The Impact of P Content in Pd Deposit for Solder Joint...
Transcript of The Impact of P Content in Pd Deposit for Solder Joint...
The Impact of P Content in Pd Deposit for Solder JointReliability and Wire Bonding Reliability of ENEPIG Deposits
Journal of the HKPCA / 2017 / Autumn / Issue No. 65
22 Technical Paper
Don Gudeczauskas and George Milad
UIC Technical Center
Southington, CT, USA
Tsuyoshi Maeda, Shinsuke Wada, Katsuhisa Tanabe, Yukinori Oda, Shigeo Hashimoto
C. Uyemura & Corporation Co., Ltd. Central Research Laboratory
Osaka, Japan
ABSTRACT
INTRODUCTION
EXPERIMENTAL AND RESULTS
Regarding Electro-less Ni/Pd/Au (ENEPIG) deposits, we
focused on the type of Pd deposit, especially different P
contents between 0 and 6% in the Pd deposit and we
compared each characteristic. As a result, we found that Pd
deposits with each P content had a best range of Pd thickness
for solder joint reliability (SJR). On the other hand, we found that
ENEPIG deposits with Pure-Pd which didn't include P contents
had slightly better wire bonding reliability (WBR) than ENEPIG
deposits with Pd-P when the Pd deposit was thicker.
Recently, it is well-known that the electroless ENEPIG process
has excellent SJR for lead free solder, and that it has the same
WBR compared to electroless Ni/Au with thicker Au (ENAG)
process, even if Au thickness is between 0.1 to 0.2um. On the
other hand, bonding wire has been thinning. Therefore, fine
circuit pattern processes have been important.
Thus, the ENEPIG process has both good SJR and WBR and it
is commonly used for wide-ranging applications. If SJR is
emphasized, it's often the case that Pd bath of Pd-P type is
selected because SJR is excellent, even if Pd thickness is thin
(around 0.05um). If WBR is emphasized, thicker Pd deposit has
an advantage because the Pd layer functions as the barrier to
the diffusion of nickel. Therefore, thicker Pd deposits have been
required more and more. In this paper, we studied each
characteristic, SJR and WBR versus Pd deposits of various P
content.
The coupons used in this study consisted of a copper-clad
laminated substrate which was copper plated to a thickness of
20um using an acid copper electroplating process. For SJR
tests, the copper-plated substrate was coated with solder
mask and imaged to form 0.5mm diameter solder ball pads.
This substrate was plated with ENEPIG by using plating
chemicals commercially available from C. Uyemura & Co., Ltd.
The ENEPIG plating process is shown in Table 1.
Table 1. ENEPIG Plating Process
Figure 1. Porosity Test Conditions
Test samples with several levels of P content were plated
followed by Table 1. The surface and cross section images
were observed by FE-SEM (Ultra55 / Carl Zeiss) and FIB
(210DB / HHS). Crystalline structure was analyzed by XRD
(RINT 2500 / Rigaku). The coverage of Pd deposit was
evaluated by porosity test as shown in Fig.1. The maximum
current density, MAX I, which was measured by this method
was compared.
Regarding the solder ball for the evaluation of SJR, 0.6 mm
diameter balls of Sn-3.0Ag-0.5Cu (M705) was used. The reflow
profile with the top temperature of 260 deg. C was applied for
mounting the solder ball as shown in Fig.2. SJR was evaluated
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23Technical Paper
by ball pull test (Dage 4000 / Dage) as shown in Table 2. The
cross section image of the intermetallic (IMC) after mounting the
solder ball was observed by FE-SEM (Ultra55 / Carl Zeiss) after
polishing by cross section polisher (CP) (SM-09010 /JEOL). The
IMC layer was analyzed by EDS (AXS / Bruker).
WBR was evaluated using a semi-auto wire bonder (HB16 /
TPT) and pull test (Dage 4000 / Dage) as shown in Fig. 3. The
condition of heat treatment prior to WBR was done for 16
hours at 175 deg. C.
The crystal orientation mapping and average grain size was
analyzed by electron backscatter diffraction (EBSD) (Digiview IV
/ TSL). EBSD condition was shown in Table 3.
