Film Capacitor Case-Resin Interface Design for High-Temperature Adhesion
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Solution Overview
Problem
Existing film capacitors experience separation at the interface between the outer case and the filling resin when used in high-temperature environments, which is not adequately addressed by controlling the amount of filler in the outer case.
Innovation Solution
The film capacitor design includes an outer case with a surface free energy of 44 mN/m or less, and a coefficient of linear expansion difference with the filling resin of 11 ppm/°C or less, to enhance adhesion and prevent separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the amount of filler (glass fiber) in the outer case is controlled, then adhesion between resin and case is improved, but separation at the interface still occurs in high-temperature environments
Solution Approach 1:
The patent changes the surface free energy parameter of the outer case material to 44 mN/m or less, which fundamentally alters the interfacial adhesion characteristics between the case and filling resin. This parameter change enables the interface to maintain bonding strength even under high-temperature conditions where conventional materials would separate.
Solution Approach 2:
The patent employs composite material design by combining the outer case material with specific surface free energy characteristics and filling resin with matched coefficient of linear expansion. This composite approach creates a synergistic effect where the material combination resists thermal separation better than individual materials alone.
2Ease of manufacture
If conventional outer case materials are used, then manufacturing is easier, but separation occurs at the interface under thermal cycling
Solution Approach 1:
The patent modifies the surface free energy parameter of the outer case to 44 mN/m or less, which can be achieved through material selection or surface treatment processes. This parameter modification maintains manufacturing feasibility while dramatically improving interface adhesion stability under thermal cycling conditions.
3Stability of the object's composition
If the coefficient of linear expansion difference between outer case and filling resin is reduced, then thermal stress is minimized, but material selection becomes more constrained
Solution Approach 1:
The patent establishes a quantitative criterion (coefficient of linear expansion difference of 11 ppm/°C or less) that provides a clear design guideline for material selection. This parameter specification enables engineers to systematically evaluate and select material combinations that minimize thermal stress while maintaining manufacturing practicality.
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 design effectively reduces or prevents separation at the interface between the outer case and filling resin, even under high-temperature conditions, ensuring durability and reliability.
Implementation Method 1
a surface free energy of an inner surface of the outer case on the side filled with the filling resin is 44 mN/m or less
Implementation Method 2
a coefficient of linear expansion difference with the filling resin of 11 ppm/°C or less
Data Source
Figure 1A~1C
Figure 2A~2B
Figure 3A~3B
AI summary
A film capacitor of the present invention includes a capacitor element having a metallized film including a resin film and a metal layer on a surface of the resin film, an outer case that houses the capacitor element, and a filling resin that fills a space between the capacitor element and the outer case, wherein a surface free energy of an inner surface of the outer case on the side filled with the filling resin is 44 mN/m or less.