AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 12

• Manufacturer Part Number: AN1902
• Description: Quad Flat Pack No-Lead (QFN)
• Manufacturer: Freescale Semiconductor / Motorola

AN1902/D
12
Several different array patterns are being recommended in this section. Smaller QFN
package sizes, such as the 3x3mm and 4x4mm, doesn’t require any thermal pad pat-
tern on PCB and stencil unless to minimize solder voids. On the larger packages,
stencil thermal openings should be segmented in smaller regions. Studies performed
for “Robust Design Guideline” suggest array designs shown in
to this study, the spacing between segments either on the stencil or on the PCB
should be 0.15mm (6mils) or more. Narrower spacing between segments can become
a manufacturing issue.
Continued efforts by Motorola to further understand and improve board attach of
QFNs has found that increasing the distance between stencil openings for the
Input/Output (I/O) pads and the exposed pad leads to fewer incidences of bridging. By
increasing the separation to greater than 30 mils, yield has improved. The resulting
stencil opening for the exposed pad is no longer close to 1:1 with the package nor
board Cu areas. However, the volume of solder paste is still sufficient to completely
wet both Cu surfaces.
From the “Motorola MAP QFN” study, the stencil opening should be approximately
50% - 80% of the total PCB thermal pad area. This stencil-PCB thermal pad ratio
would help ensure good coverage of the thermal pad area with fewer voids and less
chance of overflow bridging to the adjacent lead or terminal solder pads. The recom-
mended array design of the stencil for 4x4 and 9x9mm QFN are shown in
The opening dimension recommended for 4x4 QFN is 0.85mm x 0.85mm with
Freescale Semiconductor, Inc.
Figure 14 Segmented Stencil Openings used in Robust Design Guideline
For More Information On This Product,
Figure 13 "Scooping" Out Solder Paste During Screen Printing
Quad Flat Pack No-Lead (QFN)
Go to: www.freescale.com
Figure
14. According
MOTOROLA
Figure
15.