Help needed debugging 4-channel RS232 mPCIe card design

Thread Starter

TelluriumCrystal

Joined Nov 14, 2014
32
I have designed and assembled a 4-channel mPCIe card implemented around a FTDI FT4232H. The basis of the design is to allow communication over RS232 with up to four different microcontrollers with corresponding RS232 to TTL converters. The card will be mated to a Ventana GW5220. However, when I plugged it in for the first time something in the FT4232H mush have shorted, because it got hot enough to burn my finger in a matter of seconds. I have made many attempts to determine the cause of the short but I have had no luck so far. I would greatly appreciate any assistance with debugging this design.

Things I have tried:
- Checked all datasheed pinouts against land patterns and schematic symbols (including the mPCIe card connector) multiple times
- Checked all part numbers for errors multiple times
- Examined board post-short under a microscope for charring or visible solder bridges and found no evidence of either
- Attempted to power the board post-short with a current-limited power supply to check voltages on pins and found that the FT4232H's on-board 1.6V regulator is toast
- Checked the mPCIe standard to ensure that the voltage limits for the +3.3V output are within the limits that the FT4232H can accept, and they are

Here is the schematic (click for larger):


Here is an image of the board design (click for larger):


And an image of the assembled board (click for larger):
 

philba

Joined Aug 17, 2017
960
Did you continuity check (AKA, Buzz Out) all the traces and check for shorts. Best done before populating. Solder mask covers many evils including shorts.

A trick an old school EE taught me in the 70s is a way to fix internal shorts on a fabbed board - get a variac and apply AC across the short. Start low and slowly crank it up until the short burns out. He kept an incandescent lightbulb in series - when the light went off, the short was gone. Probably not a good idea on a populated board... ;)
 

Thread Starter

TelluriumCrystal

Joined Nov 14, 2014
32
I haven't had a chance yet to remove the FT4232H from the board, so I have not yet done a continuity check of all the traces. It's a two-layer board so if there is a shorted trace it should be visible. I did check it visually with a microscope and didn't see anything, but I may have missed a short under a component.

I have, however, heard that that PCB manufacturing errors are relatively rare nowadays so I am skeptical that a PCB fabrication error was the cause. I guess I will find out soon enough.
 
Top