Flexible Printed Board Resist Layout for Bend Root Reliability

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Solution Overview

Problem

Flexible printed boards tend to experience disconnection at the root portion when bent due to stress concentration, and providing a long second brazing material pattern to distribute stress inhibits reduction in length.

Innovation Solution

A flexible printed board design that includes a first pattern layer with a wiring end part covered by a coverlay and a resist, where the resist is provided from the covered to exposed portions of the pattern layer, relieving stress concentration without the need for a long pattern at the end part.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a long brazing material pattern is provided at the edge to distribute stress, then disconnection is prevented, but the length of the flexible printed board cannot be reduced

Engineering Contradiction:

Improveconnection reliabilityVSAvoidboard length

Core Design Contradiction:

ReliabilityVSLength of moving object

Solution Approach 1:

The patent applies local quality by providing the resist material specifically at the end part of the wiring pattern where stress concentrates during bending, rather than uniformly across the entire board. This localized reinforcement prevents disconnection at the critical stress point while allowing the rest of the board to be compact and short in length.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent transitions from a one-dimensional solution (extending the brazing material pattern along the length of the board) to a two-dimensional approach by placing the resist material at the end part of the wiring pattern in a different spatial arrangement. This dimensional change allows stress distribution without increasing the overall board length.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Length of moving object

If the flexible printed board is made shorter, then compactness is improved, but stress concentration at the root portion increases causing disconnection

Engineering Contradiction:

Improveboard lengthVSAvoidconnection reliability

Core Design Contradiction:

Length of moving objectVSReliability

Solution Approach 1:

The patent applies beforehand cushioning by providing the resist material at the end part of the wiring pattern before bending occurs. This resist material acts as a stress-absorbing element that cushions against the stress concentration that will occur during bending, preventing disconnection even when the board is made short.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Solution Approach 2:

The resist material serves as an intermediary element between the wiring pattern and the substrate. It mediates the stress concentration by absorbing and distributing the stress forces, allowing the board to be short while preventing direct stress transmission that would cause disconnection.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS12628269B2Flexible printed board

Publication Date: 2026.05.12 MITSUBISHI ELECTRIC CORP

AI summary

A flexible printed board according to the present disclosure includes a first pattern layer including, at an end part thereof, a wiring end part to be connected to a substrate and a resist and a coverlay overlaid on the first pattern layer, wherein the coverlay covers an inside of the wiring end part of the first pattern layer in a planar view, and the resist is provided from a portion of the first pattern layer covered with the coverlay to a portion of the first pattern layer exposed from the coverlay, in a planar view.