AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 16

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

Freescale Semiconductor, Inc.
AN1902/D
The spread of solder paste during reflow partially depends upon the solder paste alloy.
SnPb solder alloys spread significantly better than the lead-free pastes (ie: SnAgCu,
SnAgBiCu, etc.).
5.2.2 Component Placement
The QFN package is comparatively small in size. The high lead interconnection and
insertion density suggests that precise and accurate placement machines are pre-
ferred. To meet this tight requirement, the placement machine should be equipped
with optical recognition systems, i.e. vision system, for the centering of PCB as well as
the QFN components during the pick and place motion.
Motorola follows EIA-783 [4] standard for tape and reel orientation. See
Figure
19.
Figure 19 QFN Orientation in Tape and Reel
The identification of pin 1 on the bottom side of the QFN package varies from supplier
to supplier. If parts are supplied by more than one QFN supplier, it is necessary to cre-
ate equipment recipes for each QFN supplier. The leadframe finish of each supplier
may reflect differently in the vision system, especially between NiPdAu and SnPb.
All assemblers of QFN components are encouraged to conduct a placement accuracy
study to provide factual local knowledge about compensations needed for this pack-
age type. Motorola cannot anticipate the range of placement equipment and settings
possible for package placement and therefore cannot make a generic recommenda-
tion on how to compensate for QFN interchangeability.
5.2.3 Reflow Soldering
The purpose of the reflow process is to melt the solder particles, wet the surfaces to
be joined and at last, solidify the solder into a stronger metallurgical bond. Prior to the
melting phase, several other phases occur and the sequence of events is described in
Figure
20.
16
Quad Flat Pack No-Lead (QFN)
MOTOROLA
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