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Microstructure of Sn–1Ag–0.5Cu solder alloy bearing Fe under salt spray test
Microelectronics Reliability 54 (2014) 2044–2047
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Microstructure of Sn–1Ag–0.5Cu solder alloy bearing Fe under salt spray test N.I.M. Nordin a, S.M. Said b,⇑, R. Ramli a, M.F.M. Sabri a, N.M. Sharif c, N.A.F.N.M. Arifin d, N.N.S. Ibrahim d a
Department of Mechanical Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia Department of Electrical Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia c School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Penang 14300, Malaysia d Product Quality & Reliability Engineering (PQRE) Laboratory, Malaysian Institute of Microelectronic Systems (MIMOS) Berhad, Technology Park Malaysia, Kuala Lumpur 57000, Malaysia b
a r t i c l e
i n f o
Article history: Received 28 June 2014 Accepted 8 July 2014 Available online 15 August 2014 Keywords: SAC105 Fourth element Iron NaCl IMC Lead-free solder
a b s t r a c t Understanding the behavior of lead-free solder alloys within a high humidity environment is a serious topic in the deployment of products in various electronics applications. The work reported herein investigates this specific impact on Sn–1.0Ag–0.5Cu–0.5Fe solder alloy. Specimens were treated with 5% NaCl salt spray. All specimens showed strong resistance to corrosion. Microstructural deformations after the test were analyzed using Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/ EDX). Concerns were at the localized corroded area, as this would cause significant degradation at the solder joints. The mechanisms leading to these disadvantageous results as well as the microstructural evolution and correlation with the intrinsic properties of the solder alloy are discussed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Environmental concerns, legislation and customer preferences are driving the microelectronics industry towards the implementation of lead-free solders in packaging. This has forced the study and development of lead-free solder materials. A number of ternary alloys have been examined as candidates for replacement of the traditional leaded solder alloy. Good mechanical performance, high wettability, high thermal and electrical conductivities, low thermal expansion coefficient (CTE), ductile, creep resistant, thermal fatigue resistant, manufacturability and cost saving are desirable characteristics required for these replacement solder alloys. It has been several years since the commencement of the Restriction of Hazardous Substances Directive (RoHS) implementation. The majority of players in the electronics industry have settled on a few Sn-rich solders candidates. The Sn–Ag–Cu alloy series are the most broadly acknowledged in the industry [1–5]. Due to the high cost of the Sn– Ag–Cu alloy, owing to the inclusion of silver, many researchers are working towards low silver content alloys. This corresponds to an increased interest in the development of Sn–Ag–Cu alloy containing a fourth alloying element. The use conditions of the Sn–
⇑ Corresponding author. Tel.: +60 (03) 7967 5339. E-mail address: [email protected] (S.M. Said). http://dx.doi.org/10.1016/j.microrel.2014.07.068 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved.
1.0Ag–0.5Cu (SAC105) bearing Fe have been shown to be consistent with the conditions for conventional SAC105 solder alloys [6]. This is due to the findings that small additions of Fe may increase the elastic compliance and plastic energy dissipation ability of the solder. This specific improvement is believed to be a significant factor in better drop impact performance of the solder alloy [6,7]. Electronic products and assemblies are deployed for a variety of applications and subject to diverse extreme environment conditions. Thus far, there are still many discoveries to be made concerning the corrosion resistant behavior of low silver content lead-free alloys. Generally in most applications, the solder alloy is directly exposed to air moisture, industrial contaminant and oceanic environments (sodium chloride ions). It was also testified that during packaging, solders were exposed directly to air moisture, underfill materials and other industrial pollutants which are all corrosion agents. In order to demonstrate high reliability, solder materials must be studied for its resistance and behavior towards such corrosion agents [8,9]. Various mechanical properties will significantly be affected after prolonged exposure to this corrosive environment. Corrosivity is dependent on the presence of chloride ions which cause pit nucleation, the presence of humidity which cause galvanic corrosion and the drying phase which trap chloride ions beneath the corrosion by-products. Sodium chloride (NaCl) is identical to the properties of seawater and is commonly used to
