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Reliability and wearout characterisation of LEDs
Microelectronics Reliability 46 (2006) 1711–1714 www.elsevier.com/locate/microrel
Reliability and Wearout Characterisation of LEDs P. Jacoba,b, A. Kunza, G. Nicolettia a
Swiss Federal Labs for Materials Testing&Research, Ueberlandstr.129, 8600 Dübendorf, Switzerland b EM Microelectronic Marin SA, Rue des Sors 2-3, 2074 Marin, Switzerland
Abstract LEDs play a key role as an active element in electronic circuitry – for example in optocouplers. Their life time strongly depends on the operation conditions. Degradation usually starts by the generation of both reverse and forward bias direction pinpoint leakage paths. When the resistance of such a path becomes lower than the regular operational resistance in forward direction, it will start to act as a bypass. Then, reduced and inhomogeneous emission is the first degradation indicator. The paper describes degradation mechanisms and their characterisation.
1. Introduction In many circuits, LEDs don’t act “only” as a control indicator or display illumination, but also act as a key active element within a functional bloc. Typical examples are optocoupling devices as well as glass fibre data transfers between PCBs at different electric potentials. However, the lifetime of LEDs is much more limited compared to other active elements and often low attention is paid to their selection and careful characterisation.
2. Analysis of degradation mechanisms Usually, LEDs are not subject to a sudden death. The “normal” end of life shows as a continuous decrease of light emission. Our examinations of various LEDs, mostly those of optocoupling devices, have shown that the data given in the data sheet as normal operational conditions could be derated significantly without functional restrictions by up to a factor of 2. It is well known by reliability modelling that such derating will extend lifetime significantly [1].
0026-2714/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.microrel.2006.07.048
However, this is possibly not known to many circuitry designers, so that the LEDs are mostly used at nominal operational conditions which stress them more than necessary. Similar as known from semiconductor capacitors, we observed two basic main classes of failures: Device-border related (perimeter) failures and point failures in the inner, active LED area. A further failure class has been found considering wire-bonding problems. 2.1 Silver dendrites as perimeter failures Perimeter failures are usually more or less ohmic bypasses resulting from silver dendrite growth at the border side of the LED chip (Fig. 1). The devices are glued by means of a silver epoxy onto a small PCB from which bonded wires connect the LED chip to the outer pins. Especially if too much epoxy is used, reaching almost to the top side of the LED chip, this mechanism is promoted significantly and can be accelerated by thermal cycling and/or humidity. As long as the dendrites are still very small, failure localisation can be performed well using emission microscopy in the reverse bias direction. If the dendrite growth is so severe that bypassing current-densities become already low, emission microscopy or even optical microscopy can be used only in the forward
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P. Jacob et al. / Microelectronics Reliability 46 (2006) 1711–1714
direction: However, instead of a light emission at the failure point, in this case often the failure location displays as a shadow in the regular LED emission (Fig. 2). More useful in such cases, however, is the use of OBIRCH, allowing direct detection of bypassing metal paths in the reverse LED direction at a very low current – thus leaving the failure in his present state, too.
Fig.3: LED, showing a dark spot, which indicates the bypassed region of the emitting area. In an overall rating, the LED is still working, however, the black spot indicates that wearout has begun; image in forward direction 1,2V/ 8µA.
Fig.1: Dendrite growth along the chip border, beginning from the silver epoxy die-attach. If the dendrites reach the top metal, the LED is bypassed
Fig.2: The left side of the LED shows forward emission shadows – which is also the side where reverse bias leakage takes place (right image). Both images were made by using an emission microscope
The use of other localisation methods is less recommended. Especially liquid crystal thermography would need relatively high currents and a device decapsulation before the localisation could start. However, doing the device decapsulation before the physical failure localisation on LEDs, some failures might suffer artefacts from physical treatment or chemistry (e.g. wire bonding problems). We therefore recommend to use Emission microscopy and OBIRCH through the LED device capsulation.
