Smaller. Faster. Higher bandwidth. Better performance. Today’s leading memory products are rapidly evolving to meet the intense demands of the AI era. However, these advancements bring with them a challenge which can hinder the development of next-generation products—excessive heat generation.
To tackle this issue, SK hynix made an unprecedented breakthrough by developing a new and innovative packaging technology called MR-MUF1 that improves heat dissipation in chips. Applied to the company’s groundbreaking HBM2 products since 2019, MR-MUF has set SK hynix aside from the competition. As the only company to use MR-MUF and having received excellent client evaluations for the heat dissipation characteristics of its HBM products which apply the technology, SK hynix has risen to the position of HBM market leader.
1Mass reflow-molded underfill (MR-MUF): Mass reflow is a technology that connects chips together by melting the bumps between stacked chips. Molded underfill fills the gaps between stacked chips with protective material to increase durability and heat dissipation. Combining the reflow and molding process, MR-MUF attaches semiconductor chips to circuits and fills the space between chips and the bump gap with a material called liquid epoxy molding compound (EMC).
2High Bandwidth Memory (HBM): A high-value, high-performance product that possesses much higher data processing speeds compared to existing DRAMs by vertically connecting multiple DRAMs with through-silicon via (TSV).
This Rulebreakers’ Revolutions episode will look at how the pioneering development of MR-MUF, particularly its new materials with high thermal conductivity, solved the problem of excessive heat generation in next-generation HBM products.
The Mission: Overcome the Problem of Heat Generation
As memory products evolve, heat generation becomes an increasingly pressing issue for several reasons. For example, the miniaturization of semiconductors negatively impacts heat dissipation due to the reduced surface area and increased power density. In the case of stacked DRAM products such as HBM, thermal resistance increases due to the longer heat transfer paths, while thermal conductivity is limited by the materials between chips. Moreover, the continuous advancements in speed and capacity result in increased heat generation.
The inability to sufficiently control heat in semiconductor chips can negatively impact a product’s performance, lifecycle, and functionality. This can become a serious concern for customers and significantly impact factors including productivity, energy costs, and competitiveness. Consequently, heat dissipation, along with capacity and bandwidth, has become a key consideration during the development of advanced memory products.
Attention has therefore turned to semiconductor packaging technology as one of its main functions is heat control. Up until the second generation of HBM, HBM2, SK hynix applied the industry-standard TC-NCF3 process to its HBM products. However, with advancements in HBM that required chips to become thinner to accommodate additional chip layers, the applied packaging technology needed to control higher levels of heat and pressure. Issues such as chip warpage due to pressure and thickness limitations in densely stacked products also needed to be addressed as SK hynix planned to develop its next-generation products. At this point, the company needed to think outside the box by developing a new packaging technology for its future products.
3Thermal compression non-conductive film (TC-NCF): A method of stacking chips by applying a film-like substance between chips. Heat and pressure are applied to melt the substance so chips are glued together.
MR-MUF & Its New Materials: The Missing Pieces to the Heat Control Puzzle
As SK hynix was developing HBM2E, the third generation of HBM, controlling heat became a major focal point for improvement. Even when TC-NCF was being recognized as a packaging solution suitable for densely stacked products, SK hynix challenged the status quo and strove to develop a new packaging technology offering improved heat dissipation. After countless tests and trials, the company unveiled its new packaging technology MR-MUF in 2019 which would change the future of the HBM market.
The structural difference between TC-NCF and MR-MUF that influence heat dissipation
Developed by multiple teams at SK hynix, MR-MUF heats and interconnects all the vertically stacked chips in HBM products at once. This makes it more efficient than TC-NCF which applies a film-type material after each chip is stacked. Moreover, MR-MUF increases the number of thermal dummy bumps—which are effective at dispersing heat—by up to four times compared to TC-NCF.
Another important feature of MR-MUF is the addition of a protective material called EMC4 used to fill the spaces between chips. A thermosetting polymer with excellent mechanical and electrical insulation as well as heat resistance, EMC addressed the need for high environmental reliability and control over chip warpage. Due to the application of MR-MUF, HBM2E improved heat dissipation performance by 36% compared to its predecessor, HBM2.
4Epoxy molding compound (EMC): A heat dissipation material based on epoxy resin, a type of thermosetting polymer, that seals semiconductor chips to protect them from environmental factors such as heat, moisture, and shock.
