LED Chip Pad Structure With Aluminum Stress Buffer for Flexible Mounting
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Solution Overview
Problem
GaN-based flip-chip semiconductor diode chips experience physical stress-induced detachment when mounted on flexible substrates due to the high stress of nickel metal layers, leading to reliability issues.
Innovation Solution
The LED chip design includes a stress buffer layer with alternating aluminum and titanium metal layers, where the aluminum metal layer closest to the nickel metal layer is thicker than the others, enhancing stress buffering and adhesion, preventing detachment during substrate bending.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a nickel metal layer is used in the electrode pad to prevent tin diffusion and form eutectic connection, then the connection reliability between chip and circuit substrate is improved, but the stress of the nickel metal layer causes the LED chip to fall off during bending of flexible substrate
Solution Approach 1:
The electrode pad structure is segmented into multiple functional layers: adhesion layer, stress buffer layer, nickel metal layer, and protective layer. The stress buffer layer is further segmented into multiple aluminum metal layers with different thicknesses, where the first aluminum layer (closest to nickel) has greater thickness than the second aluminum layer. This segmentation allows each layer to perform its specific function independently, with the stress buffer layer specifically designed to mitigate the stress problem of the nickel layer.
Solution Approach 2:
The electrode pad uses a composite multi-layer structure combining different materials (titanium adhesion layer, aluminum stress buffer layers, nickel metal layer, and protective layer). Each material is selected for its specific properties: titanium for adhesion, aluminum for stress buffering, nickel for diffusion barrier and eutectic formation, and the protective layer for final protection. This composite structure resolves the contradiction by combining the benefits of different materials while mitigating their individual drawbacks.
2Reliability
If the nickel metal layer thickness is increased to prevent tin diffusion, then the diffusion barrier function is improved, but the stress-induced chip detachment problem worsens
Solution Approach 1:
The stress buffer layer composed of multiple aluminum metal layers is placed between the adhesion layer and the nickel metal layer to provide beforehand cushioning. The first aluminum layer, being thicker, is positioned closest to the nickel layer to absorb and buffer the stress generated by the nickel layer before it can transmit to the LED chip and cause detachment. This prior cushioning prevents the harmful stress effect while maintaining the necessary nickel layer thickness for diffusion protection.
3Object-affected harmful factors
If a stress buffer layer is added to buffer the nickel metal layer stress, then the stress impact on LED chip is reduced, but the existing stress buffer layer arrangement cannot achieve the best stress buffer effect
Solution Approach 1:
The stress buffer layer exhibits local quality through its multi-layer structure with varying thicknesses. The first aluminum layer has greater thickness than the second aluminum layer, creating a gradient structure where stress buffering capacity is locally optimized. The thicker first layer is positioned where stress from the nickel layer is most intense, providing enhanced local stress absorption exactly where needed, rather than using a uniform thickness throughout.
Solution Approach 2:
The stress buffer effectiveness is optimized by changing the thickness parameter of the aluminum metal layers. Specifically, the thickness of the first aluminum layer is increased relative to the second aluminum layer, creating a non-uniform thickness distribution. This parameter change allows the stress buffer layer to better accommodate and buffer the stress from the nickel layer, achieving superior stress buffer effectiveness compared to uniform thickness designs.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design effectively mitigates stress-induced detachment, improving the reliability and durability of the LED chip when used on flexible substrates by utilizing the ductility and tensile strength of aluminum metal layers.
Implementation Method 1
utilizing the ductility and tensile strength of aluminum metal layers
Implementation Method 2
the aluminum metal layer closest to the nickel metal layer is thicker than the other aluminum metal layers, so as to buffer physical stress
Implementation Method 3
the nickel metal layer can form eutectic with the tin metal layer after the reflow soldering process to realize the connection between the chip and the circuit substrate
Implementation Method 4
the aluminum metal layer closest to the nickel metal layer is thicker than that of any one remaining aluminum metal layer, so as to buffer physical stress caused by bending of the flexible substrate, avoid the chip from falling off
Data Source
AI summary
Provided are an LED chip and a light emitting device, which include a light emitting epitaxial layer, a first insulating layer covering the light emitting epitaxial layer, and a first pad and a second pad located on the first insulating layer. Each of the first pad and the second pad includes a stress buffer layer, a nickel metal layer, and a protective layer. The stress buffer layer includes aluminum metal layers; and by adjusting the thickness distribution of the aluminum metal layers, an aluminum metal layer closest to the nickel metal layer is set to be the thickest, and the aluminum metal layers are thickened, especially the thickness of the aluminum metal layer closest to the nickel metal layer is greater than or equal to the thickness of the nickel metal layer, therefore, the ductility and tensile property of aluminum metal are utilized to play a role in stress buffering.