Flexible Display Pad Area Strengthening via Local Quality
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Solution Overview
Problem
The bonding process in electronic device displays often induces tensile stress, leading to delamination of thin-film transistor layers from the flexible substrate, due to the use of adhesive materials that expand and contract during thermal cycling, which can damage the display circuitry.
Innovation Solution
Forming buffer layers on the flexible display substrate with bond pads directly on them, and applying crushed anisotropic conductive film (ACF) that experiences compressive stress during bonding, reducing the stack height in the bonding area and minimizing the risk of delamination by controlling the stress type and distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the pad area is made larger to improve bonding reliability, then the bonding strength is improved, but the flexibility and bendability of the display deteriorates
Solution Approach 1:
The pad structure is designed with non-uniform geometry where the first pad has a larger area than the second pad, creating localized bonding strength where needed. The pad area gradually decreases from the first terminal toward the second terminal, providing stronger bonding at the more flexible first end while maintaining overall device flexibility.
Solution Approach 2:
The bonding pad structure is segmented into multiple pads with different areas rather than using a single uniform pad. This segmentation allows different regions to have different bonding strengths, with larger pads at the first terminal for enhanced bonding reliability and smaller pads at the second terminal to maintain flexibility.
2Strength
If the pad area is made larger to improve bonding strength, then the bonding strength is improved, but the display device becomes more prone to cracking at the pad area
Solution Approach 1:
The pad structure implements local quality by having the first pad with larger area and the second pad with smaller area. This creates a gradient in bonding strength that matches the flexibility gradient of the display, providing strong bonding where needed while reducing stress concentration that causes cracking.
Solution Approach 2:
Instead of making all pads uniform and large for maximum bonding strength, the invention inverts the approach by making pads non-uniform with decreasing area. This inversion prevents cracking by avoiding excessive rigidity in the pad region while maintaining sufficient bonding strength through strategic pad placement and sizing.
3Strength
If the pad area is made larger to ensure sufficient bonding strength, then the bonding strength is improved, but the flexibility at the pad area is reduced
Solution Approach 1:
The pad structure is designed with local quality by varying pad areas across different locations. The first pad has larger area to provide strong bonding at the first terminal, while the second pad has smaller area to maintain flexibility at the second terminal, creating a balanced structure that satisfies both bonding and flexibility requirements.
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 reduces the likelihood of peel-off and damage to the thin-film transistor layers, enhancing the robustness of the bonding region and maintaining the integrity of the display circuitry during assembly and operation.
Implementation Method 1
While the ACF material cools from the elevated temperature level back down to room temperature, the thermal expansion/contraction of the ACF and surrounding structures may apply compressive stress to the multi-buffer layer formed a distance away from the edge of the substrate.
Implementation Method 2
While the ACF material cools from the elevated temperature level back down to room temperature, the thermal expansion/contraction of the ACF and surrounding structures may apply compressive stress to the multi-buffer layer formed a distance away from the edge of the substrate.
Implementation Method 3
Subjecting the adhesive material to thermal cycling can cause materials at the mating junction to expand and contract, resulting in tensile stress that can cause thin-film transistor layers on the flexible display substrate to be delaminated from the flexible substrate during the assembly process.
Implementation Method 4
Subjecting the adhesive material to thermal cycling can cause materials at the mating junction to expand and contract, resulting in tensile stress that can cause thin-film transistor layers on the flexible display substrate to be delaminated from the flexible substrate during the assembly process.
Data Source
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AI summary
An electronic device may have a flexible display with portions that can be bent. The display may include an array of display pixels in an active area. Contact pads may be formed in an inactive area of the display. Display circuitry in the active area may exhibit a given stack height, whereas display circuitry in the inactive area may exhibit a stack height that is less than the given stack height. In particular, the contact pads may be formed directly on a multi-buffer layer that sits directly on a flexible display substrate. Passivation material may be selectively formed only at the edges of the contact pad on the multi-buffer layer. The multi-buffer layer may be formed at a distance from the edge of the flexible display substrate to minimize cracking in the multi-buffer layer.