AN3232

Seating plane

Doc ID 17594 Rev 3

13/26

7

Seating plane

The seating plane refers to the plane determined by the bottom surface of the leads after the

package has been mounted. For design purposes, it is the distance between the bottom

surface of the package base and the bottom surface of the transistor leads. This important

dimension is specified on package outline drawings, and for through-the-board mounting of

high power packages, it indicates total depth of the pocket into which the transistor is

seated. To determine the depth of this pocket, the total thickness of the PCB, Cu traces,

solder layers, and the depth of any machined channel is considered. Some points to factor

into this calculation are shown in Figure 4.

Figure 4.

Pocket depth consideration (mm)

Care should be taken to factor in the thickness of the TIM, which is equivalent to increasing

the seating plane dimension. Along similar lines, if the PCB is fused to the heatsink by

means of a bonding film, solder layer, etc., the corresponding thickness of this layer should

be factored into the PCB thickness.

The leads of the transistor should launch perpendicular to the package body onto the traces

of the PCB to reduce mechanical stress to the package and solder joints. In some cases, the

PCB fabrication tolerance is such that the leads extend above the plane defined by the top

of the PCB; this situation is best handled with a judicious selection of solder pad preform or

paste thickness, or by a slight downward bending of the transistor leads [no more than 0.01

in (0.25 mm)].

Figure 5.

Lead bending

As with almost all power packages, in no case should the leads be bent higher than the

topside of the PCB, unless specific lead-forming techniques are applied, such as those

shown in Figure 5. Whenever non-coplanarity exists between the bottom of the device leads

and the top surface of the PCB, assembly mechanical stresses and cyclic life stresses

should be considered. Both lead-forming and a PCB pullback, such as that shown in