Gold-Tin Eutectic Solder Advantages for LED Assembly

LED arrayLEDs 2012 was another well-attended international conference, held at the Hilton on the San Diego bay. Attendance has been up year over year, with LED vendors from around the world showcased an array of LED solutions during the two-day exhibition. Of the various markets represented, industrial lighting stood out. The benefits of replacing high use lighting with LEDs has become more and more appararent for industrial use: brightness, stability, long life, and—oh yes—vastly reduced energy consumption per watt used.

New 2013 requirements are expected to better regulate this emerging industry. Many technical specifications will be needed for all publically sold LED lights. Some requirements—including estimated lifetime given a daily usage, color/temperature, lumen output and even lighting angles—will be on the back of all LED products starting next year. Product statements will be regulated with standard formats.

HBLEDMatrix paper imageHigh-power/High-brightness LEDs (HP/HB LEDs) consume only a fraction of power in comparison to incadencent bulbs, but LED applications tend to generate a lot of heat, and in a very small localized area. Temperature per unit or area is high with HP and HB LEDs, requiring an inherent cooling process if they are to last a long time. In response, some pretty interesting heat sinks and heat spreaders are available. But it all starts inside the LED, and that heat has to be removed and controlled, or the LED will self destruct. Download the "HB LED Matrix" technical paper to learn more about high-precison, automated LED assembly processes.

eutectic scrubWhile the microelectronics industry is working hard to develop low-cost LED packaging methods—such as with thermally and electrically conductive epoxies—eutectic media still dominates this lighting market. More precisely, hard eutectic solders. The coolest of all hard solders is good old 80/20 gold-tin (AuSn). Many LED manufacturers routinely back-side metalize their wafers prior to dicing. Experienced vendors also apply a very small amount of gold-tin solder to the packages that interface with the heat sinks/spreaders. The next step is bringing the substrate and die together and then applying the proper temperature profile in an inert gas environment—possibly scrubbing the interface under a specific force, and cooling the eutectic media back down to its plastic or solid point before final release. Another method if an array of LEDs reside in a single package is gang reflow. ProcessReliabilityAuSn paper image

Check out the "AuSn Eutectic Die Attach Process and Reliability Advantages" technical paper.

 

Reflowed Die Attach
Critical to the LED assembly process is a void-free eutectic solder interface between the diode and its substrate that provides the thermal and electrical connections needed to generate a stable transmission of light. Eutectic die attachments transfer the tremendous amount of heat generated by the diodes to maintain the temperature stability of the device. Controlling the eutectic attach process is critical to yield and reliability. Precision eutectic component attach includes pick-and-place of the diode; in-situ reflow of pre- form or pre-tinned devices with programmable x, y, or z-axis agitation; and programmable pulse heating or steady-state temperature. To yield the optimal thermally conducting solder interface, the temperature profile of the attachment process must be repeatable and have the capability for a high-temperature ramp rate. Once the interface is brought up to the proper eutectic temperature, the heating mechanism must maintain that programmed temperature with minimal overshoot. After the required amount of reflow time, the heating mechanism must controllably cool to minimize damage to the diode and to allow the eutectic material to reach metallurgical equilibrium. This equilibrium is reached through simultaneous application of active thermoelectric pulse heating and cooling gases.

LED Wire Bonding
Chain wire bonding for LED deviceOnce LEDs are attached, wire interconnect is completed using strings of wire bonds. Although there are several methods of wire bonding, such as ball and wedge bonding, test data indicate that chain bonding interconnects using a ball bonder achieves the best results. In standard ball/stitch bonding, a ball is placed and bonds are placed on top of the stitches to create a string of interconnected LEDs. 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.

After this first-level interconnect step, many LED packages add a phosphor to manage, enhance and encapsulate the LEDs. The result is a high-reliability, LED-based device that will last for a long, long time. LED encapsulation

HBLEDMatrix paper imageDownload the "HB LED Matrix" technical paper to learn more about high-precison, automated LED assembly processes.

There is no doubt that this market will continue to grow as we see LEDs to continuing replacing lighting in our homes, cars and other indoor/outdoor lighting venues. Stay tuned!

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Bradley Benton
Regional Account Manager, Western Americas
Palomar Technologies, Inc.