Fig.4: OBIRCH indicated reverse bias leakage. The black ring shadow of Fig.2 can now be recognized as “white” metal ring to contact the LED. The defect position is identical with the forward bias emission shadow in Fig.2
2.2 Point failures in the LED emission area Considering point failures in the active area, one starts with either emission microscopy or OBIRCH in the reverse bias direction, too (Fig.4). Only if no clear failure localisation point is indicated, the ”shadow” method, using the forward direction could be tried. Considering spots within the active area, they indicate reverse bias leakage paths, too. However, the physical visibility of such failures is rather difficult. In such cases, a basic decision must be taken between delayering and structural decoration (HC-etch or similar) – to find for example crystal failures or a local cross sectioning using FIB. In the latter case, crystal defects usually remain undiscovered, while structural setup
P. Jacob et al. / Microelectronics Reliability 46 (2006) 1711–1714
failures or metal penetration could be seen clearly. Detailed studies have been made on such crystal-failure-induced wearout; an overview article of Melanie Ott is referred under [2]. She and her team made numerous further investigations on the subject, which can be studied at the NASA web homepage. 2.3 Wirebonding problems Another frequently observed problem is the wirebonding. Fig.5 displays a wire-bond liftoff, which has been observed after decapsulation of the device. Often, this failure mechanism may already be concluded from the wearout observation of the LED: The LED shows unstable emission behaviour with a reduced and flickering emission. At the same time, the voltage-current-characteristic shows a continuously changing variation between more or less linear and diodic shapes. Considering that the wire bond current density of a healthy LED is at about 2 A/mm2 (at its normal current of around 20mA), this current density can be enhanced by a factor of 10 and more when the characteristics change to low-resistance-types. In such case, electromigration and/or electrolytic corrosion (if humidity is involved, too) start to destroy the wirebond adhesion.
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3. Discussion If we look at the degradation, in all cases the mechanisms start a slow growing, just beginning with a reduction of the regular LED emission under operational conditions. Due to the slowly growing nature of all mechanisms described here, we use a multi-step characterisation table including a new reference device. The first step starts by applying a slowly increasing forward voltage, while the DUT is observed by emission microscopy. The I-V-point at which a continuously camera-visible emission starts, is the first characterisation point. The second point marks the I-V values of the moment where the emission becomes visible by the CCD camera in the imaging mode, while the third point just uses the eye visibility (of course not applicable for infrared LEDs or laser dio-des). After these characterisation points, we change towards reverse bias direction. The –(I-V) values of the point of beginning defect emission deliver a further characterisation point (not feasible at reference devices). These data are written into a table (see table 1). The comparison of such tables to functional thres-holds of new devices allows already good estimations of beginning derating and kind of dominant mechanisms. Com- EMMI- Visibil- Eye viponent ity sible livisibil- limit of mit by ity CCD opt. camera microsc
Emission image
1,2V/ 1,5V/ 1 (bad) 1V/ 200µA 800µA 5mA
Homogene- -2,5V/ 50µA ous with shadow in region of rev.emission
1 point
2 (bad) 1,1V/ 1,2V/ 100µA 1mA
1,6V/ 10mA
Homogene- -4,5V/ 5µA ous
2 points
Good 0,8V/ 1,2V/ 1,5V/ refer- 0,1µA 0,3µA 1mA ence device
Homogene- -12V/ ous 1µA
Several changing points
Forward direction
EMMI Signal Reverse bias direction
EMMI image Reverse bias direction
Fig. 5: LED bondwire liftoff, including the pad metal
In many cases, also combinations of the different failure mechanisms have been observed. However, if we consider the increasing current density (which increases local heating) by all mechanisms described, this matter of fact becomes easily understandable.
Table 1: Emission analysis record of LEDs. Columns 2-4 consider forward biasing characterisation, while columns 5-6 look at the reverse bias direction. The dark fields indicate most significant differences in behaviour compared to the good reference device.
P. Jacob et al. / Microelectronics Reliability 46 (2006) 1711–1714
Reverse bias voltage Lenord Optokoppler
-2V/div Referenz Schlecht1 Schlecht2
200µA / div
For a better understanding of the failure mechanisms described, we can use the following electrical circuit (Fig.6), where R1 indicates the shunting resistance (for example dendrites), R2 stands for wirebondingproblems. D1 is the functional LED and D2 marks an antiparallel diode with light emission in the reverse bias direction. Since the described mechanisms promote each other (heat, high local current density etc), especially when the wearout process lasted already some time, we often observe combinations of the mechanisms described.
