For the early sudden failure analysis, according to the characteristic feature of the semiconductor device, the failure rate is higher in the early failure stage, but the failure rate decreases with the rise of the time. The device failure is caused by one or several general reasons, for the device of different variety and different craft, the perdurability and failure proportion is different during this period. Strict process operation and inspection of raw materials, semi-finished product and finished product can decrease the failure of this period. Reasonable preparation by screening can screen out the early failure device as soon as possible before being used, and make the failure rate of the device reach or close to the accidental failure level. According to the actual situation, at present, there is little early failure mechanism analysis both at home and abroad, however, since the cost of LED device is rather high, and the early failure holds a large proportion, analysis of this field should arouse the attention of people. G. Cassanelli has carried out the experimental analysis on the early sudden failure of high-power white LED, he thought that Ag2 S is generated by the reaction of the electrode Ag and the sulphur in the packaging materials, so as to increase the resistance and cause the failure of the device because of the short circuit. From the general view of reliability, most failure mechanism observed from LED device is the analysis result that the luminous flux and the electrical characteristics decay with the continuation of time. Usually, we need a long time to study of the failure mode and mechanism. Currently, the failure mechanism of white LED is mainly divided into the following several aspects:
(1) Degradation of the packing materials. The light efficiency of the packing materials decays quickly. People like Meneghesso have observed the degradation of the packing materials under high current.
(2) Ohms contact degradation. Meneghesso put LED under high DC current for aging, and observed the degradation of IV character. He thought, the reason of the series resistance increase was that P type ohms contact became vestigial under high current and high temperature.
(3) Phosphor Degradation. There are many ways to realise the way of white light, currently, the most general and most mature method is to paint the yellow fluorescent power on the blue chip, which can form the white light by combining the blue light with the yellow light.
(4) Metal Electrical Migration: P type electrode metal will get to PN junction ( LED Junction Temperature Measurement ) area and form ohms pathway along the defects and cause the degradation of the junction character.
(5) Level defects increase: Under the high temperature, level defects will increase rapidly, and even intrude into the luminous zone, and form a mass of non radiant recombination centre, severely reduce the luminous efficiency of the device.
(6) Electrostatic damage. The electrostatic discharge will cause the short circuit of PN junction, or form the junction defects in the junction and increase the leakage current ( Leakage Current Test ).
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