- Email: [email protected]
Early life field failures in modern automotive electronics – An overview; root causes and precautions
MR-12038; No of Pages 5 Microelectronics Reliability xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Early life field failures in modern automotive electronics – An overview; root causes and precautions P. Jacob Empa Swiss Federal Labs for Materials Testing and Research, Duebendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 21 June 2016 Accepted 6 July 2016 Available online xxxx Keywords: Automotive electronics reliability Early life failures in automotive electronics
a b s t r a c t Forensic failure analysis of automotive electronics deals in most cases with failures within the guarantee period. Frequently, specific operational conditions, even for a short moment, combine with specific electronic sensitivities – against ESD discharge, switching spikes, humidity ingress, vibration, undefined grounding circuitry. The paper lists impressive, partially curious examples of related failure anamnesis and analysis and tries to draw some important conclusions with respect to prevention and failure anamnesis/failure analysis methodologies. © 2016 Published by Elsevier Ltd.
1. Introduction This paper shows early life failures in automotive electronics appearing in the field and where the failed electronics worked perfectly at zero km in most cases. Only the unlucky combination of normal operation with superimposed – often short-time lasting – specific disturbing influence caused these failures. The paper highlights surprising failure mechanisms and root causes as well as useful precautions. 2. What changes, when the car leaves the manufacturing plant? Beginning from the moment of roll-out, the car is exposed to various environmental influences as dirt, dust, humidity ingress into seemingly hermetically sealed (electronic) devices, mechanical shocks and vibrations, corrosive gases (including outgasing from plastic components and rubber seals and tire rubber) and thermal cyclings, as well as environmental and climatic conditions. These influences generate effects on the electronics, which sometimes are not that obvious: In new cars, rubber hoses are clean and isolating. After a few thousand miles, they slightly collect surface dust, resulting in electrically dissipative surfaces. In modern cars, numerous electrical motors are mounted. Examples are windshield wipers, mirror and seat adjustment, and window lifts. These motors usually are equipped with carbon brushes to contact the commutator. New carbon brushes may generate high level inductive pulses until they are grinded to the collector shape. Contacts may suffer from vibration, corrosive gases and burned contact spots. E-mail address: [email protected].
Dendrite and/or corrosion in LEDs Memory loss by inductive spikes introduced into the car-internal power system. 3. Failure mechanisms and root causes 3.1. Engine sensors, throttles, air filters with sensors, flow sensors etc. (e.g. oil-/air flow-/petrol-) within rubber hoses These systems are often mounted or integrated into the rubber hose system within the engine compartment (Fig. 1). Some sensors have isolated metal pieces or caps; in case of a throttle, even a metal plate regulates the air flow. In addition, the sensors are sometimes mounted into a housing of metal. At high speed air-/gas flow, they are at risk of suffering sudden high electrostatic charging, following the principles of tribocharging [1], as long as no ground connect is applied. In such case, the discharge happens directly into the sensor, following through related cable wiring and finally damaging the control electronics. In consequence, reduced engine power, engine failure or motor control unit damage were observed. After repair, in most cases the failure never reoccurs, since surface dirt at the rubber hoses (after 5000– 10,000 km) makes them slightly electrostatic dissipative. If the only chargeable metal part is the housing, a simple copper ground strap avoids the problem from the very beginning. If internal metal parts (as for instance a sensor metal cap or a throttle) are isolated, it becomes slightly more complicated (Fig. 2). In such case, it needs to be avoided that metal pieces, exposed to the gas flow, start to collect charge and such gain voltage. If the voltage has grown enough, reaching the kVorder-of-magnitude, an electric strike discharges the collected energy into the sensor system as described above. Of course, the simplest way is to connect such parts directly to ground, too. If, however, the metal part is a part of the sensor and may not be connected to ground level,
http://dx.doi.org/10.1016/j.microrel.2016.07.015 0026-2714/© 2016 Published by Elsevier Ltd.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
2
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
Fig. 1. Electrostatic voltage measurement of an isolated metal section with electronics between rubber pipes in a car.
