Poly-p-xylylene Coating for Aeronautic Electronic Modules
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Solution Overview
Problem
Aircraft electronic modules face significant mechanical and thermal stresses, leading to issues such as differential expansion, intermetallic compound formation, and moisture ingress due to temperature variations, which can cause failures and reduce the lifespan of components.
Innovation Solution
A ceramic hybrid circuit with a thin poly-p-xylylene sealing coating and strategically chosen metals with high atomic mass and electronegativity coefficients, along with a metallurgical barrier, to limit diffusion and enhance mechanical and electrical resistance, while maintaining a low thickness to minimize stress and prevent moisture ingress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick epoxy resin coating is used to protect circuits from humidity, then protection function is improved, but differential expansion causes break in bond between epoxy and circuit
Solution Approach 1:
The patent replaces the thick epoxy resin coating with a thin film coating of poly-p-xylylene that is deposited in the vapor phase. This thin film provides moisture protection while minimizing differential expansion stresses because its small thickness reduces the magnitude of expansion/contraction during thermal cycles, preventing bond breakage between the coating and circuit.
Solution Approach 2:
The patent changes the physical and chemical parameters of the protective coating by using poly-p-xylylene deposited in the vapor phase under vacuum. This creates a coating with different thermal expansion properties and adhesion characteristics compared to epoxy resin, allowing it to withstand thermal cycling without delaminating while still providing moisture barrier function.
2Reliability
If hermetic cases under neutral gas or vacuum are used to enclose modules, then protection from differential expansion and aging is improved, but weight, size, complexity, and cost increase
Solution Approach 1:
The patent extracts the essential protective function (protection from moisture and thermal stress) and implements it through a simple thin film coating applied directly to the circuit board. This eliminates the need for complex hermetic enclosures, vacuum sealing mechanisms, and associated control systems, dramatically simplifying the overall module design while maintaining reliability.
Solution Approach 2:
The thin film poly-p-xylylene coating serves as a lightweight, simple alternative to bulky hermetic cases. The film conformally coats the circuit board and components, providing protection without requiring complex sealing structures, reducing weight, size, and operational complexity.
3Adaptability or versatility
If different materials are used for circuit components, then functionality and electrical properties are improved, but differential expansions and contractions cause stresses in assemblies
Solution Approach 1:
The patent uses a composite structure consisting of the circuit board, various electronic components, and the poly-p-xylylene coating. The coating acts as a compliant interface layer that can accommodate differential thermal expansion between dissimilar materials (circuit board, components, solder joints) while maintaining structural integrity and protecting the assembly from moisture ingress.
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 effectively reduces the impact of thermal stresses, prevents intermetallic compound formation, and extends the lifespan of wiring and connections by creating a robust metallurgical barrier that slows down diffusion, thereby enhancing the thermomechanical resistance and reliability of electronic modules.
Implementation Method 1
Poly-p-xylylene is deposited in the vapor phase under vacuum
Implementation Method 2
temperature variations cause cycles of expansion and contraction of circuit components
Implementation Method 3
Due to this small thickness, the effects induced by the differential expansions are very limited and do not generate sufficient stresses to cause the bonding of the surface components to the circuit to break
Implementation Method 4
atoms of a metal of one of the conductive elements of the circuit diffuse towards a metal of another conductive element of the circuit in such a way that the atoms of these metals combine to form an intermetallic compound
Implementation Method 5
the rupture occurred more particularly when atoms of a metal of one of the conductive elements of the circuit diffuse towards a metal of another conductive element of the circuit in such a way that the atoms of these metals combine to form an intermetallic compound of lower mechanical strength
Data Source
Figure 1~2
Figure 3
AI summary
The invention relates to a high-temperature (temperatures up to 225°C) electronic module for a piece of vehicle-borne aeronautic equipment, comprising a hybrid circuit made of thick layers covered with an impermeable poly(p-xylylène) coating.