This paper documents the feasibility study of using micrometer-scale Ag paste as a lead-free (Pb-free) die attach material for microelectronic packaging. Currently, there is no viable Pb-free die attach in the market which can pass the reliability testing regimen. Sintered Ag was explored as an interconnect material because of its relatively low processing temperature and robust joint after sintered. This report suggests a possible route for using Ag paste as a Pb-free die attach by dispensing as per current production epoxy die attach. This feasibility study reports the mechanical integrity, electrical and reliability testing of a surface mount power package with four types of dispensable Ag past‰es

1. Sintered Silver (Ag) as Lead-free Die Attach Materials
Kim S Siow
Package Innovation Development Centre
ON Semiconductor,SCG Industries Malaysia Sdn Bhd
Lot 122 Senawang Industrial Estate 70450 Seremban
Negeri SembiIan Malaysia.
Email: kimshyong.siow@onsemi.com; kimshyong@gmail.com
Abstract
This paper documents the feasibility study of using
micrometer-scale Ag paste as a lead-free (Pb-free) die attach
material for microelectronic packaging. Currently, there is
no viable Pb-free die attach in the market which can pass the
reliability testing regimen. Sintered Ag was explored as an
interconnect material because of its relatively low processing
temperature and robust joint after sintered. This report
suggests a possible route for using Ag paste as a Pb-free die
attach by dispensing as per current production epoxy die
attach. This feasibility study reports the mechanical
integrity,electrical and reliability testing of a surface mount
power package with four types of dispensable Ag past
�
s.
Separate lots of this surface-mount power package With
current die attach materials were also evaluated as controls.
This paper is expected to be of interest to companies that are
exploring alternative Pb-free die attach materials.
Keywords: Pb-free die attach,sintered Agjoint
1.0 Introduction
The search for Pb-free die attach has intensified due to
several EU directives such as the Waste Electrical &
Electronic Equipment (WEEE) 2002/96/EC, aiming to
reduce and eventually remove Pb from electronic devices.
Pb-Sn die attach materials are exempted from these
directives until 2014,but no efforts are being spared to fmd
a viable Pb-free replacement for the die attach material at an
earlier date as acquisition of these technologies will give a
competitive advantage to the owner. Current candidate
solders like Bi-alloy,Zn-alloy and Au-Sn alloys have many
limitations like poor processability, poor corrosion
resistance,and high costs [1-3].
Low temperature joining techniques (LTJT) have been
proposed as a possible alternative of lead-free die attach
materials and high temperature application mentioned above.
LTJT is a technique pioneered by Schwarzbauer and others
to produce die attach joints from micrometer-scale Ag pastes
for power electronics packaging [4]. This technique has
recently been comprehensively reviewed here [5]. Besides
having better thermal and electrical conductivities than the
commonly used Sn-Pb or Pb-free joints, sintered Ag joints
exhibits a melting temperature similar to that of bulk Ag
(961°C) once the joint is formed. This higher melting
temperature property is required by the silicon carbide
technologies which operate at very high temperatures, high
power and higher voltages than the current silicon
technologies [6].
In this report,we are looking at an approach similar to that
of dispensing Ag epoxies for die attach before being oven­
cured. This approach represents a possible "drop-in"
solution for adopting Ag paste as a Pb-free die attach
employing the well-established dispensing approach[7
, 8l.
Instead of Ag nanoparticles,the Ag pastes in this evaluation
contained micrometer-scale silver. Besides the micrometer
Ag particles, the Ag paste contains various solvent and
carrier materials to control the viscosities, affect the
tribology,and to remove the surfactants. Examples of these
solvents are cyclohexanol [9,lO],butanol [7],terpineol [11],
or ethylene glycol ether [11],or a mixture of cyclohexanol­
methanol [12].
Surfactants are added to prevent the agglomeration of the
Ag particles during processing and storage. To the author's
knowledge, there are no specific surfactants for the
micrometer-scale Ag pastes, but literature is abundant with
surfactant-dispersants used for Ag paste made from Ag
nanoparticles [5]. Most of these surfactant-dispersants
require some minimum amount of oxygen present to be
oxidized. Since these dispensable micrometer-scale Ag
pastes do not require any pressure during oven sintering,
silver organic compounds like Ag stearate or Ag oxalate are
expected to be used in the current Ag pastes to form the
silver bridges linking the micrometer-scale silver particles
during sintering [13].
