May_EDFA_Digital

edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 2 8 (a) visible light. Energy dispersive x-ray spectroscopy (EDS) can obtain qualitative and semi-quantitative elemental analysis information by working together with the SEM. In this analysis, a Hitachi SEM TM3030 with a Bruker EDS were used. As shown in Fig. 11a, whitish foreign materi- als at a hotspot area between P-GaN and N-GaN can be observed under the SEM image with 100x, 1000x, and 5000xmagnification. Further EDSmapping confirmed the foreign material is silver (Ag). SEM/EDS analysis on more failed LEDs found the same Ag dendrite at the hotspot area between P-GaN and N-GaN, as shown in Figs. 11b and c. After cleaning the Ag dendritewith sulfate, the IV curve of the failing sample becomes the same as the reference sample. The abnormal silver dendrite found at hotspot locationswas causing the high leakage current and energy efficiency drop. To identify the source of the Ag dendrite, this LED’s structure is reviewed, shown in Fig. 12. KSF (K 2 SiF 6 :Mn 4+ ) phosphor was used to achieve wider gamut. But KSF phosphor will react with moisture and generate strong HF acid: [5] Reaction Equation: KSF + H 2 O = HF + xxxx Silver bondingwires were used, which have lower cost and better color performance compared with Au wires. But silver can react with acids and generate Ag+ ions. Ag+ ions diffuse into the silicone glue and could move toward the cathode in electric field. Ag+ ions will get electrons from the cathode, restored to the Ag atom and start to accumulate as dendrites. [6] Fig. 11 SEM images and EDS results of failed LED at hotspot area. Fig. 12 Structure diagram of WLED. Fig. 13 Failure mechanism of LED Ag migration and leakage (a) original LED internal structure, (b) Ag ions migration from Ag wire, (c) Ag dendrite led to leakage between P-GaN and N-GaN. (b) (c) (a) (b) (c) (d)