Figure 2. Reflow Profile
Table 2. Test Conditions of Ball Pull
Figure 3. Wire Bonding Conditions
Table 3. EBSD Conditions
Table 4. AES conditions
Table 5. Conditions for Nano Indentation
The element analysis for each deposit was measured by Auger
electron spectroscopy (AES) (9500F / JEOL). AES condition
was shown in Table 4.
The hardness of Pd deposit was analyzed with nano
indentation (Nono hardness tester NHTX / Elionix). Conditions
of nano indentation is shown in Table.5
The result of surface and cross section observation is shown in
Fig.4 and Fig.5. From these results, it appears that the Pd
deposit with Pure-Pd was crystalline structure and Pd deposit
with P content from 3 to 6% was amorphous in structure. For
Pd deposits with P content from 1.5 to 2.5% as lower P
content, it appeared that the Pd deposit structure was between
crystalline and amorphous. Crystalline structure was analyzed
by XRD as shown in Fig.6. From XRD results, it was found that
Pd deposit with Pure-Pd was crystalline structure, Pd deposit
with P content from 1.5 to 2.5% was structure between
crystalline and amorphous and Pd deposit with P content from
3 to 6% was amorphous structure. These results are
corresponding to surface and cross section results.
CRYSTALLINE STRUCTURE
Journal of the HKPCA / 2017 / Autumn / Issue No. 65
24 Technical Paper
Figure 4. Surface Observation by FE-SEM; P=0%, 1.5-2.5%, 3-4%, 4-5%and 5-6% in Pd Deposit, Ni/Pd/Au=6um/0.2um/0.1um, After Au Strippingthe Surface.
Figure 5. Cross section observation by FIB; P=0%, 1.5- 2.5%, 3-4%, 4-5%and 5-6% in Pd deposit, Ni/Pd/Au=6um/0.4um/0.1um.
Figure 6. XRD Pattern of Various P content Pd deposits; P=0%, 1.5- 2.5%,3-4%, 4-5% and 5-6% in Pd deposit
The coverage of Pd deposit was evaluated by MAX I in porosity
test. The result is showed in Fig.7. From MAX I results, when
Pd thickness was 0.05um, the coverage of Pd deposits with
Pure-Pd and with P contents from 1.5 to 2.5% were less than
that of Pd deposits from 3 to 6%. On the other hand, if the
condition where Pd thickness was 0.2um, there was no
difference in all conditions and MAX I was low level.
SJR with the deposits as plated were evaluated by ball pull test
as shown in Fig.8 and total summary is shown in Table 4.
Figure 7. MAX I Result by Porosity Test; P=0%, 1.5-2.5%, 3-4%, 4-5% and5-6% in Pd Deposit, Ni/Pd=6um /0.2um, Without Au Plating.
Figure 8. Result of Ball Pull Test; P=0% (0.05, 0.2um), 1.5- 2.5% (0.05, 0.2um),3-4% (0.05, 0.2um), 4-5% (0.05, 0.2um) and 5-6% (0.05, 0.2um) in PdDeposit, Au Thickness=0.1um.
Table 4. Summary of Ball Pull Tests
SOLDER JOINT RELIABILITY
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25Technical Paper
Values in Table 4 show the points for failure mode after solder
ball pull testing. Points were assigned to 3 types of failure
modes. For complete broken case in the solder ball, 5 points
were assigned. If the broken interface contained less than 25%
IMC, 2.5 points was assigned. Finally, if the broken interface
contained more than 25% IMC, zero points was assigned. For
each test condition 20 balls were pulled and for the 20 broken
surface values were assigned as above. For example; if all 20
balls were broken completely at the solder without IMC
appearing, the total point results equals 100 since 5 points
were assigned for each ball. From the above data, it was found
that SJR results were different by P content of the Pd deposit
and Pd thickness. In other words, SJR is dependent on total P
content in Pd deposit and Pd bathes of various P content have
best range of Pd thickness for SJR. When the Pd deposit was
thick, the ball pull test results were bad in all conditions. It
seems that the IMC became thicker and uneven as Pd
thickness became thicker .
