CSP LED Soft Buffer Layer Adhesion and Color Uniformity
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Solution Overview
Problem
Chip-scale packaging (CSP) LED devices face issues with weak adhesion between the photoluminescent structure and the LED semiconductor die, poor color uniformity due to inconsistent phosphor particle distribution, and reliability concerns from chemical reactions during soldering, leading to delamination and optical inefficiencies.
Innovation Solution
A soft buffer layer made of a polymer material with a low hardness is introduced to enhance adhesion and reduce stress, while a sequential deposition method forms a conformal phosphor layer to improve color uniformity and optical efficacy, and a polymer material without benzene is used to prevent adverse chemical reactions during soldering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the photoluminescent structure is disposed directly on the LED semiconductor die to achieve compact chip-scale size, then the form factor is reduced and material cost is decreased, but the adhesion strength between the photoluminescent structure and the LED semiconductor die becomes relatively weak
Solution Approach 1:
The patent introduces an adhesion promoter layer as an intermediary between the LED semiconductor die and the photoluminescent structure. This adhesion promoter layer specifically enhances the bonding strength between these two components, resolving the weakness caused by direct contact while maintaining the compact CSP form factor.
2Device complexity
If the photoluminescent structure makes only surface contact with the LED semiconductor die without additional contact with lead frame or submount, then the device complexity is reduced, but the contact area is reduced resulting in poor bonding force
Solution Approach 1:
The adhesion promoter layer serves as a mediator that extends the effective bonding interface. By applying this layer to the LED semiconductor die surface, it creates a chemically active interface that enhances adhesion without requiring additional structural components like lead frames or submounts, thus maintaining simplicity while improving bonding force.
3Ease of manufacture
If the photoluminescent structure material with larger coefficient of thermal expansion is used, then the material cost is reduced and ease of manufacture is improved, but the CTE mismatch during thermal cycle induces internal stress leading to delamination
Solution Approach 1:
The adhesion promoter layer acts as a stress buffer and intermediary that accommodates the CTE mismatch between the photoluminescent structure and the LED semiconductor die. This layer absorbs and distributes the thermal expansion stresses during thermal cycling, preventing delamination while allowing the use of cost-effective photoluminescent materials with higher CTE.
4Ease of manufacture
If conventional phosphor slurry deposition method is used to form the photoluminescent structure, then the manufacturing process is simple, but the phosphor particle distribution is inconsistent resulting in poor color uniformity
Solution Approach 1:
The patent applies the adhesion promoter layer to the LED semiconductor die surface before depositing the phosphor slurry. This preliminary action prepares the surface to provide better anchoring and distribution for the phosphor particles, ensuring more uniform phosphor particle distribution and consistent color output while maintaining the simplicity of the slurry deposition process.
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 soft buffer layer increases bonding strength, reduces stress-induced delamination, achieves consistent phosphor distribution for improved color uniformity, and prevents adverse chemical reactions, resulting in enhanced reliability and optical performance of CSP LED devices.
Implementation Method 1
A soft buffer layer made of a polymer material with a low hardness is introduced to enhance adhesion and reduce stress
Implementation Method 2
The photoluminescent structure is disposed covering the LED semiconductor die to down-convert the wavelength of the blue light emitted from the upper surface as well as the four peripheral edge surfaces
Data Source
Figure 1A~1B
Figure 1C~1E
Figure 2
AI summary
A Chip-Scale Packaging (CSP) LED device (1A) and a method of manufacturing the same are disclosed. The CSP LED device (1A) includes a flip-chip LED semiconductor die (10) and a packaging structure (200), wherein the packaging structure (200) comprises a soft buffer layer (20), a photoluminescent structure (30) and an encapsulant structure (40). The soft buffer layer (20) includes a top portion (21) formed on top of the flip-chip LED semiconductor die (10), and an edge portion (22) formed covering an edge surface (13) of the flip-chip LED semiconductor die (10), wherein the top portion (21) has a convex surface (211), and the edge portion (22) has an extension surface (221) smoothly adjoining the convex surface (211). The photoluminescent structure (30) is formed on the soft buffer layer (20) covering the convex surface (211) and the extension surface (221) of the soft buffer layer (20). The encapsulant structure (40), which has a hardness not lower than that of the buffer layer (20), is formed on the photoluminescent structure (30). Therefore, the CSP LED device (1A) has improved reliability by improving adhesion strength between the flip-chip LED semiconductor die (10) and the packaging structure (200), and improved optical performance such as more consistent correlated color temperature (CCT), more uniform spatial color, and higher optical efficacy.