iLED iLED December 2012 : 10

DECEMBER 2012 ISSUE N°6 Y OLE ASKS Automated die bonding levels playing fi eld David Rasmussen, senior applications engineer at Palomar Technologies in Carlsbad, California, underlines the reliability and cost bene fi ts that the company’s die bonders provide, and how Palomar Technologies Assembly Services supports the demanding requirements of the today’s LED industry. Yole Déve loppe ment: What ’s Palomar Technologies’ history in providing automated die bonding tools to the LED industry? David Rasmussen: We’ve been working with advanced LED designs for over seven years. Palomar Technologies was one of the fi rst companies to process matrix LEDs – in modules containing anywhere from six to over 1,000 die. YD: What are the key considerations in using automated bonding in such arrays? DR: One consideration is that everything is re fl owed freestanding, so the die fl oats. Keeping the LEDs from bumping into each other is one of the secret components to the recipe. In Cree EZ1000 literature, the recommended process is to put down a dot of fl ux as a tacking agent to keep that part from moving. The amount of fl ux that you use relative to the size of the LED component you’re placing is important. Then you put down as many LEDs as required for the application, and go through the ramp and re fl ow process. We do all of the assembly using our high-accuracy Pulsed Heat System (PHS) and platform, so we don’t have to move the part after the die are in place. The entire re fl ow process is completed on that same platform, ensuring that the components do not move. However, LEDs are sensitive to temperatures in excess of 300°C, yet gold/tin re fl ows in excess of 300°C. The PHS gives us the ability to tightly control the ramp up and cool down very rapidly, which is needed to limit high-temperature exposure. YD: How have you adapted these methods to changes in LEDs packaging? DR: We have supported direct-chip attach (DCA) LED bonding or re fl ow since early 2010. The DCA process helps minimize design space by eliminating the need for wire bonds. With both anode and cathode terminations on the bottom side of the LED, controlling the amount of fl ux 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. Some ultra-high performing applications require gold/tin on both the die and the substrate to achieve optimum void-free bonds. An example is when Assembly Services takes a Cree EZ1000 LED and uses a gold/tin eutectic solder to attach them to a diamond substrate. We then take the diamond substrate and place it into a gold-plated copper package using an indium solder. With these materials, cost can become a major consideration. In order to get the best possible void-free bond, we can support gold/tin sputtered on both the diamond substrate and on the LED. YD: What are the most important process parameters LED producers mention when they’re using your tools? DR: Other than a void-free interconnect, another area of concern might be low-pro fi le interconnects or stitch bonding across LEDs. Palomar Technologies was the fi rst company to de fi ne stand-off stitch bonding, which starts by placing a ball bump on the LED – the fi rst wire bond on the substrate – that is then bonded to the bump on the LED. Stand-off stitch wire bonds enable an interconnect height above the LED of less than 75 μ m, which is critical for some applications. Palomar Technologies was also one of the fi rst companies to support chain bonding, which stitches the wires across multiple LEDs. This technique allowed for the 1,000 LED module design David Rasmussen, Senior Applications Engineer, Palomar Technologies Service industry: Palomar technologies assembly services provides customers with high-accuracy die bonding tools without a capital equipment investment. (Courtesy of Palomar Technologies) 10 i LED

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