PRP Optoelectronics

LED Arrays

Many of the array products of PRP Optoelectronics are custom designed for specific applications. Two major LED technologies are employed in their manufacture; the assembly of individual LED chips to form a hybrid structure, and the formation of monolithic arrays of LEDs on a semiconductor substrate. Hybrid technology is used to assemble individual LED die into arrays which can be relatively large in area, while specialist wafer processing techniques can provide monolithic arrays of very small, high brightness emitting areas.

The hybrid technology is used to make digital and analogue displays. Arrays can contain many thousands of individual LEDs. The monolithic technology is employed to make LED arrays which are used in optical systems, head-up displays, thermal imaging systems and also in commercial printing applications. These products can incorporate hundreds of individually addressable LEDs on a single chip, with linear dimensions of individual diodes as small as 9µm.

Drive Electronics

With both these technologies, it is often desirable, and sometimes essential, to incorporate the drive electronics in the same package as the display itself. PRP has the capability and experience to design the drive electronics in either hybrid form, or with standard or custom IC's, and then to carry out the assembly and test of the electronics in conjunction with the LED array itself. Where this is not needed PRP is ready to advise customers of the most appropriate drive scheme and implementation.

Packaging and Sealing

The choice of packaging for an LED display is a very important factor in ensuring that the resultant product will meet the required performance and environmental specification. PRP have developed products assembled onto many different substrate types including co-fired ceramic, thick film alumina, FR4 or other more specialised materials. These substrates can be supplied assembled into a metal or other material enclosure depending on the level of environmental protection required. PRP has substantial experience in the design and procurement of such substrates, packages and enclosures, and also in the selection of suitable optical filters and the appropriate assembly and sealing techniques.

In some applications a hermetically sealed product is required, while in others hermeticity is not required and a polarising plastic window can be used to enhance the contrast in high brightness environments. Important in many military programmes is the provision of windows and filters to make the display compatible with night vision equipment. Displays are regularly supplied to meet the requirements of the US NVIS specifications.

Reliability

A major reason for choosing LED technology is the reliability offered in the most demanding of environments. A carefully assembled and suitably packaged LED array can be employed in temperatures ranging from -55 to +125°C, and can withstand severe shock, vibration, and humidity. The reliability of PRP product has been demonstrated by qualification trials for a number of military programmes. PRP has participated in an AVIP (Avionics Integrity Program) for F22, demonstrating over 2000 temperature cycles that the manufacturing technology can meet the standards required. In civil aviation one LED array with 261 diodes has now been in use in a Boeing cockpit for more than twenty years, with a reported field MTBF of more than 500,000 hours.

PRP provide support with reliability analysis including MTBF to MIL-STD-217, FMECA, FTA and cause/effect. These techniques are combined with comprehensive thermal design and analysis to yield high reliability products.

Development

In addition to customer funded work, PRP Optoelectronics has carried out its own developments to acquire the necessary expertise to remain a leader in the field of custom LED displays. This has involved the evaluation of different semiconductor material technologies, the design and manufacture of display demonstrators, and the evaluation of filters and sealing techniques.

PRP Optoelectronics has been active since it’s inception in the development and use of new LED technologies. The original technology for LED arrays used GaP and GaAsP materials for the hybrid arrays and GaAsP for the monolithic ones. Subsequently, GaAlAs higher brightness die were designed into the Eurofighter warnings panel and a custom monolithic array for thermal imaging was produced using this material. More recently, much brighter LEDs using the alloy materials InGaAlP and GaInN have become available. These materials cover the entire colour range from blue to red and have allowed multi-colour hybrid displays to be developed for a number of programmes, some of which are now in production. The Company has also developed processes to produce high power custom monolithic arrays using these materials, and devices based on them are now in use in a number of industrial applications. The company is also utilising the technology to make very high output LED arrays for Photo-dynamic Therapy, a new treatment for cancer. In pursuing these developments the company has gained UK Government support through two DTI SMART awards.