A brief analysis of the packaging heat transfer standard of power adapters

　　The early industry standards for heat transfer of power adapters are mainly SEMI standards, which define the test standards for IC packages in natural convection, wind tunnel and infinite flat test environments. Today, follow the editor to understand the packaging heat transfer standard of the power adapter!

　　Since 1990, the JEDECJC51 committee has invited manufacturers and experts to formulate new industrial standards for heat transfer, and put forward a number of standards for thermal management. Compared with the SEMI standard, the new heat transfer industry standard has the same basic measurement methods and principles, but the content is more complete, and some definitions are also more clearly explained.

　　The SEMI standard defines two thermal resistance values, namely Rja and Rjc. Among them, Rja is a measure of heat transfer from the chip interface to the atmosphere in natural convection or wind tunnel environments. Since the measurements are made under standard specification conditions, and therefore have different results for different substrate designs and environmental conditions, this value can be used to qualitatively compare the ease with which the package dissipates heat. Rjc refers to the thermal resistance of heat from the chip interface to the IC package. It needs to be in contact with an isothermal surface during measurement. This value is mainly used to evaluate the performance of the heat sink.

　　With the change of package form, several measurement parameters such as Rjb, ψjt, ψjb have been added in the new JEDEC standard. Among them, Rjb is the thermal resistance from the chip interface to the test board in the environment where almost all the heat is transferred from the chip interface to the test board. This value can be used to evaluate the heat transfer efficiency of the PCB. ψjx is a heat transfer characteristic parameter, which is defined as follows.

　　(1) The definitions of ψ and R are similar, but the difference is that ψ refers to the case where most of the heat is transferred, while R refers to the total heat transfer. When the actual electronic system dissipates heat, the heat will be transferred from the top and bottom of the package or even around the package, not necessarily in a single direction, so the definition of ψ is more in line with the actual system measurement.

　　(2) ψjt refers to the thermal resistance generated when part of the heat is transferred from the chip interface to the casing above the package. This value can be used to predict the chip interface temperature from the outer surface temperature of the IC package in actual system products.

　　(3) ψjb is similar to Rjb, which refers to the thermal resistance generated when part of the heat is transferred from the chip interface to the lower test board under natural convection and wind tunnel environments, which can be used to predict the junction temperature from the board temperature.

　　Although various thermal resistance measurements in the standard can be applied to temperature prediction of actual system products, there are still great limitations in practical application. When using the standard Rja, Rjb, ψjt, ψjb and other measurement parameters to predict the temperature of the system product, it is necessary to pay attention to the test board size, copper foil layer and copper content used in the standard test conditions, and also to pay attention to the natural environment used in the measurement. Differences between convection and wind tunnel environments and actual systems.

　　Generally speaking, the thermal resistance or heat transfer parameters measured by experiments are mainly used to qualitatively compare the heat dissipation efficiency of IC packaging, that is, no matter which packaging factory is packaged, as long as it conforms to the standard method, the heat dissipation can be compared. Knowing the thermal design or heat transfer conditions of an IC package can be of great help. In addition, experimental measurements can also be used for verification and simplification of numerical simulations.

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