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Effects of soft solder materials and die attach process parameters on large
power semiconductor dies joint reliability
Conference Paper · December 2013
DOI: 10.1109/EPTC.2013.6745703
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Effects of soft solder materials and die attach process parameters on large
power semiconductor dies joint reliability
*
Vemal Raja Manikam, Samsun Paing and Amy Ang
Power Segment Development, Infineon Technologies (M) Sdn Bhd
FTZ Batu Berendam, 75350 Malacca, Malaysia
*
vemal.rajamanikam@infineon.com
Abstract
Die attach utilizing soft solders, primarily high lead (Pb)
solders for power semiconductor devices are still widely used
across the industry. These solders offer excellent mechanical,
thermal and electrical properties. Soft solders provide good
heat dissipation of typically 35 W/mK, is ductile and exhibits
desirable thermal expansion properties which tolerate that of
silicon (4.2 ppm/K) and copper (16.5 ppm/K), so as to avoid
joint failure due to thermal cyclic stresses. Typically, for large
dies, bond-line-thickness (BLT), die tilt and post-reflow void
rates are important. These factors affect the reliability and
performance of the power device. Large, localized voids
create air pockets which are detrimental to the device’s
reliability, both electrically and thermally. The experimental
work conducted here is on a large power semiconductor die
having an area of 19mm2 and Ag coated die back, using a PbSn2-Ag2.5 Type 3 soft solder on bare copper (Cu) substrates.
For large dies having unequal width and length, the flow of
solder when bond pressure is exerted and released becomes
crucial as well as challenging. It directly affects the BLT.
When reflowed, most solders undergo flux evaporation which
leads to hot slump of the die attach solder, which in turn
affects the die tilt and final outcome of void rates between the
die and substrate. Here, surface energy concepts and how well
the solder material flows and wets both the die back and
substrate surfaces need to be understood. Die back and
substrate surfaces exert varying stresses before and after
reflow. In this literature work, we examine the fundamental
concept of impact bonding utilized for soft solders on large
power devices, factors which govern the formation of an
acceptable bond line thickness, as well as influence of surface
conditions for solder paste wetting and spreading during
reflow in order to gain a reliable die-to-substrate
interconnection.
Introduction
Pb soft solder materials have formed a stable base amongst
die attach materials. For power device technology utilizing Sibased devices, the operation temperatures range between 200250˚C. One of the most used Pb soft solder materials is PbSn2-Ag2.5. Despite the high Pb content, it provides good
thermal, electrical and mechanical properties to form a reliable
joint between the die and substrate. Generally, with increasing
Pb content in a solder material, the liquidus point increases
proportionally [1].
Silver (Ag) is added to aid in surface wetting properties of
the solder material [2]. The Pb-Sn2-Ag2.5 solder material has
a solidus point of 299˚C, and liquidus point between 304309˚C [2]. Having a liquidus point which is higher than the
operational temperature of a device is important for die attach,
in particular power devices. Fig.1 lists the common die attach
materials used in the industry and academia over the years by
categories, i.e. their liquidus temperature. The Pb-Sn2-Ag2.5
solder falls into the medium range, which cover majority of
the common power device applications.
Impact bonding is a common method for soft solder die
attach, whereby the solder material is dispensed onto a
substrate, then die is then placed on the dispensed solder
followed by some exertion of pressure to enable the solder
material to come into contact with the die back. The die-die
attach solder-substrate structure is then sent for reflow
processing, whereby the solder material undergoes melting to
create a joint between the die and substrate. During reflow, the
solder material typically experiences phase changes, which in
turn affects the outcome of the soldered part, and subsequently
its reliability. Such material-process interactions are
important, and become critical for larger dies, especially ones
with irregular width and length. Concerns would be on die tilt,
void rate and final BLT. In Fig. 2, it can be seen that for a die
with unequal width and length, the material flow would be
more critical as it would differ along the longer and shorter
paths.
Fig. 1: Various die attach materials and solders, their
operation temperature ranges and application possibilities by
category. The Pb-Sn2-Ag2.5 solder falls into the medium
range [1].
x
y
x
B
A
y
A: x=y
B: x ≠ y
Fig. 2: Comparison of die size width and length; A for equal
dimensions and B for unequal dimension.
Die attach impact bonding process with regards to large
dies
Die attach is challenging for much larger dies, as several
factors need to be considered simultaneously, during setup of
the die bond equipment. These challenges become critical for
large dies with irregular width and length. In our assessment,
we used a power semiconductor die having an area of 19mm2.
The die was run using a typical die bond equipment, using PbSn2-Ag2.5 solder dispense followed by die placement with
pressure being exerted as the die was placed onto the
dispensed solder area. Fig. 3 illustrates the impact bonding
process on the Pb-Sn2-Ag2.5 solder material [3].
In Fig. 2a, the solder paste is dispensed, followed by die
placement with a bond force exerted onto the die top. In Fig.