2045
N.I.M. Nordin et al. / Microelectronics Reliability 54 (2014) 2044–2047
simulate its corrosive effects in electronics devices. Corrosion rate is proportional to the metal etched per year. The unit for the measurement is millimeter per year (mmy) or inch per year (iny). One of the methods in measuring the corrosion rate of a metal is to expose the sample to the test medium and measure the loss of weight of the material as a function of time. There are many types of corrosion processes in materials. Uniform corrosion, pitting, galvanic and intergranular corrosion are all likely to happen to binary, ternary, quaternary and other compositions of solder alloys. In the case of galvanic mechanism, this is a possible occurrence because each metal has its own electrical potential, as alloys are composed of a few metals. When the material is electrically connected and placed in an electrolyte, the metal with the higher negative potential becomes the anode and the more positive becomes the cathode. The metal that acts as the anode will corrode quickly. The flow of electric current continues until the potentials are equal between both electrodes [10,11]. A previous study has demonstrated that lead-free solders are more susceptible to corrosion than leaded solders [9]. This is despite the main element in the solder alloys, tin (Sn), is known for the passivity film that forms on its surface, and the added elements Ag and Cu are acknowledged for their stable structures [9,12,13]. These elements react with each other and form intermetallics, of Ag3Sn and Cu6Sn5. These compounds are well distributed in the matrix and also at the interface of the solder/pad joint. Generally IMCs are chemically stable, and presumably insoluble in etchants, making them stable against corrosion. However, these IMC precipitates react with the base material Sn to form galvanic mechanism. It was reported that dissolution of Sn from solder matrix was actually accelerated by the presence of Ag. The formation of Ag3Sn compound the reaction as its electrode possesses potential close to Ag’s potential value [9,14,15]. Shifting to leadfree solders are likely to create solder joint reliability issues, as the greater susceptibility of lead-free solders toward corrosion is attributed to galvanic corrosion induced by the presence of Ag (see Table 1). The use of the solder alloy is essential for the interconnection and packaging in the electronics products assembly. Extensive researches were and are still being actively conducted globally to find formulations that will work optimally in all conditions. In this work, the corrosion performance of SAC105 with minor addition of Fe was investigated. The corrosion behavior of the materials exposed in 5 wt% NaCl solution was examined. Previously, Shnawah et al. have demonstrated the improvement in the mechanical properties of these formulations compared to SAC105 and SAC305 formulation, where the addition of Fe to SAC105 was found to enhance the elastic modulus of this bulk solder alloy.
2. Experimental procedure
(a)
(c)
(b)
(d)
Element
CK OK Sn L Cu K Ag L Cl K
0hour (atm%)
96hour (atm%)
31.79 14.59 39.52 14.1
25.72 35.43 34.7 3.48 0.67
-
Fig. 1. Solder ball surface and magnified view for SAC105 (a) and (b) 0 h and (c) and (d) 96 h.
(a)
(c)
(b)
(d)
Element
Solder ball specimens of Sn–1Ag–0.5Cu (SAC105) and Sn–1Ag– 0.5Cu–0.5Fe (SAC105–0.5Fe) were used in this study. The alloys were prepared and fabricated into spheres with an average diameter of 1 mm using a laboratory-based ball fabricating apparatus. After cleaning and screening, the solder spheres were ready for the ball attachment on pads to form solder joints. The solder balls were board-assembled on 1.5 mm thick, high glass transition temTable 1 Electrode potential of elements [10].
CK OK Sn L Cu K Ag L Cl K
EDX 0hour (atm%)
96hour (atm%)
46.41 14.59 33.28 5.72 -
27.35 32.87 26.6 9.35 3.83
Fig. 2. Solder ball surface and magnified view for SAC105–0.5Fe (a) and (b) 0 h and (c) and (d) 96 h.
Metal
Electrode potential, E0 (V)
Fe Sn Cu
0.44 0.1375 +0.34
Anodic
Ag
+0.7996
Cathodic
perature (Tg), FR4 printed circuit boards with organic surface finished (OSP) copper pads. The peak in the temperature profile of soldering was around 260 °C and the reaction time above liquidus
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N.I.M. Nordin et al. / Microelectronics Reliability 54 (2014) 2044–2047
Fig. 3. Cross section optical micrograph of (a) SAC105 and (b) SAC105–0.5Fe.
