducing voltages, frequencies or shutting down. These schees can adversely effect the operation of the laptop and cause software anoalies or spontaneous rebooting. CPU Mobile Pentiu 4 Processor-M 1.2 Mobile Pentiu 4 Processor-M 2.0 Mobile Pentiu 4 Processor-M 2.6 Mobile Pentiu III Processor-M 400 MHz Mobile Pentiu III Processor-M 866 MHz Mobile Pentiu III Processor-M Mobile AMD Duron 1.0 Crusoe TM5500- Maxiu Specified Die Teperature Theral Design Power 100 C 20.8 Watts 100 C 32 Watts 100 C 35 Watts 100 C 3.8 Watts 100 C 10.1 Watts 100 C 22.0 Watts 95 C 25 Watts 100 C 8.0 Watts Table 4: Specified axiu teperature and power dissipation for CPUs The CPU teperature at altitude could be reduced by using a different fan. A replaceent fan that is designed to run at higher rotation speeds could be used to copensate for decreased air density. Other theral solutions include external fans or cooling plates upon which the laptop sits. Figure 4 shows exaples of two different cooling plates that utilize passive and active designs.

5 anufacturers indicate that there should be no proble when using electrolytic capacitors below eters (32800 feet) [Nichicon Corporation]. Another anufacture [EPCOS] specifies an altitude liit of 7000 eters (23000 feet) for standard electrolytic capacitors and offers special capacitors rated for altitudes above 7000 eters. through replaceent is not necessarily recoended as handling the laptop during rework could result in ore failures then it solves. Instead, if PSI is concerned about this failure ode, contact the laptop anufacturer and request inforation on the electrolytic capacitor anufacturers and contact the anufacturers directly. Liquid Crystal Display Figure 4: Passive and active notebook cooling pads. The passive cooling pad utilizes heat pipes, while the active cooling pad utilizes forced convection. The device shown in Figure 5 provides additional cooling by forcing air into the laptop via the PCMCIA slot. This ay not be applicable to all laptops. All these devices can provide additional cooling for less than $50.00 per laptop. Figure 5: PCMCIA fan for suppleental cooling Electrolytic Capacitors Laptops use electrolytic capacitors, but in significantly less nubers when copared to a typical desktop coputer. When capacitors are used at high altitudes, the atospheric pressure is less than the internal pressure of the capacitor and this ay cause the capacitor end seal to bulge. However, soe The liquid crystal display (LCD) is affected by teperature and air pressure changes. As the teperature decreases the liquid crystal becoes less viscous. This leads to an increase in the response tie of the display. At 0 C the response tie of the Super- Twisted Neatic (STN) LCD could be as long as 4 seconds. Thin fil transistor (TFT) LCD display response ties are not as greatly affected by low teperatures since each pixel is driven by an individual transistor. Low teperatures can also reduce the display contrast and shorten the life of the backlight. Teperature changes can also lead to coefficient of theral expansion (CTE) isatches in the LCD assebly. The teperatures in the cockpit are not expected to be below 20 C, so theral isatches and teperature related perforance degradation should not be an issue. Using the LCD beyond the anufactures specified altitude liits can cause failure through overstress, low pressure, or wearout, pressure cycling, echaniss. Low Pressure The feet altitude is well beyond the typical axiu altitude specification of feet for a LCD. At high altitudes the corresponding low air pressure can cause echanical stress in the seal. This stress ay cause icro cracks in the seal, which becoe a path for oisture ingress into the cell. Moisture ingress can cause voiding and other failures. Voids appear as black spots on a norally white display or vice versa. This is because there is no optical effect at the place of voids. Change in the static state of seal: Expansion and elasticity of the seal can create a voluetric change within the display. The seal expands through

