About i.MX thermal resistance

[tokonmei2007]

Dear Sirs,

I'm estimating the junction temperature of the iMX6ULL(MCIMX6Y2CVM08AB).

Natural convection occurs without using a heat sink.

The package top temperature is 81 °C , which was measured by a thermocouple wire.

Also, I calculated the power consumption is 1.8W.

Could you tell me is it okay to calculate the junction temperature using ψJT instead of RθJC?

Tjunction =81°C+2.3°C/W*1.8W

               =85.14°C

I am worried that if RθJC is used, the junction temperature will exceed the maximum allowable temperature(105°C) as follows.

Tjunction =81°C+19.3°C/W*1.8W

               =115.74°C

Thank you.

Best Regards,

[tokonmei2007]

@Yuri 
hello,

Thank you for your reply.

I have some doubts about the following documents.

Do I have to use RθJC even if I don't use a heatsink for this element?

>>The RθJC parameter is generally suitable for the condition that the
>>only heatsinking is attached at the top of the package

Thank you.

Best Regards,

[Yuri · Accepted solution]

@tokonmei2007 
Hello,

   It is possible to use RθJC, but in any case thermal parameters in the Datasheet
are intended for rough estimations. It may be recommended to apply to thermal
modeling, using - for example - FloTHERM.  

Regards,
Yuri.

[Yuri]

@tokonmei2007 
Hello,

  RθJC is defined as the thermal resistance between the junction to the case
surface. The RθJC parameter is generally suitable for the condition that the
only heatsinking is attached at the top of the package, where main quantity of the
heat is distributed from the top. I think it is Your situation.

 ΨJT concerns difference between the junction and reference point, not the surface,
as for RθJC. If the chip is attached to heat-sinking, then RθJC or ΨJS (junction to the
heat sink) must be used to estimate the junction temperature.

General i.MX6 thermal considerations are provided in the following
document.

  https://www.nxp.com/docs/en/application-note/AN4579.pdf

Regards,
Yuri.