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Texas Instruments QFN Layout lines Application notes
Application Report
SLOA122 – July 2006
QFN Layout Guidelines
Yang Boon Quek ........................................................................................ HPL Audio Power Amplifiers
1
Introduction
Board layout and stencil information for most Texas Instruments (TI) Quad Flat No-Lead (QFN) devices is
provided in their data sheets. This document helps printed-circuit board (PCB) designers understand and
better use this information for optimal designs.
The QFN package is a thermally enhanced standard size IC package designed to eliminate the use of
bulky heat sinks and slugs. This package can be easily mounted using standard PCB assembly
techniques and can be removed and replaced using standard repair procedures.
The QFN package is designed so that the lead frame die pad (or thermal pad) is exposed on the bottom
of the IC (see Figure 1). This provides an extremely low thermal resistance (θJC) path between the die and
the exterior of the package.
Plastic Mold Compound
IC (Silicon)
Die Attach (Epoxy)
Exposed Thermal Pad Located
Underneath the Package
Leadframe (Copper Alloy)
Figure 1. Section View of a QFN Package
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Board Layout
2
Board Layout
Figure 2 shows an example of the recommended board layout for an RGZ package.
Solder Mask Defined Thermal Pad, see Figure 4
Finger Pads, see Figure 3
Figure 2. Board Layout for an RGZ Package Finger Pads
TI recommends the use of rounded finger pads to prevent solder bridging. Surround each pad with a
0,07-mm wide solder mask. The recommended dimensions are shown in Figure 3.
Figure 3. Finger Pads Layout
2.1
Solder Mask Defined Thermal Pad
The solder mask defined thermal pad is the exposed copper area not covered by solder mask. It must be
soldered directly to the thermal pad on the bottom of the IC. Figure 2 shows an example of the
recommended dimensions.
2.1.1
Copper Areas
Copper areas on and in a PCB act as heat sinks for the QFN device. Top copper areas should be covered
with solder mask leaving only the solder mask defined thermal pad exposed. The top copper areas should
be made as large as possible.
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Board Layout
Inner or bottom layer copper planes also can be connected to thermal pad using vias and should be made
as large as possible. The thermal pad is usually tied to ground, and designers should verify the electrical
correctness when connecting the copper planes to the thermal pad.
Designers may leave the bottom copper plane exposed. However, studies have shown that this has
minimal impact on thermal performance.
2.1.2
Thermal Vias
TI recommends placing thermal vias in the solder mask defined thermal pad to transfer effectively the heat
from the top copper layer of the PCB to the inner or bottom copper layers.
TI provides the recommended layout of the thermal vias in most data sheets. The recommended via
diameter is 0,3 mm or less, and the recommended via spacing is 1 mm (see Figure 4).
Thermal Via
Figure 4. Solder Mask Defined Thermal Pad
The thermal vias should make their connection to the internal ground plane with a complete connection
around the entire circumference of the plated through hole. Place a ring of exposed copper (0,05 mm
wide) around the vias at the bottom of the copper plane.
Do not cover the vias with solder mask which causes excessive voiding.
Do not use a thermal relief web or spoke connection which impedes the conduction path into the inner
copper layer(s) (see Figure 5).
Exposed Copper –
0,05 mm Around Via
Solid Via
Recommended
Web or Spoke Via
NOT Recommended
Figure 5. Via Connection at the Bottom of the Copper Plane
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Board Layout
Thermal Via – follow spacing given and keep diameters less
than or equal to 0.3 mm
External Via
Voiding – keep
less than 50%
Solder Mask
Defined
Thermal Pad –
follow
dimensions
given
Copper Areas – make as large as possible
Rounded
Finger Pads
Figure 6. X-Ray – DRB Device
Vias may be plugged to prevent solder loss and protrusions. This often produces the best thermal
performances but is not necessary or recommended because of the increased cost of PCBs and because
solder tends to wet the upper surface first before filling the vias.
Vias also can be used in the copper area outside the solder mask defined thermal pad to help dissipate
heat through bottom or inner planes.
If thin PCBs or vias larger than 0,3 mm are used, designers may use only external vias to prevent solder
loss and protrusions (see Figure 7). Designers should note that this might reduce thermal performance
significantly and should be evaluated on their PCBs.
Top copper
area covered
with solder
mask
Only external
vias are used
Solder Mask Defined
Thermal Pad - exposed
copper area
Figure 7. Example of Using Only External Vias in a DRB Package
2.1.3
Solder Loss and Protrusions
Solder loss and protrusions result when excessive solder flowed through internal vias during reflow. These
usually happen when incorrect internal vias sizes and stencil openings are used.
Solder loss results in voiding and severely affects thermal conductivity. Designers are encouraged to x-ray
their reflowed boards to verify that at least 50% of thermal pad area is soldered (less than 50% voiding)
when using 0,127-mm thick stencils.
Protrusions might cause misalignment in stencil on the reverse side of the PCB (see Figure 8).
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Board Layout
Protrusion
Figure 8. Protrusion at the Other Side of Board
2.1.4
Stencil
Figure 9 shows an example of the recommended stencil openings and thickness. Follow stencil openings
and thickness recommended to ensure that the right amount of solder paste is used.
Figure 9. Recommended Stencil Openings and Thickness for RGZ Package
Use cross-hatching in the thermal pad stencil opening of a QFN device. This prevents excessive amount
of solder paste applied thus prevents solder bridging (see Figure 10).
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Board Layout
No solder paste applied
Solder paste applied
Cross-Hatching – Recommended
No Cross-Hatching – Not Recommended
Figure 10. Thermal Pad Stencil Opening
2.1.5
Recommended Solder Paste
TI recommends the use of type 3 or finer solder paste when mounting a QFN.
2.2
Additional Information
For detailed information on the QFN package including thermal modeling considerations and repair
procedures, see (SLUA271) QFN/SON PCB attachment.
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