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