2.0 Experimental Procedure
Four Ag-bearing pastes, herein named as A, B.l, B.2, and
C were sintered according to temperature profiles
r:commended by the suppliers. Several tests such as die­
shear tests, x-ray transmission imaging, and focused ion
beam - scanning electron microscopy (FIB-SEM) cross­
section analyses were carried out to determine the quality of
the sintering-bonding process. The die shear tests were
carried out at rate of 300f.lm/sec and other criteria as per
MIL-STD 883E.
After die-bonding with live devices, the units were
wirebonded, molded and trim-formed. The units were then
sent for electrical and reliability testing. The reliability test
was a temperature cycling test (-65°C to 150°C) with interim
results evaluated at every 500 cycles. Separate lots of this
surface mount power package with current die attach
materials were also evaluated as control experiments.
3.0 Results and Discussions
3.1 Die Shear Strength and Failure Interfaces
The die shear strength for the sintered Ag joints was
between 5 and 16 kgf, as shown in Table l. According to
MIL-STD 883E (3.2.2c), failure criteria of the current die
size (i.e. 2.8 x l.8 mm) is "5.0kgf and evidence of less than
10% of adhesion of the die attach medium" for "eutectic,
solder and other attach". Sintered Ag is a new form of die
35th International Electronic Manufacturing Technology Conference,2012

2. attach materials and its die shear strength is likely to fall
under this category of die attach materials criteria.
The die shear strength for sintered Ag joints made from Ag
paste A,B.1 and B.2 were comparable to those provided in
the Technical Data Sheets from the suppliers. (Please see
Table I). These die shear strengths also exceeded those
specified in MIL-STD 883E. All sintered Ag joints also
showed cohesive failure with more than 95% residual
coverage.
Table I: Bonding quality of the sintered Agjoints from Ag
A,B.1,B.2 ,and C (see text).
Ag pastes Die Shear Interface /
A
B.1: 9.1 ± 1.9 (18 MPa)
B
C
5.2 ± 1.0 (10 MPa)
In the case of Ag joint "C", as shown in Table I, the
measured die shear strength was 5.2 ± 1.0kgf, although the
failure interface was cohesive with more than 95% residual
on the leadframe. According to MIL-STD883E, the die
shear strength of Ag joint C was less than the minimum
stipulated die shear strength, but the areal coverage was
significantly more than that stipulated in the standard. Such
properties did not fulfill the criteria for failure,however,the
test units were submitted for subsequent electrical and
reliability testing.
Comparing these sintered Agjoints made from micrometer­
scale Ag (A, B.1, B.2 and C) with joints made from Ag
nanoparticles under similar processing conditions (Figure 1),
the die shear strengths of the former devices (made from
micrometer-scale Ag pastes) were higher than those found in
the literature (Ag nanoparticle pastes). It should be noted
here that the current micrometer-scale Ag pastes did not
require any pressure during sintering-bonding which was
highly preferable in the production. These results suggested
good sintering-bonding properties of these dispensable
micrometer-scale Ag pastes on the Ag-plated leadframes.
40 • 1 MPa, Cu (Holm, 2010)
• 1 MPa, Cu (Ide, 2005)
35 A ... 5 MPa. Cu (Ide, 2005)
<> o MPa, Ag (Wang, 2007)
30 0 1 MPa, Ag (Holm, 2010)
ro • t::. 5 MPa, Ag (Holm, 2010)
� 25
;5 20C>
0c:
2: 15 6.
U5
L..
10ro
DQ)
6..s:::.
A(f) 5
•
� •0
0 20 40 60 80 100
Nanoparticle Size (nm)
Figure 1: Shear strength of Ag nanoparticle joints for
bonding on Ag, or copper (Cu), substrates made from Ag
nano-particle sizes ranging from 8 to 100nm at different
bonding pressures [5]. [Reprinted with permission]
3.2 Transmission X-Ray Analysis
Based on the x-ray micrographs of the sintered Agjoints in
Figure 2, all void areas were less than 5%. These results
also suggested that there is minimal solvent outgassing
during the sintering step.
a) Ag paste A
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3. b.l) Ag paste B.l
b.2) Ag paste B.2
C) Ag paste C
Figure 2: Bonding quality of the sintered Ag joints from Ag
paste A, B.l, B.2 and C as shown by transmission x-ray
micrography
3.3 FIB-SEM Cross-Sections of Sintered Ag Joints
It is important to carry out this cross-section analysis to
detennine the sintering quality at the joint interfaces,i.e. Ag
paste-Ag-plated leadframe and the Ag paste-TiNiAg coated
silicon die. Earlier attempts to use mechanical cross­
sectioning with emery paper grinding were not successful
because the soft Ag smeared and prevented imaging the
sintered microstructure.