a dissipative characteristic would be sufficient to suppress electrostatic charging. This can be either achieved by using high-ohmic resistor connects to ground (b1000 MΩ) or, alternatively by using capacitors N 1 nF; the current, which the capacitor would need to become charged, is in most cases too high in relation to the charge separation rate, so that no high voltage can build up. 3.2. Electronic PCBs Due to vibration resonance frequencies, PCBs are sometimes exposed to extraordinary vibrations or bending, causing ceramic capacitor or -filter cracking (Fig. 3) [2]. Thereafter, promoted by humidity and electric potential, dendrite growth along the internal cracks generates time-delayed shorts or capacitor leakage. The root cause for such problems can be divided into three main issues: The first one is the separation of assembled PCBs from the panel (depanelization) in combination with hurting the ceramic capacitor placement rules. The second point is PCB bending during the mounting. As an example, Fig. 4 shows the pressed fixing of a small PCB into a car door grip. Superimposed to the bending from mounting, this PCB suffers severe mechanical shocks, when the door is closed in a hard manner. Third, when mounting PCBs into subsystems of the car, supporting points need to be considered to suppress vibration-frequency-dependent bending by mechanical oscillations or bending by mechanical pressure as for instance push-button switches (car keys!) or plug connectors. Besides the risk of ceramic capacitor cracks, also interconnects between PCB and silicon devices are at risk: Fig. 5 shows solder cracks (also at risk: solderball cracks in case of flipped devices), caused by mechanical vibration. Intermitting contact points like these bumps, however, may also occur within the semiconductor chip. Especially with the increasing miniaturization, the overlap between molding and pin rows became very small in many devices. In consequence, these devices are very sensitive against (thermo-)mechanical stress and bending. If – by reason of lead-free soldering- high solder temperature profiles are applied, the mold compound may suffer a small reconstruction at the pin borders,
Fig. 2. Sensor, mounted in a rubber hose (B), A = metal housing, C = metal sensor cap, D = sensor housing, E = sensor; F = terminal connector; left: sensor cap isolated, center: sensor housing at ground, cap still isolated; right: dissipative mounting with high-ohmic resistors. Yellow: potential spark risks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Bending-stress-induced cracking of a ceramic capacitor. Often the cracks are hidden under the solder caps, which need to be removed chemically.
ending up in a small micro-delamination to the pin. This allows slightly moving of pins, while bond wires are still fixed by the inner mold compound. The resulting failure signature shows up in stitch-bond cracking. Additional process sources of stitch-bond cracking are wire bonding (stitch bond geometry), molding (pressure/anneal), frame punching (cracking the mold), pin bending (mechanical overstress), soldering (temperature too high) and mechanical bending on PCBs. Corner pins are at preferred risk. Beyond these micromechanical problems, sometimes the electric grounds of subsystem PCBs are not identical to the car ground. Together with in-/outgoing wiring/cable trees, such design supports non-defined ground potentials, which may vary in their electric potentials depending from the actual load, even reaching critical potential differences. This risk will increase by future on-board power design using different batteries/potentials. 3.3. Connectors A non-negligible part of early life failures result from plug connectors. It would, however, oversize this paper to discuss in detail all possible failure mechanisms of connectors; some keywords as fretting, crimping problems, surface metallization setup, anodic corrosion, and dimensioning of connectors with respect to their current and voltage load should highlight the directions of failure mechanisms. An ESREF tutorial will deal in detail with connector problems [3]. However, considering the plug design, it is important to make sure wherever it is possible, that the ground connects of any kind of component should give the first contact in order to avoid EDS-discharging through “hot”
Fig. 4. Car door grip with a PCB mounted inside, suffering mechanical bending.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
3
Fig. 5. Solder crack of a component on PCB, causing intermitting contact.
and sensitive signal terminals. ESD impacts into subsystems frequently do not cause immediate failures but physically prepare leakage paths and cracks, which can be filled by aluminium protrusion and/or galvanic-supported effects (as for instance dendrite growth) and finally result in early life failures. 3.4. LED headlamps and luminary Humidity protection/sealing of LEDs sometimes do not match the operational environment in cars. As a result, humidity ingress in combination with electric potential (even in off-state) causes LED-internal dendrite growth (Fig. 6) which results at first in reduced emission, later in a total failure. Outgasing of aggressive chemistry from rubber seals may generate severe corrosion [4]. Sulphur is included in many rubber types and, if outgased, it may react with humidity to H2S (gas) or H2SO3 and H2SO4. Pulsed operation may superimpose additional stress, reducing LED lifetime significantly. 3.5. Locking systems/keys/RFIDs
Fig. 6. Dendrite growth within a LED, causing parallel shorts with time-dependent, decreasing resistance. Humidity ingress in combination with electric potential and high temperature significantly promotes this mechanism.
