Analysis of LED Failure
Since the emergence of LED technology, their long lifespan has been the focus of the public attention. But even though LED technology has matured, LED failure is still happening. Whenever LED failure occurs, whether in production or application, it represents a difficulty for manufacturers. Not only do they need to face the loss caused by the defective product, but the consumer’s confidence in LED products is affected. Therefore, contributing to reducing and preventing repeated occurrence of any LED product fault can help guarantee the reputation of the whole product-type, ensure individual product quality, and enhance product competitiveness. Therefore, it is necessary to conduct research and analyze the reasons for LED failure. Meanwhile, research and analysis also provides a reference for improving and enhancing enterprise technology so as to create greater economic benefit for the enterprise. In general, the common reasons for LED failure include the following:
Bonding wire breakage
To better understand the reason for LED failure, we should first confirm whether the LED is either a short circuit or open circuit. If it is open circuit, we usually then consider whether the bonding wire inside the LED is disconnected. If yes, this will cause a zero supply voltage for the LED; this is one of common reasons for a dead LED lamp. There are five places that are most likely become the off-positioning of bonding wire, as shown at Points A, B, C, D, and E in Fig. 1:
Fig.1 Diagram of off-position of bonding wire:schematic diagram
• Point A. the junction between chip electrode and the golden ball;
• Point B. the junction between golden ball and gold wire;
• Point C. the range of the arc of bonding wire;
• Point D. the junction between the second soldered support dot and gold wire;
• Point E. the junction between the second soldered support dot and cladding support structure.
The fracture position of the bonding wire can be checked from sectional analysis or glue‑dissolving of the sample by utilizing an optical microscope and scanning electron microscope (SEM), which then can contribute to further analysis and determination of the cause. In the following cases, the position of bonding wire fracture and reasons for the fracture differ.
Case study 1: The given model of failed LED is a SMD5730 encapsulation (namely the outline size is 57*30 mm). LED failure occurs after the LED is verified by a 100 cold and heat¬-circulation impact experiment. After the sectional analysis of the failed sample, there is crack on the silica gel around the first soldered dot and the second soldered dot, and Point D of the second soldered dot has been disconnected, as shown in Fig. 2 to Fig.4.
Fig.2 Cross section appearance of failure lamp sample
The difference of the thermal expansion coefficient between silicone and gold wire is considerable. As the silica gel and gold wire expand and shrink constantly during the operation of the light, after 100 times of cold and hot circulation the impact is identifiable in experiments and the bending position of the soldered dot of gold wire becomes the stress concentration point. Thus it easily causes a crack of silica gel around the soldered dot. The crack of silica gel may cause a disconnection of Point D, the weakest position of the second soldered dot of bonding wire and finally lead to the LED failure.
Solid crystal layer stripping
For a LED with vertical encapsulation structure, the stripping of the bottom of the solid crystal layer and support cladding layer is the most common cause for LED failure.
Case study 2: the failed sample is a DIP LED, from a bulk LED lights where the rejection ratio is 1.5%. After checking the cross section of the failed sample, we find that the soldered dot of gold wire is intact, as shown in Figs. 5 to 7. However, the solid crystal layer has been totally stripped from the support cladding layer and the packaging glue is also stripped from the support cup wall, as shown in Fig. 8.
According to the above phenomena, we judge that the reason for the LED failure is the stripping between the packaging glue and the support interface. The degree of stripping expands with the intensification of use, eventually causing the stripping of the solid crystal glue from the support. This ultimately leads to the LED failure of the sample. It is also possible that the cause may be poor glue adhesion between the packaging glue and the support interface.
The soldered dot burnout
In some cases, the LED failure is not caused by the LED itself. It may be triggered by the power supply used.
Case study 3: The failed samples are high power LEDs . These LEDs stopped working after the LED lamp has been used for a certain long time. After checking the result of glue dissolving experiment on multiple failed lamps, the gold wire soldered dots at both electrode-P and the electrode circuits are observed to be burnt out. However, four secondary soldering dots on the encapsulation stents, two gold wire soldering dots at both electrode-N and the electrode circuit are remained intact and are not burnt out or broken, as shown in Fig 9 and Fig. 10.
Clearly, the electrode-P burnout of chip is the direct reason for the LED failure. But what causes the electrode-P burnout of chip? We then conduct following experiment.
We pick several LED samples at random, which can be lit up normally, and we impose an instant high voltage of 20 volt on each LED. The result of the experiment shows that the lamp (after going through HV impact) produces the LED failure. After undertaking the glue dissolving process, the electrode burnout on the electrode-P of chip results in an open-circuit LED failure, as shown in Fig. 11 and Fig. 12., highly similar phenomenon as what has been found in case study 3.
Through the above inspection and verification experiments, it is not difficult to find that the fundamental reason for the LED failure of such a batch of LEDs is the excessive surge current during use. The resistance in the electrode-P area of the chip is higher than that in electrode-N area. Therefore, when the current passes by electrode-P intensively, the electrode-P is firstly burned out and results in an open-circuit LED failure. This may be linked to the surge in current when the lamp drive source is started or shut down. It may also because of a electrode-P routing defect in the chip results in a bad instant contact of the electrode-P soldered dot. When multiple LEDs are in series, HV will be accumulated on the bad contact, thus triggering high instant current and causing the bonding wire burnout and packaging glue charring.
Chip is eroded
The previous LED failure cases are caused by an open circuit. Now we will give another example caused by short circuit below.
Case study 4: the failed sample is a high power LED. During the burn-in process of the lamp, these LEDs become dead and dark. After glue dissolving of the defective products, the area with more electrodes in the chip is found to be eroded and stripped from the electrode, as shown in Fig.15 and Fig.16.
Through analyzing the element of the eroded area of the chip by using an X ray energy spectrometer, the eroded area of chip electrode is found to contain more Na, Cl and K elements, as shown in Fig. 17 and Fig. 18.
According to the chemical composition of element, we judge that chip may be polluted by NaCl and KCl. When heat and water vapor coexist, it will erode the chip electrode, causing the metal corrosion of the chip electrode and a reduction in the adhesive strength of the electrode circuit, and may even cause a partial falling off. The migration of the electrode solute causes a short circuit of the electrodes P and N and further results in LED failure.
As a summary, there are thus multiple reasons behind LED failure. The LED failure may occur at any stage from packaging, application, to use. The above cases are only for reference. How to reduce and eradicate instances of LED failure and enhance product quality and reliability is the key problem that each LED enterprise needs to face. Analysis of the reasons for LED failure is one of the most important approaches to improving quality. However, it should be noted that in addition to needing powerful equipment hardware, the failure analysis of LED products needs experience in the production of each link, from chip, packaging to application, so as to be able to properly determine the capability of equipment, thus be able to make the correct analysis to exclude difficulty and anxiety for customers.