Polyimide Coverlay CTE Mismatch Curl
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Solution Overview
Problem
Existing coverlay compositions for flexible circuits exhibit undue curl due to mismatched coefficients of thermal expansion between the polyimide adhesive layer and the substrate, leading to thermal stress and handling issues.
Innovation Solution
A two-layer polyimide-based coverlay composition with a low-Tg adhesive layer and a high-Tg cover layer, where the cover layer is designed to adjust the overall coefficient of thermal expansion to match that of the substrate, using polyamic acid precursors and ethylenically unsaturated photo-monomers, photo-initiators, and visible light sensitizers to ensure compatibility and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-layer polyimide adhesive coverlay is used, then the bonding process is simple, but the coverlay exhibits undue curl due to mismatched coefficients of thermal expansion
Solution Approach 1:
The coverlay is divided into two distinct layers: a polyimide adhesive layer (Tg: 170-250°C) and a polyimide support layer (Tg: at least 10-100°C greater than adhesive layer). This segmentation allows each layer to have optimized thermal properties, with the support layer compensating for thermal expansion mismatch to prevent curling while the adhesive layer provides bonding functionality.
2Temperature
If the adhesive layer has high Tg to improve thermal stability, then thermal resistance improves, but adhesion to the substrate deteriorates
Solution Approach 1:
Different thermal properties are assigned to different layers based on their specific functions. The adhesive layer uses polyimide with Tg between 170-250°C optimized for bonding, while the support layer uses polyimide with significantly higher Tg (at least 10-100°C greater) to provide thermal stability and control overall CTE. This local differentiation resolves the contradiction between adhesion and thermal stability.
3Strength
If the cover layer thickness is increased to improve mechanical strength, then strength improves, but the overall CTE mismatch and curling worsen
Solution Approach 1:
The patent optimizes the thickness parameters of both layers to achieve the desired balance. The adhesive layer thickness and support layer thickness are specifically controlled so that the support layer provides sufficient mechanical strength while its higher Tg polyimide composition ensures the overall composite CTE matches the substrate, preventing curling even at increased thicknesses.
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 inhibits unwanted curling by matching the thermal expansion coefficient of the coverlay with the substrate, enhancing mechanical strength and chemical resistance, and ensuring good adhesion and flatness of the flexible circuit.
Implementation Method 1
the coverlay can then be thermally cured (imidized) to provide a two-layer polyimide-based composition
Implementation Method 2
the polyamic acid of the cover layer can additionally contain from 15 to 35 percent by weight (on a dry solids basis) an ethylenically unsaturated photo-monomer
Data Source
AI summary
The present invention is directed to coverlay compositions derived from two-layer polyamic acid-based composites having a cover layer and adjacent thereto an adhesive layer wherein the cover layer comprises polyamic acid and other additives allowing the composition to be photosensitive and aqueous base developable, and where the adhesive layer can form a polyimide having a glass transition temperature between 170 and 250° C. The two-layer coverlay compositions of the present invention are used to encapsulate metal circuit traces formed on a flexible printed circuit base substrate. These two-layer compositions are particularity useful due to having an overall in-plane CTE between 10 and 40 ppm/° C. a range that is useful in avoiding unwanted curling of a flexible printed circuit when used as a polyimide-based coverlay material.