carbon brushes are new and ending planar (not grinded to the commutator's curvature), they can send significant spikes into the onboard electrical system, if the spikes are not suppressed by capacitors to ground (Fig. 8) - thus causing damage in electronics. The use of buffer capacitors directly at the commutator or using brushless electric motors avoids related failures. Inductive spikes can also be introduced by cross-talk. This applies especially where long cable trees are mounted neighboured and unshielded to cable lines carrying highly inductive loads. A typical example is shown in Fig. 9, where the starter switch is connected directly to the related magnetic relay coil – with the cables partially running within the same cable tree by which sensitive electronics has been connected, too.
Advanced packaging technologies sometimes do not withstand extraordinary thermomechanical stress, superimposed by vibration load. Bump-, TSV- or solder ball-opens or stitchbond cracking (Fig. 7) are results of such superimposition in combination with extreme miniaturization requirements. If the packaging miniaturization is more relaxed, the package may contribute to a better mechanical stability with respect to interconnects and wire bonding. This of course applies also to other electronics within the car. 3.6. ESD by rotating parts or liquid dusting Especially Hall sensors used for instance in engines or gearing mechanisms are sensitive against charging and suffer sudden charging/ discharging failures. It needs to be taken into account, that (even electrically conductive) liquids may generate charging when suffering dusting (for instance oil in a gear box). 3.7. Electric motors, sending spikes at early life into electronics, which may suffer damage DC-servo motors are used in numerous applications in a car (window lifts, pumps, seat and mirror adjustments etc.). As long as their
Fig. 7. Stitchbond crack of a device on a car key PCB. The delamination line between pin and mold compound can be seen clearly (yellow arrow). Micro-movement of the pin caused the stitchbond crack (blue arrow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
4
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
3.9. Mechanical issues Sometimes electronic failures have been caused by punching burrs, which peeled away from perforated steel panels as it can be seen in Fig. 10. If such a fine punching burr falls into open electronics, it may cause electric shorts. Since the risk of peeling off is given by vibration and mechanical shock, this mechanism applies at an early stage of life of the car. 4. Conclusion
Fig. 8. New carbon brushes at a motor commutator (top image): flat contact, generating spikes; after some time of use (centre image), the brushes are adapted to the commutator shape, resulting in a reduction of spikes; bottom images: simple spike prevention by a small capacitor.
3.8. Tire pressure sensors Tires may generate outgasing of aggressive sulphur-based chemistry, thus generating damage to internal (battery-powered) pressure sensors and electronics in case of insufficient encapsulation protection. Sulphur outgasing may also happen from rubber seals (for instance in headlights or rear-lights, causing corrosion within LEDs by humidity/ temperature-induced sulphuric acid or H2S-formation). It should also be mentioned that RF interference may cause damages to sensitive internal signal receivers, which often use so-called ISM frequencies for transmitting survey data. These frequencies are placed as an “island” into other, partially high-powered-use frequency bands. For instance, the ISM frequency band of 433 MHz is placed in the mid of the 70 cm UHF amateur radio band, reaching from 430–440 Mhz. In this band, mobile receivers with an RF output of up to 100 W output power are in operation. Neighboured to the amateur radio, other users as sports, infrastructural networks, transport logistics, police are active. Worst case, a car sensor receiver could be destroyed in case of being exposed to a nearby strong UHF transmitter. In order to avoid related failures, it should be considered to install mobile communications of any kind already in the car before its rollout, following the car manufacturer's recommendations and prescriptions.
Fig. 9. Wiring near the steering wheel: high current lines (here to engine starter relay) neighboured to sensitive control lines of an electronics-PCB may provide unexpected cross talk into sensitive electronics.
