The Impact of P Content in Pd Deposit for Solder Joint ...hkpca.org/uploadfileMgnt/02_20181810356.pdf · Pd and Pd thickness was 0.05um for Pd-P with P from 5 to 6%. This IMC study

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