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A very unusual transistor failure, caused by a solenoid
MR-12475; No of Pages 4 Microelectronics Reliability xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
A very unusual transistor failure, caused by a solenoid P. Jacob ⁎, R. Furrer Empa, Swiss Federal Laboratories for Materials Testing and Research, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 17 May 2017 Accepted 24 June 2017 Available online xxxx Keywords: System failure analysis Solenoids Heatsink degradation
a b s t r a c t By a customer providing industry electronics, we were faced to repeated failures of a small power transistor. However, nearly all samples were burnt and no conclusion could be drawn from device analysis. We took the next step forward towards system analysis. The electronic board with the transistor was mounted at the end of a big solenoid. The metal baseplate of the board holder also served as a heatsink for the power transistor. The board was mounted into a plastic housing, which has been damaged in many cases. It turned out that, when the big solenoid's core was running against its mechanical end stop, the mechanical impulse knocked the electronic board to the end of the plastic housing, thus deforming the heatsink plate in a manner that it lost the thermal contact to the transistor after some time of use. In result, the transistor overheated and failed. This case study is a nice example showing the need of a system-related failure analysis for root-cause finding. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction To control a big solenoid for industrial applications, a small electronics, including two switches, has been attached to the solenoid's housing. On the PCB, some SMT-electronics, including a small power transistor, were mounted. Since in many customer installations, the power transistor failed, we were asked to support root cause findings by providing some failure analysis on the transistor. However, nearly in all cases, the transistor only consisted of burnt leftovers and conventional failure analysis couldn't conclude towards a root cause. At this point we decided towards failure anamnesis and system failure analysis, which gave us an inside view into the subsystem level. From there, we proceeded to the root cause, as described in this paper. 2. Operation environment and statistical aspects The operation of the solenoid is within an environment, which is in most cases rather dirty and dusty, also humidity and weather have certain access. The numerous installations are both indoor and outdoor. Unfortunately, no details can be provided due to confidentiality, but they are of low significance for the failure mechanism described in this paper. From statistics point-of-view, the failures reported were in a significant level to go into deeper anamnesis and analysis. No specific countries or applications have been highlighted with respect to the ⁎ Corresponding author. E-mail address: [email protected] (P. Jacob).
occurrence of this failure. Considering the mechanical environment, it can be reported that the solenoid had to draw a mechanical load but was not damped by a rubber or a similar setup, so that the mechanical pulse of the load added itself to the solenoid core. The application of the solenoid is in a way that the system fails in case of the solenoid failure but there are no risks considering safety of persons or damaging valuable items. Since numerous systems are installed, rapid repair is no problem but is cost-intensive and needs some time. 3. Mechanical setup The solenoid consists of three sections, as shown in Fig. 1. The first section is a massive metal (steel) housing with the solenoid and a big, moveable core inside. The cylindrical hole inside the solenoid has a narrowing step, serving as a mechanical end point of the core. When the core runs against, the full mechanical impulse is transmitted without any damping by a rubber inlay or a spring. The small center section shows a plastic housing for the embedded control electronic PCB, followed by a massive third section from metal, serving as a connector holder for a very big metal plug connector. If the solenoid is power-supplied, the core moves into the inner part of the solenoid and suffers a hard mechanical stop, where the cylinder narrows itself, thus providing the mechanical end stop against the metal housing block.. This mechanical shock is transferred as an impulse through the metal housing to the plastic center, which suffers a slight reversible deformation, before the rest of the mechanical pulse hits the third section of connector holder and plug, which is also a rather massive construction (in opposite to the plastic section with the electronics inside).
http://dx.doi.org/10.1016/j.microrel.2017.06.058 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: P. Jacob, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
2
P. Jacob, R. Furrer / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 1. Mechanical setup of the solenoid.
4. Electronics and findings We now take a closer look at the electronics PCB, located in the black “plastic-housed”- section of the solenoid: at the end of the solenoid, a metal holder is fixed by two socket head screws. On this metal sheet, two small and a medium-size screw fix the PCB and - with the two small screws – also a U-shaped heatsink from copper is fixed, as shown in Fig. 2 (blue arrow). This heatsink provides the cooling of the power transistor on this PCB, which had failed frequently and device failure analysis didn't provide conclusive results. When looking at an X-ray image of the electronic section of the solenoid (Fig. 3), we can see that this heatsink sheet touches to the end of the plastic housing (yellow arrow), on which a Fig. 4. PCB setup at the head of the solenoid. The indicated screws were slightly unclenched, as found on all damaged solenoids.