When scanning all failure mechanisms on common issues, the following highlights can be concluded. First, it is very important to avoid electrical spiking within the onboard-electrical system. This can be achieved by a common ground rail (avoid capacitance sub-system earthing) and by filtering the supply lines from noise- and spike-generators as inductive loads, electrical motors. Second, considering environmental conditions, a car suffers an extreme range of climatic changes, depending from the area of operation and the local season climate. Sometimes, failure mechanisms depend on such local climate conditions. Also even local speed limits or other legal restrictions may impact the failure rates. It is important to consider these local aspects in a careful system (car)-based anamnesis including an evaluation of failure statistics and not just rely on device analysis results. Third, failures may be caused by superimposed vibrational stress (caused by the operation of the car and sometimes also the road conditions). It may become difficult to forecast, model and simulate all of these superimposed vibration and shock impacts. Fourth, outgasing of aggressive chemistry from rubber and plastic parts may provide long-term attacks to electronic systems and components. All of these circumstances need to be considered already at pre-qualification level. In addition, specific requirements apply to temperatures/ chemistry within the engine section. Electrostatic charging within the engine section is still rather unexplored but several case studies have clearly shown that this may become a severe failure issue under specific operational conditions. Last, for the failure analyst, it can be concluded that a device-focused failure analysis will hardly lead towards root-cause conclusions. Only about 10% of car failures related to electronics are caused by semiconductor manufacturers, even if in most cases, semiconductor devices failed. In most cases, a sound failure anamnesis is mandatory for root
Fig. 10. Punching burrs along a perforated steel border. Some vibration disconnects them and they will fall down – following Murphy's law onto the open electronics.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
cause findings. In this sense, 8-D-procedures need to be revised and extended towards an integral view throughout the whole supply chain and also including country- and client-specific environmental and operational conditions. References [1] S.C. Pjesky, Aerodynamic, infrared extinction and tribocharging properties of nanostructured and conventional particles(PhD work) Kansas State University, Manhattan, Kansas, 2008 (UMI 3313379).
5
[2] G. Vogel, Avoiding flex cracks in ceramic capacitors: analytical tool for a reliable failure analysis and guideline for positioning cercaps on PCBs, Microelectronics Reliability, vol. 55, Issues 9-10, August-September 2015, pp. 2159–2164, http://dx.doi.org/ 10.1016/j.microrel.2015.06.034 (proceedings of ESREF 2015). [3] Jacob, P., Failure mechanisms and precautions in plug connectors and relays, ESREF Tutorial 2016, t.b. published in Microelectronics Reliability (2016) Special Issuer ESREF 2016, Halle. [4] Vogel, G. Creeping Corrosion of Copper on Printed Circuit Board Assemblies, ESREF Tutorial 2016, t.b. published in Microelectronics Reliability (2016) Special Issue ESREF 2016, Halle.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Early life field failures in modern automotive electronics – An overview; root causes and precautions P. Jacob Empa Swiss Federal Labs for Materials Testing and Research, Duebendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 21 June 2016 Accepted 6 July 2016 Available online xxxx Keywords: Automotive electronics reliability Early life failures in automotive electronics
a b s t r a c t Forensic failure analysis of automotive electronics deals in most cases with failures within the guarantee period. Frequently, specific operational conditions, even for a short moment, combine with specific electronic sensitivities – against ESD discharge, switching spikes, humidity ingress, vibration, undefined grounding circuitry. The paper lists impressive, partially curious examples of related failure anamnesis and analysis and tries to draw some important conclusions with respect to prevention and failure anamnesis/failure analysis methodologies. © 2016 Published by Elsevier Ltd.
1. Introduction This paper shows early life failures in automotive electronics appearing in the field and where the failed electronics worked perfectly at zero km in most cases. Only the unlucky combination of normal operation with superimposed – often short-time lasting – specific disturbing influence caused these failures. The paper highlights surprising failure mechanisms and root causes as well as useful precautions. 2. What changes, when the car leaves the manufacturing plant? Beginning from the moment of roll-out, the car is exposed to various environmental influences as dirt, dust, humidity ingress into seemingly hermetically sealed (electronic) devices, mechanical shocks and vibrations, corrosive gases (including outgasing from plastic components and rubber seals and tire rubber) and thermal cyclings, as well as environmental and climatic conditions. These influences generate effects on the electronics, which sometimes are not that obvious: In new cars, rubber hoses are clean and isolating. After a few thousand miles, they slightly collect surface dust, resulting in electrically dissipative surfaces. In modern cars, numerous electrical motors are mounted. Examples are windshield wipers, mirror and seat adjustment, and window lifts. These motors usually are equipped with carbon brushes to contact the commutator. New carbon brushes may generate high level inductive pulses until they are grinded to the collector shape. Contacts may suffer from vibration, corrosive gases and burned contact spots. E-mail address: [email protected].
