Flux-Eutectic LED Submount Bonding for Void-Free Assembly
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Solution Overview
Problem
Conventional soldering techniques for mounting semiconductor light emitting devices, such as LEDs, face challenges with solder paste variability leading to weak bonds, shorting, and voids, which are particularly problematic in high-density circuit board applications.
Innovation Solution
A flux-eutectic process is used to form metal-to-metal bonds between the submount and mounting substrate, and between the LED and submount, eliminating the need for solder paste and ensuring direct contact, with the metal bonding layer thickness controlled based on the surface roughness of the mounting substrate to prevent voids and ensure strong bonds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder paste is used for mounting LED to submount and submount to mounting substrate, then electrical and physical connections are established, but solder paste variability causes weak bonds, shorting, and voids
Solution Approach 1:
The patent removes solder paste from the bonding process entirely, extracting the problematic intermediate material that causes variability. Instead, it uses direct metal-to-metal bonding between the LED pad and submount, and between submount and mounting substrate, eliminating the source of bond inconsistency, shorting, and void formation
Solution Approach 2:
The patent introduces a flux material as a controlled intermediary during the bonding process. The flux is applied in a defined amount and completely consumed during reflow, serving as a temporary mediator that facilitates clean metal-to-metal bonding without remaining as a variable element like solder paste
2Ease of operation
If conventional die attach techniques are used for flip-chip mounting, then LED is mounted substrate side up, but conductive die attach material squeezes out and short-circuits the p-n junction
Solution Approach 1:
The patent removes conductive die attach material from the bonding interface. Instead of using viscous conductive paste that can squeeze out and cause shorting, it employs direct metal-to-metal bonding with flux, eliminating the electrical isolation problem while maintaining the beneficial substrate-up mounting orientation
Solution Approach 2:
The patent changes the bonding mechanism from material-based adhesion (conductive paste) to metallurgical bonding (direct metal-to-metal). This parameter change in the bonding process prevents material extrusion and maintains electrical isolation while achieving reliable mechanical and electrical connection
3Strength
If metal-metal bonding is used with solder materials, then mechanical mounting and thermal pathway are provided, but Sn migration degrades device performance
Solution Approach 1:
The patent extracts tin from the bonding system by using flux-eutectic metal-to-metal bonding instead of solder-based bonding. This eliminates the source of tin migration that degrades metal-semiconductor interfaces and metal-metal reflective interfaces, while still providing strong mechanical bonds and thermal pathways
Solution Approach 2:
The flux material serves as a disposable intermediary that is completely consumed during the bonding process. It facilitates the metal-to-metal bonding action and then disappears, leaving no residual material that could migrate or degrade device performance like tin from solder
4Ease of manufacture
If multiple solder reflow process steps are used for mounting LED to submount and submount to mounting substrate, then electrical connections are established, but temperature management becomes complex
Solution Approach 1:
The patent merges the bonding of LED to submount and submount to mounting substrate into a single reflow process. By using flux material that facilitates bonding at one temperature stage, it eliminates the need for multiple sequential reflow steps and their associated temperature management complexity, while still establishing all necessary electrical connections
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 provides reliable, strong metal-to-metal bonds without solder paste, reducing the risk of shorting and voids, and allows for precise control of the assembled stack height, suitable for high-density circuit board applications.
Implementation Method 1
A flux-eutectic process is used to form metal-to-metal bonds between the submount and mounting substrate, and between the LED and submount
Implementation Method 2
The assembled stack is reflowed to attach the metal bonding layer of the submount to the mounting substrate and to attach the light emitting device to the submount
Data Source
AI summary
Methods of assembly for a semiconductor light emitting device package may include positioning a submount on a mounting substrate with a flux material therebetween and at least substantially free of solder material therebetween. The submount has a metal bonding layer facing the mounting substrate. A semiconductor light emitting device is positioned on a top side of the submount with a flux material therebetween to provide an assembled stack. The assembled stack is reflowed to attach the metal bonding layer of the submount to the mounting substrate and to attach the light emitting device to the submount.