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 Sembi Ian 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 Ag joint

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 (L TJT) have been proposed as a possible alternative of lead-free die attach materials and high temperature application mentioned above. L TJT 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 oven­cured. 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 cyclohexanol­methanol [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 die­shear tests, x-ray transmission imaging, and focused ion beam - scanning electron microscopy (FIB-SEM) cross­section 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

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 Ag joints 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 Ag joints made from micrometer­scale 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 20 C>

0 c:

2: 15 6. U5

L.. 10 ro D Q) 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 Ag joints 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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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 cross­sectioning 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-Ag­plated 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 paste­Ag-plated lead frame, and the Ag paste-TiNiAg coated silicon.

35th International Electronic Manufacturing Technology Conference, 2012

a) Sintered Ag joint from Ag paste B.l.

b) Sintered Ag joint 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 Ag joint 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 cross­section analysis. Note the different microscale texture in the bond line relative to that shown in figures 3 and 4.

7a

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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 Ag­epoxy based die attach because the sintered Ag was � 1 00% 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-· · ·

0.051 c · ·

0 -'-'" �

� 0.0505 : : 0.05 :

0.0495

0.049 Ag paste C Control

Die ./ttach M3Ierials

8c: RDSon of sintered Ag joint 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.

35th International Electronic Manufacturing Technology Conference, 2012

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 Ag joint.

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 Ag joint. 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). Cross­sectional 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

References

[1] K. Suganuma, S. J. Kim, and K. S. Kim, "High-temperature lead-free solders: Properties and possibilities," JOM, vol. 61, pp. 64-71,2009.

[2] Y. Takaku, I. Ohnuma, Y. Yantada, Y. Vagi, I. Nakagawa, T. Atsumi, M. Shirai, and K. Ishida, "A review of Pb-free high-temperature solders for power­semiconductor devices: Bi-base composite solder and Zn-Al base solder," ASTM Spec. Tech. Pub/., vol. 1530, pp. 27-49, 2011.

[3] V. Chidambaram, J. Hattel, and J. Hald, "High-temperature lead-free solder alternatives," Microelectron. Eng., vol. 88, pp. 981-989,2011.

[4] H. Schwarzbauer and R. Kuhnert, "Novel large area JOlmng technique for improved power device performance," IEEE Trans. Ind. App/., vol. 27, pp. 93-95,1991.

[5] K. S. Siow, "Mechanical Properties of Nano-Ag as Die Attach Materials," J. Alloys Compd., vol. 514, pp. 6-14, 2012.

[6] H. S. Chin, K. Y. Cheong, and A. B. Ismail, "A review on die attach materials for SiC-based high temperature power devices," Metal. Mater. Trans. B, vol. 41, pp. 824-832, 2010.

[7] I. J. Rasiah, "Electrically conductive thermal interface," USPTO, Ed.: US7083850B2, Honeywell International Inc., 2006.

[8] R. Kajiwara, S. Motowaki, K. Ito, T. Ishii, K. Arai, T. Nakajo, and H. Kagii, "Semiconductor device and method of manufacturing the same," US2010/0195292Al, Renesas Technology Corp, 2010.

[9] G. Palm, "Method for securing electronic components to a substrate," USPTO, Ed.: US2005/0247760Al, Semikron Elektronik GmbH, 2005.

[10] H. Schwarzbauer, "Method of securing electronic components to a substrate," USPTO, Ed.: US4810672, Siemens AG, 1989.

[11] H. Schwarzbauer, "Heat conducting adhesive joint with an adhesive-filled porous heat conductor," USPTO, Ed.: US6823915B2, Siemens AG, 2004.

[12] W. Baumgartner and J. Fellinger, "Method of fastening electronic components to a substrate using a film," USPTO, Ed. USA: US4856185, Siemens AG, 1989.

[13] C. A. Lu, P. Lin, H. C. Lin, and S. F. Wang, "Effects of metallo-organic decomposition agents on thermal decomposition and electrical conductivity of low­temperature-curing silver paste," Jpn J. App/. Phys. Part

J: Regu/. Pap. Short Notes Rev. Pap., vol. 45, pp. 6987-6992,2006.

35th International Electronic Manufacturing Technology Conference, 2012