08-10-2015 11:35 PM
I'm working for an EMS and a customer of ours uses a Virtex-5 device (XC5VLX110) on one of his boards.
In order to detect soldering shorts we measure among others the impedance (resistance) between the 1V0 VCC of the FPGA and GND. The resistance value usually is in a range of 10 - 15 ohms.
However the last batch had some boards which showed values below 10 ohm (2 - 7 ohms). Those boards are working properly in FAT so I assume there's nothing wrong with the Virtex-5 devices.
Anyway I'd like to ask if those values below 10 ohms are fine also?
Everything below 5 ohms seems unusually low to me but I have no experience if this is relevant for the proper function of the device.
We also noticed that some of the boards with values below 10 ohms changed their resistance to over 10 ohms after FAT. Probably because of the Power on sequence. Is this a normal behavior?
I'd appreciate feedback on those questions / topics. Thank you in advance!
08-10-2015 11:40 PM
@andreas318 This is normal. As long as resistance between VCC and GND is more than 1 Ohm it is good and can be used.
08-11-2015 12:34 AM
08-11-2015 07:42 AM
Resistance measurement depends on your equipment compliance voltage (the voltage used in the current source), the current used to make the measurement, the temperature, and the process corner (variability) of the Xilinx device.
The compliance voltage used may destroy the Xilinx device: the absolute maximum ratings of the data sheet must be observed to prevent damage. Use of a low ohm range ohmmeter is not recommended unless it is characterized to be safe to use (the voltages and currents are below the absolute maximum ratings).
08-12-2015 12:25 AM
Thanks for your reply!
The part that I don't fully understand is how is it possible that the resistance value changes before and after Power-on?
We did the measurement on the same board with the same XILINX device mounted using the same measuring equipment.
Resistance before Power-on between VCC and GND: 6 ohms
Resistance after Power-on between VCC and GND: 11.5 ohms
Regarding your CAUTION note:
For all in-circuit resistance measurements we use a Takaya Flying Probe. Not more than 0.2 Volts will be applied during those measurements. So I assume there is no danger of destroying the XILINX devices.
08-12-2015 06:47 AM
Yes, that is just fine (0.2v max).
The temperature is a exponential factor for static current. The temperature also affects the saturation voltage at the gate (first order linear?). So getting a different reading after it has been powered ON, even for a few moments is just telling you the temperature of the die is different as the various factors affecting the resistance have changed.
At 0.2 volts you are in the sub-threshold region, and no one really characterizes anything there, as the variation is so great, that nothing may be modeled, nor predicted.
It might be fun to take a resistance reading with a part in a temperature chamber, but there is no guarantee the next part's process corner will be similar, and you will get a different curve.
08-13-2015 12:27 AM