Reverse current
1714
Fig.7: Reverse bias I-V-characteristics of a good LED (left, showing hard Zener curve), a bad, leaky LED (right curve and a pre-damaged LED, which was still in-spec (curve in-between both other ones). X-axis: 2V/div; y-axis: 1uA/div
R2
4. Conclusion
R1
D1=LED
D2
Fig. 6: LED wearout failure circuitry Considering overall experiences, the question should be asked, why derating is mostly not used or only at a non-significant level. Considering the emission of healthy LEDs, even a reduction to less than half of the nominal forward current, would not reduce significantly the LED emission and data transfer quality. Looking at the big gradient of reverse bias blocking capability, even when using new LEDs (see Fig. 7), the question must be asked, whether a better screening in the final test would improve the reliability significantly. From the application side, it should be mentioned, that we observed often early wearout when the LEDs were used in circuitries with inductive characteristics. In such cases, switching flangues can generate very short, but high reverse bias pulses into the LED if no suitable electronic protection has been performed. Unfortunately, many of the most important applications are of such a kind, especially power converter blocks.
LED degradation is most commonly slowly beginning by reverse bias leakage current. Assembly-related root causes have been found as well as devicerelated ones. The lifetime could be significantly improved by derating, since most data sheets allow more current than it would be necessary. The analysis is best performed by using emission microscopy and reference markers as described. For applications with a high demand on reliability, a special screening measurement in reverse bias direction would be recommendable. The measurement shall point out whether a round characteristic or a Zener-type characteristic is observed at a reverse bias of around minus 20V, (Fig. 7). The ideal case would be a hard Zener-type reverse limitation at around 30-37V. In order not to damage the LEDs by such testing, a highly sensitive and fast current limitation in the test must be available. Normally we use a maximum current of 1µA.
5. References: [1] A. Birolini, “Reliability Engineering – Theory and Practice”, 3rd edition 1999, pp.135-136, Springer Verlag Berlin Heidelberg New York [2] M. Ott, “Capabilities and Reliability of LEDs and Laser Diodes”, What's New in Electronics, Vol. 20 Nr. 6, November 2000. Also published in 1997 by NASA Electronic Parts and Packaging Program, see: http://nepp.nasa.gov/photonics/pdf/sources1.pdf
Reliability and Wearout Characterisation of LEDs P. Jacoba,b, A. Kunza, G. Nicolettia a
Swiss Federal Labs for Materials Testing&Research, Ueberlandstr.129, 8600 Dübendorf, Switzerland b EM Microelectronic Marin SA, Rue des Sors 2-3, 2074 Marin, Switzerland
Abstract LEDs play a key role as an active element in electronic circuitry – for example in optocouplers. Their life time strongly depends on the operation conditions. Degradation usually starts by the generation of both reverse and forward bias direction pinpoint leakage paths. When the resistance of such a path becomes lower than the regular operational resistance in forward direction, it will start to act as a bypass. Then, reduced and inhomogeneous emission is the first degradation indicator. The paper describes degradation mechanisms and their characterisation.
1. Introduction In many circuits, LEDs don’t act “only” as a control indicator or display illumination, but also act as a key active element within a functional bloc. Typical examples are optocoupling devices as well as glass fibre data transfers between PCBs at different electric potentials. However, the lifetime of LEDs is much more limited compared to other active elements and often low attention is paid to their selection and careful characterisation.
2. Analysis of degradation mechanisms Usually, LEDs are not subject to a sudden death. The “normal” end of life shows as a continuous decrease of light emission. Our examinations of various LEDs, mostly those of optocoupling devices, have shown that the data given in the data sheet as normal operational conditions could be derated significantly without functional restrictions by up to a factor of 2. It is well known by reliability modelling that such derating will extend lifetime significantly [1].
