The medium used for component attach is key aspect of LED assembly. There is ample evidence that gold-tin (Au/Sn) yields the highest performance, although assembly costs may increase via cycle time. High-volume light engines can be assembled with silver epoxy to provide highest throughput.
Recently, Palomar Technologies has been working with Indium Corporation to evaluate Au/Sn preforms and Au/Sn solder paste. Following months of testing, several notable results for the flux and reflow on Au/Sn-backed alloys and Au/Sn preforms were found. These results are described in the technical paper “Automated Precision Assembly for High-Volume HB LEDs”.
Since uncovering these results, Palomar Technologies launched additional studies to determine the most effective process for Au/Sn solder paste and how that process compares to silver epoxy. There are several ways to reflow Au/Sn solder that includes belt furnace reflow, vacuum reflow and a Pulsed Heat System (PHS) reflow. Each method is effective, but the PHS process is much faster and limits the LED exposure beyond 300°C since most LED components degrade beyond that temperature. A traditional PHS reflow cycle is completed in approximately 30 seconds and is above 300°C for only a few seconds; a traditional belt furnace cycle is approximately 20 minutes and above 300°C for several minutes.
Traditionally, the PHS process is supported on the die attach platform so time would be lost during the reflow cycle. However, the time impact is significantly diminished by continuing component attach of one part while the first part is being reflowed.
Since the introduction of automated die and wire bonders in the 1980s, equipment manufacturers and process engineers have been challenged to balance speed with repeatability. Today, automated die bonders can perform epoxy die attach at a rate of 1.5 to 4 thousand die per hour; and automated wire bonders can interconnect complex packages at speeds of more than 10 wires per second. The advantage of automation is speed and consistency. However, a major concern with high-speed automated microelectronic packaging is that if something in the assembly process is wrong, everything will be wrong. Having a tightly regulated process flow helps avoid the risk of building a large batch of rejected product in a short period of time.
To ensure time-to-market success in high-volume production, specific 
methods to achieve throughput and quality are required. Click here to learn more about strategies and methods necessary to achieve the ultimate goal of automated precision HB LED assembly—to blend the requirements of high-reliability and high-throughput to support high-volume commercial production.
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Donald Beck
General Manager, Palomar Technologes Assembly ServicesTM
Palomar Technologies