Designing a reliable future for Ultra-Fine-Pitch LED
Fine and Ultra-Fine-Pitch LED solutions are continuing to explode in applications and orders. Last month in fact, a single display opportunity for a 1.25-mm pixel pitch LED display in the US pushed over $5 million. Opportunities for displays providing native HD and 4K resolution are emerging at an astonishing rate and in all verticals including entertainment, command control and U.S. Government systems. There is a line in the sand, Ultra-Fine-Pitch LED displays are replacing LCD walls and projection DLP systems.
LCD and projection technology currently holds a majority of the digital signage market at over 80 percent. Yet LED display image quality has begun to emerge as a rival to these other technologies in many applications and will soon surpass each with the inherent mature display solution. While the display performance design and implementation of Ultra-Fine-Pitch is now competitive, there is significant work ahead for manufacturers to realize the vast market potential of these displays.
The first key factor is reliability. The market potential will be maximized only through continuous improvements in the system design for reliability and life. Next generation Ultra-Fine-Pitch LED must target and plan to attain the reliability demonstrated by DLP projection and LCD. Expecting more from the LED display designer in order to align performance to the application requirements is absolutely imperative. And all aspects of the display should be addressed to attain maximum benefit.
Next generation designs will benefit from improvements in reliability by addressing several pieces such as the individual LED die, RGB SMD LED package, SMT processing, driver design, power supplies, components, connectors and cables. These adjustments to a final system will yield better displays with longer life and greater reliability. By addressing, implementing and promoting reliability improvements, Ultra-Fine-Pitch LED display systems can be successfully applied across the widest spectrum of the market.
Another issue of major importance when all these factors are put together is the Mean Time Between Failure or MTBF of the display.
While each physical element detailed above has an impact on the reliability of the overall display after it is installed, the RGB SMD LED package has the least favorable MTBF. The subset of the root cause of failure of the LED package can include:
In addition there are performance degradation issues including droop, overheating and primary color shift as well as luminance inconsistencies. As you might have already ascertained, LED reliability matters significantly to the display.
To dive right into MTBF and what it means to the display owner: There are over 2M RGB, SMD LED packages in an HD display, and over 8M in a 4K display. Each of these packages contains one red, green and blue LED die. So when we discuss failure rates and the MTBF, we are discussing 6M to 24M individual LED die that each has a statistical probability of failure.
Often, the industry will reference the PPM failures expected in the first year of operation for LED packages and it is not unusual to hear PPM rates quoted from 30 PPM to 300 PPM. A 300 PPM failure rate even sounds impressive. However, if indeed the PPM failure was 300 for a deployed HD display, the user could expect about 600 failures of LEDs in the first year.
Now 600 out of 2M sounds low, but keep in mind that the average viewing is less than one picture height away from the display in a conference room or tight viewing area. As a result, the user will see the failures and likely not be satisfied with that 300 PPM result. In addition if the user expects a perfect display, the maintenance cycle might be as much as every week (Failures in Time) with up to 12 modules needing replacement.
Obviously a higher standard is needed. Even 30 PPM is statistically ~60 failures per year and potentially a repair cycle each month. Should a "bright column" failure mode occur, the panel would need to be replaced before use in order to be visually acceptable to the user.
As customers see Ultra-Fine-Pitch displays replace the DLP projector or the LCD panel, the expectations from the users will likely be in the 10 PPM to meet expectations. The LED Digital Display industry must strive to produce and deploy devices with the much lower failure rates than are typical in our industry for the 1010 or 0808 packages, and as the industry delivers this type of reliability the market will expand rapidly without pause.
Some easy initial solutions to the failure rate would be to eliminate the infant mortality (early life failures) issues. This can be completed through effective burn-in and early stress screening. In fact, designers can incorporate HASS (Highly Accelerated Stress Screening) and HALT (Highly Accelerated Life Testing) to verify design and manufacturing processes to assure the right reliability performance in the field. As failures occur it is also critical that the manufacturers perform Failure Modes and Effects Analysis (FMEA) steps to determine and eliminate the root cause of the failures; continuous improvement is required.
The goal is the right reliability performance for the right application and Ultra-Fine-Pitch requires high reliability standards throughout the process. One additional factor is temperature. A hot display will fail earlier if all other factors are the same. Designers must keep package (and therefore junction) temperatures well below 70 degrees Celsius in UHP; recent providers are exceeding those temperature levels.
Ultra-Fine Pitch LED displays have a great current and rapidly growing future market, and the industry will exceed even those expectations if reliability is addressed appropriately in the design and LED device specification phase. Long-term reliability means the lowest maintenance costs and it creates important realizable selling features and value in the market.
Both the LED display industry and company will benefit in reputation and sales by maximizing the reliability feature in the overall specification. The need and the capability to address this opportunity for a very successful product and market result has been put in place by companies incorporating reliability in the design phase.