3.3.1 Ag paste A
Images of the FIB cross-sectioned Ag joint A are shown in
following figures. Figure 3.a shows good sinteringlbonding
at the centre regions of both interfaces: the Ag paste-Ag­
plated lead frame, and the Ag paste-TiNiAg deposited
silicon backmetal. In figure 3b,separation was observed at
the edge of the silicon dies which was likely to be caused by
the metal shrinkage when cooled to room temperature.
Voids, as large as the thickness of the Ag joint, were also
visible in Figure 3c. These voids were likely to be caused by
the unsuitable rheological properties of the Ag paste which
resulted in improper dispensing and dispersal.
(3a)
(3b)
���--""�-=--I�...�a�............
Cu L/F
���. _·--'.l,"-,'.:-'...::J"-i
(3c)
Figure 3: FIB-SEM cross-section Ullage of a sintered Ag
joint made from Ag paste A.
3.3.2 Ag paste B.l and B.2
Figure 4 shows the results of Ag pastes B.1 and B.2, that
fonned good sintering-bonding interfaces at the Ag paste­
Ag-plated lead frame, and the Ag paste-TiNiAg coated
silicon.
35th International Electronic Manufacturing Technology Conference,2012

4. a) Sintered Agjoint from Ag paste B.l.
b) Sintered Agjoint from Ag paste B.2.
Figure 4: FIB-SEM images of the surface mount power
package with sintered Ag joints from Ag paste B.l (a) and
B.2 (b)
Further FIB-SEM analysis, as shown in figures 5 and 6,
showed that the sintered Ag joint B.l has a higher
percentage of porosity and more unevenly distributed void
space than the sintered Ag joint made from Ag paste B.2.
This difference was likely caused by an unsuitable reflow
profile which was not sufficient to densify the sintered Ag
joint material. Differential scanning calorimetry (DSC)
studies confirmed that this postulation was the reason for the
higher and more uneven porosity in the sintered Agjoint A.l
as compared to sintered Ag joint A.2 (DSC results were not
shown in this paper). In general, a well sintered joint has
significantly higher density and lower porosity than those
joints that were poorly sintered.
Fig 5: Cross-sectional analysis of the FIB-prepared units
showing the percentage of porosity for Ag paste B.l.
Fig 6: Cross-sectional analysis of the FIB-prepared units
showing the percentage of porosity for Ag paste B.2.
3.3.3 Supplier C
As shown in figures 7a-c, the as-built surface mount
package showed good interfacial bonding at both interfaces:
the Ag-plated leadframe,and the TiNiAg coated silicon dies.
No visible voids or delamination were visible in this cross­
section analysis. Note the different microscale texture in the
bond line relative to that shown in figures 3 and 4.
7a
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5. 7b
Cu L/F
7c
Fig 7: FIB-SEM cross-section image of sintered Agjoint
made from Ag paste C
3.4 Electrical Testing
The parameter to quantify the electrical performance of this
Ag sintered silicon dies was RDSon which measured the
electrical resistance through the die attach materials. To
establish a baseline of RDSon measurements,all units tested
in this paper referred to pre-retlow components, meaning
they did not undergo any thermal profiles. Additional testing
would be carried out on units post retlow and discussed in a
follow-on paper. Preliminary data did indicate significant
differences between pre-retlowed vs post-retlow units for
RDSon performance. Data supplied from at least one of the
vendors on post-retlow testing, showed an improved results
for Ag sintered materials over that of Ag epoxy based die
attach.
For the purpose of this paper, only one test parameter was
discussed here; the testing conditions were Vgs (gate-source
voltage) of 10 V and ID (drain current) current of 3 A.
Separate controls lots were built for each batch of Ag pastes.
T-test analyses including Tukey Kramer criteria were carried
out to confirm the statistical significance of this result.
Intuitively, devices with a sintered Ag joint were expected
to have a lower RDSon value than those devices with Ag­
epoxy based die attach because the sintered Ag was �100%
Ag (with certain percentages of pores/voids filled with air).
