NOVEL DIE ATTACH ADHESIVE FOR THIN QUAD FLAT PACKAGE
Hee Yeoul Yoo, Byung Hoon Moon, Jae Sung Kwak.1
Cheol Woo Kwak, Ji Young Lee & Thomas J Borghard 2
1
Material & Process Development Department, R&D Center, Anam Semiconductor, Inc.
Seoul, Korea
2
Ablestik Laboratories
Rancho Dominguez ,California
ABSTRACT
Thin plastic packages, such as TQFPs, are more sensitive to
stress and warpage. As a result, warpage in TQFPs may
lead to severe delamination or die cracking. The assembly
process and the choice of packaging materials can strongly
influence the stress that develops in these large thin
packages. In particular, the die attach adhesive plays a
significant role in controlling stress or warpage in the
package. Die attach material properties, such as modulus,
CTE, adhesion strength, and Tg, directly affect stress
generated in a package and, as a result, can have application
limitations in terms of die size usage.
For larger die sizes (>300 mil square), low modulus
adhesives are used to decrease the stress between a die and
leadframe having a large CTE mismatch. These types of
adhesives show low die warpage and stress at room
temperature. However, die attach materials that have
comparatively low modulus and high elastic properties at
elevated temperature (above Tg) can cause problems at wire
bonding process temperatures. In other words, at wire bond
temperatures these materials have spring-like properties with
relatively less stiffness. This reduces the effective power at
the Au/Al interface, resulting in poor ball bond adhesion
strength. This phenomena gets worse as the die size
decreases. In the case of small die size (<100 mil sq.), ball
bond shear strength becomes unacceptable.
For small die, high modulus adhesives have typically been
used for their strength at wire bond temperatures. In the
case of high modulus adhesives, the rigidity supports the die
even above Tg so the ball bond shear strength is acceptable
when very small die is used. Although these adhesives are
suitable for small die, high modulus adhesives do not absorb
the stress induced by the CTE mismatch of the leadframe
and larger die. This poor stress-absorbing property leads to
warpage problems and limits die size usage.
This paper presents a novel die attach material for TQFPs.
A unique additive was added into a silver-filled die attach
adhesive system to increase the adhesion strength and to
enhance anti-delamination performance. This approach
allows for applications to a wide range of die sizes providing
low warpage and stress, and good wire bonding
performance. The resin system of this material has so called
"ideal modulus" property. That is, it has modulus high
enough for improved wire bonding performance and low
enough modulus at room temperature to absorb stress from
CTE mismatches. For the moisture resistance test (MRT)
performance of this material, it passed JEDEC level 3
followed by 3X 235oC IR reflow condition without
delamination at any interfaces on 144 lead TQFP (on 400
sq. mil die) and 100 lead TQFP (98 sq. mil die). Other
material used in this package are SUMITOMO 7320CR
epoxy molding compound and copper leadframe. In this
paper, details of assembly process results such as warpage,
wire bond ball shear strength, and MRT performance will be
discussed.
INTRODUCTION
After die attach, the adhesive is cured to bond the chip to the
substrate. During this cure process, expansion of the die and
leadframe take place.
The thermosetting adhesive
crosslinks and hardens during cure and thermal stress is
developed upon cooling down. The CTE mismatch or the
contraction of the leadframe relative to the Si die causes this
stress.
Material
Si
Cu
CTE (ppm/K)
3.0
16.2-17.6
During cooling to room temperature stress is induced and
delamination can occur. Modulus at room temperature is
used to compare the rigidity or flexibility of adhesives. The
modulus is a good indicator of what type of adhesive to use
in different semiconductor packages.
To resolve this problem of warpage-related delamination
with large dies, low modulus die attach materials are
utilized. These epoxy die attach materials usually are very
MODULUS AND WIREBONDABILITY
A typical high modulus or high stress adhesive is one whose
modulus is 2.5-4x 109 Pa in the temperature range of 0 to
100°C and rapidly reduces to 2-4x 108 Pa above 150°C. A
low modulus adhesive is on the order of 2.5-4.5x 109 Pa at
~~--~ooc-and-mpldIYreduces to 1-4x107 Pa at 70°C. Intermediate
modulus or medium stress adhesives are those that have a
modulus in the range of2-4x109 Pa at 0°C and 7x107-3x108
Pa above 150°C.
Generally high modulus adhesives have high crosslinking
density and low flexibility.
This property prevents
vibrations, which interfere with wire bonding of small die.
However, the adhesive is relatively high in warpage because
of the large difference in coefficient of thermal expansion
between the adhesive layer and the semiconductor chip and
paddle. Thus, the high modulus adhesives are suitable for
mounting semiconductor chips as small as 10Ox100 mir in
size.
