Misuse Of Thermal Numbers

By Javier DeLaCruz
So many of us in the semiconductors realm are guilty of using JEDEC thermal data incorrectly. I often get questions such as “how much power can this package handle” or “what’s the thermal efficiency of this package.” Unfortunately, in almost all situations these questions cannot be generally answered.

The numbers we throw around for thermal performance come from the JEDEC JESD51 thermal standard. This spec was really created to compare one package to another. Many folks have used this to determine the thermal performance of their package in their system, which is not correct. The following statement is contained in the twelfth chapter in the spec, and is commonly overlooked:

“While standardized thermal test information cannot apply directly to the many specific applications, the standardized results can help compare the relative thermal performance of different packages. A more meaningful comparison is possible if the test conditions are understood along with the factors affecting package thermal performance…

Several factors affect the thermal performance of a device in a user’s application. These include power dissipation in the component; airflow velocity, direction and turbulence level; power in adjacent components; two-sided vs. one-sided active component mounting; printed circuit board (PCB) orientation; and adjacent boards and their power dissipation.”.

Therefore, even the JEDEC committee that compiled this standard did not intend for these theta-ja and theta-jc numbers to be used the way they commonly are. These really are intended to compare one package to another. They do happen to be helpful indicators of approximate performance.

Let’s take a common example of how data may be misused in an application. Assume you have a device that is going into a handheld device such as a cell phone. Most folks will assume that theta-ja (thermal resistance from junction to ambient air) without airflow would make the most sense. This is actually a poor indicator of performance. First of all, the circuit boards used in a handheld device are generally smaller than the JEDEC 100mm x 100mm test boards, and hence have a much lower ability to absorb heat (lower thermal mass.) More importantly, the JEDEC condition of no airflow is actually more accurately described as no forced-airflow.

The JEDEC condition does allow for natural airflow to occur since the board is in a 300mm x 300mm x 300mm cube, so heat can rise from the device (shown in purple in the image,) cool off as it rises and then fall again, causing a natural air circulation in this large enclosure. This would not be the environment inside of a handheld device (unless your target market had really large hands). Therefore, there are several reasons why theta-ja is not a good indicator of thermal performance in this particular system.

This particular example has an interesting thermal solution. I’m sure most people reading this have had long conversations on a mobile phone when they notice their ear gets hot. Rest assured this is not caused by radiation. Instead, mobile phone system designers use one of the few components inside of their system that can spread heat effectively, which is the LCD display housing inside of the phone. Many of the most power hungry devices are located there and are in contact with the display to dissipate heat from the sealed interior of the phone to the exterior. Unfortunately, your ear is generally in contact with this same LCD display, so your ear ends up being the heat sink for these power devices. This is just another good reason to use a hands-free set while on a long discussion.

–Javier DeLaCruz is the semiconductor packaging director at eSilicon