Fig. 2. a (left) and b (right): Electronic PCB. The blue double arrow shows front- and backside of the heat sink, which is fixed - together with one of the two switches - to the silver-colour base plate with the paper label (right) by two screws (small yellow arrows). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. X-ray analysis of the electronic. The yellow arrow shows that the heatsink touches the end of the plastic housing. The blue arrow shows a neighboured screw which fixes the metal housing of the plug connector. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
big metal plug connector follows. This X-ray image was taken from a new reference solenoid. On all defective solenoid units we've examined, we found that the two screws, indicated by yellow arrows in Fig. 2 were unclenched, also the medium size screw bottom from center of the PCB in Fig. 2 left image, while the 2 big socket head screws on the solenoid body still were fixed. Thus, the PCB and the heatsink as well had some clearance along the solenoid axis. Every time when the core of the solenoid knocks against its mechanical end position, the mechanical pulse is transmitted to the PCB and from there via the heatsink end – to the connector housing. This results at first in unclenching the PCB-fixing screws, allowing the PCB to move also perpendicular to the solenoid axis. In order to study the problem more in detail, we installed a high-speed camera setup, which allowed us to record the relative movements of the components to each other when the solenoid core is giving its mechanical pulse. To observe the points of interest, small windows had been cut in the plastic housing of the PCB. Fig. 4 shows the PCB with the plastic housing behind, Fig. 5 shows the measurement points of interest.
Fig. 5. Ready for high-speed-camera recording. The points of interest are marked. Most interesting is the upper right point, where the heatsink touches the plug connector housing. The fixation of the setup was made accordingly to the original place of use. The green fields are just to hide labels due to confidentiality of our customer, who agreed to the publication if brands remain anonymous. (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, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
P. Jacob, R. Furrer / Microelectronics Reliability xxx (2017) xxx–xxx
3
Fig. 9. EOS (“End of Story”) by missing cooling.
Fig. 6. Showing the relative movements of the observation points of Fig. 5 to each other in both X- and Y-direction. Point 4 shows a huge relative movement in Y direction.
Fig. 10. Due to mechanical shock, the resistor (4700) of Fig. 9 has been knocked-off.
Fig. 7. Footprint of the heatsink touching to the neighboured screw head (arrow).
The results were somehow surprising. Fig. 6 shows the relative movements of the observation points to each other in both X- and Y- direction. It can be seen clearly that only very small relative movements happen in X-direction, while in Y-direction, test point 4 shows a significant deviation. This is the point top right of Fig. 5, the point of the end of the heatsink. The high perpendicular (to the solenoid axis) amplitude of
Fig. 8. The power transistor lost its thermal contact to the heatsink. The gap is clearly visible (arrow).
the swinging concludes that the heatsink touches the head of the neighboured screw (marked in blue in Fig. 3). A further careful inspection of the heatsink of all defective solenoids confirmed this conclusion, since footprints of the touch at the heatsink were clearly visible, as shown in Fig. 7. Finally, the hard knocking against the heatsink during solenoid operations results in some deformation of the heatsink, so that it loses its thermal contact to the power transistor. Thereafter, the transistor suffers thermal runaway and ends up in an internal short. The gap between the transistor and its heatsink can be clearly seen in Fig. 8. With the heatsink missing, the transistor overheated, but still worked for a certain time. In this time, the surrounding PCB became
Fig. 11. Causal chain from mechanical problem to electrical failure.
Please cite this article as: P. Jacob, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
4
P. Jacob, R. Furrer / Microelectronics Reliability xxx (2017) xxx–xxx
dark until the transistor's life ended by melting in many cases. Fig. 9 shows the end of such scenario. In some cases, the hard mechanical shocks even ended up in the additional knocking off of bigger SMD components as shown in Fig. 10. The big resistor (4700), which can be seen in Fig. 9 top left side of the image, was mechanically knocked off in the example of Fig. 10.
present, the approach of most failure analysts is still a strongly deviceoriented one. Hence, the goal of this paper is not to describe complex failure analysis methods or difficult applications of instruments, but it is to sensibilize the failure analysis community towards a system – instead of device-oriented view on a failure and its possible root causes.
5. Conclusion
Acknowledgement
Overall, the causal chain shows up as displayed in Fig. 11. A relatively simple mechanical construction issue caused the failure of a related electronics. In conclusion, this case study illustrates in a very comprehensive manner that often device failure analysis cannot provide the root cause. To obtain root cause analysis, a deep understanding of the operational environment in the customer application on system level is mandatory [1]. Successful 8-D-reports should involve the manufacturing chain and not just focus on a failed device and its manufacturer. At
The authors thank their customer who agreed in publishing this case study under the precondition that all brands, applications or labels remain anonymous. References [1] P. Jacob, Failure analysis and reliability on system level, Microelectron. Reliab. 55 (9– 10) (August–September 2015) 2154–2158.
