Electronics unlike mechanical devices are perceived to rarely wear out. The reliability of digital components is best described by three situations with the vast majority of failures occurring during early phases of operation often referred to as “burn in”. “Useful life” is the stage where a predictable but relatively low failure rate is realized and most noted failures are a result of external factors. The “end of life” phase is sort of a grey area with a significant number of component replacements due to advancements or enhancements incorporated on newer equipment rather than higher than normal failures.
Wear out can still occur and is not infrequent in components such as cathode ray tubes (CRT) used as displays in earlier electronic flight instrument systems (EFIS). Yes, the old electron beam projecting on a phosphorescent screen tends to lose its luminance over time and often fails to meet the design specification when the display is sent in for evaluation. In addition, Electromigration is the motion of atoms in response to electron wind and can lead to circuit failure through metal line resistance increase and possibly open circuits typically on metal traces on circuit cards that are subject to routine high current operations.
Many electronic devices used on aircraft including some flight management computers and passenger entertainment systems employ technology where an internal memory is kept alive by a small, usually well-concealed battery. CMOS (complementary metal-oxide-semiconductor) is a term frequently used to describe the small amount of memory on a computer motherboard that stores the basic input output settings (BIOS) and removal or loss of power to CMOS can result in clearing this memory and a reset of the BIOS. Some CMOS batteries will last the lifetime of a motherboard, possibly up to 10 years, but will sometimes need to be replaced earlier. Unfortunately, many manufacturers incorporating these memory backup batteries do not always alert equipment users of their presence and simply plan to replace as the equipment is sent to servicing facilities for repair or upgrade. Incorrect or slow system date and time and loss of BIOS settings are major signs of a dead or dying CMOS battery. It is a worthwhile endeavor to include inquiries about equipment incorporating backup batteries along with their estimated life expectancy anytime new devices are planned to be installed in the aircraft.
What to watch for:
There are a number of factors that can have a significant negative impact on the longevity of a digital circuit. Should the barrier protecting the component from the environment become compromised, outside factors such as humidity and oxygen can accelerate the aging of the component and cause it to fail much sooner. Solder joints provide the main means of contact between a component and a circuit and have their fair share of failures. Using the wrong type of solder with a component or PCB can lead to electromigration of the elements in the solder that form brittle layers called intermetallic layers. These layers lead to broken solder joints and often elude early detection. Thermal cycles are also a prime cause of solder joint failure, especially if the thermal expansion rates of the materials (component pin, solder, PCB trace coating, and PCB trace) are different. As all of these materials heat up and cool down, massive mechanical stress can form between them which can break the physical solder connection, damage the component, or delaminate the PCB trace. Tin splinters on lead-free solders can also be a problem as they tend to grow out of joints resulting in bridged contacts or shorts.
Heat is a mortal enemy of electronic circuits with high temperatures causing over 50 percent of electronic equipment failures, as reported in a study by the U.S. Air Force Avionics Integrity Program. The majority of heat-related component fatalities are not a result of temperatures occurring during normal day-to-day operations but in fact may, in part, be induced either by maintenance personnel or flight crews. Often, during maintenance events it is not uncommon for aircraft to remain powered for extended periods of time. Frequently cooling airflow may not be present as a result of the aircraft environmental systems not operating. Placement of cooling fans or opening of electronics bays can be a worthwhile practice to combat the effects of heat. In addition, managing which circuits need to be energized during power on conditions will result in non-required systems to remain dormant and therefore not producing additional heat or exposing the system to unnecessary hazard. It is essential to monitor the proper operation of cooling equipment while making sure any air filters are unobstructed along with free airflow in designated supply and exhaust ducts. Accumulation of dust and other foreign particles are another concern as they can form a thermal blanket resulting in inadequate heat dissipation.
The majority of semiconductor devices operate efficiently in a range of -40 C to +85 C. Low temperatures also impact electronics. Damage to an integrated circuit (IC) at low temperatures while unpowered would be due to mechanical effects; differences in thermal expansion coefficients between the epoxy and framework materials. Problems with operation would be due to increased resistance (semiconductors' temperature coefficient of resistance is negative). When the temperature and doping concentration is low enough, semiconductors will essentially become insulators and not conduct at all, causing unspecified operation. Some ICs will operate just fine at cryogenic temperatures but they must start up warm to allow voltage references to stabilize prior to system boot. Devices with sensitivity to cold weather such as liquid crystal displays (LCD) may incorporate internal thermal sensors and in some cases heaters to allow reasonable operating temperatures are obtained prior to energizing critical circuitry. This means if an aircraft with temperature protected LCDs is exposed to extreme cold weather for an extended time, the electronic flight deck displays may take some time after initial power up before providing any useful information. Unfortunately, this protection is often not mentioned in operating documents and the dark displays after power up often result in an assumption of failure.
Vibration and humidity are attributed to cause an additional 20 percent of failures. One interesting and pertinent fact: blower fans used to circulate air in avionics compartments, when mounted to the same racks as the electronic circuits, are a known contributor to accelerated circuit card failure as a result of vibrations. Such fans should frequently be checked for proper airflow as well as smooth operation.
As electronic devices became faster and smaller, their sensitivity to electrostatic discharge (ESD) increased. Today, ESD impacts productivity and product reliability in virtually every aspect of today's electronics environment. Estimated average electronics damage due to static discharge is reported to be up to 33 percent of total failures.
Several major avionics companies report that 25 percent of all identified electronic part failure is due to ESD. As ESD control programs improve, a resulting decrease in unidentified field failures and ”no fault found” returns should occur. Good practices to ensure ESD protection include the use of bonding straps when handling sensitive equipment as well as storage procedures utilizing proper ESD approved bags or containers.
There is no magic formula to predict when a digital circuit will fault but understanding the leading causes of component failure should help create and implement guidelines for ensuring the various hazards can be significantly reduced resulting in long trouble-free operation.
Jim Sparks has been maintaining aircraft for almost 40 years with the majority of the time involving Business Aviation activities. Jim’s endeavors have placed him on six of the seven continents contending with numerous situations from routine flight dispatch to critical AOGs. His career includes maintainer, avionics/electrician, educator, tech rep, and director of aircraft maintenance. In addition to other activities he is engaged with ASTM assisting in the global development of criteria defining the Next Tech for NEXTGEN. You can reach him at [email protected].