Flexible Printed Circuit Adhesive Containment Design
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Solution Overview
Problem
In flexible printed circuits (FPCs), excessive adhesive exudation during bonding can lead to poor connections with electronic components, reducing yield and potentially causing stress concentration when the FPC is bent or stretched, due to the adhesive spreading over connector areas.
Innovation Solution
The design incorporates a second circuit substrate with an opening and a depressed portion deeper than the opening edge, allowing the adhesive to exude and spread within a controlled offset range, preventing it from reaching the connector area and distributing stress effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If adhesive layers are used to bond circuit substrates together, then bonding strength is improved, but adhesive exudation spreads over connector areas causing poor connections
Solution Approach 1:
The connector region is differentiated from other regions by removing the adhesive layer specifically in this area. This local modification allows the connector region to have different properties (no adhesive) compared to the bonding regions (with adhesive), thereby preventing adhesive contamination of connectors while maintaining strong bonding elsewhere.
Solution Approach 2:
The adhesive layer is completely removed from the connector region through etching or other removal processes. This extraction of the adhesive material from the problematic area eliminates the source of contamination that would otherwise spread to connectors and cause poor electrical connections.
2Reliability
If adhesive is applied generously to ensure bonding, then bonding reliability is improved, but adhesive exudation increases spreading to connector areas
Solution Approach 1:
The adhesive exudation, which is normally a harmful effect, is redirected into a beneficial function by having it fill the depressed portion. The excess adhesive that would otherwise contaminate connectors is now contained and utilized to fill voids in the depressed region, converting a manufacturing defect into a feature that enhances bonding reliability.
Solution Approach 2:
The depressed portion is nested within the circuit substrate structure, creating a contained space that captures and holds the adhesive exudate. This nested structure prevents the harmful spread of adhesive while maintaining the bonding function, effectively containing the potential harm within a designated area.
3Strength
If adhesive layer is placed between circuit substrates, then bonding is achieved, but stress concentration occurs when FPC is bent or stretched
Solution Approach 1:
The depressed portion is pre-formed in the circuit substrate before bonding, creating a cushioning space that anticipates and absorbs the stress that will occur during bending or stretching. This beforehand preparation allows the adhesive to be contained in a region that can accommodate dimensional changes, preventing stress concentration at the connector area.
Solution Approach 2:
The depressed portion introduces a vertical dimension (depth) to the otherwise planar bonding interface. This three-dimensional feature allows the adhesive to be distributed in a volumetric manner rather than just a planar layer, providing additional space to accommodate stress and deformation without concentrating forces at critical connector regions.
Data Source
AI summary
According to one embodiment, a flexible printed circuit has a first circuit substrate and a second circuit substrate extending over the first circuit substrate. The first circuit substrate has a connector area. The second circuit substrate has an opening and an adhesive layer. The opening allows the connector area to be exposed. The opening has an opening edge which has a predetermined shape and defines the opening. The second circuit substrate has in the opening a depressed portion depressed deeper than the opening edge. Among various constituents of the second circuit substrate, at least the adhesive layer is depressed from the opening edge at the depressed portion.