Power impedance between

Hello Abhishek,

    I'm not able to duplicate your measurements with a unpopulated LTM2881-3.  My measurements between B6-7 and B3-5 are closer to a meg ohm.  This could indicate your device is damaged or it could be there are some subtle differences in how we are measuring the part.

Here are several points which may be useful to consider:

1.   The LTM2881-3 is intended to be supplied from a nominal 3.3V Vcc; however, a 5V Vcc is within the operating range all of the internal parts and while not recommended won't necessarily damage the part.  The transformer turns ratio for the isolated supply will create a higher input voltage to the LDO which will result in increased power dissipation (and a lower max output current in order to remain within the LDO's thermal limits).  The LDO has internal thermal protection which, if triggered, should prevent immediate damage, but the parts lifetime will be reduced.  If you have a 5V supply rail you really should be using the LTM2881-5 part.

2.  The LTM2881 can and will be damaged by over voltage transients which exceed its ABSMAX ratings on the Vcc pin.  The LTM2881 includes a low ESR capacitor internally for the Vcc supply and so a step application of voltage through an inductance (such as test leads/wires or power supply traces) can result in a resonant 2x voltage transient which will damage the part.  This can be avoided by dampening this resonant circuit such as by including a higher ESR electrolytic capacitor near the LTM2881.

3. Its not clear how you are measuring the device, but given that you've correctly identified the different internal connections for the GND node (B3-5 are the actual GND pins and B1 and B2 are pins which must be connected to B3-5), I'm guessing you're measuring the part after its been removed from the board since these connections are apparent from the bottom view of the part.   

Please note that the mounting/removal of these parts must be performed exactly following the guidelines in this document: Assembly Considerations for Analog Devices uModule LGA & BGA Packages.  In particular the guidelines regarding part/board drying and temperatures must be followed precisely to prevent damage to the module.  Failure to abide by these guidelines can result in the trapped/absorbed moisture forming steam which can internally crack/delaminate the module structure which in turn allows the liquified solder internal to the module to wick between the internal PCB and mold epoxy creating internal short circuits.  Thus, if you did remove the module to make the measurements, if the guidelines were not followed during the removal, the module may have been damaged.  It is also possible for an initial delamination to occur during the removal, but the solder shorts had not yet formed and so the part would still appear to operate correctly following its removal.  If this part were then to be re-installed, it is highly likely that the solder shorts could now form during the re-installation solder reflow due to the prior delamination damage to the module.  Therefore, even if the part does test good after removal, we would not suggest reusing it.

Eric

Hello Abhishek,

    I'm not able to duplicate your measurements with a unpopulated LTM2881-3.  My measurements between B6-7 and B3-5 are closer to a meg ohm.  This could indicate your device is damaged or it could be there are some subtle differences in how we are measuring the part.

Here are several points which may be useful to consider:

1.   The LTM2881-3 is intended to be supplied from a nominal 3.3V Vcc; however, a 5V Vcc is within the operating range all of the internal parts and while not recommended won't necessarily damage the part.  The transformer turns ratio for the isolated supply will create a higher input voltage to the LDO which will result in increased power dissipation (and a lower max output current in order to remain within the LDO's thermal limits).  The LDO has internal thermal protection which, if triggered, should prevent immediate damage, but the parts lifetime will be reduced.  If you have a 5V supply rail you really should be using the LTM2881-5 part.

2.  The LTM2881 can and will be damaged by over voltage transients which exceed its ABSMAX ratings on the Vcc pin.  The LTM2881 includes a low ESR capacitor internally for the Vcc supply and so a step application of voltage through an inductance (such as test leads/wires or power supply traces) can result in a resonant 2x voltage transient which will damage the part.  This can be avoided by dampening this resonant circuit such as by including a higher ESR electrolytic capacitor near the LTM2881.

3. Its not clear how you are measuring the device, but given that you've correctly identified the different internal connections for the GND node (B3-5 are the actual GND pins and B1 and B2 are pins which must be connected to B3-5), I'm guessing you're measuring the part after its been removed from the board since these connections are apparent from the bottom view of the part.   

Please note that the mounting/removal of these parts must be performed exactly following the guidelines in this document: Assembly Considerations for Analog Devices uModule LGA & BGA Packages.  In particular the guidelines regarding part/board drying and temperatures must be followed precisely to prevent damage to the module.  Failure to abide by these guidelines can result in the trapped/absorbed moisture forming steam which can internally crack/delaminate the module structure which in turn allows the liquified solder internal to the module to wick between the internal PCB and mold epoxy creating internal short circuits.  Thus, if you did remove the module to make the measurements, if the guidelines were not followed during the removal, the module may have been damaged.  It is also possible for an initial delamination to occur during the removal, but the solder shorts had not yet formed and so the part would still appear to operate correctly following its removal.  If this part were then to be re-installed, it is highly likely that the solder shorts could now form during the re-installation solder reflow due to the prior delamination damage to the module.  Therefore, even if the part does test good after removal, we would not suggest reusing it.

Eric