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J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 2 ( 2 0 0 1 ) 9 3 ± 9 7
Warpage behavior of LOC-TSOP Memory Package
MINJIN KO, DONGSUK SHIN, MYUNGSUN MOON, INHEE LIM,YONGJOON PARKLG Chemical Ltd./Research Park, Advanced Materials Research Institute,Taejon, KoreaE-mail: [email protected]
This paper describes a warpage study on LOC-TSOP memory devices. The main objectives ofthis study are to evaluate the impacts of epoxy mold compounds on package warpage withdifferent sized dies. It was found that the balance of the bending between the edge sideregion and the die attach region controls the package warpage. The package tends to bendconvex upwards in the edge side region and concave downwards in the die attach region.The various mold compounds were prepared to study the affect on the components,particularly the resin and the ®ller. It has been shown that the optimum mold compound canbe changed according to the inner structure of the LOC-TSOP. The effect of post-mold cure onpackage warpage was also examined based on the cure rate.# 2001 Kluwer Academic Publishers
1. IntroductionDue to the ratio of a chip size to the package size and low
pin count of many DRAM designs, lead on chip (LOC) is
a packaging technique that has been used over the last 10
years for memory products [1, 2]. The technique consists
of attaching leadframe ®ngers directly on the chip
surface via a double-sided adhesive tape, thus the chip
density can be increased up to 90% compared to 65% in
conventional packages. Other advantages in the LOC
design include faster current speed associated with
shorter Au wires resulting from an array of pads scattered
on the center of the chips rather than periphery bonds.
Plastic packaging in the electronic industry has been
extensively used in the past. One of the reasons is its
lower cost compared to ceramic packaging. Epoxy resins
are most often used in plastic packages due to their low
melt viscosity that facilitates injection and minimizes
leadframe and gold wire deformation. It is known that the
internal stress in epoxy resins cured at high temperature
is produced by the volumetric shrinkage during the
cooling process from the cure temperature to room
temperature. The internal stress can reduce adhesion
strength and occasionally induces die cracking if a tensile
stress is built up in the die, particularly in thin packages.
Along with internal stress, excess bending of the package
can cause yield loss in many downstream assembly
processes, such as the trim and form since lead
coplanarity is necessary for device testing and for surface
mount assembly [3]. Residual warpage is often used by
package manufacturers as a check on the quality of the
molding process [4].
As the package increases and the chip decreases in
size, maintaining package planarity becomes more
dif®cult particularly in an asymmetric structure. Since
the degree of package warpage depends on the package
design, materials selection, and assembly process,
several methods can be used to reduce package warpage.
Among these approaches, optimization of the mold
compound is the least costly and most manufacturable
solution although the package design would be a more
effective way to minimize package warpage. It is
important to know how sensitive the warpage of a
particular package is to change in material properties. To
date, most studies have centered on the geometric
modi®cation of package design and ®nite element
analysis [5]. This paper discusses a study evaluating
the effect of mold compounds on the package warpage of
LOC-TSOP devices. The warpage behavior of LOC-
TSOP is discussed based on experimental results.
2. Experimental2.1. Package DescriptionIn our study, LOC-TSOP was evaluated for package
warpage using different mold compounds. All test
vehicles were 400 mil JEDEC 54 lead packages with a
body size of 10.106 22.22 mm and a body thickness of
1.0 mm. The 54-lead device is currently the standard
TSOP in 64/128/256M SDRAM production. The basic
construction of the packages is shown in Fig. 1. The
typical die attach and an alloy 42-leadframe with down-
set depth, 50 mm, are 0.1 mm and 0.125 mm, respectively.
In our experiment different sized dies with 0.28 mm die
thickness were used.
24 LOC-TSOP samples (2 leadframes of 12 packages
each) were assembled for each mold compound. After
wire bonding, the units were transfer-molded at 175 �C in
a conventional mold chase with uneven top and bottom
cavity thickness. The samples were then post-mold cured
at 175 �C for 5 h in a conventional oven. After the samples
were cooled in air to room temperature, they were
measured for warpage using an optical projector. The 24
0957±4522 # 2001 Kluwer Academic Publishers 93
packages of each test were inspected with scanning
acoustic microscopy to check for delamination and die
tilt. Any defective package was eliminated from the test.
2.2. Mold Compound CharacterizationTMA (therm-mechanical analysis): The glass transition
temperature �Tg�, and the coef®cient of thermal
expansion (CTE) below Tg �a1� and above Tg �a2� of
the epoxy molding compounds were measured with a
TMA/SS-100 Seiko therm-mechanical analyzer. The test
specimens (56 56 15 mm) were heated from 0 �C to
about 250 �C at a rate of 2 �C minÿ 1.
DSC measurement: The calorimetric measurement was
conducted using a TMA/SS-100 Seiko differential
scanning calorimeter with a microprocesser controller.
The temperature and power calibration of the DSC were
optimized for the temperature of 20±300 �C using high-
purity indium. For the isothermal cure, the DSC was ®rst
equilibrated at the present cure temperature and then the
sample was introduced into the DSC cell.
