06-07-2019 09:24 AM
We are using LTC2997 to monitor the Artix-7 FPGA Die temperature using the on-chip Temperature-Sensing Diode pins (DXP_0, DXN_0). In the board, the Temperature measurement from the LTC2997 analog voltage conversion is pretty much accurate upto +/-2'C compared with the actual FPGA die temperature.
So far we have seen any issue with the temperature conversion in many boards that we have built. But recently with one board, when the ambient temperature transitioned from -40'C to +70'C, the Temperature Monitoring IC (LTC2997) reads the FPGA Die temperature >+200'C where the actual Die Temerature was below +60'C.
We have reflowed the FPGA, replaced the Temperature Monitoring IC (LTC2997), and it still reads the FPGA Die temperature >+200'C during the board temperature transition from -40'C to +70'C. We have inspected the FPGA BGA Pin-N9, N10 using X-ray and found no issues with the soldering joints.
The Temperature Monitoring IC (LTC2997) converts the temperature based on the Thermal Diode Parameters such as Ideality Factor, Forward Current Gain, and Series Resistance.
Need help to understand what is going on with this particular FPGA On-Chip Thermal Diode when the board ambient temperature is around +35'C during the thermal cycle -40'C to +75'C. How the thermal diode parameters are affected which shows the Die temperature as +220'C where the actual FPGA die temperature is +50'C.
06-07-2019 10:45 AM
Looks like a transient (not a real temperature). Does the sysmon also record a max value of 200C? If not, looks like you have a faulty LT device.
It is possible there is a cracked solder joint acting like a switch (opens) at a given temperature. I have seen this.
06-19-2019 11:09 AM
Thanks for your reply.
I have replaced the LT device with new device and also reflowed the FPGA. It is doing the same.
06-19-2019 11:38 AM
Do you have more than one board?
Is this unique to one pcb assembly? I have seen scracked solder joints interrupt, invisible on x-ray inspection.
06-19-2019 11:40 AM
Yes, this is happening in only one in PCB. Is there any other way to find the cracked solder joints..?
06-19-2019 11:52 AM
one method of checking for cracked joints use a G clamp to push the chip on,
leave the clamp on and cycle .
a cracked pad will normaly hold up with a large force holding it on,
strange it happens at one temprature same each time.
id be tempted to put board in bin to use as spares ..
06-19-2019 01:34 PM