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The target products were 12-inch
silicon wafers (~775µm) + Cu Pillar +
SnAg µBump (~45µm). Shallow edge
trim was used on these wafers to clean
the wafer edge and allow for better
clamping of the wafers during the mold
process and prevent any leakage of
resin material. Consequently, the thin
resin layer of 30μm is molded onto
the wafers, while leaving the top of
the solder ball exposed over the entire
wafer (Figure 1b).
Types of materials. We wanted to
study the effect of the resin type on
both the warpage and mold thickness
uniformity. Therefore, both granular
Table 1: Resin chemical and physical properties. and liquid compounds are used for this
evaluation (Table 1). Liquid resin has
a number of advantages including a
lower melt viscosity that allows better
flow properties and easily fills tighter
pitches. Granular compounds, on the
other hand, have a longer shelf life,
lower material cost, and lower material
shrinkage [8]. To see the effect of
resin cut size on the flow properties
and consequently on the final mold
characteristics, different granular resin
cut sizes were used.
Types of process skews. Based on
previous experiments by TOWA and
reported in earlier studies [6], the
processing temperature, press pressure
and the clamp press speed were seen
to be the main parameters to control
the final molded wafer properties.
Table 2: Compression molding process skews. Different processing skews were
selected as shown in Table 2. Each of
the parameters was tweaked to test their
effect on the post-mold wafer warpage
and mold uniformity. Because the liquid
resin mold showed incomplete coverage
(discussed f ur ther in the section
entitled, “Results and discussion”),
these skews were all carried out on the
granular mold compound.
Material characterization
The following sections discuss wafer
mold coverage, warpage measurements,
mold height measurements, and various
scanning electron microscope (SEM)
measurements.
Wafer mold coverage. Optical
microscopy was used to verify full
wafer coverage by mold material using
a Keyence V K-3000 microscope.
Images captured at the center and edge,
post mold, were used to confirm proper
encapsulation of the solder balls within
Table 3: Wafer processing skews and wafer mold coverage.
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