OPA445BM Datasheet, PDF (12/25 Pages) Texas Instruments – High Voltage FET-Input OPERATIONAL AMPLIFIER

OPA445
SBOS156B − MARCH 1987 − REVISED APRIL 2008
PowerPAD THERMALLY-ENHANCED
PACKAGE
In addition to the SO-8, DIP-8, and TO-99 packages, the
OPA445 also comes in an SO-8 PowerPAD. The SO-8
PowerPAD is a standard-size SO-8 package where the
exposed leadframe on the bottom of the package can be
soldered directly to the PCB to create an extremely low
thermal resistance. This architecture enhances the
OPA445’s power dissipation capability significantly and
eliminates the use of bulky heatsinks and slugs
traditionally used in thermal packages. This package can
be easily mounted using standard PCB assembly
techniques. NOTE: Since the SO-8 PowerPAD is
pin-compatible with standard SO-8 packages, the
OPA445 can directly replace operational amplifiers in
existing sockets. Soldering the PowerPAD to the PCB is
always required, even with applications that have low
power dissipation. Soldering the device to the PCB
provides the necessary thermal and mechanical
connection between the leadframe die pad and the PCB.
The PowerPAD package is designed so that the leadframe
die pad (or thermal pad) is exposed on the bottom of the
IC; see Figure 13. This design provides an extremely low
thermal resistance (q_JC) path between the die and the
exterior of the package. The thermal pad on the bottom of
the IC can then be soldered directly to the PCB, using the
PCB as a heatsink. In addition, plated-through holes (vias)
provide a low thermal resistance heat flow path to the back
side of the PCB.
Leadframe (Copper Alloy)
IC (Silicon)
Die Attach (Epoxy)
Mold Compound (Plastic)
Leadframe Die Pad
Exposed at Base of the Package
(Copper Alloy)
Figure 13. Section View of a PowerPAD Package
GENERAL PowerPAD LAYOUT GUIDELINES
The OPA445 is available in a thermally-enhanced
PowerPAD package. This package is constructed using a
downset leadframe upon which the die is mounted. This
arrangement results in the lead frame being exposed as a
thermal pad on the underside of the package. This thermal
pad has direct thermal contact with the die; thus, excellent
thermal performance is achieved by providing a good
thermal path away from the thermal pad.
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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 must 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 either
a ground plane or other heat-dissipating device. Soldering
the PowerPAD to the PCB is always required, even with
applications that have low power dissipation. Follow these
steps:
1. The PowerPAD must be connected to the most
negative supply voltage on the device, V−.
2. Prepare the PCB with a top-side etch pattern. There
should be etching for the leads as well as etch for the
thermal pad.
3. Place recommended holes in the area of the thermal
pad. Recommended thermal land size and thermal via
patterns for the SO-8 DDA package is shown in
Figure 14. These holes should be 13 mils in diameter.
Keep them small, so that solder wicking through the
holes is not a problem during reflow. The minimum
recommended number of holes for the SO-8
PowerPAD package is five.
4. Additional vias may be placed anywhere along the
thermal plane outside of the thermal pad area. These
vias help dissipate the heat generated by the OPA445
IC. These additional vias may be larger than the 13-mil
diameter vias directly under the thermal pad. They can
be larger because they are not in the thermal pad area
to be soldered; thus, wicking is not a problem.
5. Connect all holes to the internal power plane of the
correct voltage potential (V−).
6. When connecting these holes to the 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, makeing 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 OPA445 PowerPAD package
should make the connections to the internal plane with
a complete connection around the entire
circumference of the plated-through hole.
7. The top-side solder mask should leave the terminals
of the package and the thermal pad area exposed.
The bottom-side solder mask should cover the holes
of the thermal pad area. This masking prevents solder
from being pulled away from the thermal pad area
during the reflow process.
8. Apply solder paste to the exposed thermal pad area
and all of the IC terminals.