Flexible PCB Air Layer Impedance Control
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Solution Overview
Problem
Flexible printed wiring boards face challenges in maintaining consistent characteristic impedance when bent, leading to signal quality deterioration due to varying capacitance between signal and ground layers.
Innovation Solution
Incorporating air layers between signal and ground layers in bent regions, with strategically placed ground openings and guard ground wirings to maintain impedance within a target range, and a manufacturing method involving multiple metal clad laminated boards and laser processing to form conductive holes and vias.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a flexible printed wiring board is bent, then bending resistance is improved, but characteristic impedance varies due to changing distance between signal line and ground layer
Solution Approach 1:
The ground layer is segmented by introducing ground openings that interrupt the continuous ground pattern in the bent region. This segmentation allows the ground layer to flex without creating excessive capacitance variation, thereby maintaining characteristic impedance consistency while enabling bending.
Solution Approach 2:
The ground layer structure is made non-uniform by providing ground openings specifically in the bent region while maintaining continuous ground in non-bent regions. This local modification allows the wiring board to maintain proper impedance control in bent areas without compromising ground continuity in straight sections.
2Adaptability or versatility
If conductor layers are provided in a flexible printed wiring board, then wiring functionality is improved, but stress during bending increases leading to metal fatigue and disconnection risk
Solution Approach 1:
Air layers are pre-formed between the conductor layers at the bent region to act as a cushion that absorbs bending stress. This beforehand preparation of void spaces prevents direct contact between opposing conductor layers during bending, reducing stress concentration and metal fatigue risk.
Solution Approach 2:
Air layers are introduced as an intermediary substance between the signal line and ground layer in the bent region. This air medium acts as a stress-absorbing buffer that prevents direct mechanical contact between conductor layers during bending, thereby reducing fatigue and disconnection risk.
3Stability of the object's composition
If ground layer is provided continuously in flexible printed wiring board, then ground stability is improved, but characteristic impedance varies when bent due to capacitance changes
Solution Approach 1:
The continuous ground layer is segmented by introducing ground openings in the bent region. This segmentation allows the ground layer to maintain electrical stability in non-bent areas while reducing capacitance variation in bent areas, thereby maintaining characteristic impedance control during bending.
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 suppresses variation in characteristic impedance during bending, enhancing signal quality and bending resistance while maintaining a 50Ω impedance range.
Implementation Method 1
when the flexible printed wiring board is bent, a distance between a signal line and a ground layer varies in the hollow portion, and accordingly, a capacitance component between the signal line and the ground layer varies, resulting in variation in the characteristic impedance
Implementation Method 2
irradiating the conformal mask with laser light to form a conductive hole in which the signal line is exposed on a bottom surface
Implementation Method 3
performing plating processing on the conductive hole to form a via electrically coupling the signal line and the second metal foil
Data Source
AI summary
A flexible printed wiring board according to an embodiment is a flexible printed wiring board having a bent region provided with air layers, including a signal line passing through the bent region; and ground layers provided in a layer different from the signal line via an insulating layer, in which the ground layers are not provided in at least a part of a portion overlapping the signal line in the bent region when the flexible printed wiring board is viewed in a thickness direction.