Please cite this article as: P. Jacob, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
A very unusual transistor failure, caused by a solenoid P. Jacob ⁎, R. Furrer Empa, Swiss Federal Laboratories for Materials Testing and Research, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 17 May 2017 Accepted 24 June 2017 Available online xxxx Keywords: System failure analysis Solenoids Heatsink degradation
a b s t r a c t By a customer providing industry electronics, we were faced to repeated failures of a small power transistor. However, nearly all samples were burnt and no conclusion could be drawn from device analysis. We took the next step forward towards system analysis. The electronic board with the transistor was mounted at the end of a big solenoid. The metal baseplate of the board holder also served as a heatsink for the power transistor. The board was mounted into a plastic housing, which has been damaged in many cases. It turned out that, when the big solenoid's core was running against its mechanical end stop, the mechanical impulse knocked the electronic board to the end of the plastic housing, thus deforming the heatsink plate in a manner that it lost the thermal contact to the transistor after some time of use. In result, the transistor overheated and failed. This case study is a nice example showing the need of a system-related failure analysis for root-cause finding. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction To control a big solenoid for industrial applications, a small electronics, including two switches, has been attached to the solenoid's housing. On the PCB, some SMT-electronics, including a small power transistor, were mounted. Since in many customer installations, the power transistor failed, we were asked to support root cause findings by providing some failure analysis on the transistor. However, nearly in all cases, the transistor only consisted of burnt leftovers and conventional failure analysis couldn't conclude towards a root cause. At this point we decided towards failure anamnesis and system failure analysis, which gave us an inside view into the subsystem level. From there, we proceeded to the root cause, as described in this paper. 2. Operation environment and statistical aspects The operation of the solenoid is within an environment, which is in most cases rather dirty and dusty, also humidity and weather have certain access. The numerous installations are both indoor and outdoor. Unfortunately, no details can be provided due to confidentiality, but they are of low significance for the failure mechanism described in this paper. From statistics point-of-view, the failures reported were in a significant level to go into deeper anamnesis and analysis. No specific countries or applications have been highlighted with respect to the ⁎ Corresponding author. E-mail address: [email protected] (P. Jacob).
occurrence of this failure. Considering the mechanical environment, it can be reported that the solenoid had to draw a mechanical load but was not damped by a rubber or a similar setup, so that the mechanical pulse of the load added itself to the solenoid core. The application of the solenoid is in a way that the system fails in case of the solenoid failure but there are no risks considering safety of persons or damaging valuable items. Since numerous systems are installed, rapid repair is no problem but is cost-intensive and needs some time. 3. Mechanical setup The solenoid consists of three sections, as shown in Fig. 1. The first section is a massive metal (steel) housing with the solenoid and a big, moveable core inside. The cylindrical hole inside the solenoid has a narrowing step, serving as a mechanical end point of the core. When the core runs against, the full mechanical impulse is transmitted without any damping by a rubber inlay or a spring. The small center section shows a plastic housing for the embedded control electronic PCB, followed by a massive third section from metal, serving as a connector holder for a very big metal plug connector. If the solenoid is power-supplied, the core moves into the inner part of the solenoid and suffers a hard mechanical stop, where the cylinder narrows itself, thus providing the mechanical end stop against the metal housing block.. This mechanical shock is transferred as an impulse through the metal housing to the plastic center, which suffers a slight reversible deformation, before the rest of the mechanical pulse hits the third section of connector holder and plug, which is also a rather massive construction (in opposite to the plastic section with the electronics inside).
http://dx.doi.org/10.1016/j.microrel.2017.06.058 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: P. Jacob, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
2
P. Jacob, R. Furrer / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 1. Mechanical setup of the solenoid.
4. Electronics and findings We now take a closer look at the electronics PCB, located in the black “plastic-housed”- section of the solenoid: at the end of the solenoid, a metal holder is fixed by two socket head screws. On this metal sheet, two small and a medium-size screw fix the PCB and - with the two small screws – also a U-shaped heatsink from copper is fixed, as shown in Fig. 2 (blue arrow). This heatsink provides the cooling of the power transistor on this PCB, which had failed frequently and device failure analysis didn't provide conclusive results. When looking at an X-ray image of the electronic section of the solenoid (Fig. 3), we can see that this heatsink sheet touches to the end of the plastic housing (yellow arrow), on which a Fig. 4. PCB setup at the head of the solenoid. The indicated screws were slightly unclenched, as found on all damaged solenoids.