#.
HMA:
3-0imensional
LMA:
Flexible
3-Dimensional
Structure
Flexibilizers can be used to reduce stress or the modulus.
Typically low modulus adhesives are low in warpage and are
suitable for mounting semiconductor chips as large as
400x400 mir in size. These types of adhesives are low in
crosslinking density, but high in flexibility .Therefore, they
tend to be soft at high temperatures and have relatively high
vibrations with small die leading to a high nonsticking rate
of the bonding wire.
The "ideal" modulus or medium stress adhesives have
crosslinking densities, which are between the high and low
crosslinking densities, thus showing intermediate flexibility .
The "ideal" modulus adhesive is rigid enough to allow for
relatively little or no vibration with small die, making the
nonsticking rate of the bonding wire similar to hIgh modulus
adhesives. Their warpage is also intermediate in magnitude
and allows successful bonding of die from 10Ox100 mir to
400x400 mil2 in size.
To simulate this problem with vibration during wire
bonding, Dynamic Mechanical Thermal Analysis was used.
After a high modulus and low modulus adhesive was bonded
to a lead frame, a downward force of 5 Newtons, was
applied every 0.6 second to simulate actual wire bonding
(see Figure 3).
The results show both the high modulus adhesive (HMA)
and the low modulus adhesive (LMA) have minimal
vibration on large die. However, the LMA has the highest
vibration on small die (Figure 4).
This vibration will result in poor intermetallic connections
between the Au wire ball and the Al pad on the silicon die.
This high nonstick or poor adhesion will be obvious by the
low wire bond ball shear strength.
The high modulus adhesive had a ball shear strength of 45
grams. The low modulus adhesive was significantly lower,
30 grams, related to the effects of the vibration during wire
bonding.
RP323, the “ideal” modulus adhesive,
demonstrated ball shear strength of 44 grams, similar to the
high modulus adhesive (see Figure 5). This would indicate
good wire bond capability for small die.
Wire Bond Ball Shear Comparison
Input Force
Grades
HM A
RP 323
LMA
Average
45.27
44.38
30.43
5
23.9
36
22.5
54.1
48.9
35.6
STD
4.77
3.11
3.29
Average ball shear values for epoxy types
Average ball shear values for epoxy types
50.00
50.00
45.00
45.00
45.27
45.27
44.38
44.38
40.00
40.00
35.00
35.00
0.0
Ball shear(g)
Ball shear(g)
Force, N
0. 6
Min
Max
)
Time(sec
30.43
30.43
30.00
30.00
25.00
25.00
20.00
20.00
15.00
15.00
10.00
10.00
5.00
5.00
Downward force simulates wire bonding
0.00
0.00
LM ISR4
HMA
LM ISR4
RP323-1
RP
323
RP323-1
Epoxy
types
Epoxy type
Epoxy type
JM 2500AN
LMA
JM 2500AN
Figure 5. RP323 has wire ball shear strength
Figure 3 Simulation of wire bonding to test for
the vibration effect
similar to HMA
DMTA Simulation of Bouncing Behavior
LMA / Small die
LMA / Large die
Figure 6. TQFP
HMA / Small die
HMA / Large die
Figure 4. Die vibration simulation results
MOISTURE RESISTANCE TESTING (MRT)
Typically, surface-mounting type semiconductor packages
comprise leadframes where semiconductor chips are bonded
on paddles using adhesives. Figure 6 shows a thin quad flat
package (TQFP). In the TQFP seen in Figure 6, a
semiconductor chip (2) is mounted on a semiconductor chip
paddle (3a) of a lead frame (3) via an adhesive layer (6).
This thin type of package is very prone to cracking and
delamination when exposed to VPS or IR reflow conditions.
With large die, good adhesion and low stress in the die
attach play a critical role in preventing delamination and
cracking. RP323 needs to have stress similar to a LMA,
which is presently acceptable in TQFP. Using a Tencor
Surface Profile to measure warpage 460 mil diagonally
across a die surface, we see RP323 has stress properties
similar to the LMA (see Figure 7).
Given both of these properties, low stress and high adhesion
strength, moisture resistance testing was performed to
determine the reliability of RP323 in a difficult anti-popcorn
package, TQFP. Both small and large packages were tested
to determine the suitability for small to large die.
Test
Vehicle
TQFP
Leads
Thickness
Die Size
3
Large Package
14 x 14mm
100
1.0mm
2.5 x 2.5
0.3mm
20 x 20mm
144
1.4mm
9.7 x 10.4 x
0.4mm
x
RP323, LMA (control for large die) and HMA (control for
small die) adhesive were tested in the above mentioned
packages. Die attach, wire bonding, and molding were
performed. Packages were preconditioned as follows:
2.5
2
Surface profle(mil)
Small Package
HM A
RP323
LMA
1.5
1
1.
