AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 15

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

5.0 PCB ASSEMBLY
MOTOROLA
4.2.2.3 PCB Land and Aperture Opening for 9x9 QFN
Figure 17 (L) Land Pattern on PCB, (R) Stencil Aperture Opening on 9x9mm QFN
5.1 Assembly Process Flow
A typical Surface Mount Technology (SMT) process flow is depicted in
5.2 QFN Assembly Guideline
5.2.1 Screen Printing: Solder Paste Material
Solder paste is one of the important materials in SMT assembly process.It is a homog-
enous mixture of metal alloy, flux and viscosity modifiers.The metal alloy particles are
made in specific size and shape. Flux has a direct effect on soldering and cleaning
and it is used to precondition the surfaces for soldering by removing minor surface
contamination and oxidation. There are two different flux systems commonly avail-
able. The first type requires cleaning such as standard rosin chemistries and water
soluble chemistries. Standard rosin chemistries are normally cleaned with solvents,
semi-aqueous solutions or aqueous / saponifier solutions while the water soluble
chemistries are cleaned with pure water. The second flux system type requires no
cleaning but normally some residue will remain on the PCB after soldering.
The QFN package has essentially no standoff height. Therefore, post reflow cleaning
will be difficult. It is recommended to use a no-clean solder paste.
Freescale Semiconductor, Inc.
Solder Screen
0.28mm
Printing
For More Information On This Product,
Quad Flat Pack No-Lead (QFN)
0.69mm
Go to: www.freescale.com
Component
Placement
Figure 18 SMT Process Flow
PCB Land
Aperture
opening
PCB
land
Soldering
Reflow
0.28mm
0.69mm
Figure
Inspection
AN1902/D
Stencil
18.
15