The THS403x is available in a thermally-enhanced DGN package, which is a member of the PowerPAD™ family of packages. This package is constructed using a downset lead frame upon which the die is mounted [see Figure 7-9(a) and Figure 7-9(b)]. This arrangement results in the lead frame exposed as a thermal pad on the underside of the package [see Figure 7-9(c)]. Because this thermal pad has direct thermal contact with the die, excellent thermal performance can be achieved by providing a good thermal path away from the thermal pad.

The PowerPAD™ package allows for both assembly and thermal management in one manufacturing operation. During the surface-mount solder operation (when the leads are being soldered), the thermal pad can be soldered to a copper area underneath the package. Through the use of thermal paths within this copper area, heat can be conducted away from the package into a ground plane or other heat-dissipating device.

The PowerPAD™ integrated circuit package represents a breakthrough in combining the small area and ease of assembly of surface mount with the more-recent, awkward mechanical methods of heat sinking.

A. The thermal pad is electrically isolated from all pins in the package.

Figure 7-9 Views of Thermally-Enhanced DGN Package

Although there are many ways to properly heat sink this device, the following steps show the recommended approach.

Figure 7-10 PowerPAD™ PCB Etch and Via Pattern

1. Prepare the PCB with a top-side etch pattern as shown in Figure 7-10. There must be etch for the leads as well as etch for the thermal pad.
2. Place five holes in the area of the thermal pad. These holes must be 13 mils (0.3302 mm) in diameter. The reason to keep the holes small is to discourage solder wicking through the holes during reflow.
3. Additional vias can be placed anywhere along the thermal plane outside of the thermal pad area. This action helps dissipate the heat generated by the THS403x device. The additional vias can be of any diameter because wicking is not a concern outside of the thermal pad area.
4. Connect all holes to the internal ground plane.
5. When connecting these holes to the ground plane, do not use the typical web or spoke via connection methodology. Web connections have a high thermal-resistance connection that is useful for slowing the heat transfer during soldering operations. This makes the soldering of vias that have plane connections easier. In this application, however, low thermal resistance is desired for the most efficient heat transfer. Therefore, the holes under the THS403x package must connect to the internal ground plane with a complete connection around the entire circumference of the plated-through hole.
6. The top-side solder mask must leave the pins of the package and the thermal pad area with the five holes exposed. The bottom-side solder mask must cover the five holes of the thermal pad area, which prevents solder from pulling away from the thermal pad area during the reflow process.
7. Apply solder paste to the exposed thermal pad area and to all the device pins.
8. With these preparatory steps in place, the THS403x device is placed in position and run through the solder reflow operation as any standard surface-mount component. This results in a part that is properly installed.