Dendrite and/or corrosion in LEDs Memory loss by inductive spikes introduced into the car-internal power system. 3. Failure mechanisms and root causes 3.1. Engine sensors, throttles, air filters with sensors, flow sensors etc. (e.g. oil-/air flow-/petrol-) within rubber hoses These systems are often mounted or integrated into the rubber hose system within the engine compartment (Fig. 1). Some sensors have isolated metal pieces or caps; in case of a throttle, even a metal plate regulates the air flow. In addition, the sensors are sometimes mounted into a housing of metal. At high speed air-/gas flow, they are at risk of suffering sudden high electrostatic charging, following the principles of tribocharging [1], as long as no ground connect is applied. In such case, the discharge happens directly into the sensor, following through related cable wiring and finally damaging the control electronics. In consequence, reduced engine power, engine failure or motor control unit damage were observed. After repair, in most cases the failure never reoccurs, since surface dirt at the rubber hoses (after 5000– 10,000 km) makes them slightly electrostatic dissipative. If the only chargeable metal part is the housing, a simple copper ground strap avoids the problem from the very beginning. If internal metal parts (as for instance a sensor metal cap or a throttle) are isolated, it becomes slightly more complicated (Fig. 2). In such case, it needs to be avoided that metal pieces, exposed to the gas flow, start to collect charge and such gain voltage. If the voltage has grown enough, reaching the kVorder-of-magnitude, an electric strike discharges the collected energy into the sensor system as described above. Of course, the simplest way is to connect such parts directly to ground, too. If, however, the metal part is a part of the sensor and may not be connected to ground level,
http://dx.doi.org/10.1016/j.microrel.2016.07.015 0026-2714/© 2016 Published by Elsevier Ltd.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
2
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
Fig. 1. Electrostatic voltage measurement of an isolated metal section with electronics between rubber pipes in a car.
a dissipative characteristic would be sufficient to suppress electrostatic charging. This can be either achieved by using high-ohmic resistor connects to ground (b1000 MΩ) or, alternatively by using capacitors N 1 nF; the current, which the capacitor would need to become charged, is in most cases too high in relation to the charge separation rate, so that no high voltage can build up. 3.2. Electronic PCBs Due to vibration resonance frequencies, PCBs are sometimes exposed to extraordinary vibrations or bending, causing ceramic capacitor or -filter cracking (Fig. 3) [2]. Thereafter, promoted by humidity and electric potential, dendrite growth along the internal cracks generates time-delayed shorts or capacitor leakage. The root cause for such problems can be divided into three main issues: The first one is the separation of assembled PCBs from the panel (depanelization) in combination with hurting the ceramic capacitor placement rules. The second point is PCB bending during the mounting. As an example, Fig. 4 shows the pressed fixing of a small PCB into a car door grip. Superimposed to the bending from mounting, this PCB suffers severe mechanical shocks, when the door is closed in a hard manner. Third, when mounting PCBs into subsystems of the car, supporting points need to be considered to suppress vibration-frequency-dependent bending by mechanical oscillations or bending by mechanical pressure as for instance push-button switches (car keys!) or plug connectors. Besides the risk of ceramic capacitor cracks, also interconnects between PCB and silicon devices are at risk: Fig. 5 shows solder cracks (also at risk: solderball cracks in case of flipped devices), caused by mechanical vibration. Intermitting contact points like these bumps, however, may also occur within the semiconductor chip. Especially with the increasing miniaturization, the overlap between molding and pin rows became very small in many devices. In consequence, these devices are very sensitive against (thermo-)mechanical stress and bending. If – by reason of lead-free soldering- high solder temperature profiles are applied, the mold compound may suffer a small reconstruction at the pin borders,
Fig. 2. Sensor, mounted in a rubber hose (B), A = metal housing, C = metal sensor cap, D = sensor housing, E = sensor; F = terminal connector; left: sensor cap isolated, center: sensor housing at ground, cap still isolated; right: dissipative mounting with high-ohmic resistors. Yellow: potential spark risks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Bending-stress-induced cracking of a ceramic capacitor. Often the cracks are hidden under the solder caps, which need to be removed chemically.
