AX8052F151 datasheet(49/51 Pages) ONSEMI

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QFN40 Package Information

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AX8052F151

8.3. QFN40 Recommended Pad Layout

PCB land and solder masking recommendations are shown in Figure 11.

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 + 0.1mm

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

Figure 11 PCB land and solder mask recommendations

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.

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.

8.4. Assembly Process

Stencil Design & Solder Paste Application

Stainless steel stencils are recommended for solder paste application.

A stencil thickness of 0.125 – 0.150 mm (5 – 6 mils) is recommended for screening.

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 12.

The aperture opening for the signal pads should be between 50-80% of the QFN pad area as

shown in Figure 13.

Optionally, for better solder paste release, the aperture walls should be trapezoidal and the

corners rounded.

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.

No-clean flux is recommended since flux from underneath the thermal pad will be difficult to

clean if water-soluble flux is used.

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