Ultra-Fine-Pitch LED Display Chip Structure

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Wire-bonding LED chips, vertical LED chips, and flip-chip LEDs are three different structures of LED chips. Different structures of LED chips influence the selection of lead-free solder pastes.

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1. LED Chip Definition

The LED chip is a semiconductor device that realizes the photoelectric conversion function. It is made by processing semiconductor chips on epitaxial wafers. This semiconductor chip fabrication process is to transfer metallic organic compounds to a homogeneous or heterogeneous substrate at a suitable temperature and to grow semiconductor thin film materials with specific photoelectric properties through chemical reactions.

2. LED Chip Lighting Principle

Each pixel of the LED display screen is composed of a set of RGB (red, green, and blue) chips, which belongs to the self-luminous display technology. RGB LED luminescent materials are different according to the various sizes of the band gaps between the conduction bands and the valence bands of the materials. Thus, the energy released is different when the materials are excited, which is reflected by the different wavelengths of electromagnetic waves. The human eye receives different light colors of electromagnetic waves with different wavelengths. The core part of the LED is the p-n junction.

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3. LED Chip Structure

There are three main types of LED chip structures: wire-bonding LED chips, vertical LED chips, and LED flip-chips.

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The wire-bonding LED chip electrodes are above the light-emitting surface. The components from top to bottom are P-GaN, light-emitting layer (PN junction), N-GaN, and substrate. The vertical chip structure adopts Si, Ge, Cu, and other materials to replace the sapphire substrate, improving heat dissipation efficiency. The vertical LED electrodes are on both sides of the LED epitaxial layer to avoid local current congestion and high temperature. Flip-chip structure consists of N-GaN, light-emitting layer (PN junction), P - GaN, metal electrodes, and bumps.

The wire-bonding structure mainly uses blue-green LED chips. The vertical structure applies red LED chips. The flip chip is suitable for red, blue, and green LED chips.

For the previous fine-pitch LED (1.0mm≤Pitch≤2.5mm) packaging, wire-bonding blue-green LED chips, and vertical red LED chips are used. With the rise of mini-LED and micro-LED and the shrinking of chip size and pixel pitch, the wire-bonding structure is limited by space due to the need for bonding wires when the pixel pitch is less than P0.78 (data from experts). Therefore, it is required to adopt a LED flip-chip structure.