LED Package Pad Geometry for Thermal Stress Relief
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Solution Overview
Problem
Existing LED packages face challenges with mechanical robustness and electrical reliability due to differences in the coefficient of thermal expansion between the package substrate and the printed circuit board, leading to stress and defects in solders.
Innovation Solution
The LED package design includes a package substrate with an LED chip and external connection pads arranged in specific geometric configurations to reduce stress and improve reliability, featuring asymmetric and chamfered shapes with stress relief features.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional LED package design is used, then manufacturing is simple, but mechanical robustness and electrical reliability deteriorate due to stress from thermal expansion differences
Solution Approach 1:
The external connection pad is designed with an asymmetric shape where the first side extending in the first direction has a length longer than that of the second side extending in the second direction. This asymmetric configuration optimizes the stress distribution pattern to better accommodate thermal expansion differences between the package substrate and printed circuit board, thereby improving mechanical robustness and electrical reliability
Solution Approach 2:
Corner portions of the external connection pad are rounded with a radius of curvature rather than sharp corners. This curvature design reduces stress concentration at the corners, preventing crack initiation and improving the overall mechanical robustness of the solder connection
2Reliability
If symmetric pad design is used, then manufacturing is easier, but stress relief is insufficient leading to solder defects
Solution Approach 1:
The external connection pad employs asymmetric dimensions with the first side being longer than the second side, creating an optimized stress distribution pattern that reduces solder defects while maintaining manufacturability through standard PCB fabrication processes
3Reliability
If standard rectangular pad shape is used, then manufacturing precision is easier to achieve, but stress concentration occurs at corners causing solder cracks
Solution Approach 1:
Corner portions of the external connection pad are designed with rounded profiles having a specified radius of curvature. This curvature eliminates sharp corners that would act as stress concentration points, preventing solder cracks and improving mechanical robustness while remaining compatible with standard manufacturing tolerances
Data Source
AI summary
A light-emitting diode (LED) package includes a package substrate, an LED chip disposed on a first surface of the package substrate, and a first external connection pad and a second external connection pad disposed on a second surface of the package substrate opposite the first surface. The first external connection pad includes a first side, a second side, a third side and a fourth side, the first side being parallel to the second side, and the third side being parallel to the fourth side. The first side is spaced farther from a center of the package substrate than the second side. A length of the first side is shorter than a length of the second side.