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I just got my first 4 layer newly assembled board from PCBway.

When USB cable is connected, the board is not powering on.

When I checked the vbus it's showing 0.45v. After this I checked the MOSFET q1 and q2 and they seem to be damaged. How could this have happened with just powering the board for the first time?

Because this board is not getting powered I can't flash the firmware ofc

enter image description here

On the PCB, I soldered a wire directly to vbus to check the voltage. From the source itself it's now working.

Another question: Is it ok to just bypass the q1 and q2 and directly connect a jumper from micro USB vbus to mcu usb_vbus?

Also, when I connected a ST-Link probe to the JTAG, it didn't detect although it is Renesas board. So I wasn't that hopeful about my punctuation. Are there any other ways to get the MCU detected by pc?

Once I can get power through USB then only after pulling down MD can I flash the firmware (until then I can't do anything).

Please suggest something should I desolder the USB board, and/or resolder?

To me right now USB port seems faulty but it's a new board and ports like this shouldn't get damaged easily in first boot. Another thing is that I can power the board through can port after which everwhere else voltage seems fine but then i still can't flash firmware without usb so please someone suggest something this already got me panicing.[sic]

Other this I suspect maybe I got a wrong footprint and soldered the port completely wrong? But then the ratsnest and ERC checks would have shown this.

enter image description here

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1 Answer 1

Reset to default
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TL;DR; not all SOT23 transistors use the same pinout. The transistors you have selected do not match the pinout you have used in your layout.

Assuming red is the top layer, your footprint for the BSS123 transistor is incorrect.

From the datasheet, the pinout is as follows:

Pinout of device

Note that the source is pin 2, which in the schematic you connect to ground. However in the layout, it is the drain, pin 3, which is connected to ground.

Pin 1 is the gate for the transistor which your schematic shows connected to VBus. But for your layout pin 2, the source, is connected to VBus.

Layout incorrect pinout

That means your transistor has source connected to Vbus, and drain connected to GND. This means the body diode is forward biased and conducting, shorting out VBus to ground.

You will have the same issue everywhere on the board that this transistor is used.

You also have the same issue with the PMOS. From the datasheet its pinout is:

PMOS pinout

Again this doesn't match your layout.

Depending on how good your solding skills are, you could potentially desolder the existing transistors and resolder them at a 45 degree angle as shown below. This would allow you to get up and running without having to respin your board. Alternatively, try and find a suitable replacement part with the desired footprint (if they exist).

Soldering at an angle

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  • \$\begingroup\$ Thanks for the this answer stupid me just use the default kicad footprint for mosfet. For vbus let’s say mosfet was shorting but even after i desoldered the mosfet then vbus didn’t had any mosfet in its path and still i was showing 0.4v does that means port and the whole board is damaged permanently? \$\endgroup\$
    – Sumit Khobragade
    Commented Sep 4 at 22:13
  • \$\begingroup\$ @SumitKhobragade Have you removed both Q1 and Q2? \$\endgroup\$
    – Tom Carpenter
    Commented Sep 4 at 22:22
  • \$\begingroup\$ No just the q2 , please suggest a way so that i can still salvage the rest of pcb without being in loss. Ofc next time i have to fix this but for now atleast it should be working somehow😭. And just to be use can you once tell me how to check the MOSFET for damage using dmm feels like i might be doing that wrong too. \$\endgroup\$
    – Sumit Khobragade
    Commented Sep 4 at 22:35
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    \$\begingroup\$ @SumitKhobragade Q1 is the one shorting to GND, so I'm not surprised you still read .4V without removing it. See the diagram at the end for how you can solder the transistors back on at an angle to salvage the PCB. You'll need to do this for all of the ones on the board, not just the two shown. \$\endgroup\$
    – Tom Carpenter
    Commented Sep 4 at 23:38
  • \$\begingroup\$ Your previous solution has been helpful, but I need further clarity. This PCB can be powered by both USB and CAN port.When powered through the CAN port,it works perfectly, and the MCU turns on.However, when powered via USB, power seems to go everywhere,but the MCU doesn’t turn on,and the LDO outputs 0.45V.Oddly, when I touch pin 1 of the LDO, the MCU turns on.Shorting pins 2 and 4 of the LDO also turns on the MCU instantly.Could this be due to a MOSFET issue, a damaged LDO, or is there something wrong in the schematic? Should I explore ORing the power supply more, and where should I start? \$\endgroup\$
    – Sumit Khobragade
    Commented Sep 5 at 15:32

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