Micro-LED Module Gap Fill Layer for Thermal Expansion Mismatch
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Solution Overview
Problem
The misalignment between micro-LED and submount substrates due to their different coefficients of thermal expansion during flip-bonding leads to uneven gaps, causing connection failures and defects such as electrical shorting, making it difficult to bond high-resolution displays with fine pixel pitches.
Innovation Solution
A micro-LED module with a gap fill layer formed between the micro-LED and submount substrate, using a bonding strength-enhancing material like epoxy or silicone adhesive, to stabilize the connection members and prevent gap unevenness, ensuring reliable connection of electrode pads to electrodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If flip-bonding is performed at high temperature to melt solder bumps, then electrical connection between electrode pads and electrodes is achieved, but misalignment occurs due to large differences in thermal expansion between sapphire substrate and Si-based submount substrate
Solution Approach 1:
A buffer layer is introduced between the sapphire substrate and the Si-based submount substrate. This buffer layer has a coefficient of thermal expansion that is intermediate between sapphire and silicon, thereby reducing the thermal expansion mismatch during heating and cooling cycles. The buffer layer acts as a mediator that absorbs differential expansion stresses, preventing misalignment of the micro-LED with the submount substrate while maintaining reliable electrical connection through solder bumps.
2Strength
If high melting point solder is used for flip-bonding, then strong electrical connection is achieved, but misalignment of 5 to 6 μm per 1 cm occurs at bonding temperature of 260°C
Solution Approach 1:
The buffer layer serves as a thermal expansion mediator that allows the use of high melting point solder materials without suffering from severe misalignment. By absorbing the differential thermal expansion between sapphire and silicon substrates, the buffer layer enables strong electrical connections through high-temperature soldering while maintaining bonding precision within acceptable limits.
3Reliability
If sapphire substrate with high coefficient of thermal expansion is used for micro-LED, then good LED cell performance is achieved, but large misalignment occurs when bonding to Si-based submount substrate
Solution Approach 1:
The buffer layer is positioned between the sapphire substrate (which provides good LED cell performance) and the Si-based submount substrate. It mediates the large coefficient of thermal expansion difference between these two materials, allowing the micro-LED to maintain its performance characteristics while preventing misalignment during the bonding process to the silicon substrate.
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
The gap fill layer effectively prevents gap unevenness between the micro-LED and submount substrate, enhancing the reliability of connection members and preventing defects like electrical shorting, thereby enabling successful bonding of high-resolution displays with fine pixel pitches.
Implementation Method 1
there are large differences in expansion and contraction strains between the Si-based submount substrate and the sapphire substrate upon heating and cooling during flip-bonding because the coefficient of thermal expansion of the Si-based submount substrate is significantly different from that of the sapphire substrate
Implementation Method 2
The micro-LED is flip-bonded to the mount substrate through solder bumps so that the electrode pads of the micro-LED are connected to the electrodes of the submount substrate. For flip-bonding of the micro-LED to the submount substrate, a solder constituting at least a portion of each solder bump should be heated to a temperature around its melting point.
Data Source
AI summary
A micro-LED module is disclosed. The micro-LED module includes: a micro-LED including a plurality of LED cells, each of which includes a first conductive semiconductor layer, an active layer, and a second conductive semiconductor layer; a submount substrate mounted with the micro-LED; a plurality of electrode pads formed on the micro-LED cells; a plurality of electrodes formed corresponding to the plurality of electrode pads on the submount substrate; a plurality of connection members through which the plurality of electrode pads are connected to the corresponding plurality of electrodes; and a gap fill layer formed in the gap between the micro-LED and the submount substrate and having a bonding strength to the micro-LED and the submount substrate.