AN1256

MTTF (mean time to failure) enhanced package

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MTTF (mean time to failure) enhanced package

A major problem inherent to high power RF transistors is their high operating junction

temperatures. If careful consideration is not taken to remove the heat generated by the

junction, degradation in device performance and reduction in its operating lifetime are the

consequences. The design of the SD2933 has solved this problem by using a thermally

enhanced package, labeled a non-pedestal (NP) package, having lower thermal resistance

than the widely used pedestal (P) packages. A finite-element analysis was performed on a

similar NP package and the results showed a 10 °C lower peak temperature than the P

package under the same thermal conditions. The SD2931-10 and a similar ST device were

then chosen to further demonstrate the enhanced thermal properties of the NP package

over the P package under normal operating conditions. These devices use the same

transistor die and also share the same overall mechanical dimensioning. The major

difference between the devices is that the SD2931-10 is a NP package and the other was a

P package. Both devices were operated under RF conditions and their corresponding die

junction temperatures were measured using an infrared (IR) imaging unit. Once again the

results were in favor of the NP package with a 25% improvement in thermal resistance

which corresponds to an operating life improvement of approximately 400%.

Figure 6.

SD2933 MTTF for various currents as a function of junction temperature

Electromigration is another temperature-enhanced failure mechanism. Electromigration

failures are related to current density in the metallization as well as other metallization

material properties. A graph of the SD2933 mean-time-to-failure, for various currents as a

function of junction temperature, based on the electromigration activation energy derived

from Arrhenius plots for the SD2933, is shown in Figure 6. It can be seen that the typical

lifetime is greater than 1 million hours.

It should be noted that in comparison, the SD2933 is a larger device than the SD2931-10,

but the overall structure of the SD2933 package and the SD2931-10 package are the same

and both devices use the same transistor die. For a complete thermal analysis of the P vs.