High Brightness Matrix LED Packaging Challenges and Solutions

In recent years, the application of light emitting diodes ( LEDs ) has been growing, covering a wide range of markets, including automotive lighting applications such as indicator lights, spotlights and headlights, photographic functions such as display backlights and camera flashes, consumer products such as LED display backlights and projection systems, architectural applications such as building feature lighting and signs, and many other applications. LEDs have high brightness , high luminous efficiency and fast response. Due to their low energy consumption, long service life, low heat release and ability to emit colored light, they have replaced incandescent lamps in many aspects.

As LED efficiency continues to improve, the amount of lumens produced per watt continues to increase, and the use of LEDs for general lighting is becoming more and more practical. For example, in 2003, a fluorescent tube equivalent to 3,000 lumens required more than 1,300 LEDs with an efficiency of 30 lumens per watt to achieve the same effect. But by 2005, the number of LEDs required to achieve the same fluorescent tube lighting effect was reduced by 20 times to only about 50, each with an efficiency of 50 lumens per watt or more and a luminous intensity of 60 lumens.

LED lighting Horizontal

There are four stages, or areas, in LED production. The first stage is called Product Stage 0, which refers to the production of the device itself. The second stage, Product Stage 1, is the primary packaging, which refers to connecting the device to the power source through die attach and wire bonding to form a surface mount package. The third stage, Product Stage 2, refers to the secondary packaging. Multiple primary packages are put together to form the light output required for applications such as external signals or outdoor lighting. The fourth stage, Product Stage 3, is the system packaging of the entire system or solution.

Primary LED packaging includes packaging of single LEDs and complex LED matrices. In a standard LED array, each LED is connected to a substrate electrode. The LEDs can be processed separately or connected together. Most of this type of packaging uses epoxy to adhere the chip. For high-brightness LED applications, such as outdoor lighting or tail projection screen lighting, a matrix structure of LEDs is required. In this structure, the LEDs are arranged in tight rows and columns to obtain as much light as possible. Figure 1 is a diagram of a matrix of LEDs, which together can emit a huge amount of lumens. The number and tightness of the LEDs require good thermal conductivity of the chip attach material to keep the LEDs as cool as possible.

Figure 1 LED matrix diagram

Matrix LED packages are the basis of many systems in production. Their recent popularity is due to the fact that this structure can achieve more lumens per watt of power . However, compared with single-chip packages, matrix LED packages pose great challenges for die adhesive and wire bonding. High-brightness LED applications require maximum heat transfer to meet performance requirements.

Packaged high brightness LED

The matrix LED process steps include material preparation, chip placement, pulse reflow, cleaning, wire bonding, and testing . The following discussion will focus on the pulse reflow (low temperature eutectic bonding) and wire bonding steps. The example is a 9 8 290μm LED matrix using AuSn bonding. The LEDs are electrically connected together in the column direction. The goal is to use metallurgical eutectic interconnects to connect the LEDs and substrate together, and arrange the LEDs as closely as possible based on component tolerances (about 1 mil gap). Figure 2 shows the 290μm LED matrix.

Figure 2 290 micron LED attached to AuSn before wire bonding

Pulse reflow

The key to LED packaging is to avoid holes in the eutectic solder between the diode and its substrate, and to create thermal and electrical connections for stable light transmission . The eutectic die adhesive transfers the huge heat generated by the diode to maintain the thermal stability of the device. Controlling the eutectic bonding process is the key to high yield and reliability.

Precise eutectic part bonding involves diode pick and place, in-situ reflow of pre-molded or pre-tinned devices with programmable x, y or z-axis stirring, and programmable pulsed heating or steady-state temperature. To achieve an optimized heat conduction solder interface, the temperature profile of the bonding process must be repeatable and capable of high temperature ramp rates. When the interface temperature rises to the appropriate eutectic temperature, the heating mechanism must maintain the set temperature with minimal temperature overshoot. After a required reflow time, the heating mechanism must be able to control cooling to minimize damage to the diode and allow the eutectic material to reach metallurgical equilibrium. This balance is achieved by the simultaneous application of active thermoelectric pulse heating and cooling gases.

LED matrix packaging is a very temperature sensitive process that requires careful control during the packaging process. The reflow temperature profile of the on-site eutectic die bonding process is designed to provide a constant melt and void-free bonding interface. This is necessary to stably transfer heat away from the diode and maintain temperature stability when the LED is operating.

This example uses wire reinforcement pulse heat reflow. During the pulse heating cycle, the temperature is ramped from the preheat temperature to the reflow temperature using a servo system controlled ramp profile, which results in very low temperature overshoot compared to conventional heating systems. The repeatability of the temperature profile is critical to the process, which allows for proper eutectic wetting with minimal voiding and no damage to the LEDs. The required temperature profile depends on the material used for the substrate, the size of the substrate and the composition of the solder. Wetting is performed using a point-and-click programmable profile to form a temperature command profile. The system captures the actual temperature profile during the wire bonding process for process traceability. Pulsed heat profile control allows batch reflow of LED matrices, reducing overall cycle time and keeping the high temperature time as low as possible to protect temperature sensitive LED devices.

Wire Bonding

After the LEDs are bonded, wirebonds are used to complete the interconnection. High-density, high-frequency LED matrix formats require that the LEDs be interconnected using metal wires. Although there are a variety of wire bonding methods, such as ball bonding and wedge bonding, test data shows that chain bonding interconnection using a ball bonder gives the best results. For standard ball/pin bonding, a ball is formed first, and then the wire is pulled to the pin to bond to form the LED interconnection. Chain bonding is a variation of ball/pin bonding, where the pin is not a terminal, and a coil-pin combination is formed on it to complete the chain wire bond set. Figure 3 shows a chain bond using a wire bonder, setting up a ball-wire arc-intermediate pin-wire arc-intermediate pin-wire arc. The last is a wire arc pin, and then a ball pin is formed on each terminal pin to ensure the connection. This is not a new technology, but it has been further developed through material selection and software tools. Chain bonding allows for higher yields because it forms a standard ball bond without the need to form an air-free sphere. In addition, due to the chain-shaped solder pin pattern, there is less light occlusion, and the tensile test results prove that it has better pull-out strength.

Figure 3 Chain welding with safety connection

in conclusion

Assembling LEDs in a matrix can achieve higher density and higher brightness LEDs. This structure poses a challenge to packaging due to the high concentration of heat and the requirement for high-frequency wire bonding connections. Bonding wires must be placed accurately in areas where LEDs are densely packed, and the connection has a stable wire arc shape. Due to large thermal disturbances, the connection strength should also be strong enough to withstand mechanical shock and stress. There are three critical steps in the packaging process. The first step is to pick and place the chips with high precision to achieve matrix LED applications within the geometric tolerance of the LED. Second, it is necessary to apply pulse heating controlled batch eutectic reflow chip bonding processes for assembly production, LED protection, and good thermal conductivity, while providing high quality and low-risk performance. Third, chain connections provide excellent array electrical and mechanical connections for all LEDs. These packaging processes can achieve high brightness effects while also achieving heat dissipation and maximum light extraction efficiency.