2b, the movement of the die creates a “volcano effect” which
relates to the flow of the solder material out from the bulk
solder volume which has been dispensed [3]. Subsequently, in
Fig. 2c, the thin crevice created by the die and substrate
clearance enables capillary forces to act, whereby the
“volcanoes” collapse and the solder volume flows back
between the die and substrate. This in turn forces the die
upward, using what we can call a spring-back force, which
forms the bond-line-thickness.
For larger dies, the flow of die attach material is important,
in particular at the edges. If the solder material’s flow back is
poor with regards to bond force, incomplete coverage will
take place. It would lead to corner delamination issues. Fig. 4
depicts such a scenario with incomplete solder coverage for a
large die, followed by a through-scan image in Fig. 5.
The mechanism presented in Fig. 3, followed by failures
indicate how severe the die attach process itself is, as it
promotes and controls the material flow back to achieve
acceptable solder material coverage on the die back. In
subsequent sections, the mechanism presented here for impact
bonding will also be discussed with regards to surface energy
of both the die back material and substrate.
a
b
c
d
Fig. 3: Schematic diagram of impact die attach bonding
for the Pb-Sn2-Ag2.5 soft solder.
a.
a.
b.
b.
Fig. 4: Optical images: a.) Incomplete solder at die edge; b.)
Poor solder volume flow at edges, unable to form fillet height.
Fig. 5: Through-scan images for post-reflowed dies. The
arrows indicate delamination occurrences at the die edges.
(i) Assessment of die attach solder material during and
after reflow
Most die attach solder materials composition play an
important role in obtaining good BLT and die tilt. These are
inherent properties of the solder material which translate to
performance indicators of the die attach process itself. Any
solder material constitutes of solder particles and fluxes. The 2
components combine to yield an easily handled solder paste
[4]. Solder deposition on a substrate affects bond-linethickness value. Solder volume fraction, V, is a representation
of the solder volume being deposited, whereby in this
experimental work would be via dispensing as shown in Eq. 1
[4]:
In Eq. 1, x represents the metal content (%w/w), ds is the
density of the solder alloy, and df is the density of the solder
flux. By obtaining the solder alloy and flux densities, we are
able to generate a relationship for solder volume fraction and
metal weight percent content in the solder, as depicted in Fig.
6. In our experiment, the Pb-Sn2-Ag2.5 solder paste had 90%
solder alloy weight content. When converted to volume
fraction, this amounts to approximately 44.6%. This would
mean, during solder paste dispensing for die attach, a solder
alloy volume fraction of 44.6% only is present, and the
balance 55.4% would constitute of flux material. This flux
material would evaporate and burn off during reflow.
Therefore, the bond-line-thickness theoretically would be
reduced after reflow, and can affect the die tilt properties too.
To demonstrate this concept, a simple experiment was
conducted. The Pb-Sn2-Ag2.5 soft solder was dispensed on
Cu substrates and 5 specific points were measured before and
after reflow. As mentioned earlier, the maximum reflow
temperature was 400˚C, with a ramp rate of 2˚C/sec. A total of
7 units were measured and compared. The statistical analysis
is shown in Fig. 7. There was a significant collapse in material
height after reflow, which was estimated at about 70% of the
original thickness. This fits the earlier theoretical assumption
presented in Fig. 6. The loss of volume after reflow is crucial
in establishing good bond line thickness, especially for large
dies which could yield poor die tilt results as well. Therefore,
controlling the bond height and its impact during die attach of
these solders is important, when the loss of solder materialpost reflow is taken into consideration.
PbSn2Ag2.5 volume versus weight %
for metal particles
The relationship between these 3 interfacial tensions can
be represented by Eq. 2, whereby cos θ is the contact angle of
the molten solder droplet on the substrate [4,5]:
PbSn2Ag2.5 volume %
100
80
60
44.6%
40
20
0
20
40
60
80 90% 100
PbSn2Ag2.5 weight %
Fig. 6: Volume fraction percentage relationship with solder
alloy weight percent content for Pb-Sn2-Ag2.5 solder paste.
1
4
5
3
1
4
5
2
3
Pattern
height
before
reflow
Pattern
height
after
reflow
2
Fig. 7: Oneway analysis of dispensed pattern height (micron)
by measured points for the Pb-Sn2-Ag2.5 solder paste before
and after reflow.
(ii) Influence of die back and substrate surfaces on solder
spreading and wetting
Molten solder spreading and wetting is an important
criteria towards achieving joint reliability for die attach [5]. If
the material is unable to interact with the contact surfaces, a
strong bond cannot be achieved. During solder reflow, the
interaction between die back and substrate surfaces can have
an impact on bond-line-thickness formation. This relationship
will be discussed with regards to surface energy as well for the
contact surfaces. To begin with, the interaction for a molten
solder droplet on a Cu substrate can be represented by Fig. 8.
In Fig. 8, using Young’s equation, the solder droplet
undergoes 3 different interfacial tension; ΦLG or liquid-gas
interfacial tension, ΦSG or solid-gas interfacial tension and
finally ΦSL or solid-liquid interfacial tension.
