Designing with surface mount high-brightness LEDs for electronic sign applications
With the increasing demand for timely clear communication of relevant and critical information in a variety of environments the use of LED technology in the design and manufacturing of Electronic Signs and Signals (ESS) has grown exponentially over the past several years.
This increasing demand has challenged manufacturers of LED based products to design smaller, less expensive, higher efficiency, lower power (energy saving), and environmentally tolerant solutions to meet the demands of the ESS industry.
Even with the widespread adoption of LED technology the drive to innovate in Signs and Signals continues.
Figure 1: Based out of Spain, Imago Screens has adopted Avago’s SMT LED lamp technology for variable message signs such as the lane control sign above.
Today, the standard is for Through-Hole (TH) technology. Unfortunately, Through Hole (TH) technology available today has technical limitations because the TH LED leads necessitate more space on a Printed Circuit Board (PCB) than Surface Mount Technology (SMT).
Through Hole LED’s limit the design since they only enable implementation of 2-layer PCBs, thus increasing the size, weight and cost of LED-based Screen solutions.
Current SMT LED’s have been limited in the luminosity provided and in moisture sensitivity. In response to this need for the more versatile SMT ESS LED’s, Avago Technologies, recently announced the industry’s first surface mount technology (SMT) high brightness oval and round LED lamps for use in outdoor and indoor electronic sign applications.
These products were developed as a result of intense collaboration with our customers who design, manufacture and service large screens for entertainment (stadium scoreboards), Infotainment Display displays (Times Square displays), and Outdoor applications like VMS (Variable Message Signs), with graphic RGBY area and/or Alphanumeric Area, SMS (Single Message Signs) for Lane Control Signs, Variable Speed Limit and Warning Signs.
The Avago round ALMD-xx3D and oval ALMD-Lx36 LEDs (available in Amber, Red, Green and Blue colors) provide similar optical performance when compared to conventional high brightness through-hole LED lamps and can be assembled using common SMT assembly processes.
Figure 2: Avago’s SMT LEDs enable designers to implement multiple-layer PCBs reducing the number of PCBs needed.
To provide the customer with increased uniformity each reel is shipped from a single intensity and color bin resulting in consistent and more uniform delivery of information to the sign viewer, from multiple viewing angles.
Key Features include:
• Compact form factor with well defined spatial radiation pattern.
• Viewing angles of 30 degrees for round and 40 by 100 degrees oval LED’s.
• Tinted lenses.
• High brightness material.
• Compatible with industrial reflow soldering processes.
• Advanced optical grade epoxy providing Moisture Sensitivity Level (MSL) 2 A.
One of our partner customers, Imago Screens has adopted Avago’s SMT LED lamp technology, and demonstrated the latest Variable Message Signs at the International Road Safety and Equipment Exhibition last October in Madrid, Spain.
“We’ve been working with Imago for over a decade on the development of innovative variable message and full color signs for their customers,” says Fatt-Lun Ho, vice president and general manager, Optoelectronics Products Division, Avago Technologies.
“During this period, they have found innovative methods to integrate our SMT LED’s into their electronic sign products, and have become one of our most valued customers. As a result of this relationship, we believe Imago’s commitment to using SMT lamps will mark the beginning of a technology component transition that will be adopted by more electronic sign suppliers throughout the industry.”
According to Tony Batllo, CEO Imago Screens, the collaboration between Avago and Imago Screens included a specific set of challenges to the Avago design engineers.
The project had the following objectives:
• Reduce the assembly cost (SMD against Through Hole Lamps).
• Avoid lens using the LED cover to reduce the usual SMD angle.
• Improve contrast using tinted LEDs which reduce reflection.
• Develop a “spider” LED, a lamp with a square encapsulation.
• Reach similar optical performance with less power consumption.
The Avago engineers were able to meet most of Mr. Batllo’s requirements, including all of his technical requirements.
According to Mr. Batllo, “with the pace of product innovation and introductions continuing at an unabated pace in the variable message and full color sign market, our partnership with Avago will prove to be a great asset to us and enable us to stay at the forefront of our industry in supplying innovative electronic sign products.”
High brightness, low power and superior outdoor reliability performance are key attributes for ESS applications. To achieve high brightness while maintaining very low driving currents, the light ray from the LED chip must be efficiently directed to achieve a viewing angle of 30deg Round, or 40x100deg in the Oval shapes.
The semiconductor platform and leads (lead frame) must be designed with the reflector cup in close proximity to the LED chip itself. Additionally an optical lens must be encapsulated to create the complete package. The combination of a reflector cup at the lead frame and epoxy encapsulated lens, will normally result in a high thermal-mechanical stress combination.
This results in technical challenges that affect reliability in an outdoor application. It is also more challenging to make the LED lens compatible with industrial reflow soldering processes while maintaining appropriate moisture sensitivity level (MSL).
To meet these challenges for the ESS industry and to deliver a high MSL rating the Avago engineers made several technological breakthroughs, for which they were granted several patents including, breakthroughs in the physical shape/geometry of the lens to maximize luminosity, development of a unique lead frame design to enable the reflector to be tightly integrated, and development of special epoxy materials to provide the moisture control needed in SMT LED lamps used outdoors.
The Avago SMT devices are compact in terms of form factor and spatial radiation pattern. The combination of package design and epoxy materials are the first of their kind in the world that meet or exceed all of the industry requirements while achieving the performance characteristics needed in outdoor signs and signals.
From a sign maker’s point of view, the sheer number of LEDs used in Electronics Sign Applications per display panel is extremely high, necessitating the need for Lamp package designs that are simple, cost effective and manufacturing friendly.
The signage system designer and manufacturing engineers can utilize familiar processes to install the Avago devices such as utilizing solder reflow technology resulting in faster, more efficient, and cost-effective board assembly.
These patented technology improvements enable the customer greater ease of assembly through the use of a pick and place reflow soldering processes, providing designers faster cycle time with higher output. When utilizing the older Through-Hole technology, designers are limited to two PCB layers.
Avago’s SMT LED’s enable designers to implement multiple layer PCBs reducing the number of PCBs needed. With SMT the entire front of the PCB can contain display LED’s allowing the drivers, controllers and other components to be placed on the back of the board. Thus achieving a much higher component density which in turn reduces the thickness, weight and cost of the overall LED based sign.
About the author: Shi Khee Yong is ESS Product Marketing Manager, Avago Technologie.