2.
3.
Dry baked @ 125oC for 24 hours
Moisture soak @ 30oC/60% RH for 192 hours
3x 219oC VPS or 235oC IR reflow
0.5
0
0
100
200
300
400
Scan length(mil)
Figure 7. Die warpage versus cured adhesive
Another important factor is the adhesion strength. For small
die, high adhesion is required for wire bonding while for
large die good hot wet adhesion is needed for antidelamination performance in popcorn testing. With the new
additive system in RP323, the hot adhesion is improved over
an LMA, and more so over an HMA (see Figure 8). This is
a significant improvement because LMA typically have very
poor adhesion strength.
SAT Comparison
TQ 14*14 , 100LD, 1.0T, die size 2.5x2.5x0.3 (mm)
Epoxy
10 kg DSS after 85/85 on 500 x 500 mil die
9
8
7
6
5
4
3
2
1
0
323
Once exposed to VPS or IR reflow to simulate soldering of
packages to the PWB, the packages were checked for
delamination using Scanning Acoustic Tomography (SAT).
Figure 9 shows a 98x98 mil2 semiconductor die mounted on
5x5 mm2 paddles. Figure 10 shows a 415x381 mil2
semiconductor die mounted on 11.5x11.5 mm2 paddles. In
these figures, dark portions represent the portions where
interface delamination occurs. While Figures 9a and 10a are
tomographs of the packages whose external leads were
subjected to vapor phase soldering (VPS) at 215oC, Figures
9b and 10b are tomographs of the packages which were
subjected to infrared reflow at 235oC.
HMA
Level 3
VPS
215’C
HMA
LMA
FORMULATION
Level 3
IR
235’C
Figure 8. RP323 shows improved hot wet adhesion
Figure 9. Small die
323
LMA
SAT Comparison
TQ 20*20 , 144LD, 1.4T, die size 9.7x10.4x0.4 (mm)
Epoxy
HMA
323
LMA
Level 3
VPS
215’C
Level 3
IR
235’C
Figure 10. Large die
The HMA gave good results, that is, little interface
delamination to the packages in which small size
semiconductor chips were mounted, as shown in Figures 9
and 10 while giving bad results, that is, great interface
delamination, to the packages in which large size
semiconductor chips were mounted, as shown in Figures 9
and 10. Irrespective of chip sizes, the packages, which were
subjected to VPS at 215oC, are better in interface
delamination resistance than those which were subjected to
IR flow at 235 C. From these results, the HMA is apparently
suitable for small size packages only.
In the case of the LMA, the interface delamination occurred
in most of the packages in which small size semiconductor
chips were mounted, as shown in Figure 9. On the other
hand, there is little interface delamination in the
semiconductor packages in which large size semiconductor
chips were mounted, as shown in Figure 10. These results
demonstrate that the LMA is suitable for large size packages
with large size semiconductor chips only. Irrespective of
chip sizes, the packages, which were subjected to VPS at
215oC, are better in interface delamination resistance than
those that were subjected to IR flow at 235oC. This suggests
that the higher the treatment temperature, to a limit, the
easier the interface delamination occurs.
RP323 with the new additive system, gave excellent results.
That is, no interface delamination, to the VPS-treated
packages in which small size semiconductor chips were
mounted, as shown in Figure 9, while giving good results to
the IR reflow-treated packages, as shown in Figure 9. In all
of the semiconductor packages in which large size
semiconductor chips were mounted, excellent results were
also obtained with RP323 shown in Figure 10. Therefore,
RP323 allowed relatively excellent results irrespective of
semiconductor chip sizes, so that it can be used for
semiconductor chips in a wide range of sizes.
CONCLUSION
Based on the testing, it appears that although low modulus
adhesives have low stress properties required for bonding
large die, but they have poor wire bonding capability. This
is due to the softening of the adhesive at high temperatures,
and results in absorption of ultrasonic energy from the wire
bonding process. This creates a “bouncing effect” which
results in poor wire ball bond adhesion. On the contrary,
high modulus adhesives are rigid to begin with at room
temperature and remain fairly rigid at wire bond
temperatures. This property prevents the bouncing or
absorption of energy and allows for good wire bondability
of small die. However, due to their inflexibility they are not
suitable for large die applications.
With a medium modulus, RP323 with a new additive
system, we are able to use one die attach for small and large
die applications. An adhesive with this type of broad
package application has many advantages. Customers using
a number of adhesives can use one to cover the majority of
their needs. This means, easier inventory control and less
clean up between assembly of different packages.