A similar behavior might be expected for a luminaire system, but because LED technology is still rather new, we should also be conscious of another contributor to failure: design flaws. Though not really a part of estimating product lifetime, design flaws are a reality of the current state of the art that needs to be addressed. Thus, we have divided reliability issues for discussion in this guide into three main categories:
1. Design flaws.
As the first LED lighting products appeared on the market, many design flaws were evident. The most common, initially, was poor design (or no design) of heat removal from the chips. This problem often resulted in overheated chips for which the luminous output depreciated quickly, leading to short-lived products. It was further exacerbated by claims that were essentially based on the LED lumen depreciation data. Design flaws, while perhaps less common and more subtle than they were only a short time ago, still exist: poor thermal management; using incompatible chemicals which degrade the optics or the chips; poorly matching the driver to the LED requirements; overdriving the chips; poor seals allowing moisture penetration, and so forth. Problems of this sort should largely diminish as designers become more familiar with the technology, so that claims more closely match performance. Choices of drive current and operating temperature, especially, will affect the design life of a product—an important concept that is appropriate engineering for cost control. Slipshod design leads to unpredictable design life; a product with a predictable design life that is advertised accurately and appropriately priced will satisfy a customer need far better than one with excessive and almost surely unmet claims of “lasting forever.”
2. Manufacturing defects.
These will always be with us. Even with a well-designed product, excursions from process control occur from time to time. Usually, these defects result in early failures. They may be partially covered by warranties, but that may still be unsatisfactory if the incidence of failure is too high. At present this does not seem to be an overly serious issue, at least with the major manufacturers, but as the volumes rise and as less experienced manufacturers enter the market, it is important that close attention be paid to quality controls. Factory testing and “burn-in” can also help. These can lead to additional costs, but can also minimize the customer seeing the early failures.
3. End of life.
A well-designed system operated under normal conditions within specifications will, nonetheless, eventually fail. There may be two or three critical modes of failure that eventually make the system unusable. With well-made components, the time of this failure should be fairly predictable, at least within a range, and this is what lighting users have come to expect is the “lifetime” of the luminaire. Understanding how to evaluate a system and predict end of life accurately is very important for market acceptance of solid-state lighting. It is the focus of the discussions in this guide.
There is no standardized method to determine lifetime, but for many electronic systems it can be and is typically estimated using the predicted lifetime of individual components at the anticipated operating conditions, which are then statistically combined. We do not have sufficient information today on all of the components and interactions of a luminaire to make these predictions. However, that should ultimately be our goal because for now, absent that data, the only remaining option to estimate system MTTF or B50 is to do a full LM-79 luminaire test on a population of product, which poses a conundrum. For many manufacturers a full LM-79 test may be too expensive and time-consuming. We recommend the full LM-79 test to establish lifetime, taking into account all failure mechanisms, but to address this difficulty we also suggest a number of alternative approaches to provide some indications of reliable design, if not of true lifetime