Place 10,000 µ-LEDs in less than Two Minutes

By Dr. Subodh Kulkarni, President and CEO CyberOptics and Justin Wendt, Chief Technology Officer, Rohinni. As published in Global SMT and Packaging.

Mini (m-) and micro (µ-) LEDs are poised to usher in a new generation of display and specialty lighting technologies. Both offer many advantages over current technologies, including higher brightness, blacker blacks, wider color gamut, greater energy efficiency, resistance to moisture and oxygen and absence of image burn-in. The primary barrier to wide-spread adoption of m-and µ-LED based products is manufacturability. Both are based on conventional LED fabrication processes, but at greatly reduced sizes. Conventional LEDs measure a millimeter or more on a side. Mini LEDs are between 1mm and 100 µm (a grain of table salt), and micro LEDs between 100µm and 1µm (the width of a human hair). Compared to other integrated circuits, LED structures are relatively simple, and the fabrication process is well understood. One of greatest challenge arises because, in most applications, m- and µ-LEDs must be separated and placed individually on the final product substrate with placement accuracies that are a small fraction of the LED size and with enough speed to be economically viable. In addition, manufacturers need measurement and inspection capability that allow them to control the process, ensure quality and hit yield targets. One solution already in use in high volume manufacturing combines a novel placement technology (Pixalux/Rohinni®), capable of placing mini LEDs at rates greater than 50/sec, with a 3D inspection and measurement technology (MRS™/CyberOptics®), widely used to monitor the placement of electronic components using surface mount technology (SMT).

LEDs

All LEDs (light emitting diodes) use similar technology to create light. When current flows through a forward biased diode of appropriate materials, electrons and holes recombining near the junction between the p- and n-type semiconductors that comprise the diode emit light. The wavelength of the light can range from infrared to ultraviolet, encompassing the entire visible range. The light is not coherent or monochromatic (like a laser), but it does have a relatively narrow wavelength distribution. The wavelength is determined by the difference in the energy levels of the conduction and valence bands of the semiconductor materials. White LEDs are made by applying a phosphorescent coating to a blue LED, such that the yellow emissions of the coating combine with the blue light of the LED to create a broad-spectrum white light. Color displays combine red, green, and blue LEDs to generate colors across a wide color gamut. An important difference between conventional LEDs and m-/µ-LEDs is the absence of packaging in the latter. Packaging provides protective encapsulation and electrical connections for conventional LEDs.