Micro-LED Flip-Bonding via Laser Heating for Submount Alignment

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Solution Overview

Problem

The difference in coefficient of thermal expansion between a sapphire substrate and a Si-based submount substrate during flip-bonding of micro-LED modules leads to misalignment and electrical issues such as shorting, making precise bonding challenging.

Innovation Solution

The method involves using laser beams to locally heat and melt solders between the micro-LED and submount substrate, allowing for precise bonding without applying heat to the substrates, thus minimizing thermal expansion and contraction effects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional heating methods are used to melt solder during flip-bonding, then solder bonding is achieved, but substrate misalignment occurs due to differential thermal expansion

Engineering Contradiction:

Improvesolder bonding reliabilityVSAvoidsubstrate alignment precision

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent replaces conventional thermal heating methods with laser beam irradiation to melt the solder. The laser beam is focused on the solder bump to locally heat and melt it, enabling flip-bonding without heating the entire substrate. This substitution of heating method eliminates differential thermal expansion between substrates with different coefficients of thermal expansion, thereby preventing misalignment while achieving reliable solder bonding.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Strength

If high melting point solder is used to achieve strong bonding, then bonding strength is improved, but substrate misalignment worsens due to higher required heating temperature

Engineering Contradiction:

Improvebonding strengthVSAvoidsubstrate alignment precision

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

The patent uses laser beam irradiation instead of conventional thermal heating to melt the high melting point solder. The laser provides localized heating only at the solder bump, allowing the solder to reach its melting point and achieve strong bonding without heating the entire substrate to high temperatures. This eliminates the differential thermal expansion problem that would otherwise occur at high temperatures, maintaining alignment precision while achieving strong bonding.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The laser beam is focused to heat only the local region where the solder bump is located, rather than heating the entire substrate. This localized heating approach allows the solder to be melted and bonded without causing thermal expansion of the substrates, thereby maintaining alignment precision while achieving the bonding strength required for high melting point solder.

Inventive Principle:

Principle #3Local quality

3Reliability

If uniform heating is applied to melt solder, then complete solder melting is achieved, but substrate warping and misalignment occur

Engineering Contradiction:

Improvesolder bonding completenessVSAvoidsubstrate flatness

Core Design Contradiction:

ReliabilityVSShape

Solution Approach 1:

The patent applies laser beam irradiation that is focused on the solder bump to achieve local heating. This localized heating melts the solder completely for reliable bonding without heating the entire substrate, thereby preventing substrate warping and maintaining substrate flatness. The selective heating of only the solder region eliminates the differential thermal expansion that would cause misalignment.

Inventive Principle:

Principle #3Local quality

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 effectively prevents misalignment and ensures reliable electrical connections by hardening the solders to secure the micro-LED to the submount substrate, improving bonding precision and reducing defects.

Implementation Method 1

heating the plurality of solders by a laser

Methodology Applied

Scientific EffectLaser heating: Laser

Implementation Method 2

locally heating the plurality of solders by a plurality of laser beams

Methodology Applied

Scientific EffectLocal heating: Heating

Implementation Method 3

heating the plurality of solders by a laser... heating the plurality of solders by a plurality of laser beams

Methodology Applied

Scientific EffectMelting: Melting

Implementation Method 4

hardening the solders to secure the micro-LED to the submount substrate

Methodology Applied

Scientific EffectHardening:

Implementation Method 5

large differences in expansion and contraction strains between the Si-based submount substrate and the sapphire substrate upon heating and cooling... coefficient of thermal expansion of the sapphire substrate amounts to about 2.5 times that of the Si-based submount substrate

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentUS10847504B2Micro-LED module and method for fabricating the same

Publication Date: 2020.11.24 LUMENS CO LTD

AI summary

A method for fabricating a micro-LED module is disclosed. The method includes: preparing a micro-LED including a plurality of electrode pads and a plurality of LED cells; preparing a submount substrate including a plurality of electrodes corresponding to the plurality of electrode pads; and flip-bonding the micro-LED to the submount substrate through a plurality of solders located between the plurality of electrode pads and the plurality of electrodes. The flip-bonding includes heating the plurality of solders by a laser.