Fig. 4. Cross X-ray diffraction of (a) SAC105–0.5Fe.
was about 40 s. For the pretreatment, the salt spray test was compliant to JESD22-A107B JEDEC standard [16]. The test specimens were placed in an enclosed chamber to provide exposure towards 5 wt% of sodium chloride (NaCl) aqueous solution at 35 °C for 96 h. The salt deposits on the surface of the specimens were removed by a gentle rinse of deionized water and left to dry. The as-soldered specimens were mounted in cold-setting epoxies for 24 h. Samples were cross-sectioned and polished to reveal the solder/pad interface. The interfacial microstructures and compositional information were examined by a Scanning Electron Microscope (SEM) equipped with an Energy-Dispersive X-ray (EDX) system. 3. Result and discussion After 96 h of treatment, the surface of the solder spheres demonstrated a degree of surface roughness. Numerous corrosion pits and penetrated areas were observed (see Figs. 1 and 2). Different incidences and patterns were observed on the surfaces of the base
(SAC105) and element added alloy. SAC105 possessed thicker tinoxide layer compared to the Fe added SAC105. This information can be seen by referring to the EDX analysis results (Figs. 1 and 2). The Cl penetration through the material surface was higher in Fe added SAC105. As the ion concentrations detected were higher than those detected in SAC105. Through visual inspection, it is evident that superficial quality degradation of the element added alloys was more significant than the base alloy. Lead-free solders are known for its susceptibility towards corrosion. This is due to the inherent potential difference between all metals within the alloy compositions. This fact therefore explained the impact of corrosion as more significant on the Fe added SAC105. Numerous small holes due to material spalling were seen on the surface of Fe added alloys (see Fig. 2(c)). These incidents were clearly observed from the top view of the samples. EDX analysis detected Sn, O, C, Ag and Cl within the corrosion spot scanned. The atomic concentrations before and after the test measured by EDX were displayed in Fig. 2. The presence of carbon may be due to the accumulation of contaminants. After exposure in NaCl for 96 h and exposure to air for 24 h, Sn content decreased by 20%, which can be due to the material falling off; Ag content were detected about 9%, indicating that the corrosion attacks tin while Ag remained within the solder bulk. As reported, the Ag3Sn remained in the matrix where it was initially located. This leaves Sn as the only element which is affected and corroded away from the solder bulk region [12]. Thus literally, the earlier-mentioned scenario could be similar to the IMCs formed for the investigated Fe added alloy, where the hard IMC and more noble elements stayed in locations where they were initially formed. This could suggest the degraded surface performance of the element added alloy studied in this paper. Since addition of Fe was proven to modify the IMC formation and the main metal of the base alloy (see Fig. 3). As referred to Table 1, Ag and Cu are inert than Sn and Fe. The similar scenario applied to their IMCs.
Fig. 5. Cross-section from an adjacent SAC105–0.5Fe treated at 96 h, arrows indicate corroded area.
