Anti-Reflective Underfill for Flip Chip Imager Assembly
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Solution Overview
Problem
In flip chip imager assemblies, underfill material can cause image artifacts due to insufficient or excessive underfill, leading to light reflection and contamination of the image sensor pixel array, which affects image quality and is exacerbated by shrinking die sizes.
Innovation Solution
The use of anti-reflective underfill materials, such as carbon black pigments, and physical or chemical discontinuities like dam-like structures or trenches on the image sensor die to prevent underfill overflow onto the pixel array, combined with reduced interconnect sizes to maximize active area usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If underfill material is introduced around the connectors to reinforce interconnection, then mechanical strength and drop protection are improved, but light reflection off the underfill material causes image artifacts on the pixel array
Solution Approach 1:
The underfill material incorporates carbon black pigment to change its optical properties, making it non-reflective to prevent light from reflecting onto the pixel array and causing image artifacts
Solution Approach 2:
The harmful reflective property of the underfill material is converted into a beneficial non-reflective property by adding carbon black pigment, transforming the underfill from a source of image artifacts into a protective element that absorbs light
2Area of stationary object
If the width or diameter of electrically conductive connectors is reduced to allow higher percentage of active area, then the active area of the image sensor is increased, but the interconnection reinforcement may be insufficient
Solution Approach 1:
The underfill material is selectively placed around the electrically conductive connectors rather than uniformly across the entire substrate, providing localized reinforcement exactly where the connectors need mechanical support while leaving the pixel array area clear
Solution Approach 2:
The underfill material acts as an intermediary substance that transfers and distributes mechanical stress around the connectors, reinforcing the interconnection between the image sensor and substrate without requiring larger connector dimensions
3Object-generated harmful factors
If dam-like structures or trenches are created to prevent underfill overflow onto the pixel array, then image artifacts are reduced, but the device complexity increases
Solution Approach 1:
The harmful reflective property is extracted from the underfill material by incorporating carbon black pigment, eliminating the need for additional dam-like structures or trenches to prevent light reflection artifacts
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 approach reduces image artifacts by minimizing light reflection and contamination, allowing for increased scaling of CMOS image sensor packaging without introducing unwanted artifacts, thereby enhancing image quality and maximizing pixel array area.
Implementation Method 1
the underfill may include carbon black pigment, which may reduce or eliminate the reflection of incoming light off of the underfill material and onto the active-pixel image sensor
Implementation Method 2
a chemical or physical discontinuity on the integrated circuit may, during assembly, prevent the underfill material from entering an area framed by the discontinuity
Data Source
AI summary
A CMOS imager assembly may include an integrated circuit (IC) having an active-pixel image sensor that is mounted on a printed circuit board (PCB) substrate using flip chip packaging technology. The IC and the PCB may be physically and electrically connected to each other through multiple electrically conductive connectors. An underfill material (which may include an anti-reflective material) may, during assembly, be introduced around the connectors in the space between the IC and the PCB. A chemical or physical discontinuity on the integrated circuit may, during assembly, prevent the underfill material from entering an area framed by the discontinuity, which may include the pixel array of the image sensor. The discontinuity may include a dam-like structure built up on the IC, a trench-like structure created on the IC, or a low surface tension material that has been applied to the surface of the IC.