- Prepare the PCB with a
top-side etch pattern, as shown in the attached Thermal Land Pattern
mechanical drawing. Use etch for the leads as well as etch for the thermal
land.
- Place the recommended number
of holes (or thermal vias) in the area of the thermal pad as shown on the
attached Land Pattern mechanical. Use holes that are 13 mils in
diameter. Keep the holes small so that solder wicking through the holes is
not a problem during reflow.
- Best practice is to place a
small number of the holes under the package and outside the thermal pad
area. These holes provide additional heat path between the copper land and
ground plane and are 25 mils in diameter. The holes can be larger because
the holes are not in the area to be soldered, so wicking is not a
problem.
- Connect all holes, including
those within the thermal pad area and outside the pad area, to the internal
ground plane or other internal copper plane.
- When connecting these holes
to the ground plane, do not use the typical web or spoke via connection
methodology; see Figure 7-5. Web connections have a high thermal resistance that is useful for
slowing the heat transfer during soldering operations. This heat-transfer
slowing makes the soldering of vias that have plane connections easier.
However, in this application, low thermal resistance is desired for the most
efficient heat transfer. Therefore, connect the holes under the PowerPAD
package to the internal ground plane with a complete connection around the
entire circumference of the plated through hole.
- On the top-side solder mask,
leave exposed the terminals of the package and the thermal pad area. On the
thermal pad area, leave the 13 mil holes exposed. Cover the larger 25 mil
holes outside the thermal pad area with solder mask.
- Apply solder paste to the
exposed thermal pad area and all of the package pins.
- With these preparatory steps
in place, the PowerPAD IC package is simply placed in position and run
through the solder reflow operation, as with any standard surface-mount
component. This procedure results in a part that is properly installed.
For detailed information on the PowerPAD IC package, including thermal modeling
considerations and repair procedures, see the
PowerPAD Thermally Enhanced
Package technical brief, available at www.ti.com.