LED Package Pad Shape Design for Thermal Stress Reduction
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Solution Overview
Problem
Conventional light emitting device packages experience stress and reliability issues due to residual thermal expansion differences between layers, leading to initial failure and cumulative fatigue when bonded at high temperatures.
Innovation Solution
The light emitting device package design includes pads with rounded or chamfered edges and corners, positioned asymmetrically to minimize stress, with varying radii of curvature and different thermal expansion coefficients between pads and contact layers, reducing shear and normal stresses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pads are bonded to light emitting elements at high temperatures (200°C or higher), then electrical connection is achieved, but residual stress from thermal expansion differences causes initial failure and cumulative fatigue
Solution Approach 1:
The pad structure employs asymmetric design with different edge configurations (first edge, second edge, third edge, fourth edge) where at least one edge is positioned at a different distance from the center of the light emitting element. This asymmetric arrangement creates non-uniform stress distribution that reduces concentrated stress at any single location, thereby mitigating residual stress effects while maintaining bonding reliability at high temperatures.
Solution Approach 2:
The pad structure incorporates rounded corners or chamfered edges instead of sharp 90-degree angles. This curvature modification at the pad edges reduces stress concentration points that would otherwise form at sharp corners during thermal expansion and contraction cycles. The rounded or chamfered geometry allows stress to distribute more evenly across the pad structure, preventing initial failure and reducing cumulative fatigue.
2Ease of manufacture
If pads have sharp corners and straight edges, then manufacturing is simple, but stress concentration occurs at corners leading to package destruction
Solution Approach 1:
The pad structure incorporates rounded corners or chamfered edges instead of sharp 90-degree angles. This curvature modification at the pad edges reduces stress concentration points that would otherwise form at sharp corners during thermal expansion and contraction cycles. The rounded or chamfered geometry allows stress to distribute more evenly across the pad structure, preventing initial failure and reducing cumulative fatigue.
Solution Approach 2:
The pad structure applies different geometric characteristics to different edges (first edge, second edge, third edge, fourth edge) with at least one edge positioned at a different distance from the center. This local variation in geometry optimizes stress distribution at specific high-stress locations while maintaining overall manufacturing feasibility through standardized fabrication processes.
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
This design minimizes stress and enhances reliability by altering the pad shapes, reducing shear and normal stresses, thereby preventing initial failure and cumulative fatigue, and improving the overall performance of the light emitting device package.
Implementation Method 1
at least one of the first or second pad may include at least one of a rounding portion or a chamfer portion, wherein the rounding portion or the chamfer portion may be positioned at at least one of the first-second edge or the second-second edge
Implementation Method 2
the first pad and the first contact layer may have different coefficients of thermal expansion, and the second pad and the second contact layer may have different coefficients of thermal expansion
Data Source
AI summary
A light emitting device package according to an embodiment comprises: a light emitting device comprising a light emitting structure including a first conductive semiconductor layer, an active layer, and a second conductive semiconductor layer; first and second lead frames disposed to be spaced apart from each other; first and second solder portions disposed on the first and second lead frames, respectively; and first and second pads disposed between the first and second solder portions and the first and second conductive semiconductor layers, respectively, wherein at least one of the first or second pad comprises at least one of a rounding portion and a chamfer portion, wherein the first pad comprises a first-first edge and a first-second edge being positioned farther than the first-first edge from the center of the light emitting device, wherein the second pad comprises a second-first edge and a second-second edge being positioned farther than the second-first edge from the center of the light emitting device, and wherein the rounding portion or the chamfer portion is positioned at at least one of the first-second edge or the second-second edge.