Bismuth-Indium Solder Connection for Transparent Electrodes

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Solution Overview

Problem

The challenge lies in achieving a reliable and stress-reduced connection between light-transparent electrodes and metal electrodes in circuit members, particularly in thin substrates, where existing methods like anisotropic conductive films (ACF) face issues with high temperature expansion and pressure-induced deformation, leading to increased contact resistance and residual stress.

Innovation Solution

A connection structure and method utilizing a resin portion and a solder portion with a bismuth-indium alloy, where the light-transparent electrode contains indium and tin, and the solder portion contains bismuth and indium, allowing for thermal compression to form a reliable electrical connection while minimizing residual stress through a low-melting-point bismuth-indium alloy and a thermosetting resin adhesive.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a light-transparent electrode is coated with metal plating to improve wettability with solder material, then connection reliability is improved, but manufacturing cost increases

Engineering Contradiction:

Improveconnection reliabilityVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The invention extracts and removes the metal plating layer from the light-transparent electrode structure. By using a solder material containing bismuth and indium that has high wettability with ITO, the patent eliminates the need for additional metal plating, thereby reducing manufacturing steps and cost while maintaining reliable solder connections.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The invention changes the compositional parameters of the solder material to contain bismuth (40-70 wt%) and indium (30-60 wt%). This parameter change in solder composition provides high wettability with the light-transparent electrode without requiring metal plating, thus resolving the contradiction between connection reliability and manufacturing cost.

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If anisotropic conductive film (ACF) is used for inter-electrode connection, then connection process is simplified, but residual stress and contact resistance increase at high temperatures

Engineering Contradiction:

Improveconnection process complexityVSAvoidconnection stability at high temperature

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The invention utilizes the phase transition (melting and solidification) of the bismuth-indium solder material at controlled temperatures. The solder material melts during the connection process to form reliable electrical connections, then solidifies to create stable, low-resistance joints that maintain integrity at high operating temperatures, unlike ACF which degrades under thermal stress.

Inventive Principle:

Principle #36Phase transitions

3Ease of manufacture

If conventional solder material is used with light-transparent electrode, then connection process is simple, but wettability is poor and connection state is unsatisfactory

Engineering Contradiction:

Improveconnection process simplicityVSAvoidwettability and connection quality

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The invention changes the compositional parameters of the solder material to contain bismuth (40-70 wt%) and indium (30-60 wt%). This specific composition provides excellent wettability with the light-transparent electrode (ITO) while maintaining a simple connection process without additional plating steps, thus resolving the contradiction between process simplicity and connection quality.

Inventive Principle:

Principle #35Parameter changes

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 results in a connection structure with reduced residual stress and enhanced reliability, maintaining contact area integrity even at high temperatures, and is suitable for thin substrates, offering a cost-effective alternative to ACF methods.

Implementation Method 1

At least one of the first circuit member and the second circuit member is heated while compressing the second circuit member to the first circuit member to melt the solder material

Methodology Applied

Scientific EffectMelting: Melting

Implementation Method 2

The adhesive is cured to form a resin portion that bonds the first circuit member and the second circuit member

Methodology Applied

Scientific EffectCuring: Chemical Bonding

Data Source

PatentUS9999123B2Connection structure of circuit member, connection method, and connection material

Publication Date: 2018.06.12 PANASONIC INTELLECTUAL PROPERTY MANAGEMENT CO LTD

AI summary

A connection structure of circuit members includes a first circuit member, a second circuit member, and a joint portion. The first circuit member has a first main surface on which a light-transparent electrode is provided. The second circuit member has a second main surface on which a metal electrode is provided. The joint portion is interposed between the first main surface and the second main surface. The joint portion includes a resin portion and a solder portion. The solder portion electrically connects the light-transparent electrode and the metal electrode. The light-transparent electrode contains an oxide that includes indium and tin, and the solder portion contains bismuth and indium.