ΦLG
ΦSG
θ
ΦSL
Contact surface
Fig. 8: Common analogy of differing forces acting for a liquid
droplet on a contact surface [4].
To have good wetting and spreading, the value of θ has to
be as small as possible. Most leaded molten solders, like the
Pb-Sn2-Ag2.5 soft solder have low contact angles on bare Cu
substrates. The wetting and spreadability is almost always
much more superior compared to Pb-free soft solder solutions
[2]. When surface energy comes into play, it is seen that there
are 2 interacting surfaces here; the Ag die back and the Cu
substrate. Both these surfaces display varying degrees of
surface energy with regards to the surface tension of the solder
paste before reflow and during reflow, i.e. creation of a molten
state.
Before reflow, the solder paste molecules would be
attracted to the surface of the Ag die back as the die bond
force in Fig. 3d is released. This is because the Ag die back
surface is more energetically favorable, or has a higher surface
energy than the solder paste which makes it suitable for
wetting as compared to the Cu substrate. The Cu substrate
prior to reflow would tend to have a much more oxidized
surface than that of the Ag die back, which makes it less
energetically favorable to be wet. Therefore, coupled with the
solder paste’s interfacial attraction to Ag surfaces and the
material spring-back as the bond force is released, the bondline-thickness obtained would be rather high. Here, it would
depend also on the amount of bond force being used during
impact bonding.
When reflow starts, and the flux in the solder paste
removes the oxides from the surface, rendering it more
suitable for wetting, slumping would occur as the solder paste
material spreads across the substrate. The volume loss as
depicted in Fig. 6 together with the spread of the solder
material would cause the bond-line-thickness to reduce
significantly. As it can be seen from this discussion, there are
several factors which affect the creation of a good BLT. If a
good BLT can be obtained, it would also translate to lower die
tilt, and controlled or acceptable rates of voiding after reflow.
In essence, the reliability of the joint from controlled process
parameters is improved.
(iii) Solder paste reflow profile considerations
The solder paste reflow profile ultimately plays an
important role too with regards to obtaining a reliable solder
joint for large dies. With respect to the irregular die width and
length used in this study, the solder flux material will
evaporate along the shortest path possible. This loss of flux
from the solder paste after reflow was proven earlier with
measured dispensed solder paste material height. The concern
however would be on the longer path. In this study, the peak
reflow temperature was kept at 400˚C under N2/forming gas
conditions, which illustrates a fully molten or liquidus stage
exists for the solder material, bearing in mind the liquidus
point of the material is between 304-309˚C [2]. Post-reflow
examinations were able to showcase good coverage of PbSn2-Ag2.5 solder material under the die. However, for such
large dies in mass manufacturing operations, the concern lies
in how the BLT is controlled as the flux volatiles evaporate. It
could cause uneven BLT, in particular at the corners or worse
still severe voiding when the flux material expands without
proper escape paths. The key would be to select a suitable flux
material with acceptable activation energy which reflects the
ramp rate of the solder profile. Too fast a ramp rate coupled
with high flux activation energy can lead to trapped flux
volatiles and subsequently voiding [4-5]. These factors need to
be balanced.
Conclusions
To obtain a reliable solder die attach joint using Pb-Sn2Ag2.5 soft solder on large power dies, we have studied
varying material and process factors. These factors are interrelated and cannot act alone. Die attach is a critical process in
electronic packaging, as it relates to BLT, voiding and die tilt
which are all possible failure mechanisms. For high power
applications, the severity of failure can be catastrophic due to
the high-end systems which utilize it for automotive,
aerospace and military applications. Fig. 9 summarizes these
interacting factors for soft solder impact bonding.
Incomplete coverage
under die
Tilted die
Large dies after reflow
Hot slump and solder material
collapse due to flux evaporation
Cold slumping of material
Higher BLT
Large dies before reflow
Spring-back of solder to die
back due to surface energy
interactions and bond force
Surface energy interaction of
solder with post-fluxed
substrate
Lower BLT
Voiding
Fig. 9: Interaction between pre and post-reflow soft solder die
attach factors which lead to obtaining a reliable solder joint.
Acknowledgments
The authors would like to acknowledge the support given
by the Infineon Failure Analysis and Power Segment
Engineering Development teams in Malacca, Malaysia, and
Neubiberg, Germany, respectively.
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References
1. V.R. Manikam and K.Y. Cheong: IEEE Transac. Comp.
Packag. Manuf. Vol. 1 (2011) pp. 457-478.
2. K. Suganuma: Lead Free Soldering in Electronics:
Science, Technology, and Environmental Impact. Marcel
Dekker (2004).
3. F. Joho: Semicon Singapore (2006) pp. 1-3.
4. N.C.
Lee:
Reflow
Soldering
Processes
and
Troubleshooting SMT, BGA, CSP and Flip Chip
Technologies. Butterworth-Heinemann (2002).
5. F.G. Yost, F. M. Hosking and D. R. Frear: The Mechanics
of solder alloy wetting and spreading. Van Nostrand
Reinhold (1993).