Light emitting diode (LED)-based applications are growing, and cover a broad range of markets. LEDs are bright, efficient, and quick to react. They have become a substitute for light bulbs in many applications because they use less power, have longer lifetimes, produce little heat, and emit colored light.
LEDs are a special type of diode, which is a common electronic circuit component used in all electronic devices. Like all diodes, an LED passes electric current in one direction only. All diodes dissipate some energy when the electric current passes through; common diodes convert this energy to heat, but LEDs convert the energy substantially to light.
In this special series of blogs, we are going to dive into various elements and characteristics of LEDs, from A to Z. Hopefully, by the time we make it to “Z” we will be well versed in LED knowledge—so let’s get started!
A - Attach process
High brightness matrix LEDs involve traditional ball-stitch (crescent) bonding where a ball is placed on the previous stitch to create a chain-like string of wires. This method requires a traditional flame-off sequence to form a free air ball for every interconnection which reduces throughput.
Chain bonding (wedge bond emulation) is a variant on ball/stitch bonding where the stitch is not terminated and another loop-stitch combination is repeated to complete a chain bond wire set. Chain bonding enables higher throughput since there is no need to create free air balls for standard ball bonding. Additionally, less light occlusion exists due to chain bond stitch geometry, and pull test results indicate better pull strength. Chain bonding starts with a ball bond and then forms a loop then stitch. Chain bonding is a promising interconnect technology for some applications which can start with a ball bond and it can help reduce cost. Once LEDs are attached, wire interconnect is completed using strings of wire bonds. The high-density, high-frequency LED matrix format requires LEDs to be interconnected using wires. Although there are several methods of wire bonding, such as ball and wedge, test data indicates that chain bonding interconnects using a ball bonder achieves the best results.
In addition to LED wire bond processes, Palomar Technologies Assembly Services has also supported precision direct-chip attach (DCA) LED bonding for several years. Direct Chip attach light emitting diodes (LEDs) are the next generation of solid-state LED emitters. They deliver superior value for consumer products and markets that include TV backlighting, camera flashes, and a variety of general illumination needs. The bond-pad down design of direct attach LEDs eliminates the need for wire bonds, yet improves the thermal management.
Palomar Technologies has developed a precision eutectic process for direct attach LEDs. The Pulsed Heat System allows precision die attach systems to control LED exposure much more effectively than most other processes. Both the 3800 and 6500 Die Bonder offer PHS for eutectic die bonding.
With both anode and cathode terminations on the bottom side of the LED, controlling the amount of flux becomes even more important. You cannot have any bridging between the anode and cathode. Assembly Services has perfected the precision and repeatability of the DCA process, allowing us to support designs with high numbers of LEDs per part.
B – Boule
In an LEDs Magazine article, Steven Chen of KLA-Tencor explains: "The main substrate utilized in the production of LEDs is sapphire, gallium arsenide, or silicon carbide. There are also other initiatives for alternative substrates such as gallium nitride and silicon. With any substrate, a crystalline boule is produced in a similar fashion as the semiconductor process in the integrated circuit industry. The boule is normally sliced into very thin wafers with a diamond saw, and after slicing, they are polished via a rigorous process before being shipped to the LED manufacturer for further processing."
Stay tuned next week for more on LEDs!
Pulsed Heat System
3800 Die Bonder
6500 Die Bonder
LED Journal white paper: http://www.ledjournal.com/main/category/white/,
A Critical Advance In MR16 Lamps, http://www.soraa.com/public/docs/A_Critical_Advance_In_MR16_Lamps-Benya.pdf.
LEDs Magazine article: “Improve LED manufacturing via in-line monitoring and SPC”, http://ledsmagazine.com/features/10/7/10.
Palomar Technologies, Inc.