I'm a R&D engineer, recently I'm doing some evaluation on thermal
performance of exposed die paddle.

The package outline for the exposed die paddle package remain the same, the
only changes is deeper the downset of the die paddle until the bottom of the
die paddle is exposed. People claimed that this will improve the thermal
performance of the package, i.e. the junction to ambient temp should be
lower (theta Ja). However, through the computer simulation (FEA) and also
practical measurement, it show a worst off result, the junction to ambient
temperature is increased (higher theta Ja).

My explanation to this is when the die paddle is exposed, the distance
between the inner leadframe with the die is far compare to the standard
package which is almost parallel. The outer lead of the package would
actually acts as an extended surface to help to dissipate heat from the
package, in addition, the lead is connected to a PCB, and the copper trace
layer of the PCB has a very good thermal conductivity (about 2.5X of
leadframe), so the outer lead not only dissipated heat through convection
but also conduction.

When the die paddle is exposed, it actually degrade the thermal performance
of the leadframe. Even  though, the heat dissipation through the die paddle
is improved, but the surface area of the exposed die paddle compare to the
exposed surface of the leadframe is not significant, especially for the
package with high leadcount. In addition, the heat dissipated from the
exposed die paddle is only through convection compare to leadframe by
convection and conduction. The convection rate of the exposed die paddle
also limit by the convection coefficient of the still air.  So the overall
thermal performance of the exposed die paddle is being degrade, so a higher
Theta Ja is expected.

Please share with me your opinion regarding this issue.

Thanks.














Regards
Cindy Hoe
HC Hoe (Cindy)
R&D Engineer (Package Analysis Section)
E-Mail  : [log in to unmask]
Tel       : 605-5262333 ext 304
Fax      : 605-5265333

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