1. Introduction:
1.1
Quad Flat Pack - SMD SMT packages
are plastic OR CERAMIC SMD SMT die encasements with lead
contact distribution around the perimeter of the package and can
be referred to as "Gull Wing" packages due to the shape of the
very fine contact leads.
Great care must be exercised in handling, not to distort or bend
the leads of any smt smd , any distortion effects the "Coplanarity"
of the overall component lead plane to the contact pads on the
board and this distortion can effect the ease of which a chip is
placed, for example if just one lead is lower in plane than the
others, the whole chip begins to pivot around the lower lead and
a swivel sliding effect is found during placement. If any lead
is higher than it's companions then that lead might not be
effectively soldered during reflow for repair and rework of
these smd smt devices. So it's important to handle new smd smt
packages with caution, no matter what the lead count.
1.2
Removing a smt smd is relatively straightforward, as long as
the component is no longer required, the two processes discussed
in this procedure, cover "Eutectic" removal and "Reflow"
removal.
1.3
Eutectic removal for rework and repair of smt and smd
removes the component at eutectic condition, which means the
minimum time and temperature to pull the component from the
contact pads when the solder securing the last leads changes to
a molten state, as the component is subject to upwards tension
during the process, by an Automatic Vacuum Pickup Assembly a
small amount of solder can be dragged up with the component
leads, leaving reverse icicles on the contact pads, which need
to be leveled prior to component replacement. Also a number of
the last leads to be released may be bent due to the drag
tension. With the Chipmaster this is reduced due to the linear
heating throughout the nozzle area, but this does not compensate
for the thermal differences in the overall circuit board mass,
so that new solder and leveling may be necessary during the
rework and repair of smd smt devices.
1.4
Reflow removal not only allows the removed chip to be
reused but removes the necessity of dressing the contact pads
with new solder or having to Hot Air Level between stages of
removal and placement, thus reducing time to effect a rework or
repair and also reducing possible errors in applying new solder,
which is a skilled and somewhat difficult process. With Reflow
removal, the operation is delayed until a total reflow condition
is reached and the component is removed without drag or transfer
of solder from the contact area to the component leads, thereby
leaving all contact pads complete with clean coplanar solder.
(Figure
#2)
1.5
The following procedure is designed around the Chipmaster High
Power Low Temperature operation, (Figure 2) working at reduced
temperatures, which does not degrade solder integrity or
original quality, i.e. does not reach the critical temperature
level, which begins to accelerate the aging process and
crystallizes the tin (SN) and lead (PB) molecules, visible by a
grayish pitted appearance. Rework and repair of smt and smt with
the Chipmaster results in mirror surface joints, providing the
original solder has not been degraded through high temperature
rework or repair (Figure 2a). The rework and repair of the smd
smt - QFP procedure discussed is for lead counts of 100 leads,
(Figure 1) or greater, however the process is identical for all
QFP components, excluding those, which are Conforrnally Coated.
Figure #1