0026-2714/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.microrel.2006.07.048
However, this is possibly not known to many circuitry designers, so that the LEDs are mostly used at nominal operational conditions which stress them more than necessary. Similar as known from semiconductor capacitors, we observed two basic main classes of failures: Device-border related (perimeter) failures and point failures in the inner, active LED area. A further failure class has been found considering wire-bonding problems. 2.1 Silver dendrites as perimeter failures Perimeter failures are usually more or less ohmic bypasses resulting from silver dendrite growth at the border side of the LED chip (Fig. 1). The devices are glued by means of a silver epoxy onto a small PCB from which bonded wires connect the LED chip to the outer pins. Especially if too much epoxy is used, reaching almost to the top side of the LED chip, this mechanism is promoted significantly and can be accelerated by thermal cycling and/or humidity. As long as the dendrites are still very small, failure localisation can be performed well using emission microscopy in the reverse bias direction. If the dendrite growth is so severe that bypassing current-densities become already low, emission microscopy or even optical microscopy can be used only in the forward
1712
P. Jacob et al. / Microelectronics Reliability 46 (2006) 1711–1714
direction: However, instead of a light emission at the failure point, in this case often the failure location displays as a shadow in the regular LED emission (Fig. 2). More useful in such cases, however, is the use of OBIRCH, allowing direct detection of bypassing metal paths in the reverse LED direction at a very low current – thus leaving the failure in his present state, too.
Fig.3: LED, showing a dark spot, which indicates the bypassed region of the emitting area. In an overall rating, the LED is still working, however, the black spot indicates that wearout has begun; image in forward direction 1,2V/ 8µA.
Fig.1: Dendrite growth along the chip border, beginning from the silver epoxy die-attach. If the dendrites reach the top metal, the LED is bypassed
Fig.2: The left side of the LED shows forward emission shadows – which is also the side where reverse bias leakage takes place (right image). Both images were made by using an emission microscope
The use of other localisation methods is less recommended. Especially liquid crystal thermography would need relatively high currents and a device decapsulation before the localisation could start. However, doing the device decapsulation before the physical failure localisation on LEDs, some failures might suffer artefacts from physical treatment or chemistry (e.g. wire bonding problems). We therefore recommend to use Emission microscopy and OBIRCH through the LED device capsulation.
Fig.4: OBIRCH indicated reverse bias leakage. The black ring shadow of Fig.2 can now be recognized as “white” metal ring to contact the LED. The defect position is identical with the forward bias emission shadow in Fig.2
2.2 Point failures in the LED emission area Considering point failures in the active area, one starts with either emission microscopy or OBIRCH in the reverse bias direction, too (Fig.4). Only if no clear failure localisation point is indicated, the ”shadow” method, using the forward direction could be tried. Considering spots within the active area, they indicate reverse bias leakage paths, too. However, the physical visibility of such failures is rather difficult. In such cases, a basic decision must be taken between delayering and structural decoration (HC-etch or similar) – to find for example crystal failures or a local cross sectioning using FIB. In the latter case, crystal defects usually remain undiscovered, while structural setup
P. Jacob et al. / Microelectronics Reliability 46 (2006) 1711–1714
failures or metal penetration could be seen clearly. Detailed studies have been made on such crystal-failure-induced wearout; an overview article of Melanie Ott is referred under [2]. She and her team made numerous further investigations on the subject, which can be studied at the NASA web homepage. 2.3 Wirebonding problems Another frequently observed problem is the wirebonding. Fig.5 displays a wire-bond liftoff, which has been observed after decapsulation of the device. Often, this failure mechanism may already be concluded from the wearout observation of the LED: The LED shows unstable emission behaviour with a reduced and flickering emission. At the same time, the voltage-current-characteristic shows a continuously changing variation between more or less linear and diodic shapes. Considering that the wire bond current density of a healthy LED is at about 2 A/mm2 (at its normal current of around 20mA), this current density can be enhanced by a factor of 10 and more when the characteristics change to low-resistance-types. In such case, electromigration and/or electrolytic corrosion (if humidity is involved, too) start to destroy the wirebond adhesion.
