Image Sensor Interconnect CTE Mismatch Stress Reduction
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Solution Overview
Problem
Conventional image sensors face issues due to mismatches in coefficients of thermal expansion (CTE) between different materials, leading to stress-induced cracking and delamination of solder joints, particularly under thermal shock conditions.
Innovation Solution
The implementation of methods and devices that minimize thermally-induced stresses by using techniques such as increasing solder joint stand-off height, employing softer polymer core solder balls, underfill materials, adjusting PCB materials to lower CTE, and inserting intermediate interposer PCBs, along with strategic coupling locations and materials selection to reduce CTE mismatch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional image sensors use multiple materials with different CTEs (ceramic/plastic carrier substrates, solder pads, PCBs), then the system achieves functional integration and electrical connectivity, but thermal shock causes CTE mismatch stresses that lead to solder joint cracking and delamination
Solution Approach 1:
The patent introduces an intermediate carrier substrate (reel) with a coefficient of thermal expansion (CTE) that matches the CTE of the image sensor die. This intermediate substrate acts as a mediator between the image sensor die and the final PCB assembly, distributing and reducing thermal stress during temperature transitions. The reel carrier substrate has a CTE of approximately 7-9 ppm/°C, which closely matches the image sensor die CTE, thereby preventing solder joint cracking and delamination caused by CTE mismatch stresses.
Solution Approach 2:
The patent modifies the CTE parameter of the carrier substrate by selecting materials with specific thermal expansion properties. The reel carrier substrate is designed with a CTE that matches the image sensor die (7-9 ppm/°C), while the final PCB uses materials with different CTE characteristics. This parameter optimization ensures that during thermal cycling, the stress differential is minimized at critical interfaces, preventing solder joint failure.
2Reliability
If the stand-off height of solder joints is increased to reduce stress concentration, then thermal shock resistance improves, but the device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent increases the stand-off height of solder joints by utilizing the vertical dimension of the reel carrier substrate structure. The solder joints connect the image sensor die to the reel carrier substrate at a greater vertical distance, which reduces stress concentration during thermal expansion and contraction. This dimensional approach allows for improved thermal shock resistance without significantly complicating the manufacturing process, as the increased height is achieved through the natural structure of the reel carrier substrate rather than additional components.
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
These approaches effectively reduce or eliminate cracking and structural failures in image sensor systems by mitigating thermal stress, enhancing their thermal shock performance and reliability.
Implementation Method 1
Conventional image sensors often include a plurality of different materials having different respective coefficients of thermal expansion (CTE). For example, image sensor substrates can be coupled to ceramic or plastic carrier substrates, which could be coupled by way of Land Grid Array (LGA) solder pads to a printed circuit board.
Implementation Method 2
For example, if the forces due to the CTE mismatch are strong enough (e.g., due to thermal shock), then the solder joints attaching sensor LGA pad to the printed circuit board can crack and/or delaminate.
Data Source
AI summary
The present disclosure relates to optical systems and methods of their manufacture. An example system includes a printed circuit board assembly (PCBA) and an image sensor package coupled to the PCBA by way of a plurality of bond members. The system additionally includes a sensor holder coupled to the PCBA. The image sensor package and the sensor holder are coupled to the PCBA so as to minimize thermally-induced stresses in at least one of: the plurality of bond members, the PCBA, the sensor holder, or the image sensor package.