Although MR-MUF was also used for HBM2E’s successor, the 8-layer HBM3, SK hynix elevated the MR-MUF process to another level when developing the 12-layer HBM3 in 2023. As the DRAM chips had to be 40% thinner than the chips used in the 8-layer HBM3 in order to maintain the product’s overall thickness, chip warpage became a significant issue. SK hynix actively responded by developing Advanced MR-MUF, introducing the industry’s first chip control technology5 and new protective materials that improved heat dissipation. In this process, SK hynix once again achieved innovation in materials as the new EMC applied in Advanced MR-MUF offered a 1.6-time improvement in heat dissipation properties compared to the EMC for the original MR-MUF.
5Chip control technology: The application of a momentary burst of high heat to each chip as it is stacked, causing the bump under the top chip to fuse to a thin pad on top of the bottom chip. The pad holds the chip together and protects it from warpage.
With Heat Control, SK hynix Mass-Produces Highest Level of HBM
A timeline of HBM’s multiple generations and progress in heat dissipation
Starting with the development of HBM2E, the application of MR-MUF and the subsequent Advanced MR-MUF enabled SK hynix to produce the industry’s highest standards of HBM products. Fast-forward to 2024, SK hynix became the first company to mass-produce HBM3E, the latest HBM product which boasts the highest standards of performance. HBM3E saw a 10% improvement in heat-dissipation performance compared with its previous generation, the 8-layer HBM3, following the application of Advanced MR-MUF to become the in-demand memory product in the AI era. Looking ahead, the company is set to maintain its HBM leadership as it has announced plans to bring forward the mass production of the next-generation HBM4 to 2025.
Rulebreaker Interview: Kyoung-Moo Harr, HBM Package Product
To find out more about the original approach which led to the development of MR-MUF and advancement of HBM, the SK hynix newsroom spoke with Technical Leader Kyoung-moo Harr of HBM Package Product. Having actively supported the development of MR-MUF through exploration, testing, and verification of new materials, Harr discusses the impact of this innovative process.
How significant was the successful development of HBM products with MR-MUF for SK hynix? What are the major breakthroughs of MR-MUF and Advanced MR-MUF in terms of material innovation?
“MR-MUF has propelled us to the top of the HBM market and enabled us to secure HBM leadership. Ever since we made the calculated risk of applying MR-MUF to HBM2E rather than TC-NCF like other companies in the industry, SK hynix has been outpacing its competitors. Enabling the mass production of unprecedented HBM products with increasingly more layers, MR-MUF is a true testament to the company’s persistent pursuit of innovation.
“In terms of material innovation, MR-MUF features EMC which has stronger heat dissipation qualities than NCF. This played a key role in improving the heat control capability of MR-MUF and enhancing the environmental reliability compared to TC-NCF. For Advanced-MUF, SK hynix took its EMC a step further by creating a new version with improved heat dissipation properties.”
What are some behind-the-scenes efforts during the development of MR-MUF that you would like to highlight?
“Behind these highly advanced technologies lies a continuous cycle of tests and evaluations for verifying and enhancing the qualities of new materials that would be used in the packaging process.
“When developing Advanced MR-MUF, it was crucial that the new EMC was continuously applied to a universal test vehicle6 (UTV) for reliability testing. A UTV with the same specifications of a HBM product undergoes WLP7 to become a sample. It then proceeds to a look ahead reliability8 (LAR) test to identify defects. Only materials that pass the test and receive necessary improvements are applied to the final HBM products.”
6Universal test vehicle (UTV): Samples produced in a product’s early development stage to test and establish its specifications and standards.
7Wafer-level package (WLP): Technology that produces end products by packaging and testing a wafer all at once before the wafer is diced. It differs from the conventional packaging method of processing a wafer and dicing each chip.
8Look ahead reliability (LAR): A preliminary test before quality evaluation that seeks to set countermeasures for defections found during the test. These countermeasures need to be applied during quality assessment to fix defects.
How did SK hynix’s rulebreaker spirit encourage employees to break convention with the development of MR-MUF?
“Our company has a “rulebreaking” culture of encouraging everyone to choose challenging goals instead of settling for easier ones. In addition, all members no matter their department are committed to one-team collaboration and strive to be the best role players they can possibly be for the team.
“This was clear during the development of MR-MUF, when members from various departments collaborated on the project to ensure its success. It truly was a company-wide effort as members came together to make this innovation possible. My role also involved significant collaboration as I supported engineers in their development of the process. This was on top of my main duties of conducting preliminary risk evaluations of materials, drawing up technical verification plans, monitoring competitors, and identifying customer needs ahead of time.
“At SK hynix, we are all rulebreakers because we believe our joint efforts allow us to reach previously unimaginable heights.”
