Silicon Labs have an application note about thermal, that is AN765, you can download from  https://www.silabs.com/documents/public/application-notes/an765.pdf

JESD have specified the thermal so there is official documents in JESD official website, you can download directly

JESD15-3  Two-Resistor Compact Thermal Model Guideline - Junction-to-Case

JESD51-8   Integrated Circuit Thermal Test Method Environmental Conditions — Junction-to-Board

JESD51-12 Guidelines for Reporting and Using Electronic Package Thermal Information

https://www.jedec.org/  (need to registration if you don't have an account, free download)

Junction-to-board thermal resistance (θJB ) 
This parameter is measured in a ring cold plate fixture (see JESD51-8). This test fixture is designed to
ensure that all the heat generated in the package is conducted to the cold plate via the board.
The metric is defined as:
JB ( J B ) H θ = T − T / P
where θJB = thermal resistance from junction-to-board (ºC/W)
TJ = junction temperature when the device has achieved steady-state after application
of PH (ºC)
TB = board temperature, measured at the mid point of the longest side of the package no
more than 1mm from the edge of the package body (ºC)
PH = heating power which produced the change in junction temperature (W)
It is important to note that the θJB metric includes a contribution from the thermal resistance of the test
board. Therefore, the thermal conductivity of the board affects the measurement results. The JESD51-
8 standard requires that the metric be measured on a 2s2p board defined in JESD51-7, 9, 10, or 11.
Measurement of the board temperature very close to the edge of the package body is also intended to
minimize the contribution from the board.

Junction-to-case thermal resistance (θJCtop )

The metric is measured in a top cold plate fixture and is defined as:
( ) JCtop J Ctop H θ = T −T / P
where θJCtop = thermal resistance from junction-to-case (ºC/W)
TJ = junction temperature when the device has achieved steady-state after application
of PH (ºC)
TCtop = case temperature, measured at center of the package top surface (ºC)
PH = heating power in the junction that causes the difference between the junction
temperature TJ and the case temperature TCtop; this is equal to the power passing
through the cold plate (W)

Note: for more JEDEC standards, you can also search in this website.