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FPGAs have a lot of different voltage rails, and at high speeds, there are many constraints that must be satisfied with the PCB design for them. Microcontrollers can also have a lot of pins and need to interface with many different peripherals.

Must FPGA and microcontroller circuit PCBs always fit onto four or more layer boards or can they also fit onto two-layer boards?

I understand that the question is kind of vague. So assuming that we have an FPGA with only few dozen pins in a QFP package and needs to connect to perhaps less than a handful of peripheral ICs and a similar number of LEDs, can it be fit onto a two-layer board and work reliably? Or is such an expectation foolish?

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    \$\begingroup\$ Simplicity and relatively low speed. (Ground and power planes become important for high speed signal integrity). These are more easily achieved with MCU than FPGA. Note there is probably a higher risk of requiring modifications to mitigate problems with the 2-layer board, so ... simplicity, relatively low speed, and a schedule tolerant of some technical risk. \$\endgroup\$
    – user16324
    Aug 6, 2021 at 15:08
  • \$\begingroup\$ "Under what circumstances...", the "stupid" answer would be, when it cannot be done on a 2 layer PCB. It depends on schematic complexity, pitch required, minimum track width/spacing of copper and lots of other things so there can be no clear answer to this question as "it depends". \$\endgroup\$ Aug 6, 2021 at 15:09
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    \$\begingroup\$ Number of pins and speed. You need planes for speed \$\endgroup\$
    – DKNguyen
    Aug 6, 2021 at 15:09
  • \$\begingroup\$ Problem with a 2 layer PWB is that you don't have the room needed to route signals. A 4-layer stack up gives you two routing layers, X & Y, and 2 layers for power and returns. Even this stack up is insufficient for all but the simplest of designs. \$\endgroup\$
    – SteveSh
    Aug 6, 2021 at 15:11
  • \$\begingroup\$ One problem is the fan-out. For QFP it isn't such a problem because all pins are on the side of the packge. But with pins underneath the IC like BGA it becomes almost impossible with just two layers. \$\endgroup\$
    – mais
    Aug 6, 2021 at 15:15

3 Answers 3

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FPGA and microcontroller circuits do not need to use 4 layers.

While most current FPGA chips use BGA packages, there are other packages available.

The simplest FPGA package I could find in Mouser is the QFN-32, which is a QFN configuration with 32 pins. By the image it is easy to see that even a single layer allows routing, specially if you don't use all I/O pins, which depends on the project.

QFN-32 package

Also, newer low-pincount FPGAs may not need all those voltage rails. This data sheet lists only 2 VCC and 2 GND pins. Some may include embedded flash, which reduces the number of pins needed for the configuration memory.

For microcontrollers, it is even easier to see that only a single layer is the minimum needed, since there are packages with as few as 6 pins, such as the SOT-23-6:

SOT-23-6 package

With that in mind, you can see that even a single-layer project is possible, albeit only simpler ones. Your project may more complex and require components with harder routing or more pins, which may only be possible to route with more layers, like the BGA packages.

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There is absolutely no requirement that either FPGA or MCU PCB designs use a specific number of layers. PCB designers nearly always try to use the minimum number of layers to save money. Some designs might even use a single-sided PCB.

But different design constraints may cause 4 layer or even more layers to be used in order to meet them.

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  • \$\begingroup\$ on simple boards it's usually either the package or controlled impedance - getting the right impedance for USB or ETH on a two layer board can require track width which is nigh impossible ti fit \$\endgroup\$
    – jaskij
    Aug 7, 2021 at 16:21
  • \$\begingroup\$ It's often way easier to pass EMC testing with a four layer board with ground/power planes on the internal layers - much less chance of accidentally creating a nasty return path. \$\endgroup\$ Aug 7, 2021 at 19:46
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Under the DIY circumstances

DIY PCB

This is my solar MPPT charge controller. I etched the board by hand using two-sided copper clad board from ebay.

You can see that it has a MCU in an LQFP-32 as well as a USB UART in a QFN package (top left, not fitted as on this occasion I destroyed the tracks to the USB connector and so used an external debug board to the TTL lines instead). There are some surface mount op-amps on the back.

Note that the MCU has quite a lot of stuff built in, the flash, SRAM, voltage regulators and numerous peripherals such as ADC. This greatly reduces the amount of connectivity actually required.

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