The electrical test results, shown in figure 8, showed
unexpectedly the opposite result. These results could be
attributed to the quality of the sintering-bonding at the
interface and the percentage of porosity in the sintered Ag.
0.053
0.052
0.051
� 0.05
i�0.049 r i
I +-
0.048 T"" I
I
0.047 I
,-
0.046
Ag paste A Control
Die �ach M3Ierials
8a: RDSon of sintered Ag joint A was comparable to control
lot.
0.05 ·
•
•
•
·
�
....
•
0.049 . •
c :0
:'" �
0 .L
� ..... ·
·
•
0.048 •
•
·
•
0.047
Ag paste 8.1 Ag paste 8.2 Control
Die ./ttach M3Ierials
8b: RDSon of sintered Ag joint B.l was comparable to
control lot. RDSon of sintered Ag joint B.2 was higher than
control lot.
0.052 · :
·
0.0515 ..L.. 1-
· ·
·
0.051c · ·
0 -'-
'" �
� 0.0505 : :
0.05 :
0.0495
0.049
Ag paste C Control
Die ./ttach M3Ierials
8c: RDSon of sintered Agjoint C was lower than control lot
Figure 8: RDSon measurements of three lots made with Ag
pastes A,B.l,B.2 and C.
As shown in figure 3, the sintered Ag joint A suffered from
delamination cracking at the edge of the silicon dies which
increased the resistance across the die attach regions.
35
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6. Figures 4-6 showed the higher than expected porosity in the
sintered Ag joint B.l and B.2 which hindered the flow of
current through these die attach regions. Only sintered Ag
joint C showed a significant lower resistance (RDSon) than
the control lot.
These electrical results showed that die shear strength
alone, as shown in section 3.1, did not provide a good
prediction of the electrical properties. A higher die shear
strength did not necessarily translate to higher electrical
conductivity or lower RDSon. Instead, the porosity of the
sintered Ag and bonding quality at the interfacial regions
played a crucial role affecting the electrical properties of the
sintered Agjoint.
3.5 Reliability Testing (Temperature Cycling)
Temperature cycling tests were carried out on the surface
mount package units with sintered Ag joints together with
control devices. All sintered Ag joints A, B.1, B.2 and C
exceeded the minimum requirements of 1500 cycles of -65°C
to 150°C.
4.0 Conclusions
This study reported the mechanical (i.e. die shear and
thermal cycling),and electrical,properties of silicon devices
bonded in surface-mount power packages with four types of
Ag-based pastes. The die shear strength of sintered Ag
joints made from Ag pastes A, B.l and B.2 exceeded the
minimum requirement stipulated in MIL-STD883E while
sintered Ag joint C results were marginal. All failure
interfaces were cohesive with void densities of less than 5%,
based on x-ray micrographs,before die-shearing. In tenns of
electrical testing,the RDSon of the Ag joints A,B.l and B.2
did not show better perfonnance than control units because
the interfacial regions of the Ag joints dominated the overall
electrical properties of the Agjoint. Cross-sectional analysis
of as-built Ag joint showed voids and porosity, and
separations between Ag die attach and the silicon dies for Ag
pastes B.l, B.2 and A, respectively. Only Ag joints C
showed better RDSon perfonnance than joints made from
standard die attach materials (control devices). Cross­
sectional analysis of the sintered Ag joints C showed good
interfacial bonding and an absence of voids. The reliability
of all tested Ag joints,in terms of thermal cycling resistance,
exceeded the TC1500X required to qualify this package.
It should be noted here that the mechanical and electrical
properties of these Ag joints reflect one particular set of
processing condition used in this evaluation. Other
unpublished data showed that these properties could change
substantially with different processing conditions. There
were also additional reliability-related benefits of using Ag
sintered joints which would be discussed in a follow-on
paper.
Acknowledgments
The author would like to thank his colleagues ET Tan,
Harmit Singh, Sharifah Syed Mokhtar, Khairil Fitry
Khairuddin,Redentor Canoza,John Parsey and Cheah Fook
Nyen for assistance rendered during this evaluation. The
author also would like to express his gratitude to the
materials suppliers and his managers in On Semiconductor,
Shutesh Krishnan and CH Chew for supporting his work
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35th International Electronic Manufacturing Technology Conference,2012