When it comes to the flash memory medium type for SSDs, users are no longer faced with the forced transition from MLC to TLC and the “no choice but to buy” scenario. Unbeknownst to many, QLC SSDs have been on the market for nearly a decade. In the past two years, QLC SSDs have gradually transitioned from being an additional option for users to becoming the mainstream in the market.

Just as TLC replaced MLC, QLC is also accelerating its adoption, though some users are reluctant. This is precisely the evolution taking place within the supply chain. We need to understand more about flash memory technology and the trends in SSDs over the coming period. The chart below shows the QLC consumer-grade client SSDs released this year, most of which adopt the PCIe 4.0 x4 interface and are designed with a four-channel, DRAM-less cache architecture.

The Density Advantage of QLC

As users gain a deeper understanding of QLC, there is no longer a need to start from scratch explaining the concepts and principles of TLC and QLC. With a 30% increase in storage density compared to TLC, this is the confidence behind QLC. A higher storage density not only means larger SSD capacities but also lower costs and higher cost-effectiveness.

Currently, mainstream TLC flash memory has a single die capacity of 1 Tbit, while QLC can achieve 2 Tbit per die. The image below shows Kioxia/Western Digital's 2 Tb BiCS8 QLC flash memory die, which, through 16-die packaging, can provide up to 4 TB of storage capacity in a single flash memory chip. In addition to Kioxia/Western Digital, Yangtze Memory Technologies and Micron have also launched next-generation 3D QLC flash memory with a single die capacity of 2Tb.

The single die capacity of current 3D TLC flash memory remains at the 1Tb level. When applied to SSDs, under identical conditions, using QLC flash memory can provide twice the storage space of TLC flash memory. This is why QLC SSDs can easily reach 8TB, 16TB, or even hundreds of TB—who wouldn't love that? Major flash memory manufacturers are all heavily promoting QLC, not because it's “low-end,” but because the industry believes QLC is a future direction for technological development. They want to showcase their breakthroughs in cutting-edge technology.

QLC SSDs Can Also Achieve Bus Saturation

Four-channel design is the current mainstream for SSDs. Given the limited number of flash memory channels, improving the speed of the flash memory interface has become the key to accelerating SSD performance.

Interface speed enhancement is an inevitable trend in flash memory technology development. Kioxia recently announced the commencement of sampling for its ninth-generation BiCS flash memory. Although the current samples are of the TLC type, it is evident that by separating the CMOS and storage array processing on existing BiCS5 technology, SSD performance can be significantly enhanced. Both TLC and QLC can benefit from flash memory interface speed enhancements.

The figure above shows the architecture of Kioxia's BiCS5 flash memory, which originally belonged to the CNA design. Compared to the CUA-designed BiCS6, it is one generation behind. However, after applying CBA technology, the CMOS circuit is manufactured on a separate wafer, no longer constrained by the high-temperature processes of the storage array section, resulting in a flash memory interface speed of 3600 MT/s, the same as BiCS8.

Contemporary QLC now nearly matches TLC in interface speed, enabling bus saturation—commonly referred to as “full-speed read.”

Take KingSpec's XG7000 and VP14000 SSDs as examples; both use 3D QLC flash memory and achieve bandwidth saturation under PCIe 4.0 and PCIe 5.0 interfaces, respectively. In consumer-grade application environments where reading is the primary use case, their cost-effectiveness advantage is further highlighted.

Solutions for QLC’s Write Limitations

In the past, it was widely believed that no matter how much QLC SSDs improved, their write performance shortcomings could not be ignored. While SLC caching can maximize write performance, for QLC emulating SLC write operations, it requires four times the user space, and performance degradation after the cache is exhausted is also severe.

The recently launched Micron 7100 introduces a new multi-layer caching mechanism for QLC SSDs. The AWT adaptive write technology leverages the flexibility of flash memory, combining SLC and TLC caching to achieve smoother write performance while maintaining the cost advantages of QLC.

QLC Lifespan Is No Longer a Concern

Thanks to advances in flash memory technology and LDPC error correction algorithms, the write endurance of SSDs has actually been steadily improving in recent years. High-quality 3D QLC flash memory now has a write endurance cycle count that far exceeds that of the original 2D TLC flash memory. For example, the Floating Gate architecture 3D QLC flash memory used in the KingSpec XG7000/VP14000 has a write endurance cycle lifespan of 3,000 cycles, more than twice that of 2D TLC flash memory.

Regarding the Floating Gate architecture, it was the mainstream technology during the 2D flash memory era. With the advent of 3D flash memory, most flash memory manufacturers shifted to the Charge Trap architecture due to manufacturing process constraints. Intel remained the only company to continue using the Floating Gate architecture. Later, Intel's flash memory business was sold to SK Hynix, which is now known as Solidigm. Solidigm currently focuses on the enterprise-grade SSD market and does not favor one architecture over the other; both types of flash memory are used.

According to Intel's documentation from that time, the Floating Gate architecture has a significant advantage over the Charge Trap architecture in terms of data retention time after power loss.

In terms of data retention time, industry standards stipulate that after the rated write endurance is exhausted, consumer-grade SSDs should be able to retain data for one year at 30 degrees Celsius without data loss.

KingSpec's newly launched XG7000 and VP14000 series SSDs go beyond industry standards, achieving BOL performance of seven years of data retention at 43°C without errors, and EOL performance designed for one year of error-free operation at 43°C. This means there is no longer a need to store important data exclusively on HDDs.

Here, BOL and EOL stand for Beginning of Life (BOL) and End of Life (EOL), respectively, referring to the initial state and the state after the nominal lifespan has been exhausted. According to the JESD218 specification, consumer-grade SSDs should meet the requirement of storing data for one year at 30°C under power-off conditions in the EOL state. However, the higher the temperature during power-off storage, the greater the flash memory error rate. KingSpec achieves error-free storage for one year at 43°C, which is significantly more challenging than the JEDEC specification. This highlights the advantages of Floating Gate (FG) architecture flash memory.

Users Won’t Be Forced to Choose

From an industry trend perspective, leading storage manufacturers are actively developing QLC SSDs, particularly for industry-specific products. TLC will not disappear in the short term, and due to the significant increase in capacity, PLC flash memory will not enter practical use anytime soon. In the foreseeable future, TLC SSDs will coexist with QLC SSDs for an extended period, and QLC SSDs may develop at a faster pace.

Take KingSpec as an example: the company has already unveiled the XG7000 and VP14000 at CES 2025/IFA 2025, offering brand-name PCs with larger capacity and faster SSDs to enhance the overall user experience. Additionally, KingSpec plans to launch enterprise-grade QLC SSDs in the future, providing more options for servers and workstations.

In addition to OEM models, QLC SSDs are also expanding into retail brands. However, we can rest assured that both SSD manufacturers and media will prioritize user preferences, ensuring that users can make precise choices based on their needs, whether it be SLC, TLC, or QLC. The key lies in transparent flash memory medium labeling and comprehensive product reviews, which provide users with valuable reference information to make decisions that suit their needs.