Flip-Chip LED Underfill via Compression Molding

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Solution Overview

Problem

The existing underfill techniques for flip-chip LEDs face challenges in precisely delivering the right amount of material, are time-consuming, and often damage the LEDs due to high viscosity or thermal expansion issues, which can lead to cracking during the solder reflow process.

Innovation Solution

A compression molding process is used to fill the gap between the LED chip and submount with underfill material, creating a vacuum and using a mold with cavities to ensure complete filling without pressure injection, allowing for a wider range of viscosities and materials, including polyimide with a glass transition temperature suitable for solder reflow temperatures.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If high pressure injection is used to fill the narrow gap between LED and submount, then the gap can be filled, but the LED may be damaged

Engineering Contradiction:

Improvegap filling precisionVSAvoidLED damage

Core Design Contradiction:

Manufacturing precisionVSObject-affected harmful factors

Solution Approach 1:

Instead of injecting underfill material from below the submount upward through high pressure, the patent inverts the approach by placing the LED chip on a temporary carrier first, then injecting underfill material from the top side of the LED chip downward into the gap between the chip and submount. This reversal of injection direction reduces pressure concentration on the LED and prevents damage while achieving complete gap filling.

Inventive Principle:

Principle #13The other way round (Inversion)

2Measurement precision

If underfill material is injected sequentially on each LED, then precise control is possible, but the process is time-consuming

Engineering Contradiction:

Improveunderfill amount controlVSAvoidprocessing speed

Core Design Contradiction:

Measurement precisionVSProductivity

Solution Approach 1:

The patent merges multiple individual LED underfilling operations into a single batch processing step. By mounting multiple LED chips on a temporary carrier in an array configuration and using a single injection nozzle to supply underfill material to all chips simultaneously through capillary action, the process achieves both precise material control and high productivity, eliminating the need for sequential processing of each LED individually.

Inventive Principle:

Principle #5Merging (Combining)

3Strength

If high viscosity underfill material is used, then structural support is improved, but the material cannot fill narrow gaps effectively

Engineering Contradiction:

Improvestructural supportVSAvoidgap filling capability

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

The patent utilizes capillary hydraulic action to deliver underfill material into the narrow gap between LED chip and submount. The injection system controls the flow of underfill material through pressure regulation, allowing the material to be pushed into the gap initially, then relying on capillary forces to distribute it uniformly. This hydraulic approach enables high viscosity materials to fill narrow gaps effectively without requiring excessive injection pressure that would damage the LED.

Inventive Principle:

Principle #29Pneumatics and hydraulics

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 method reduces processing time significantly, minimizes damage to LEDs, and allows for a variety of underfill materials with improved thermal stability, ensuring reliable encapsulation and support during substrate removal and soldering.

Implementation Method 1

a vacuum is created within the mold

Methodology Applied

Scientific EffectVacuum: Vacuum

Implementation Method 2

the vacuum and the liquid material pressure causes the material to completely fill the cavities

Methodology Applied

Scientific EffectPressure differential: Pressure Gradient

Implementation Method 3

The liquid material is then cured by heat or UV light to harden the underfill material

Methodology Applied

Scientific EffectPhotopolymerisation: Photopolymerisation

Data Source

PatentEP2266149B1Underfill process for flip-chip LED

Publication Date: 2017.08.23 KONINKLIJKE PHILIPS NV

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AI summary

An underfill technique for LEDs (10, 12) uses compression molding (50) to simultaneously encapsulate an array of flip-chip LED dies mounted on a submount wafer (22). The molding process causes liquid or softened underfill material (41) to fill the gap between the LED dies and the submount wafer. The underfill material is then hardened, such as by curing. The cured underfill material (54) over the top and sides of the LED dies is removed using microbead blasting (58). The exposed growth substrate (12) is then removed from all the LED dies by laser lift-off (60), and the underfill supports the brittle epitaxial layers (10) of each LED die during the lift-off process. The submount wafer is then singulated. This wafer-level processing of many LEDs simultaneously greatly reduces fabrication time, and a wide variety of materials may be used for the underfill since a wide range of viscosities is tolerable.