AlN Focal Plane Array CTE Mismatch Compensation
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Solution Overview
Problem
Existing large format imaging focal plane arrays face challenges in accommodating a large number of imaging dies due to size limitations and inefficient heat dissipation, as well as mismatched thermal expansion coefficients between components, which restricts the number and efficiency of connections and thermal management.
Innovation Solution
A multi-layer, optically flat aluminum nitride (AlN) circuit card assembly is used, which is highly thermally conductive and has a coefficient of thermal expansion matching silicon imaging dice, combined with a flexible connector array and land grid array connectors to compensate for thermal expansion differences, allowing for a dense array of connections and efficient heat dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional FR-4 circuit card material is used, then manufacturing cost is reduced, but thermal conductivity is insufficient to dissipate heat from power-consuming imaging dies
Solution Approach 1:
The patent employs aluminum nitride (AlN) as a composite material for the circuit card substrate. AlN provides superior thermal conductivity compared to conventional FR-4 materials, enabling effective heat dissipation from the imaging dies while maintaining structural integrity. This material substitution directly addresses the thermal management challenge without compromising manufacturability, as AlN can be fabricated into multi-layer circuit card assemblies using established processes.
Solution Approach 2:
The patent changes the thermal conductivity parameter of the circuit card material from the low value typical of FR-4 to the high value characteristic of aluminum nitride. This parameter change enables the substrate to actively dissipate heat from the imaging dies, transforming the circuit card from a purely electrical interconnection medium to a thermal management component as well.
2Area of stationary object
If the imaging plane board size is reduced, then the perimeter-to-area ratio improves for connection efficiency, but the coefficient of thermal expansion mismatch with silicon imaging dies becomes more problematic
Solution Approach 1:
The patent uses aluminum nitride, a composite ceramic material, whose coefficient of thermal expansion can be engineered to match that of silicon imaging dies. This material selection resolves the thermal expansion compatibility issue, allowing the imaging plane board to be reduced in size without causing reliability problems due to CTE mismatch between the substrate and the mounted dies.
Solution Approach 2:
The patent changes the coefficient of thermal expansion parameter of the circuit card material to match that of silicon. By selecting aluminum nitride with appropriate compositional and structural parameters, the CTE is tuned to be compatible with silicon imaging dies, enabling reliable thermal and mechanical coupling even in compact configurations.
3Measurement precision
If hundreds of imaging dies are integrated in a gigapixel camera system, then image resolution is improved, but the number of required connections and thermal management complexity increase significantly
Solution Approach 1:
The patent makes the aluminum nitride circuit card substrate multi-functional by integrating thermal conduction, electrical interconnection, and mechanical support functions into a single component. This universal substrate handles heat dissipation from all imaging dies simultaneously while providing the electrical connection network, thereby reducing overall system complexity despite the high number of integrated dies.
Solution Approach 2:
The patent merges the thermal management function and electrical interconnection function into a single aluminum nitride circuit card assembly. Instead of having separate heat sinks and connection substrates, the AlN material simultaneously performs both functions, simplifying the integration of hundreds of imaging dies in a gigapixel camera system.
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 configuration enables the integration of hundreds of imaging dies in a gigapixel camera system with improved thermal management and increased connection density, overcoming size and thermal expansion issues, while maintaining structural stiffness and optical flatness.
Implementation Method 1
is highly thermally conductive which enables it to dissipate large amounts of heat
Implementation Method 2
the coefficient of thermal expansion of the AlN closely matches that of imaging dice which are constructed out of silicon wafers. However, the aluminum nitride board has substantially lower coefficient of thermal expansion (CTE) than conventional FR-4... The flexible connector allows compensation for the difference in the coefficient of thermal expansion
Data Source
AI summary
A composite focal plane assembly with an expandable architecture has a multi-layer, double-sided aluminum nitride (AlN) substrate and vertical architecture to achieve the dual function of focal plane and electronics backplane. Imaging dice and other electrical components are mounted and wire bonded to one surface and then direct backplane connectivity is provided on the opposing surface through a matrix of electrical contacts. In one embodiment, a flexible connector is sandwiched between the AlN focal plane and a FR-4 backplane is used to compensate for differences in coefficient of thermal expansion (CTE) between the AlN and commercially available high density circuit card connectors that are commonly manufactured from materials with CTE properties more closely approximating FR-4. In an alternate embodiment, the FR-4 and flexible connectors are eliminated by using high density circuit card connectors that are fabricated out of materials more closely matching the CTE of AlN.