1713
3. Discussion If we look at the degradation, in all cases the mechanisms start a slow growing, just beginning with a reduction of the regular LED emission under operational conditions. Due to the slowly growing nature of all mechanisms described here, we use a multi-step characterisation table including a new reference device. The first step starts by applying a slowly increasing forward voltage, while the DUT is observed by emission microscopy. The I-V-point at which a continuously camera-visible emission starts, is the first characterisation point. The second point marks the I-V values of the moment where the emission becomes visible by the CCD camera in the imaging mode, while the third point just uses the eye visibility (of course not applicable for infrared LEDs or laser dio-des). After these characterisation points, we change towards reverse bias direction. The –(I-V) values of the point of beginning defect emission deliver a further characterisation point (not feasible at reference devices). These data are written into a table (see table 1). The comparison of such tables to functional thres-holds of new devices allows already good estimations of beginning derating and kind of dominant mechanisms. Com- EMMI- Visibil- Eye viponent ity sible livisibil- limit of mit by ity CCD opt. camera microsc
Emission image
1,2V/ 1,5V/ 1 (bad) 1V/ 200µA 800µA 5mA
Homogene- -2,5V/ 50µA ous with shadow in region of rev.emission
1 point
2 (bad) 1,1V/ 1,2V/ 100µA 1mA
1,6V/ 10mA
Homogene- -4,5V/ 5µA ous
2 points
Good 0,8V/ 1,2V/ 1,5V/ refer- 0,1µA 0,3µA 1mA ence device
Homogene- -12V/ ous 1µA
Several changing points
Forward direction
EMMI Signal Reverse bias direction
EMMI image Reverse bias direction
Fig. 5: LED bondwire liftoff, including the pad metal
In many cases, also combinations of the different failure mechanisms have been observed. However, if we consider the increasing current density (which increases local heating) by all mechanisms described, this matter of fact becomes easily understandable.
Table 1: Emission analysis record of LEDs. Columns 2-4 consider forward biasing characterisation, while columns 5-6 look at the reverse bias direction. The dark fields indicate most significant differences in behaviour compared to the good reference device.
P. Jacob et al. / Microelectronics Reliability 46 (2006) 1711–1714
Reverse bias voltage Lenord Optokoppler
-2V/div Referenz Schlecht1 Schlecht2
200µA / div
For a better understanding of the failure mechanisms described, we can use the following electrical circuit (Fig.6), where R1 indicates the shunting resistance (for example dendrites), R2 stands for wirebondingproblems. D1 is the functional LED and D2 marks an antiparallel diode with light emission in the reverse bias direction. Since the described mechanisms promote each other (heat, high local current density etc), especially when the wearout process lasted already some time, we often observe combinations of the mechanisms described.
Reverse current
1714
Fig.7: Reverse bias I-V-characteristics of a good LED (left, showing hard Zener curve), a bad, leaky LED (right curve and a pre-damaged LED, which was still in-spec (curve in-between both other ones). X-axis: 2V/div; y-axis: 1uA/div
R2
4. Conclusion
R1
D1=LED
D2
Fig. 6: LED wearout failure circuitry Considering overall experiences, the question should be asked, why derating is mostly not used or only at a non-significant level. Considering the emission of healthy LEDs, even a reduction to less than half of the nominal forward current, would not reduce significantly the LED emission and data transfer quality. Looking at the big gradient of reverse bias blocking capability, even when using new LEDs (see Fig. 7), the question must be asked, whether a better screening in the final test would improve the reliability significantly. From the application side, it should be mentioned, that we observed often early wearout when the LEDs were used in circuitries with inductive characteristics. In such cases, switching flangues can generate very short, but high reverse bias pulses into the LED if no suitable electronic protection has been performed. Unfortunately, many of the most important applications are of such a kind, especially power converter blocks.
LED degradation is most commonly slowly beginning by reverse bias leakage current. Assembly-related root causes have been found as well as devicerelated ones. The lifetime could be significantly improved by derating, since most data sheets allow more current than it would be necessary. The analysis is best performed by using emission microscopy and reference markers as described. For applications with a high demand on reliability, a special screening measurement in reverse bias direction would be recommendable. The measurement shall point out whether a round characteristic or a Zener-type characteristic is observed at a reverse bias of around minus 20V, (Fig. 7). The ideal case would be a hard Zener-type reverse limitation at around 30-37V. In order not to damage the LEDs by such testing, a highly sensitive and fast current limitation in the test must be available. Normally we use a maximum current of 1µA.
5. References: [1] A. Birolini, “Reliability Engineering – Theory and Practice”, 3rd edition 1999, pp.135-136, Springer Verlag Berlin Heidelberg New York [2] M. Ott, “Capabilities and Reliability of LEDs and Laser Diodes”, What's New in Electronics, Vol. 20 Nr. 6, November 2000. Also published in 1997 by NASA Electronic Parts and Packaging Program, see: http://nepp.nasa.gov/photonics/pdf/sources1.pdf