1. Introduction:
1.1 Quad
Flat Pack packages are plastic 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,
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. So it’s important to handle new
packages with caution, no matter what the lead count.
1.2 Removing
a QFP 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, 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 Pick Up 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.
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 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.
The following
procedure is designed around the Chipmaster High Power Low Temperature
operation, (Illustration # 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 with the Chipmaster results in mirror surface joints,
providing the original solder has not been degraded through
high temperature rework (Illustration # 2a). The QFP procedure
discussed is for lead counts of 100 leads, (Illustration # 1)
or greater, however the process is identical for all QFP components,
excluding those, which are Conformally Coated
Illustration # 1