Mechanical test: Mechanical characterization of the mold
compounds was carried out using a Zwick universal
testing machine with a high-temperature chamber oper-
ated from room temperature to 350 �C. The ¯exural
modulus was measured by a three-point bending test with
span interval of 64 mm and bending speed of 1 mm minÿ 1
according to ASTM-D-790. The test specimens
(56 12.76 60 mm) were prepared by transfer-molding
at 175 �C for 120 s and cured in an oven at 175 �C.
3. Results and Discussion3.1. Warpage modeAs seen in Fig. 1, in the LOC-TSOP it is very dif®cult to
obtain a perfect balance between top and bottom sides of
the package due to the complex cavity geometry, inner
leadframe structure, size and location of the device. In
particular, the volume of the compound is intrinsically
uneven at the side edge region for the ¯at-type LOC
package. The chip occupation area will also be an
important factor. In order to investigate the warpage
mode, two kinds of TSOP, one with silicon dies and the
other with no silicon die, were tested after post-mold
cure. The typical warpage modes are shown in Fig. 2.
The package without a die is relatively uncomplicated
since it is comprised of only the metal leadframe and the
mold compound. The warpage of this package would be
easy to predict. As seen in Fig. 2a, the package without a
silicon die bends convex upwards, relative to the middle
of the package. This result is obvious in that the total
volume of the mold compound below the leadframe is
larger than that above the leadframe.
However, the warpage mode changes by the insertion
of the die, and the shape depends on the die size shown in
Fig. 2b and C; a gull shape mode for a small-sized die
and a concave mode for a large sized die. It is interesting
to see that insertion of die can make the package warpage
concave. The same trend was obtained even with no
down-set leadframe. This phenomenon is probably due to
the different interface layer at the top and bottom side of
the die. The leadframe and passivation layer contacted
with the mold compound at the top interface should be
more amenable to bend than the rigid silicon die at the
bottom side. Thus the package will tend to warp concave
downwards when the volume shrinkage between the top
and the bottom are the same. The change of warpage
mode from the gull shape to the concave appears for the
die size around 35±40%.
3.2. Mold compound evaluationWe ®rst evaluated two different grades of new-
generation, low-stress mold compounds developed for
thin plastic IC packages. The mold compounds,
identi®ed as ``EMC Grade A'' and ``EMC Grade B'',
were formulated based on a biphenyl epoxy and a high
performance conventional epoxy, which passed the
JEDEC Level I MRT (moisture resist test). Fig. 3
Figure 2 Typical warpage modes: mode A for no die, mode B for a
small-sized die, and mode C for a large-sized die.
Figure 3 TMA plot of dimension versus temperature for ``EMC Grade
A'' and ``EMC Grade B''.
Figure 1 Cross-section view of 54-lead LOC-TSOP package.
94
shows the TMA plots of ``EMC Grade A'' and ``EMC
Grade B'' from 0 �C to 250 �C at 10 �C minÿ 1, with
a1 � 0:91 ppm �Cÿ 1 and a2 � 4:1 ppm �Cÿ 1, and with
a1 � 1:15 ppm �Cÿ 1 and a2 � 5:4 ppm �Cÿ 1, respec-
tively. CTE, Tg and ¯exural modulus values are
summarized in Table I.
The average warpage value for each compound is
shown in Fig. 4. It is seen that the warpage of the samples
molded with ``EMC Grade A'' is about 30 mm for a
large-sized die and 78 mm for a small-sized die while the
warpage of the samples molded with ``EMC Grade B'' is
62 mm for a large-sized die and 64 mm for a small-sized
die. The ratio of the chip size to package size is about
60% for a large die and 30% for a small die, respectively.
By using the conventional compound, the package
warpage increased to 100% for a large-sized die, but
decreased to about 20% for a small-sized die. This is
quite interesting in that the ``EMC Grade A'' with higher
®ller content does not always give better warpage
perform-ance.
To understand these phenomena, we have tested
several model compounds. It is known that the package
warpage results primarily from internal stresses caused
by volumetric shrinkage of the resin during the curing
process and thermal expansion coef®cient mismatches
between the package components as the devices cool
from the curing temperature to room temperature. Since
volumetric shrinkage and CTE mainly depend on the
type of resin systems and the amount of ®ller in mold
compounds, we ®rst evaluated the warpage of the model
compounds designed to have different Tg. The Tg of the
compounds was changed by varying the base resin
system with ®xed ®ller content. Fig. 5 shows the TMA
plots of the three model compounds. Volumetric
shrinkage and modulus are varied only by the resin
matrix during thermal cooling, and the CTE of the
compounds is not changed signi®cantly. Warpage test
results showing the effect of the glass transition are
presented in Fig. 6. It is seen that the low-Tg resin system
helps to decrease warpage for a large-sized die, while the
high-Tg resin does this for a small-sized die. In general,
the magnitude of thermal stress, s, is dependent on both
the elastic modulus and the coef®cient of thermal
expansion;
s � k
Z T2
T1
E ? adT
Since the ratio of elastic modulus below Tg to above
Tg�E1=E2� should be much greater than the ratio of CTE
above Tg to below Tg�a2/a1�, thermal stress below Tg
would be greater than that above Tg. The stress level of
the model compounds estimated with the CTE and
¯exural modulus is shown in Fig. 7. Therefore, a low-Tg
resin system would decrease thermal stress during
volumetric shrinkage resulting in less warpage for a
T A B L E I Material properties of ``EMC Grade A'' and ``EMC
Grade B''
Material properties Units EMC A EMC B
Base resin matrix Biphenyl Conventional
Elastic modulus at 25 �C kg mmÿ2 2300 1950
Elastic modulus at 240 �C kg mmÿ2 100 140
Thermal expansion below Tg 10ÿ5 �C 0.91 1.15
Thermal expansion above Tg 10ÿ5 �C 4.1 5.4
Glass transition �C 110 147
Figure 4 Warpage of the TSOP package with different die size molded
with ``EMC Grade A'' and ``EMC Grade B''.
