High-brightness Matrix LEDs
09/01/2007
Packaging Challenges and Solutions
Light-emitting diode (LED)-based applications are growing, and cover a broad range of markets including automotive lighting applications such as indicators, spot utility, and headlamps; camera functions like display backlights and camera flashes; consumer products such as LCD display backlight and projection systems; architectural uses like accent lighting for buildings, and signs; and many others. LEDs are bright, efficient, and quick to react. They have become a substitute for light bulbs in many applications because they use less power, have longer lifetimes, produce little heat, and emit colored light.
Using LEDs for general lighting is becoming more practical as they become increasingly more efficient, generating more lumens per watt. For example, a fluorescent tube equivalent of 3,000 lumens would have required over 1,300 LEDs using 30 lumens/watt at 2 to 3 lumens per LED in 2003. However, by 2005, there was a 20X reduction in the number of LEDs required for the same florescent tube, to around 50, using 50 lumens/watt or higher at 60 lumens per LED.
LED Lighting Levels
There are four different levels - or areas of involvement - in the manufacture of LEDs. The first level is called product level 0, and refers to the device itself. Product level 1 is first-level assembly. This involves connecting the device to the source of electricity through die-attach and wire bonding methods, creating a surface-mountable package. Product level 2 involves second level assembly. The package is put into a structure in multiples to create light output for applications like external signage or outdoor lighting. Product level 3 is system assembly for the whole system or solution.
Level 1 LED packages range from a single LED to a complex matrix of LEDs. In a standard-array LED, each LED is wire bonded to a substrate pad. LEDs can be addressed individually or tied together. Many of these type assemblies are die-attached with epoxy. For high-brightness LED applications, such as outdoor lighting or rear projection screen lighting, a matrix configuration of LEDs is used. In this configuration, the LEDs are placed in tightly packed rows and columns to generate the most light. Figure 1 shows a matrix of LEDs ganged together to produce a massive amount of lumens. The number and closeness of LEDs requires good thermally conductive die attach to keep the LEDs as cool as possible.
 Figure 1. Matrix LED |
Matrix LED assemblies are the basis of many systems found in production. Their emerging popularity is derived from their ability to get more lumens per watt from this configuration. However, matrix LED packages present challenges for both die attach and wire bonding compared to single-die packages. High-brightness LED applications require maximum thermal transfer to achieve performance requirements.
Packaging High-brightness LEDs
Matrix LED process steps include material preparation, pick-and-place, pulsed reflow, clean, wire bond, and test. This discussion will focus on pulsed-reflow (eutectic attach) and wire-bonding steps. The example is a 9×8 matrix of 290-µm LEDs using AuSn attachment. LEDs are electrically connected together in rows. The goal is to place LEDs as close as part tolerances allow (~1mil gap) using a metallurgical eutectic interconnect of the LEDs to the substrate. Figure 2 shows the 290-µm LED matrix.
 Figure 2. 290 micron LED attachment to AuSn before wire bond. |
Pulsed Reflow
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 matrix assembly is an extremely temperature-sensitive process that requires careful control during assembly. The reflow profile during an in-situ eutectic die-attach process is engineered to provide consistent melting and a void-free attach interface. This is necessary for consistent heat transfer from the diode and contributes significantly to temperature stabilization during LED operation.
In this example, a ganged pulse-heat reflow was used. During the pulsed heat cycle, temperatures were ramped from a pre-heat temperature to the reflow temperature using a servo-controlled ramp profile with low overshoot compared to traditional heater stage systems. The reflow temperature is held for a prescribed duration and then the cooling profile is commanded using both servo-control temperature and cooling gas. Temperature profile repeatability is critical to the process to allow proper eutectic wetting with low voids without damaging the LEDs. Required temperature profiles depend on the substrate materials, geometry, and solder composition. Programmable point-and-click profiling was used for establishing the temperature command profile. The system also captures actual temperature profiles during bonding for process traceability. Pulsed-heat profile control allows batch reflow of the LED matrix for reduced overall cycle time and minimum time at temperature for the protection of temperature-sensitive LED devices.
Wire Bonding
Once LEDs are attached, wire interconnect is completed using strings of wire bonds. The high-density, high-frequency LED matrix format requires LEDs to be interconnected using wires. 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 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. Figure 3 shows chain bonding using a wire bonder to place a ball-loop-intermediate stitch-loop-intermediate stitch-loop. It ends with a loop-end stitch which is followed by a security ball bump on the end stitch. This is not new technology, but it has been further developed through materials selection and software tools. 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.
Conclusion
Placing LEDS in a matrix configuration can result in high intensity, brighter LEDs. This configuration creates challenges in assembly because of the high concentration of heat and the need for high-frequency wire-bonded connections that must be accurately placed in tight areas, have consistent loop structure, and have a connection strong enough to withstand mechanical shock and stresses due to large thermal variations. Three steps in the assembly process are key. The first step involves high accuracy die pick-and-place for matrix LED applications to within LED geometric tolerances. Second, using a pulse-heat-controlled batch eutectic reflow die-attach process is necessary for assembly throughput, LED protection, and high thermal conductivity while providing high quality, low risk performance. Third, chain bonding provides excellent electrical and mechanical connection of the arrays across the LEDs. Adhering to these assembly processes results in high brightness while achieving thermal dissipation and maximum light extraction.
 Figure 3. Chain bonding with security bond |
DAN EVANS, senior scientist, may be contacted at Palomar Technologies, Inc 2728 Loker Ave. West, Carlsbad CA 92010-6033; 760/931-3600; E-mail: [email protected].