Pressing small ICs down when soldering them?

[I14R10]

Hi

Do you usually press down very small IC's with pads under the package, like WSON package when soldering them with hot air? I'm asking specifically about that part when the solder melts and the solder surface tension holds the chip where it should be. Pressing down will likely move the chip, but I've been reading about TMP117 temperature sensor where it's recommended to leave bottom pad unsoldered but in contact with PCB. Doesn't seem like the bottom pad would be in contact with the pad when the solder on outer pads keeps it floating.

 

[KlausST]

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

 

[andre_luis]

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

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

 

[FvM]

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.

 

[barry]

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.

 

[I14R10]

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

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

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

oh, ok then.

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.

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.

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.

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

 

[FvM]

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.