AX−SFUS, AX−SFUS−API

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19

QFN40 Recommended Pad Layout

1. PCB land and solder masking recommendations

are shown in Figure 7.

Figure 7. PCB Land and Solder Mask Recommendations

A = Clearance from PCB thermal pad to solder mask opening, 0.0635 mm minimum

B = Clearance from edge of PCB thermal pad to PCB land, 0.2 mm minimum

C = Clearance from PCB land edge to solder mask opening to be as tight as possible

to ensure that some solder mask remains between PCB pads.

D = PCB land length = QFN solder pad length + 0.1 mm

E = PCB land width = QFN solder pad width + 0.1 mm

2. Thermal vias should be used on the PCB thermal

pad (middle ground pad) to improve thermal

conductivity from the device to a copper ground

plane area on the reverse side of the printed circuit

board. The number of vias depends on the package

thermal requirements, as determined by thermal

simulation or actual testing.

3. Increasing the number of vias through the printed

circuit board will improve the thermal

conductivity to the reverse side ground plane and

external heat sink. In general, adding more metal

through the PC board under the IC will improve

operational heat transfer, but will require careful

attention to uniform heating of the board during

assembly.

Assembly Process

Stencil Design & Solder Paste Application

1. Stainless steel stencils are recommended for solder

paste application.

2. A stencil thickness of 0.125 – 0.150 mm

(5 – 6 mils) is recommended for screening.

3. For the PCB thermal pad, solder paste should be

printed on the PCB by designing a stencil with an

array of smaller openings that sum to 50% of the

QFN exposed pad area. Solder paste should be

applied through an array of squares (or circles) as

shown in Figure 8.

4. The aperture opening for the signal pads should be

between 50−80% of the QFN pad area as shown in

Figure 9.

5. Optionally, for better solder paste release, the

aperture walls should be trapezoidal and the

corners rounded.

6. The fine pitch of the IC leads requires accurate

alignment of the stencil and the printed circuit

board. The stencil and printed circuit assembly

should be aligned to within + 1 mil prior to

application of the solder paste.

7. No−clean flux is recommended since flux from

underneath the thermal pad will be difficult to

clean if water−soluble flux is used.

Figure 8. Solder Paste Application on Exposed Pad