Composite Double-Sided Copper Foil Substrates for Flexible PCBs
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Solution Overview
Problem
Current double-sided copper foil substrates for flexible printed circuit boards face challenges such as high production costs, yield rate issues, and warpage due to asymmetric thermal expansion coefficients, limiting bending cycles and R angle to less than 0.8 mm.
Innovation Solution
A composite double-sided copper foil substrate is developed, comprising a polymer layer with a first copper foil, an adhesive layer, and a second copper foil, where the adhesive layer is sandwiched between the polymer and copper foils, with a thickness range of 12 to 25 μm, and a glass transition temperature above 170°C, using materials like epoxy resin and polyimide, to enhance thermal resistance and flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a double-sided copper foil substrate is manufactured by laminating copper foils on polyimide layers with different thermal expansion coefficients, then the substrate can be formed with more circuits within limited space, but asymmetric thermal expansion coefficients cause non-uniform stress leading to warpage and inability to achieve bending R angle less than 0.8 mm
Solution Approach 1:
The patent applies asymmetry principle by intentionally designing the substrate structure with a recessed portion on one surface, creating an asymmetric configuration that compensates for the asymmetric thermal expansion coefficients of the polyimide layers. This asymmetric design allows the substrate to accommodate thermal stress differently across layers, reducing warpage while maintaining high circuit density.
Solution Approach 2:
The patent changes physical parameters by controlling the thickness of polyimide layers and copper foils within specific ranges, and by creating a recessed portion with controlled depth. These parameter adjustments optimize the balance between thermal expansion compensation and mechanical flexibility, enabling both high circuit density and resistance to warpage.
2Ease of manufacture
If conventional double-sided copper foil substrates are manufactured with standard thickness specifications, then manufacturing processes are simplified, but the bending R angle cannot be reduced below 0.8 mm and the number of bending cycles is limited
Solution Approach 1:
The patent changes the thickness parameters of the substrate components (polyimide layers and copper foils) to optimized values within specific ranges, and introduces a recessed portion with controlled depth. These parameter changes enable the substrate to achieve bending R angles less than 0.8 mm and withstand higher bending cycles while maintaining ease of manufacture through standardized production processes.
3Reliability
If thermoplastic polyimide films are provided on each surface of a thermosetting polyimide film with copper foils, then a double-sided substrate structure is formed, but the manufacturing cost increases due to source limitation and soaring costs of materials from American or Japanese manufacturers
Solution Approach 1:
The patent changes the material specification parameters by defining specific thickness ranges for polyimide layers and copper foils, and by specifying performance parameters (such as glass transition temperature above 170°C) rather than relying on specific brand names. This approach maintains structural reliability while enabling cost-effective manufacturing by allowing selection from multiple suppliers.
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 composite substrate achieves a higher number of bending and sliding cycles with a low R angle of 0.65 mm, reducing warpage and delamination, while maintaining effective thermal resistance and cost control, as demonstrated by the test results showing improved performance in bending and sliding tests.
Implementation Method 1
an adhesive layer formed on the second surface of the polymer layer for the polymer layer sandwiched between the adhesive layer and the first copper foil
Implementation Method 2
since the thermal expansion coefficients of the polyimide layers 201 and 202 are asymmetric, a non-uniform stress may cause warpage
Data Source
AI summary
A double-sided copper foil substrate, which comprises: a polymer layer; a first copper foil; an adhesive layer formed on the polymer layer such that the polymer layer is sandwiched between the adhesive layer and the first copper layer; and a second copper foil causing the adhesive layer to be sandwiched between the second copper foil and the polymer layer, wherein the polymer layer and the adhesive layer have a total thickness of from 12 to 25 μm. The present invention further provides a flexible printed circuit board structure utilizing the composite double-sided copper foil substrate of the present invention, wherein the second double-sided copper foil substrate has a trench for exposing a portion of the adhesive layer. The double-sided copper foil substrate of the present invention are lower in rebound and satisfying the demand for the greater number of bending and sliding cycles under a lower R angle, and particularly is suitable for thin and flexible electronic products.