N.I.M. Nordin et al. / Microelectronics Reliability 54 (2014) 2044–2047
The minor addition of Fe, though maintaining the formation of Ag3Sn and Cu6Sn5, will introduce FeSn2 and form larger b-Sn (see Fig. 3(b)). Previously, it was proven that the electrode potential of Ag3Sn was similar to Ag. This produced reaction that accelerates the dissolution of Sn into the corrosive media, from the solder matrix [12]. The dissolution reactions were further enhanced when Fe addition into SAC105 formed large b-Sn. A condition that enlarges the area of reactions between Ag and its IMC with tin. In addition, Cu6Sn5 which were identified to form a fine interbranch spacing were suspected to provoke localized strain at the boundaries with the Sn-rich matrix. This feature could be related to the susceptibility of this alloy to corrosion [14] (see Fig. 2(c) and (d)). Conforming to the results from EDX, XRD detected the phases discussed earlier (Fig. 4). There were many FeSn2 phase detected within the scanned area together with other IMCs common to SAC105 solder alloy. Occurrence and intensity of Sn detected were also indicative that the elements were as mentioned, large in size. It is crucial to study the corrosion impact of solder alloys. Localized corrosion which penetrate the material surface are likely to induce crack propagation. The microstructure of most lead-free solder materials consist of grains boundaries, which are significantly different from leaded solder that is constructed of lamellar microstructure. The initiated crack will potentially propagate throughout the bulk solder or in a more detrimental case, could enter the solder joint area and cause joint reliability issues. The localized corroded region is in close proximity to the joint at the pad (see Fig. 5). In this specific condition, if the material experiences continuous stress such as thermal or vibration excitations, this may lead to joint discontinuity. 4. Conclusions Even though Sn is protected by a stable passivation layer of SnO and exhibits relatively good resistance to corrosion, there is still potential for localized corrosion to occur. A mechanism to prove the corrosion path follows a certain Sn grain orientation and the opportunities to investigate the corrosion within SAC105 with element addition are still widely open. Some important causes to be considered are galvanic coupling and intergranular effects. The structure of SAC solder alloy is Sn with the precipitation and inclusion of Cu and Ag that form or suppress IMCs. Literally, certain IMC could accelerate dissolution of Sn into the corrosive solution due to galvanic coupling or create instability between grains through intergranular state. Pretreatment by 5% NaCl salt spray is an aggressive humidity acceleration test condition. Even though the study had a short test duration, it managed to highlight the implications of the potential risk factors caused by a corrosive atmosphere. Further investigations on the end-use conditions and environment is important for a long-term reliability of interconnect using this materials. Specifically: (1) Superficial quality degradations were seen on element added alloy compared to the base solder alloy.
2047
(2) Principally, the high humidity condition test the overall solder alloy corrosion performance, as different alloys behave differently due to the reaction between the main metal, its alloying components and also the formed IMCs. (3) SAC105–0.5Fe displayed a rough surface by top view inspection on solder bump. Numerous small areas of material spalling were observed. Fe addition maintains Ag3Sn whilst forming larger b-Sn. This combination of elements encourages material dissolution.
Acknowledgements The authors acknowledge the financial supports provided by UMRG Fund (RP003A-13AET and RP003D-13AET) and High Impact Research (Grant No. UM.C/25/1/HIR/MOHE/ENG/29). Authors would also acknowledge PQRE, MIMOS for providing the testing facility. References [1] Abtew M, Selvaduray G. Lead-free solders in microelectronics. J Mater Sci Eng R 2000;27:95–141. [2] Frear DR, Vianco PT. Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys. J Metall Mater Trans 1994;25(7):1509–23. [3] Glazer J. Microstructure and mechanical properties of Pb-free solder alloys for low-cost electronic assembly: a review. J Electron Mater 1994;23:693–700. [4] Kang SK, Sarkhel AK. Lead (Pb)-free solders for electronic packaging. J Electron Mater 1994;23:701–7. [5] Deng X, Piotrowski G, Williams JJ, Chawla N. Influence of initial morphology and thickness of Cu6Sn5 and Cu3Sn intermetallics on growth and evolution during thermal aging of Sn–Ag solder/Cu joints. J Electron Mater 2003;32(12):1403–13. [6] Shnawah DA, Said SM, Sabri MFM, Badruddin IA, Che FX. Novel Fe-containing Sn–1Ag–0.5Cu lead-free solder alloy with further enhanced elastic compliance and