OHMs between VSS and VDD falls down to minimum value.and CPU RUN led turned off within few minuits.

[KN.2]

we are developing HMI(ethernet,sdcard,4.3 inch tft touch display) using STM32F746 microcontroller with 6 layer pcb,we already did one pcb and its working perfectly.and we started 2nd one almost at the end of the process found random auto reset,cpu run indication led(normally blink) completely turned off.so that i just check the short circuit test between vcc and ground using multimeter i got short with minimum ohms(2ohms sometime 0).and the value is slowly increasing after some time.and i turn on the board again same problem occured again.and we start with new pcb again assembled the section one by one and dump the test code for each section.after sometime(6-7 hours) again i got the same issue.the time period(6-7hrs) of last 2 pcb are similar its working around 6-7 hours properly after that the problem is arriving.i need some clarficationds regarding this.

[Tesla DeLorean]

Have an FAE review the schematic and layout.

​

Check ground and power connectivity. Check the netlist for islands that should have been connected.

Check placed capacitors and voltages on VCAP pins.​

[KN.2]

thank you for your response,

in a previous board with the same problem,we removed the controller ic and powered for few mins and turned off.measured the ohmes value across the power supply(3.3.v regulator output side) there is no short circuit,and i soldered the same ic,and measured the ohm value its 5ohms and i didnt powerd it.my confusion is one board is working so my thought is no schematic issue and pcb issue am i right??

[Tesla DeLorean]

So start by focusing on the manufacture of the PCB, soldering of the board and the components placed.

​

If you have boundary scan methods, use those. Test integrity of all nets on the board.​

​

Damage to the IC will occur if large currents traverse inappropriate paths.

If you have islands of power​, you can have pads at different potentials, and current via protection diodes, or other board level components.

​

Check signal levels from external interfaces/circuits, make sure they are within operational maximums, and have bonded grounds.

​

Review design with all involved engineers, and FAE assigned to the account.​

[KN.2]

i will check all those and back to you,is any chance to consider that particular ic batch is defective??

[TDK]

It’s far far more likely you have a pcb error than a defective IC. An infrared camera can be useful to figure out where the short is. Make sure the ic is correctly oriented. If it saw more than 3v3 it may be dead.

[KN.2]

Hi sir,

If I have error in pcb how my 1st prototype is working,any idea???​

[TDK]

Could be any number of individual pcb defects. Solder bridge, esd damage, over voltage blown diode. I would investigate rather than guess if it were my project.