Flip-Chip LED Electrode Structure for Strong Tin Eutectic Bonding
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Solution Overview
Problem
Conventional flip-chip LED packaging on flexible substrates suffers from low bonding strength and eutectic bonding issues due to high thermal expansion coefficients and reduced available bonding area, leading to separation of electrodes and reduced reliability, especially with smaller pixel pitches.
Innovation Solution
A light-emitting device design featuring a carrier substrate with flip-chip LEDs and an electrode unit comprising multi-layered connecting electrodes, including a stress relieving sublayer, stress transition sublayer, and a binding layer with a tin eutectic system, to enhance bonding strength and reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional connecting electrodes are bonded to flexible substrate, then flexibility is achieved, but bonding strength deteriorates due to high coefficient of thermal expansion
Solution Approach 1:
The connecting electrode uses a composite multi-layer structure comprising a first connecting electrode layer (Cu or Cu alloy), a first intermediate layer (Ni or Ni alloy), and a first binding layer (Sn or Sn alloy). This composite structure addresses the thermal expansion mismatch between flexible substrate and rigid electrode by distributing thermal stress across layers with different mechanical properties, preventing delamination while maintaining flexibility.
Solution Approach 2:
The patent specifies controlled thickness parameters for each layer: the first connecting electrode layer is 1-10 μm, the first intermediate layer is 0.1-5 μm, and the first binding layer is 0.1-5 μm. These parameter optimizations ensure adequate bonding strength while accommodating thermal expansion differences, resolving the contradiction between flexibility and bonding strength.
2Manufacturing precision
If LED chip size is reduced for smaller pixel pitch, then display resolution is improved, but eutectic bonding area is reduced leading to weaker bonding
Solution Approach 1:
The multi-layer composite electrode structure (Cu layer + Ni intermediate layer + Sn binding layer) increases the effective bonding interface area and provides multiple bonding mechanisms (metallic bonding, diffusion bonding, eutectic bonding). This compensates for the reduced total bonding area in smaller LED chips, maintaining strong connections despite reduced pixel size.
Solution Approach 2:
The Ni intermediate layer acts as a mediator between the Cu connecting electrode and the Sn binding layer, facilitating controlled diffusion and forming a stable transition zone. This intermediate layer ensures reliable eutectic bonding even when the overall bonding area is reduced due to smaller chip dimensions.
3Strength
If multi-layer electrode structure is implemented to improve bonding, then device complexity increases
Solution Approach 1:
The electrode is segmented into functionally distinct layers: Cu layer for electrical connection, Ni intermediate layer for diffusion control and mechanical transition, and Sn binding layer for eutectic bonding. This segmentation assigns specific functions to each layer, optimizing performance while keeping the overall structure manageable through clear functional division.
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 multi-layered electrode structure improves die bonding and shear strength, preventing tin diffusion and maintaining interfacial bonding, thereby enhancing the reliability and performance of LED devices, particularly in high power applications.
Implementation Method 1
One of the flip-chip packaging techniques involves applying a solder paste between connecting electrodes of the LED chips and the substrate (such as a packaging substrate or circuit board), followed by subjecting the LED chips and the substrate to heating in a reflow oven, so as to achieve eutectic bonding.
Implementation Method 2
the binding layer includes a first portion that is adjacent to the carrier substrate and that forms an eutectic system with tin
Data Source
AI summary
A light-emitting device includes a carrier substrate, a flip-chip light-emitting diode (LED) mounted onto the carrier substrate, and an electrode unit disposed between the carrier substrate and the flip-chip LED. The electrode unit includes first and second connecting electrodes that have opposite conductivity. Each of the first and second connecting electrodes includes an intermediate metal layer and a binding layer that are sequentially disposed on the flip-chip LED in such order. The binding layer includes a first portion being adjacent to the carrier substrate and forming an eutectic system with tin, and a second portion located between the first portion and the intermediate metal layer.