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resin needs to be dispensed as one because the resin compound volume and an incomplete fill (Figure 6b)
point at the center. Numerous other is too small, it starts to harden before shows an example of a spiral pattern
dispensing patterns like the spiral one reaching to the end of the wafer while with incomplete fill). In contrast, we
were evaluated (Figure 6a). However, creating an uneven mold thickness observed that a center point dispensing
creates an even thickness of the resin
as it travels from the center to the edge
of the wafer. However, this pattern
gives us the largest distance between
the compound dispense and the wafer
edge. The flow of the compound isn’t
fast enough to help ensure a complete
fill till the wafer edges. Figure 6c
shows a wafer with a center dispensing
pattern with incomplete coverage at the
edges. Adding additional liquid resin
material was one option, but that, in
turn, led to lower control of the mold
thickness and resulted in similar edge
coverage issues.
Ef fects of d i f ferent mold cut
s i z e s a nd t e m p e r a t u r e . T h r e e
types of different mold cut sizes of a
granular compound were evaluated
and all showed a fully-molded wafer
appearance (Figure 7a.). The granular
cut size of 0.7-1.0mm was the easiest
to dispense most evenly over the entire
molding area. By using this cut size,
the margin of compound filling can
be expected to be wider. A granular
cut size <2mm contains a wide range
of particle sizes and much more size
variability, ranging from smallest
(~ 0.7m m) to the largest (~2m m).
Figure 5: a) Bump mold encapsulation at center; b) the edge of an incomplete filled wafer; c) resin bleed-out So even though it is possible to be
at the edge of the wafer; and d) granular resin distribution on the bottom chase pre-compression molding. dispensed evenly and provide full
coverage, the performance stability
becomes a potential concern.
Because temperature will influence
compound melt and cure speed, we
tried two different temperatures to
confirm the influence on compound
filling. However, not a lot of differences
were observed in the compound filling
or warpage with different molding
temperatures for various resin cut sizes.
We were able to lower the temperature
to 130°C and still successfully achieve
molded wafers with full coverage.
Effect of process skews. One of the
parameters that should be considered
is the impact of press speed. Different
press speeds will lead to different
compound flow speeds. Consequently,
this can result in the lack of control
of compound filling. When the press
speed was set to a lower speed (Figure
7b), incomplete fill was observed. The
reason can be attributed to the longer
Figure 6: a-b) Spiral dispense pattern pre-mold and final post-mold coverage; and c-d) center dispense filling time, which caused the melt
pattern pre-mold and final post-mold coverage.
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