AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 17

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

MOTOROLA
Temperature profile is the most important control in reflow soldering and it must be
fine tuned to establish a robust process. QFN components probably would be among
the smallest devices on any PCB. In such case, place thermocouples under the heavi-
est thermal mass device on the PCB to monitor the reflow profile. Generally when the
heaviest thermal mass device reaches reflow temperatures, all other components on
the PCB will reach reflow temperatures.
For all devices on the PCB, the solder paste needs to be taken into account for the
reflow profile. Every paste has a flux, and the flux dominates the reflow profile for
steps like soak time, soak temperature, and ramp rates. Peak reflow temperature is of
course the melting temperature of the metals in the paste plus a “safety” margin to
ensure that all solder paste on the PCB reflows.
The reflow profile should follow the paste supplier’s “recommended” profile. Deviation
from the recommended profile should be first evaluated using a copper (Cu) coupon
test. The horizontal size for a typical solder paste volume is measured as either a
diameter or as “x” and “y” lengths. The Cu-coupon is then reflowed and the solder
paste volume is measured for either diameter or “x” and “y.” The goal is have a reflow
profile with the most horizontal spread. For best results, the Cu coupon should be
lightly sanded before use to remove Cu-oxide build up.
5.2.4 Inspection
Unlike traditional leaded components, the solder joints of QFN are formed underneath
the package. The conventional visual inspection technique to check the quality of the
solder joint is time consuming. Whenever possible, optical inspection and X-ray
inspection are recommended to verify any open or short circuit after reflow soldering.
5.2.4.1 Optical Inspection
The following photos show the solder joint formation of each QFN package styles.
The observed solder wettings are considered acceptable.
Freescale Semiconductor, Inc.
For More Information On This Product,
Solvent
evaporation
I
Quad Flat Pack No-Lead (QFN)
Go to: www.freescale.com
Flux
reduces
metal oxide
Figure 20 General Solder Reflow Phases
II
Solder balls
melt, wetting
and wicking
begin
III
Solder
completely
melted
IV
Cool down
phase,
solder
solidifies
V
AN1902/D
17