System-In-Package Vs. eNVM

The booming automotive and IoT markets are driving increasing demand for microcontrollers (MCUs). Recent forecasts project that the overall MCU compound annual growth rate (CAGR) will reach 4% over the next five years, and in particular the automotive MCU CAGR could reach close to 14%.

Non-volatile memory (NVM) is a critical element of MCUs, as it is needed not only to store the code, but also to store the operating data throughout the product lifetime.

Two NVM solutions
There are two NVM solutions commonly used to build MCUs: NVM directly embedded in the system-on-chip (SoC) or a separate, external NVM chip assembled with a logic chip as a system-in-package (SiP) solution. MCUs with embedded NVM (eNVM) are fabricated in a special logic process that includes eNVM, and everything needed for MCU operation is created within that single chip. For MCUs that use a SiP solution, a NOR Flash chip and a logic chip are packaged together. Code and data are therefore stored off the logic chip on a standalone NOR Flash chip.

Top MCU providers primarily use an eNVM solution in their products, but a SiP solution may be an attractive option for smaller companies. Those companies may realize a shorter time to market, partly because using a standard, readily-available logic process may simplify and shorten the design cycle. However, a SiP solution may not meet all the requirements of many IoT and automotive applications. Using eNVM will often be the superior solution given the the cost, power, speed, security, stability, and reliability requirements of high growth MCU applications.

Choosing the best solution
To choose the best solution for an application, consider the following comparison of these two solutions based on key end market requirements of power consumption, power-up time, speed, security, reliability and cost:

• • Power Consumption: eNVM offers more than 30% lower active power consumption than SiP because SiP flash requires constant IO toggle. Therefore, GF recommends eNVM for battery powered IoT applications that require low power. GF and eVaderis are co-developing a low-power MCU using 22FDX and eMRAM.
  • Power-up Time: eNVM offers a 20x faster time to power up and access first data than SiP (5µs vs. 100µs) because eNVM is XIP, whereas with SiP flash, the system needs to copy the data to on-chip SRAM. Therefore, for normally-off applications that require very fast power-up and read times, GF recommends embedded eNVM.
  • Speed: eNVM offers a 2x faster read speed than SiP (400MB/sec vs. 200MB/sec), since the eNVM macros have x32 to x128 bit IO bus width, whereas the SiP uses x4 or x8 bit Therefore, GF recommends eNVM for high speed/high bandwidth applications.
  • Security: eNVM offers higher security than SiP because the eNVM macro can be customized and the SoC can use IP such as PUF to enhance security. In contrast, SiP flash is a standard offering in the market, and extra security cannot be added. So, GF recommends eNVM for high security applications.
  • Reliability: eNVM offers higher reliability because it is qualified as a single SoC by the required reliability level, whereas SiP flash can only achieve high reliability by adding strict test screening on known good die (KGD) and the package. CMOS Embedded STT-MRAM arrays in 2x nm Modes for GP-MCU applications.
  • Cost: To compare the cost of the two solutions, several factors must be taken into account:
    • NVM memory density and full chip size, which determines the Gross-Die-Per-Wafer with or without eNVM
    • Wafer price, with or without eNVM
    • Extra on-chip SRAM density for SiP solution, which is used to download the code from external Flash during power up
    • Flash KGD price for SiP solution
    • Wafer testing cost, with or without eNVM
    • Additional factors like wafer yield, with or without eNVM, SiP solution FT yield loss, management cost

Comparing cost
The following cost comparison includes six typical NVM memory densities (2Mb, 4Mb, 8Mb, 16Mb, 32Mb, 128Mb) implemented both on GF’s 40nm LPx platform with eFlash, and also on the 22FDX (22nm FD-SOI) platform with eMRAM.

Because every company has a different power up methodology for SiP solutions, various on-chip SRAM densities are used to “shadow” the external Flash. The following results assume the ideal case for SiP, where a SiP solution and eNVM solution utilize an identical SRAM size. Note that most common SiP solutions increase the SRAM size for code shadowing from the external Flash.

Source: GLOBALFOUNDRIES, 2018

The graph above shows that the 40nm platform with eNVM (eFlash) is lower cost when the NVM density is less than 16Mb, while the SiP solution is lower cost when the NVM density is equal to, or higher than 16Mb.

For a design utilizing 22nm FDX platform, the eNVM solution (eMRAM) is lower cost when the NVM density is less than 32Mb, while the SiP solution is lower cost when the NVM density is equal to or higher than 32Mb.

Comparing the two platforms, the 22FDX eNVM solution (eMRAM) has lower cost at all the NVM densities versus the 40nm SiP solution. In addition, for the 22nm platform the additional cost for eMRAM at higher densities (32Mb+) is 4% or less, while also outperforming a SiP solution in power, speed, security, and reliability.

For even larger SRAM densities, the advantages of an eNVM solution are even greater.

So, which solution is best for my MCU?
In summary, both eNVM and SiP solutions are viable methods to combine logic and NVM. However, eNVM is often a better choice for MCUs based on the superior power, speed, security and reliability. With respect to cost, eNVM is often a lower cost than SiP, especially below 32Mb NVM densities. As MCU makers consider all the trade-offs for their products, GF stands ready to assist clients in selecting the appropriate solution to win in their market.

In a recent Tech Talk video GF talks about the pros and cons of embedded non-volatile memory versus system in package.