Pressing small ICs down when soldering them?

Hi,

I don´t press down SMD ICs that have pads under the package.
If the pads, solder, temperature is correct, then they center themselves.

Klaus

oesn't seem like the bottom pad would be in contact with the pad when the solder on outer pads keeps it floating

Click to expand...

As stated on device's datasheet, it is not expected to leave an air gap inbetween, but rather filled with an thermal conductive paste.

The question is very specific to TMP117, it's exposed pad should not be soldered to rigid PCB because the mechanical stress causes additional temperature errors. I don't see an explicite recommendation that the exposed pad should be "in contact" with the PCB. I believe that there will be always a certain gap defined by the amount of solder paste applied to the other pads which can't be reduced to zero, even by pressing the IC down. If you want to further reduce the thermal resistance, you can try to apply a small amount of thermal paste.

If you read the data sheet you'll see:

The package thermal pad is not connected to the device ground and should be left unsoldered for best measurement accuracy.

KlausST said:

Hi,

I don´t press down SMD ICs that have pads under the package.
If the pads, solder, temperature is correct, then they center themselves.

Klaus

Click to expand...

That's what I did most of the time as well.

andre_luis said:

As stated on device's datasheet, it is not expected to leave an air gap inbetween, but rather filled with an thermal conductive paste.

Click to expand...

oh, ok then.

FvM said:

The question is very specific to TMP117, it's exposed pad should not be soldered to rigid PCB because the mechanical stress causes additional temperature errors. I don't see an explicite recommendation that the exposed pad should be "in contact" with the PCB. I believe that there will be always a certain gap defined by the amount of solder paste applied to the other pads which can't be reduced to zero, even by pressing the IC down. If you want to further reduce the thermal resistance, you can try to apply a small amount of thermal paste.

Click to expand...

somehow I got the impression that it shouldn't be soldered, but it should be in contact. My bad.
I just need accurate temperature readings, not quick response to temperature change.

barry said:

If you read the data sheet you'll see:

The package thermal pad is not connected to the device ground and should be left unsoldered for best measurement accuracy.

Click to expand...

yes, I saw that, but I thought it should be somehow in contact with the pad under on the PCB.

The datasheet discusses two scenarios:
- air temperature measurement
- contact temperature measurent through the PCB
For the latter, a good thermal coupling to PCB bottomside is wanted, including e.g. thermal vias below the sensor. In this case, a direct contact or usage of coupling media between exposed pad and center land would be preferred. The datasheet doesn't say how, thermal paste is explicitely mentioned for the interface between PCB bottom side and the measured object, not for the sensor pad. Nevertheless it seems a possible solution, but you need to assure that the soldered terminals are not polluted.