Fig. 2. a (left) and b (right): Electronic PCB. The blue double arrow shows front- and backside of the heat sink, which is fixed - together with one of the two switches - to the silver-colour base plate with the paper label (right) by two screws (small yellow arrows). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. X-ray analysis of the electronic. The yellow arrow shows that the heatsink touches the end of the plastic housing. The blue arrow shows a neighboured screw which fixes the metal housing of the plug connector. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
big metal plug connector follows. This X-ray image was taken from a new reference solenoid. On all defective solenoid units we've examined, we found that the two screws, indicated by yellow arrows in Fig. 2 were unclenched, also the medium size screw bottom from center of the PCB in Fig. 2 left image, while the 2 big socket head screws on the solenoid body still were fixed. Thus, the PCB and the heatsink as well had some clearance along the solenoid axis. Every time when the core of the solenoid knocks against its mechanical end position, the mechanical pulse is transmitted to the PCB and from there via the heatsink end – to the connector housing. This results at first in unclenching the PCB-fixing screws, allowing the PCB to move also perpendicular to the solenoid axis. In order to study the problem more in detail, we installed a high-speed camera setup, which allowed us to record the relative movements of the components to each other when the solenoid core is giving its mechanical pulse. To observe the points of interest, small windows had been cut in the plastic housing of the PCB. Fig. 4 shows the PCB with the plastic housing behind, Fig. 5 shows the measurement points of interest.
Fig. 5. Ready for high-speed-camera recording. The points of interest are marked. Most interesting is the upper right point, where the heatsink touches the plug connector housing. The fixation of the setup was made accordingly to the original place of use. The green fields are just to hide labels due to confidentiality of our customer, who agreed to the publication if brands remain anonymous. (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, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
P. Jacob, R. Furrer / Microelectronics Reliability xxx (2017) xxx–xxx
3
Fig. 9. EOS (“End of Story”) by missing cooling.
Fig. 6. Showing the relative movements of the observation points of Fig. 5 to each other in both X- and Y-direction. Point 4 shows a huge relative movement in Y direction.
Fig. 10. Due to mechanical shock, the resistor (4700) of Fig. 9 has been knocked-off.
Fig. 7. Footprint of the heatsink touching to the neighboured screw head (arrow).
The results were somehow surprising. Fig. 6 shows the relative movements of the observation points to each other in both X- and Y- direction. It can be seen clearly that only very small relative movements happen in X-direction, while in Y-direction, test point 4 shows a significant deviation. This is the point top right of Fig. 5, the point of the end of the heatsink. The high perpendicular (to the solenoid axis) amplitude of
Fig. 8. The power transistor lost its thermal contact to the heatsink. The gap is clearly visible (arrow).
the swinging concludes that the heatsink touches the head of the neighboured screw (marked in blue in Fig. 3). A further careful inspection of the heatsink of all defective solenoids confirmed this conclusion, since footprints of the touch at the heatsink were clearly visible, as shown in Fig. 7. Finally, the hard knocking against the heatsink during solenoid operations results in some deformation of the heatsink, so that it loses its thermal contact to the power transistor. Thereafter, the transistor suffers thermal runaway and ends up in an internal short. The gap between the transistor and its heatsink can be clearly seen in Fig. 8. With the heatsink missing, the transistor overheated, but still worked for a certain time. In this time, the surrounding PCB became
Fig. 11. Causal chain from mechanical problem to electrical failure.
Please cite this article as: P. Jacob, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058
4
P. Jacob, R. Furrer / Microelectronics Reliability xxx (2017) xxx–xxx
dark until the transistor's life ended by melting in many cases. Fig. 9 shows the end of such scenario. In some cases, the hard mechanical shocks even ended up in the additional knocking off of bigger SMD components as shown in Fig. 10. The big resistor (4700), which can be seen in Fig. 9 top left side of the image, was mechanically knocked off in the example of Fig. 10.
present, the approach of most failure analysts is still a strongly deviceoriented one. Hence, the goal of this paper is not to describe complex failure analysis methods or difficult applications of instruments, but it is to sensibilize the failure analysis community towards a system – instead of device-oriented view on a failure and its possible root causes.
5. Conclusion
Acknowledgement
Overall, the causal chain shows up as displayed in Fig. 11. A relatively simple mechanical construction issue caused the failure of a related electronics. In conclusion, this case study illustrates in a very comprehensive manner that often device failure analysis cannot provide the root cause. To obtain root cause analysis, a deep understanding of the operational environment in the customer application on system level is mandatory [1]. Successful 8-D-reports should involve the manufacturing chain and not just focus on a failed device and its manufacturer. At
The authors thank their customer who agreed in publishing this case study under the precondition that all brands, applications or labels remain anonymous. References [1] P. Jacob, Failure analysis and reliability on system level, Microelectron. Reliab. 55 (9– 10) (August–September 2015) 2154–2158.
Please cite this article as: P. Jacob, R. Furrer, A very unusual transistor failure, caused by a solenoid, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.06.058