Laser Bonding of Conductive Joints With Low-Heat Solder Melting

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Solution Overview

Problem

Reflow soldering is limited by heat effects on substrates and components, especially with high-strength solders having high melting points, and laser light absorption by metal electro-conductive portions is low, making it difficult to melt bonding materials without melting the conductive portions.

Innovation Solution

Irradiate laser light with a wavelength of 550 nm or shorter on the electro-conductive portion to melt a bonding material with a lower melting point through thermal conduction, followed by cooling to solidify and bond the conductor and electro-conductive portion.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If reflow soldering is used to bond conductor and electro-conductive portion, then bonding strength is achieved, but heat adversely affects substrates and components

Engineering Contradiction:

Improvebonding strengthVSAvoidheat effect on substrates and components

Core Design Contradiction:

StrengthVSObject-affected harmful factors

Solution Approach 1:

The patent replaces the conventional thermal reflow soldering process with a laser-based bonding method. Instead of heating the entire assembly to melt solder, a laser beam is focused on the bonding material to locally melt and bond the conductor to the electro-conductive portion, thereby achieving strong bonding without subjecting substrates and components to adverse heat effects

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The laser beam is focused to a specific spot to melt only the bonding material at the bonding interface, rather than heating the entire assembly. This localized heating approach ensures that the conductor and electro-conductive portion are strongly bonded while the surrounding substrates and components remain unaffected by excessive heat

Inventive Principle:

Principle #3Local quality

2Strength

If high melting point solder is used for high strength bonding, then bonding strength is improved, but heat influence is enhanced

Engineering Contradiction:

Improvebonding strengthVSAvoidheat influence

Core Design Contradiction:

StrengthVSTemperature

Solution Approach 1:

The patent changes the bonding method from thermal heating to laser irradiation. By using laser energy directly to melt the bonding material, the process eliminates the need for high melting point solders and high temperature heating, achieving strong bonding with reduced heat influence on the overall assembly

Inventive Principle:

Principle #35Parameter changes

3Strength

If laser light is irradiated on electro-conductive portion to melt bonding material, then bonding is achieved, but electro-conductive portion may melt due to low laser absorption

Engineering Contradiction:

Improvebonding reliabilityVSAvoidcontrol of melting process

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

The patent introduces a coating layer on the electro-conductive portion as an intermediary that enhances laser light absorption. This coating layer has high absorption characteristics for the laser wavelength used, allowing the laser energy to be effectively absorbed and transferred to the bonding material for melting, while the electro-conductive portion itself remains protected from direct laser exposure and potential melting

Inventive Principle:

Principle #24Intermediary (Mediator)

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

Achieves a reliable and efficient bonding method that avoids melting the electro-conductive portion, ensuring a stable bonded state with minimal heat damage.

Implementation Method 1

irradiating laser light having a wavelength of 550 [nm] or shorter on a site of the bonded part opposite to the bonding material to melt the bonding material by thermal conduction at the bonded part

Methodology Applied

Scientific EffectThermal conduction: Conduction (thermal)

Implementation Method 2

The laser light typically in the infrared wavelength region is, however, less absorbed by a metal material that composes the electro-conductive portion

Methodology Applied

Scientific EffectLight absorption: Absorption (EM radiation)

Implementation Method 3

a solidification step of cooling the molten bonding material to solidify, thereby electrically connecting the conductor and the electro-conductive portion through the bonding material

Methodology Applied

Scientific EffectSolidification: Freezing

Data Source

PatentEP4624086A1Joining method and laser processing device

Publication Date: 2025.10.01 FURUKAWA ELECTRIC CO LTD

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AI summary

Provided is a bonding method including: a bonding material melting step of, upon arranging a first face that faces a first direction and that is included in a conductor provided on a circuit board and a bonded part of an electro-conductive portion in the first direction and upon interposing a bonding material having an electric conductivity and having a melting point lower than melting points of the conductor and of the electro-conductive portion between the first face and the bonded part, irradiating laser light having a wavelength of 550 [nm] or shorter on a site of the bonded part opposite to the bonding material to melt the bonding material by thermal conduction at the bonded part; and a solidification step of cooling the molten bonding material to solidify, thereby electrically connecting the conductor and the electro-conductive portion through the bonding material, for example.