6 environental stressors like pressure, teperature and huidity and its static state changes. Copensation for this voluetric change coes fro the foration of vacuu voids in the LC aterial. Pressure Cycling If subjected to repeated flights, the COTS laptop will experience changes in pressure during its lifetie. This pressure cycling ay cause cheical breakdown of the LC aterial. A color shift in the display background is often an indication of such a breakdown. Pressure cycling can also cause breakdown of the LCD seal [3, 5], which is the sae echaniss affected by low pressure. Liquid crystal displays are sealed and using the beyond their specified liits ay daage the LCD. Typical axiu operational altitude for a LCD is between and feet. Laptop LCD s rated at the altitudes specified ay not be obtainable. The laptop LCD should be cycled to pressures equivalent to the field environent and then inspected to verify operation and seal integrity. Batteries The battery type used in ost current laptops is lithiu ion. The use of lithiu ion batteries in consuer electronics on airplanes has led to soe incidents and is a serious safety concern. In response to this issue, the United Nations released a Recoendation on the Transport of Dangerous Goods (ST/SG/AC.10/11/Rev.3), which recoended that lithiu-ion batteries with a equivalent lithiu content of greater than 8 gras be capable of surviving six hours at feet in unpressurized conditions. This recoendation was by adopted by the International Civil Aviation Organization (ICAO), responsible for all aspects of international civil aviation, and the International Air Transport Association (IATA), a trade association representing airlines, in the for of the A45 special rule. Eight gras of equivalent lithiu content is approxiately equivalent to 96 W-hours. This is below the typical content of a lithiu-ion battery for coercial laptops, which tend to have a capacity of 20 to 40 W-hours. As a result, there is no guarantee of battery survivability at feet. Not all anufactures provide altitude operational specifications, but Aerican Power Conversion (APC) specifies a axiu operating altitude of feet for its IBM ThinkPad replaceent batteries. If the lithiu-ion battery is susceptible to high altitudes, the failure ode would ost likely be shorting of the cells followed by a potential fire or explosion. While the likelihood of this failure echanis ay be less than that of single event upsets or hard drive failures, the potential consequences are severe enough that PSI ay wish only consider those laptops with batteries rated to feet. The selection of the laptop based on the battery, rather than selecting an appropriate battery, is due to the constraint laptop anufacturers place on the availability of alternative sources of batteries. Hard Disk Drive The hard disk drive is a ajor coponent of the coputer syste and is known to be sensitive to atospheric pressure changes. The ajor parts of a hard drive are: Flying head/agnetic edia syste Servo positioning voice coil actuator Read/Write channel electronics Spindle and Spindle otor Servo control channel electronics Containation control syste Soe typical hard drive specifications are: Max operating altitude ft Max huidity 95 % Min huidity 5 % Max change 20%/hr Max tep 55 C Min tep 5 C Max change 20 deg C/hr Flying Head Several specific parts of the drive are sensitive to the high altitude environent. Disk drives are designed for environents which are relatively benign copared to the environent found aboard a ilitary aircraft. The un-pressurized airborne environent will exceed the disk drive specification. The concern is that while it takes a significant aount of tie for the huidity in the drive to reach the abient environent the pressure is allowed to reach equilibriu rather quickly. The change in air density can effect the operation of the drive. The air in the hard drives creates an air bearing as the disk spins on which the agnetic head is supported. The head flying height is deterined by this air

7 bearing, and is proportional to the rotational surface speed of the disk and the air teperature and pressure (air density). If the head flies below its design liit for prolonged periods, the head ay coe into contact with the disk and cause head crashes or excessive head/edia wear [2]. Newer head designs fly ore consistently (less variation in gap) over a range of pressures than older designs. This is a cobination of design iproveents in heads, suspensions, and edia vibration. Drive crashes (catastrophic head or disc daage) occur less frequently with every new generation. This iproveent in fly height control is necessary to eet arket deands for increased reliability with lower fly-heights. [Industry Source] Bearings and Electronics There are various bearing surfaces in the disk that require lubrication. Low pressure conditions in the disk drive ay cause outgassing of these lubricants changing their viscosity. This change can create functional probles and affect the perforance of the drive. In general, soe copanies design for operation to 10,000 ft and non-op to 40,000 ft and do not generally test a drive beyond the 10K liit. The issue is the head-disc interface. The air bearings can not support the head as the air thins. At higher altitudes, the head ay start to contact the disc ore frequently. This disrupts the head and causes decreased perforance or in the extree, lead to catastrophic daage of the head or disc surface. There is variability fro productto-product in the air bearing design that will affect the altitude argin. Soe products will run at 20K feet, but the overall failure rate ay suffer, beyond specification liit failure rate tests are rarely conducted. There ay also be an acceleration of soe failure echaniss if the drive were required to continuously operate at that altitude. The solution to any of the probles with hard disk drives is to eploy a sealed disk cartridge design. Replacing the hard drive with a solid state device will eliinate the altitude proble associated with hard disk drives. Discussion and Conclusion In conclusion, the failure echaniss that ay be of greatest concern are failure of the liquid crystal display (LCD) seal, battery leakage, and lack of sufficient air cushion for the hard drive. Initial tests by PSI indicate no iediate failure of the LCD at 35,000 feet. This should indicate liited issues during early use. The LCD seal will ost likely leak after repeated pressure cycles; however the nuber of cycles required for failure is unknown and could exceed the expected lifetie. While the probably of battery leakage is unknown, this should be seriously considered as a design liitation due to the potential for fire and explosion. References 1. J. F. Ziegler et al. IBM experients in soft fails in coputer electronics ( ). IBM 1. RES. DEVELOP. VOL. 40 NO. 1, JANUARY Kaseta, Robert G. Ruggedized Disk Drives for Coercial Airborne Coputer Systes 3. Revels A.R., Void Forations in Liquid Crystal Displays: a case study, Proceedings SPIE International Society of Optical Engineering, vol. 2462, pp Dishoungh T.J., Dube M., Pecht M., Wyler J., Failure Analysis of Liquid Crystal Display due to Indiu Tin Oxide Breakdown, Journal of Electronic Packaging, vol. 121, June 1999, pp Burnette, Keith T., "Preliinary Analysis of Potential Void Foration Mechaaiss in ARC-164 Radio Control Head LCDs", April 1994, SeClfion 4.2 (ref. Brunette) 6. Resburg, Ralph. Advanced Theral Design of Electronic Equipent. Chapan and Hall 1998 Flash eory devices are typically used to replace hard disk drives and can be specified to an operational altitude of feet. The typical cost per egabyte for a 1.2 gigabyte drive is about 4 dollars copared to 0.20 dollars for a standard notebook hard drive. Flash eory is also known to be resistant to soft failures due to cosic ray ipacts.