Multilayer Flexible PCB Layout for Low-Stress Fold Hinges
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Solution Overview
Problem
The reduction in the radius of curvature and increase in stress on flexible printed circuit boards in foldable electronic devices due to repeated folding, along with increased path loss and difficulty in maintaining a designed shape, especially in the folding area where all layers are bonded together.
Innovation Solution
A multilayer flexible printed circuit board design with bonding layers confined only to non-flexure areas, allowing adjacent pattern layers in flexure areas to slide relative to each other, maintaining the desired shape and reducing stress and path loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If all layers of the flexible printed circuit board are bonded together, then the structural integrity is improved, but the stress applied to the circuit board increases during folding operations
Solution Approach 1:
The flexible printed circuit board is divided into multiple layers that are not fully bonded together. Specifically, the first and second flexible circuits are disposed between rigid circuits such that they are not bonded to each other, creating separable segments that can independently deform during folding operations, thereby reducing stress while maintaining overall structural integrity.
Solution Approach 2:
Different regions of the circuit board have different bonding characteristics. The rigid circuits provide structural support in non-flexure areas, while the flexible circuits remain unbonded in flexure areas to allow bending. This local differentiation of bonding quality enables the board to maintain strength where needed while reducing stress in folding regions.
2Stress or pressure
If the radius of curvature of the flexible printed circuit board is increased, then the stress applied to the circuit board is reduced, but the space required in the folding area increases
Solution Approach 1:
The circuit board is segmented into rigid and flexible portions, allowing the flexible circuits to bend with smaller radii of curvature without increasing overall stress. The rigid circuits maintain structural integrity while the flexible circuits accommodate the folding geometry, enabling compact folding area design without compromising stress reduction.
Solution Approach 2:
The circuit board structure transitions from a static fully-bonded configuration to a dynamic multi-layer configuration where flexible circuits can independently deform. This dynamic arrangement allows the board to adapt its shape during folding operations, achieving smaller folding areas while maintaining acceptable stress levels through the unbonded flexible portions.
3Adaptability or versatility
If the length of the flexible printed circuit board is increased to accommodate folding operations, then the folding functionality is improved, but the path loss increases
Solution Approach 1:
The circuit board is segmented into rigid and flexible sections, with flexible circuits positioned to follow the folding path. This segmentation allows the flexible portions to accommodate the increased length required for folding operations while the rigid portions maintain stable electrical connections, thereby enabling folding functionality without excessive path loss in critical signal paths.
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
An electronic device according to an embodiment of the disclosure may include a first housing in which a first printed circuit board is disposed, a second housing in which a second printed circuit board is disposed, a hinge configured to rotatably connect the first housing and the second housing, and a multilayer flexible printed circuit board configured to connect the first printed circuit board and the second printed circuit board, wherein the multilayer flexible printed circuit board may include a plurality of pattern layers and a bonding layer configured to bond a portion between at least two pattern layers of the plurality of pattern layers, wherein in the folding area, the multilayer flexible printed circuit board may include a flexure area and a non-flexure area between flexure areas, and in the folded area, at least one bonding layer may be formed at least in part only on the non-flexure area.