Image Sensor Package Structure for Thermal Stress on Bonding Wires
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Solution Overview
Problem
Image sensor packages, particularly those used in autonomous vehicles, face reliability issues due to stress-induced cracks in bonding wires, which are not adequately addressed by existing technologies.
Innovation Solution
The implementation of a stress reducing layer with specific material properties, such as a glue adhesive, covering the bonding wires to mitigate stress and prevent cracks, combined with a dam and encapsulant design that manages thermal expansion and contamination, enhances the reliability of the image sensor package.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bonding wires are used to electrically connect the image sensor chip to the substrate, then electrical connectivity is achieved, but stress-induced cracks occur in the bonding wires due to thermal expansion differences
Solution Approach 1:
A stress reducing layer is formed over the bonding wires before final packaging to proactively cushion and distribute thermal stress before it can cause cracks. This preventive measure addresses the thermal expansion mismatch between the image sensor chip, substrate, and encapsulant by providing a compliant intermediate layer that absorbs stress during temperature cycling.
Solution Approach 2:
The stress reducing layer acts as an intermediary element between the rigid bonding wires and the surrounding structures experiencing thermal expansion. This intermediate layer with appropriate mechanical properties mediates the stress transfer, preventing direct stress concentration on the bonding wires while maintaining electrical connectivity.
2Object-affected harmful factors
If a transparent cover is placed over the image sensor to protect it, then protection is provided, but stress is transmitted to the bonding wires during thermal cycling
Solution Approach 1:
The stress reducing layer is selectively positioned at specific locations where bonding wires are most vulnerable to stress, rather than uniformly distributing protection throughout the entire package. This localized approach targets the critical regions experiencing maximum stress concentration while maintaining overall package integrity.
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 solution significantly reduces stress on bonding wires, thereby minimizing the likelihood of cracks and improving the overall reliability of the image sensor package, making it suitable for demanding applications like autonomous vehicles.
Implementation Method 1
a stress reducing layer with specific material properties, such as a glue adhesive, covering the bonding wires to mitigate stress and prevent cracks
Implementation Method 2
a dam and encapsulant design that manages thermal expansion and contamination
Data Source
Figure 1A
Figure 1B
Figure 2A
AI summary
Provided are an image sensor package with improved reliability and a method of fabricating the same. The image sensor package includes a package substrate, an image sensor chip mounted on the package substrate, a transparent cover on the image sensor chip, an encapsulant encapsulating the image sensor chip and covering a side surface of the transparent cover, a dam on a surface of the image sensor chip and surrounding a portion of the upper surface of the image sensor, the transparent cover on the dam, a bonding wire connecting a chip pad of the image sensor chip to a substrate pad of the package substrate, the dam covering a first end of the bonding wire connected to the chip pad, and a stress reducing layer covering a second end of the bonding wire connected to the substrate pad, the stress reducing layer including substantially the same material as the dam.