ending up in a small micro-delamination to the pin. This allows slightly moving of pins, while bond wires are still fixed by the inner mold compound. The resulting failure signature shows up in stitch-bond cracking. Additional process sources of stitch-bond cracking are wire bonding (stitch bond geometry), molding (pressure/anneal), frame punching (cracking the mold), pin bending (mechanical overstress), soldering (temperature too high) and mechanical bending on PCBs. Corner pins are at preferred risk. Beyond these micromechanical problems, sometimes the electric grounds of subsystem PCBs are not identical to the car ground. Together with in-/outgoing wiring/cable trees, such design supports non-defined ground potentials, which may vary in their electric potentials depending from the actual load, even reaching critical potential differences. This risk will increase by future on-board power design using different batteries/potentials. 3.3. Connectors A non-negligible part of early life failures result from plug connectors. It would, however, oversize this paper to discuss in detail all possible failure mechanisms of connectors; some keywords as fretting, crimping problems, surface metallization setup, anodic corrosion, and dimensioning of connectors with respect to their current and voltage load should highlight the directions of failure mechanisms. An ESREF tutorial will deal in detail with connector problems [3]. However, considering the plug design, it is important to make sure wherever it is possible, that the ground connects of any kind of component should give the first contact in order to avoid EDS-discharging through “hot”
Fig. 4. Car door grip with a PCB mounted inside, suffering mechanical bending.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
3
Fig. 5. Solder crack of a component on PCB, causing intermitting contact.
and sensitive signal terminals. ESD impacts into subsystems frequently do not cause immediate failures but physically prepare leakage paths and cracks, which can be filled by aluminium protrusion and/or galvanic-supported effects (as for instance dendrite growth) and finally result in early life failures. 3.4. LED headlamps and luminary Humidity protection/sealing of LEDs sometimes do not match the operational environment in cars. As a result, humidity ingress in combination with electric potential (even in off-state) causes LED-internal dendrite growth (Fig. 6) which results at first in reduced emission, later in a total failure. Outgasing of aggressive chemistry from rubber seals may generate severe corrosion [4]. Sulphur is included in many rubber types and, if outgased, it may react with humidity to H2S (gas) or H2SO3 and H2SO4. Pulsed operation may superimpose additional stress, reducing LED lifetime significantly. 3.5. Locking systems/keys/RFIDs
Fig. 6. Dendrite growth within a LED, causing parallel shorts with time-dependent, decreasing resistance. Humidity ingress in combination with electric potential and high temperature significantly promotes this mechanism.
carbon brushes are new and ending planar (not grinded to the commutator's curvature), they can send significant spikes into the onboard electrical system, if the spikes are not suppressed by capacitors to ground (Fig. 8) - thus causing damage in electronics. The use of buffer capacitors directly at the commutator or using brushless electric motors avoids related failures. Inductive spikes can also be introduced by cross-talk. This applies especially where long cable trees are mounted neighboured and unshielded to cable lines carrying highly inductive loads. A typical example is shown in Fig. 9, where the starter switch is connected directly to the related magnetic relay coil – with the cables partially running within the same cable tree by which sensitive electronics has been connected, too.
Advanced packaging technologies sometimes do not withstand extraordinary thermomechanical stress, superimposed by vibration load. Bump-, TSV- or solder ball-opens or stitchbond cracking (Fig. 7) are results of such superimposition in combination with extreme miniaturization requirements. If the packaging miniaturization is more relaxed, the package may contribute to a better mechanical stability with respect to interconnects and wire bonding. This of course applies also to other electronics within the car. 3.6. ESD by rotating parts or liquid dusting Especially Hall sensors used for instance in engines or gearing mechanisms are sensitive against charging and suffer sudden charging/ discharging failures. It needs to be taken into account, that (even electrically conductive) liquids may generate charging when suffering dusting (for instance oil in a gear box). 3.7. Electric motors, sending spikes at early life into electronics, which may suffer damage DC-servo motors are used in numerous applications in a car (window lifts, pumps, seat and mirror adjustments etc.). As long as their
Fig. 7. Stitchbond crack of a device on a car key PCB. The delamination line between pin and mold compound can be seen clearly (yellow arrow). Micro-movement of the pin caused the stitchbond crack (blue arrow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
4
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
3.9. Mechanical issues Sometimes electronic failures have been caused by punching burrs, which peeled away from perforated steel panels as it can be seen in Fig. 10. If such a fine punching burr falls into open electronics, it may cause electric shorts. Since the risk of peeling off is given by vibration and mechanical shock, this mechanism applies at an early stage of life of the car. 4. Conclusion
Fig. 8. New carbon brushes at a motor commutator (top image): flat contact, generating spikes; after some time of use (centre image), the brushes are adapted to the commutator shape, resulting in a reduction of spikes; bottom images: simple spike prevention by a small capacitor.
