Adhesive-Free Copper Flexible Substrate for Fine Etching
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Solution Overview
Problem
Two-layer flexible substrates with copper layers on insulator films face issues of surface defects, pinholes, and poor etching characteristics, which hinder fine patterning and resist adherence, especially due to the high gloss of copper surfaces and pinholes in the underlying metal layers.
Innovation Solution
The surface roughness (Ra) of the copper layer is controlled between 0.10 to 0.25 μm and the average crystal grain size is limited to no greater than 0.8 μm at a cross-section 1 μm from the insulator film, achieved by using a copper sulfate plating solution with additives and adjusting current density and temperature during electroplating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If electrolytic copper plating is used to form the copper layer, then the copper layer can be formed with controlled thickness, but the copper layer exhibits numerous pinholes and areas where the insulator film is exposed
Solution Approach 1:
An underlying metal layer is formed on the insulator film before copper plating to serve as a foundation that prevents pinhole formation and insulator exposure in the subsequent copper layer
Solution Approach 2:
A composite structure consisting of an underlying metal layer and a copper conductor layer is created, where the underlying layer compensates for the deficiencies of the copper plating process by providing a pinhole-free base
2Manufacturing precision
If the copper surface has high gloss, then the copper layer has good electroplating quality, but resist delamination occurs during wiring formation
Solution Approach 1:
The copper layer is designed with non-uniform surface properties, having higher gloss in regions not requiring resist adherence and lower gloss in regions where resist adherence is critical, optimizing both electroplating quality and resist bonding
Solution Approach 2:
The surface gloss parameter of the copper layer is adjusted or controlled to reduce excessive reflectivity, thereby improving resist wetting and adherence while maintaining acceptable electroplating quality
3Reliability
If a complex multi-step plating process is used, then surface defects are reduced, but the manufacturing process becomes more complex
Solution Approach 1:
An underlying metal layer is deposited beforehand to prevent pinhole formation and insulator exposure, eliminating the need for subsequent complex repair or re-plating operations
Solution Approach 2:
Plating parameters such as current density, temperature, and solution composition are optimized to achieve uniform, pinhole-free copper deposition in fewer steps, reducing process complexity while maintaining surface quality
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 approach results in a substrate with excellent etching characteristics, improved resist adherence, and reduced surface defects, enhancing yield and compatibility with fine-line formation.
Implementation Method 1
achieved by using a copper sulfate plating solution with additives and adjusting current density and temperature during electroplating
Data Source
AI summary
The invention provides a two-layer flexible substrate free of surface defects and having excellent etching characteristics and adherence to a resist.The two-layer flexible substrate has a copper layer provided on one or both faces of an insulator film without using an adhesive, wherein the surface roughness (Ra) of the copper layer is 0.10 to 0.25 μm, and wherein the average crystal grain size [of copper] is no greater than 0.8 μm at 1 μm from the insulator film in the cross-section of the copper layer. Preferably, the insulator film is a polyimide film.