"Three Keys" Solve the Problem of Matrix LED Packaging Technology

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, camera functions such as display backlights and camera flashes, like LED displays. Consumer products such as backlights and projection systems, such as buildings with distinctive lighting and signs, and many other applications. The LED has high brightness, high luminous efficiency and fast response. Incandescent lamps have been replaced in many ways due to their low energy consumption, long service life, low heat release and the ability to emit colored light.

As the efficiency of LEDs continues to increase, the amount of lumens per watt that is generated continues to increase, and the use of LEDs for general illumination becomes more and more practical. For example, in 2003, a fluorescent tube equivalent to 3000 lumens required more than 1300 LEDs with an efficiency of 30 lumens per watt to achieve comparable results. However, by 2005, the number of LEDs required to achieve the same fluorescent tube illumination was reduced by a factor of 20, requiring only about 50, each LED having a luminous efficiency of 50 lm/W or higher and a luminous intensity of 60 lumens.

LED lighting level

There are four links in LED production, or four areas. The first link is called product link 0, which refers to the production device itself. The second link of product link 1 is the first-level package, which refers to the method of connecting the device to the power supply through chip bonding and wire bonding to form a surface mount package. The third link of product link 2 refers to the secondary package. Put multiple primary packages together to create the light output required for an external signal or outdoor lighting application. The fourth link, product link 3, is to systematically package the entire system or solution.

The primary LED package includes a single LED and a complex LED matrix package. In a standard LED array, each LED is connected to a substrate electrode. LEDs can be handled separately or connected together. Most of this type of package utilizes epoxy adhesive chips. For high-brightness LED applications, such as outdoor lighting or tail-projection screen illumination, a matrix-structured LED is required. In this configuration, the LEDs are arranged in tight rows and columns to obtain as much light as possible. Figure 1 is a matrix diagram of LEDs that together emit a large amount of lumens. The number and alignment of the LEDs requires good thermal conductivity of the die attach material to keep the LEDs as cold as possible.

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

Package high brightness LED

Matrix LED process steps include material preparation, chip pick and place, pulse reflow, cleaning, wire bonding, and testing. The following discussion will focus on pulse reflow (low temperature eutectic bonding) and wire bonding steps. An example is a 98 290 祄 LED matrix using AuSn bonding. The LEDs are electrically connected together in the column direction. The goal is to connect the LEDs to the substrate using metallurgical eutectic interconnects, placing the LEDs as close as possible based on component tolerances (a gap of about 1 mil).

Pulse reflow

The key to the LED packaging process is to avoid holes in the eutectic solder of the diode and its substrate, and the thermal and electrical connections required to produce stable light transmission are accomplished by solder. The eutectic chip adhesive transfers the enormous thermal energy generated by the diode to maintain the thermal stability of the device. Controlling the eutectic bonding process is the key to achieving high yield and reliability.

Accurate eutectic component bonding includes diode pick-and-place, on-site reflow of pre- or pre-tinted devices using programmable x, y or z-axis agitation, as well as programmable pulse heating or steady-state temperatures. To achieve an optimized heat transfer solder interface, the temperature profile of the bonding process must be repeatable with the ability to increase at high temperatures. When the interface temperature rises to the appropriate eutectic temperature, the heating mechanism must be maintained at the set temperature and the temperature overshoot should be as small as possible. After a necessary reflow time, the heating mechanism must be able to control the cooling, so that the damage to the diode is as small as possible, so that the eutectic material can reach the metallurgical balance. This balance is achieved by applying both active thermoelectric pulse heating and cooling gas.

LED matrix packages are temperature-sensitive processes that require careful control during the packaging process. The reflow profile of the in-situ eutectic die bonding process is designed to provide a constant melt and non-porous bonding interface. This is necessary to keep heat out of the diode and to maintain temperature stability while the LED is operating.

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