3.8. Tire pressure sensors Tires may generate outgasing of aggressive sulphur-based chemistry, thus generating damage to internal (battery-powered) pressure sensors and electronics in case of insufficient encapsulation protection. Sulphur outgasing may also happen from rubber seals (for instance in headlights or rear-lights, causing corrosion within LEDs by humidity/ temperature-induced sulphuric acid or H2S-formation). It should also be mentioned that RF interference may cause damages to sensitive internal signal receivers, which often use so-called ISM frequencies for transmitting survey data. These frequencies are placed as an “island” into other, partially high-powered-use frequency bands. For instance, the ISM frequency band of 433 MHz is placed in the mid of the 70 cm UHF amateur radio band, reaching from 430–440 Mhz. In this band, mobile receivers with an RF output of up to 100 W output power are in operation. Neighboured to the amateur radio, other users as sports, infrastructural networks, transport logistics, police are active. Worst case, a car sensor receiver could be destroyed in case of being exposed to a nearby strong UHF transmitter. In order to avoid related failures, it should be considered to install mobile communications of any kind already in the car before its rollout, following the car manufacturer's recommendations and prescriptions.
Fig. 9. Wiring near the steering wheel: high current lines (here to engine starter relay) neighboured to sensitive control lines of an electronics-PCB may provide unexpected cross talk into sensitive electronics.
When scanning all failure mechanisms on common issues, the following highlights can be concluded. First, it is very important to avoid electrical spiking within the onboard-electrical system. This can be achieved by a common ground rail (avoid capacitance sub-system earthing) and by filtering the supply lines from noise- and spike-generators as inductive loads, electrical motors. Second, considering environmental conditions, a car suffers an extreme range of climatic changes, depending from the area of operation and the local season climate. Sometimes, failure mechanisms depend on such local climate conditions. Also even local speed limits or other legal restrictions may impact the failure rates. It is important to consider these local aspects in a careful system (car)-based anamnesis including an evaluation of failure statistics and not just rely on device analysis results. Third, failures may be caused by superimposed vibrational stress (caused by the operation of the car and sometimes also the road conditions). It may become difficult to forecast, model and simulate all of these superimposed vibration and shock impacts. Fourth, outgasing of aggressive chemistry from rubber and plastic parts may provide long-term attacks to electronic systems and components. All of these circumstances need to be considered already at pre-qualification level. In addition, specific requirements apply to temperatures/ chemistry within the engine section. Electrostatic charging within the engine section is still rather unexplored but several case studies have clearly shown that this may become a severe failure issue under specific operational conditions. Last, for the failure analyst, it can be concluded that a device-focused failure analysis will hardly lead towards root-cause conclusions. Only about 10% of car failures related to electronics are caused by semiconductor manufacturers, even if in most cases, semiconductor devices failed. In most cases, a sound failure anamnesis is mandatory for root
Fig. 10. Punching burrs along a perforated steel border. Some vibration disconnects them and they will fall down – following Murphy's law onto the open electronics.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015
P. Jacob / Microelectronics Reliability xxx (2016) xxx–xxx
cause findings. In this sense, 8-D-procedures need to be revised and extended towards an integral view throughout the whole supply chain and also including country- and client-specific environmental and operational conditions. References [1] S.C. Pjesky, Aerodynamic, infrared extinction and tribocharging properties of nanostructured and conventional particles(PhD work) Kansas State University, Manhattan, Kansas, 2008 (UMI 3313379).
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[2] G. Vogel, Avoiding flex cracks in ceramic capacitors: analytical tool for a reliable failure analysis and guideline for positioning cercaps on PCBs, Microelectronics Reliability, vol. 55, Issues 9-10, August-September 2015, pp. 2159–2164, http://dx.doi.org/ 10.1016/j.microrel.2015.06.034 (proceedings of ESREF 2015). [3] Jacob, P., Failure mechanisms and precautions in plug connectors and relays, ESREF Tutorial 2016, t.b. published in Microelectronics Reliability (2016) Special Issuer ESREF 2016, Halle. [4] Vogel, G. Creeping Corrosion of Copper on Printed Circuit Board Assemblies, ESREF Tutorial 2016, t.b. published in Microelectronics Reliability (2016) Special Issue ESREF 2016, Halle.
Please cite this article as: P. Jacob, Early life field failures in modern automotive electronics – An overview; root causes and precautions, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.015