The cross section of the IMC was observed in order to study
the difference of SJR by P content in Pd deposit and Pd
thickness. Pd-P deposits with P content from 1.5 to 2.5% and
from 5 to 6% were selected for IMC observation because the
difference between 0.05 and 0.20um was clear in Table 4. The
result is shown in Fig.9. It was found that IMC compounds
formed contained (Cu,Ni) Sn and Ni+Ni P . Pd deposits with
Pd thicknesses at 0.2um for Pd-P with P content from 1.5 to
2.5%, and Pd deposit with Pd thickness at 0.05um for Pd-P
with P content from 5 to 6% formed thinner and uniform IMC.
Therefore, it is considered that SJR is excellent. (Ball pull point
score was 100.)
Pd deposits with Pd thickness at 0.05um for Pd-P with P
content from 1.5 to 2.5% had worse ball pull points, even
though there was not an obvious defect in Fig.9. It may relate
with poor coverage of Pd deposit in Fig.7. Pd deposit with Pd
thickness at 0.2um for Pd-P with P content from 5 to 6%
formed very thick and non-uniform IMC. It's possible that higher
P contents in total Pd deposit may cause these IMC and poor
SJR. On the other hand, it was found that there was a big
difference for SJR between Pd deposit without P (Pure Pd) and
with P (Pd-P). SJR of Pure Pd was much less than that of Pd-P.
In order to study the difference of SJR between Pure-Pd and
Pd-P deposits, the IMC cross sections were observed and
analyzed for composition of the IMC by EDS for conditions
1
1 ,2 ,3
6 5 3
where the Pd thickness was 0.05um for Pure-Pd and for Pd
thickness of 0.05um for Pd-P with P from 5 to 6%. The result is
shown in Fig.10. From this result, obvious differences between
Pure-Pd and Pd-P deposits could not be found. Next, crystal
orientation distribution and average grain size was analyzed by
EBSD for conditions where Pd thickness was 0.05um for Pure-
Pd and Pd thickness was 0.05um for Pd-P with P from 5 to
6%. This IMC study by EBSP was reported in past days4. The
result is shown in Fig.11. From this result, it was found that
there was a difference of average grain size between Pure-Pd
and Pd-P bath. The average grain size of IMC for ENEPIG with
Pure-Pd bath was smaller than that of Pd-P bath. This may
relate with the difference of SJR.
Figure 9. IMC Observation; P=1.5- 2.5% (0.05, 0.2um), 5-6% (0.05, 0.2um) inPd deposit
Figure 10. IMC Observation and Analysis of IMC Composition by EDS [at%];P=0% (0.05um), 5-6% (0.05um) in Pd Deposit, Au Thickness =0.1um.
Journal of the HKPCA / 2017 / Autumn / Issue No. 65
26 Technical Paper
Figure 11. Crystal Orientation Distribution and Average Grain Size by EBSD;P=0% (0.05um), 5-6% (0.05um) in Pd deposit, Au thickness=0.1um.
Figure 12. Wire pull test results; P=0% (0.05, 0.2um), 1.5- 2.5% (0.05, 0.2um),3-4% (0.05, 0.2um), 4-5% (0.05, 0.2um) and 5-6% (0.05, 0.2um) in Pddeposit, Au thickness=0.1um, as plated
The strength and failure mode of wire pull test for the samples
as plated with each Pd contents in Pd deposit are shown in
Fig.12, while the results after heat treatment were shown in
Fig.135. First, for the sample as plated, Pd deposits with Pd
thickness of 0.05um for Pure-Pd and Pd-P with P from 1.5 to
2.5%, were worse than other conditions. It may relate with poor
coverage of Pd deposit as shown in Fig.7. In other conditions,
WBR was not influenced by P content in Pd-P deposit. For the
sample after heat treatment, W/B strength of condition with Pd
thickness of 0.2um for Pure-Pd was slightly higher than other
conditions. Generally, W/B strength is decreased by heat
treatment. Therefore, if this condition, it was found that the
decreasing of W/B strength can be prevented. As the result, it
was indicated that WBR of Pure-Pd without P was slightly
better than that of Pd-P.
The wide scan and depth profile by AES were analyzed as
Fig.14 and Fig.15 in order to investigate the impact of heat
treatment. Results showed the Pd peak was detected for both
Pure-Pd and Pd-P deposits after heat treatment, and the Pd
peak of Pure-Pd had a tendency to be bigger than that of Pd-P.
From the depth profile, it was found that Pd diffused into Au
layer for both Pure-Pd and Pd-P after heat treatment.
