metrology LEDs
Unveiling structural anomalies in LEDs
The performance of LED light bulbs is impaired when delamination occurs within or between the parts that make up a packaged device. These deficiencies cannot be detected by widely used characterisation techniques such as X-ray and infrared microscopy, but they are exposed with acoustic imaging at very high frequencies, according to Tom Adams, a consultant to Sonoscan.
W
e are on the cusp of a lighting revolution. Although today’s homes
are generally lit with a mix of incandescents and fluorescents, LED light bulbs are starting to make inroads, and they will dominate by the end of this decade.
If you look hard today, you’ll find that manufacturers are already offering products at eye-watering prices. These will come down, but how far and how fast suppliers drop their prices will not be the only factors determining their success – the quality and amount of light will also play a major role in the relative popularity of the various brands of LED bulb.
As LED sales rise and consumers learn more about their virtues, it will become clear how much homeowners are willing to pay for their 75 W- equivalent bulbs and how long they expect them to last. What is sure to infuriate them is if a bulb fails
abruptly due to an electrical failure in its power supply or control circuitry. They will hope that the multiple LED chips in the bulb’s array will work for tens of thousands of hours. Slow dimming during that time is acceptable, while outright failure of a significant proportion of the chips will not be tolerated. A household LED bulb is generally considered to have fallen below an acceptable level of performance when its total output dips below 70 percent of its initial brightness.
Electrical failures of individual LED chips often stem from either structural anomalies in the packaging processes and materials of a semiconductor die, or in the epilayers built up on the die. These imperfections, which may have had their origin in assembly processes or in handling, include non-bonds, delaminations, cracks and voids.
Figure
1.Sonoscan’s S-CAM acoustic imaging tool reveals gap-type defects (shown in red) in a LED wafer with a sapphire base
22
www.compoundsemiconductor.net October 2012
In general, these weaknesses cause two types of problem: Die to overheat and fail, due to gaps at interfaces between die, die attach, substrate and heat sink that can block all the heat that needs to be removed from the assembly; and LED light output to diminish, due to the formation of
unintended insulators resulting from gaps among the contact
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131