Figure 5 TMA plot of dimension versus temperature for three model
compounds with different Tg.
Figure 6 Effect of resin matrix on package warpage.
95
large-sized chip. In the case of the small-sized die,
however, the larger thermal stress above the die causing
the bending downwards at the center region might
suppress the opposite bending of the leadframe at the side
edge region, resulting in less warpage.
Among many components in mold compounds that
can have an in¯uence on reducing the thermal stress and
lowering chemical shrinkage, the high loading of ®llers
has been known as a most effective way [9, 10]. It is
reported that the CTE of mold compounds is a major
factor in package warpage [11]. Fig. 8 shows the
relationship between ®ller content and package warpage.
We tested model compounds designed to have different
®ller content with the same resin system. Biphenyl-type
epoxy and xylok-type hardener were used in this
experimental. Unlike the resin effect, the package
warpage increased as the ®ller content of the compounds
decreased for both a large- and small-sized die. It is
related to both the lowered CTE of the compound and the
reduced chemical or curing shrinkage due to the reduced
resin part. By adding the 4% silica ®ller the warpage was
reduced about 25% for a small-sized die and 30% for a
large-sized die.
3.3. Postmold cure effectNearly all IC mold compounds require a post-cure
treatment to ensure the package reliability. The mold
compound does not reach 100% chemical conversion in
the high production rate molding process. The reaction
rate becomes signi®cantly slower at the later stages of
conversion, making elimination of post-cure unlikely.
PMC for 5 h has been adopted in the industry to achieve
``complete'' conversion. To evaluate the PMC effect on
package warpage the three model compounds are
formulated to have similar gel time around 22 sec. It is
seen in Fig. 9 that the package warpage is reduced after
PMC by about 4 mm up to 30 mm, depending on the
compounds used. Since the extent of warpage variation
seems to be related to the degree of conversion before
and after PMC, we assess the cure rate of each compound
using an isothermal DSC measurement. Fig. 10 shows
that the cure rate of the mold compounds passes through
a maximum point and then decreases. The compound A
reaches the maximum rate earlier than other compounds.
Figure 7 Estimated stress level (CTE6E) for each compounds.
Figure 8 Effect of ®ller content on package warpage.
Figure 9 Effect of post-mold cure on package warpage.
Figure 10 Plots of cure rate and conversion versus time for the mold
compounds at 130 �C.
96
It also provides a plot of the conversion as a function of
time. The DSC data show that the compound A is cured
fastest compared to the compounds B and C. Thus the
conversion gap before and after PMC would be smaller
for the package molded with the compound A than that
with compounds B and C, resulting in a narrow variation
of material properties. The gel time has no close relation
with the warpage difference before and after PMC. Fig.
11 also shows that the ultimate warpage value can be
reached within 2 h post-cure, indicating that the lengthy
post-mold cure process often adopted in the industry may
not be necessary from the viewpoint of package warpage.
4. ConclusionThe experiments were carried out to study the impacts of
mold compounds on the package warpage of 54-lead
LOC-TSOP with different sized dies. The following
conclusions are drawn:
1. The package without a silicon die warps convex
upwards, but the warpage mode changes by the insertion
of the die, and the shape depends on die size; a gull shape
mode for a small-sized die and a concave mode for a
large-sized die. The change of warpage mode from the
gull shape to concave appears for die sizes around 35±
40%.
2. Since package warpage results primarily from
internal stress caused by volumetric shrinkage of the
resin during the curing process, the mold compound
effect has been studied. A low-Tg resin system helps to
decrease warpage for a large-sized die, while the high-Tg
resin does this for a small-sized die. However, the
package warpage increased as the ®ller content of the
compound decreased for both a large die and small die
due to the decrease in chemical shrinkage.
3. PMC is necessary after transfer-molding to reduce
package warpage. The extent of warpage improvement
relates not to gel time but to the cure rate of the mold
compound. However, the ultimate value of the warpage
can be reached within 2 h post-cure.
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Received 20 November 2000and accepted 19 January 2001
Figure 11 Package warpage with various post-cure time.
97