WIRE BONDING RELIABILITY
Figure 13. Wire pull test results; P=0% (0.05, 0.2um), 1.5- 2.5% (0.05, 0.2um),3-4% (0.05, 0.2um), 4-5% (0.05, 0.2um) and 5-6% (0.05, 0.2um) in Pddeposit, Au thickness=0.1um, after heat treatment
Figure 14. Surface Scan Results by AES; P=0% (0.2um), 3-4% (0.2um), AuThickness=0.1um.
Figure 15. Depth Scan Results by AES; P=0% (0.2um), 3-4% (0.2um), AuThickness=0.1um
The ratio of Pd and Au intensity integrated from top surface to
60nm was plotted based on result of depth profile as shown in
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27Technical Paper
Fig 16. From this result, Pd ratio became higher as heating time
increased, and Pd ratio of Pure-Pd deposit was higher than that
of Pd-P deposit. Therefore, it is considered that the diffusion
into Au layer of Pure-Pd deposit is easier than that of Pd-P
deposit. From this phenomenon, there is a possibility that the
diffusion of Pure-Pd deposit with crystalline structure is easier
than that of Pd-P deposit with amorphous structure as shown
Fig.4. Although the diffusion of Pure-Pd was easier than that of
Pd-P, W/B strength of Pure-Pd was slightly higher than Pd-P.
Therefore, it is considered that there are other factors for this
phenomenon, except Pd diffusion.
The hardness of each Pd deposit with various P content was
measured by nano indentation. The result is showed in Fig.17.
From this result, it was found that Pure-Pd deposit was softer
than other Pd-P deposits, and there was not obvious difference
by P content in Pd-P deposit. Regarding Pure-Pd, the sample
after heat treatment was softer than the sample as plated. In
Figure 16. The Ratio of Pd and Au Intensity from Depth Profile by AES;P=0% (0.2um), 3-4% (0.2um), After Heat Treatment (175deg.C-16hrs), AuThickness=0.1um.
contrast, the sample of Pd-P after heat treatment was harder
than that as plated. Therefore, the difference of Pd hardness
between Pure-Pd and Pd-P became wider after heat treatment.
As a result, it is guessed that W/B strength of Pure-Pd became
higher than that of Pd-P because of this difference in Pd
hardness.
SJR of ENEPIG deposit was dependent on total P content in
Pd deposit and Pd deposits of various P contents have
optimum ranges of Pd thickness for excellent SJR. If P content
is from 1.5 to 2.5%, optimal Pd thickness is 0.2um. If P
content is from 3 to 5%, optimal Pd thickness is from 0.1um to
0.2um. If P content is from 5 to 6%, optimal Pd thickness is
0.05um. Pure-Pd deposits generally showed lower SJR
compared to Pd-P deposits.
WBR of Pure-Pd deposit without P was slightly better than that
of Pd-P deposit after heat treatment. It is considered that the
factor of WBR is not only Pd diffusion into Au layer, but also the
hardness of Pd deposit. This effect of Pd hardness was obvious
only when Pd thickness is thicker. As a result, Pure-Pd with
softer deposit was better for WBR than Pd-P with harder
deposit.
1. Yukinori Oda, Masayuki Kiso, Akira Okada, Kota Kitajima,
Shigeo Hashimoto, George Milad, Don Gudeczauskas, 41st
International Symposium on Microelectronics, Providence,
RI, Nov 2008
2. Donald Gudeczauskas et al, 39th International Symposium
on Microelectronics, October 8-12, 2006, San Diego
3. V. Vuorinen, T Laurila, H. Yu, and J. K. Kivilahti. J. appl.
Phys. 99, 023530, 2006
4. Chien-Fu Tseng, Cheng-Ying Ho, Joseph Lee, Jenq-Gong
Duh, Journal of Alloys and Compounds 600, 2014, 21-28
5. Tatsushi Someya, Katsuhisa Tanabe, Shigeo Hashimoto,
ESTC, "Relation of wire bonding reliability between the
deposit composition and the type of the bonding wire",
2014.
CONCLUSION
REFERENCES
Figure 17. Vickers Hardness of Pd Deposit; P=0%, 1.5- 2.5%, 3-4%, 4-5%and 5-6% in Pd Deposit