The LEDs They Are a-Changin’

Dr Giles Humpston, Applications Manager
Cambridge Nanotherm

LEDs have been around a surprisingly long time. Science project curiosities appeared in the swinging 60′s, and then a decade later the first mass-produced and affordable devices became commercially available. Monochrome LEDs are now manufactured in wavelengths that span the deep UV to far infrared, and the power of white LEDs is beginning to resemble an artificial sun. These accomplishments have been achieved by impressive innovations in LED semiconductor physics and optics.

Bob_Dylan_in_Toronto2However, what has not changed is LED packaging technology. Irrespective of whether it is a simple indicator LED or the latest offering from a company in China you’ve never heard of, LED packages are pretty standard affairs. The LED is attached to a base that provides a modicum of heat spreading and a facility for making electrical connections, while a glob of plastic over the top does a bit of beam shaping, wavelength conversion and provides an element of protection from the environment.

There are two problems with this approach. First, there are multiple interfaces between the semiconductor and the heat sink. LEDs and heat are not good bedfellows, and there exists a plethora of failure modes that will cut short the life of any hot LED.

More problematic is the method of manufacture. LED packaging involves a discrete assembly process. Each die is individually picked from the semiconductor wafer then placed in a package. While machines exist that can do this many times per second, when attempting to build hundreds of millions of LEDs, the time adds up (along with the cost).

All of these challenges are neatly circumvented by the move towards chip-scale packaging (CSP). In a CSP, the materials that form the package are applied to the semiconductor while it is still in wafer form. The wafer is then diced to free packaged LEDs. This approach has multiple advantages. Whole wafer processing eliminates the handling of individual LEDs. There is less material wastage because the LED and package have the same plan area dimensions, and thermal management is improved because the CSP provides a short thermal path between the LED and the circuit board. With these cost and performance advantages, it is unsurprising CSP LEDs are on target to achieve 10 percent market penetration of packaged LEDs within their first year of commercial availability.

The availability of CSPs is also a huge boon to the LED light engine manufacturers. To build products, CSP LEDs are simply soldered to a thermally conductive PCB. However, the high thermal dissipation requirements of clusters of CSP LEDs means only the best-in-class metal-in-board PCBs have sufficiently low thermal resistance to be useable. Fortunately, there exist products that are good enough for the task. The high-power density of CSP LEDs means attention must be given to managing heat spreading in the copper tracking layer of the metal-in-board PCB, but the simplicity of the structure makes finite element modelling a synch.

Just as Dylan oversaw a social revolution back in the 60′s, so LEDs have ushered in a revolution in lighting technology. But while Dylan ‘going electric’ was an evolution too far for some, the continued evolution of the LED, from its humble roots in the 60′s to the development of CSP technology today, is proving to be as revolutionary as ever.

CN_HumpstonAbout the Author 
Dr. Giles Humpston is a metallurgist by profession and has a doctorate in alloy phase equilibria.  He is a cited inventor on more than 250 patents and has co-authored over 150 papers as well as several text books.  Dr Humpston currently works as the Field Applications Manager for Cambridge Nanotherm on thermal substrate technologies.



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