plastic energy dissipation ability for mobile products. J Microelectron Reliab 2012;52:2701–8. [7] Zhang L, He CW, Guo YH, Han JG, Zhang YW, Wang XY. Development of SnAgbased lead free solders in electronics packaging. J Microelectron Reliab 2012;52:559–78. [8] Dezhi L, Paul PC, Changqing L. Corrosion characterization of tin–lead and lead free solders in 3.5% NaCl solution. J Corros Sci 2008;50:995–1004. [9] Liu B, Lee TK, Liu KC. Impact of 5% NaCl salt spray pretreatment on the longterm reliability of wafer-level packages with Sn–Pb and Sn–Ag–Cu solder interconnects. J Electron Mater 2011;40:2111–8. [10] CRC handbook. Chemistry and physics; 1990. [11] Harsyanyi G. Irregular effect of chloride impurities on migration failure reliability: contradictions or understandable? J Microelectron Reliab 1999;39:1407–11. [12] Fubin S, Ricky L. Corrosion of SnAg–Cu lead-free solders and the corresponding effects on board level solder joint reliability. In: Proceeding of the 56th electronic components and technology conference; 2006. p. 891–8. [13] Wu BY, Chan YC, Alam MO. Electrochemical corrosion study of Pb-free solders. J Mater Res 2006:21–62. [14] Osorio WR, Peixoto LC, Garcia LR, Garcia A, Spinelli JE. The effects of microstructure and Ag3Sn and Cu6Sn5 intermetallics on the electrochemical behavior of Sn–Ag and Sn–Cu solder alloys. Int. J Electrochem Sci 2012:6436–52. [15] Hayes SM, Chawla N, Frear DR. Interfacial fracture toughness of Pb-free solders. J Microelectron Reliab 2009;49:269–87. [16] JESD22-A107B JEDEC. Salt atmosphere; 2004.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Microstructure of Sn–1Ag–0.5Cu solder alloy bearing Fe under salt spray test N.I.M. Nordin a, S.M. Said b,⇑, R. Ramli a, M.F.M. Sabri a, N.M. Sharif c, N.A.F.N.M. Arifin d, N.N.S. Ibrahim d a
Department of Mechanical Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia Department of Electrical Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia c School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Penang 14300, Malaysia d Product Quality & Reliability Engineering (PQRE) Laboratory, Malaysian Institute of Microelectronic Systems (MIMOS) Berhad, Technology Park Malaysia, Kuala Lumpur 57000, Malaysia b
a r t i c l e
i n f o
Article history: Received 28 June 2014 Accepted 8 July 2014 Available online 15 August 2014 Keywords: SAC105 Fourth element Iron NaCl IMC Lead-free solder
a b s t r a c t Understanding the behavior of lead-free solder alloys within a high humidity environment is a serious topic in the deployment of products in various electronics applications. The work reported herein investigates this specific impact on Sn–1.0Ag–0.5Cu–0.5Fe solder alloy. Specimens were treated with 5% NaCl salt spray. All specimens showed strong resistance to corrosion. Microstructural deformations after the test were analyzed using Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/ EDX). Concerns were at the localized corroded area, as this would cause significant degradation at the solder joints. The mechanisms leading to these disadvantageous results as well as the microstructural evolution and correlation with the intrinsic properties of the solder alloy are discussed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Environmental concerns, legislation and customer preferences are driving the microelectronics industry towards the implementation of lead-free solders in packaging. This has forced the study and development of lead-free solder materials. A number of ternary alloys have been examined as candidates for replacement of the traditional leaded solder alloy. Good mechanical performance, high wettability, high thermal and electrical conductivities, low thermal expansion coefficient (CTE), ductile, creep resistant, thermal fatigue resistant, manufacturability and cost saving are desirable characteristics required for these replacement solder alloys. It has been several years since the commencement of the Restriction of Hazardous Substances Directive (RoHS) implementation. The majority of players in the electronics industry have settled on a few Sn-rich solders candidates. The Sn–Ag–Cu alloy series are the most broadly acknowledged in the industry [1–5]. Due to the high cost of the Sn– Ag–Cu alloy, owing to the inclusion of silver, many researchers are working towards low silver content alloys. This corresponds to an increased interest in the development of Sn–Ag–Cu alloy containing a fourth alloying element. The use conditions of the Sn–
⇑ Corresponding author. Tel.: +60 (03) 7967 5339. E-mail address: [email protected] (S.M. Said). http://dx.doi.org/10.1016/j.microrel.2014.07.068 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved.
1.0Ag–0.5Cu (SAC105) bearing Fe have been shown to be consistent with the conditions for conventional SAC105 solder alloys [6]. This is due to the findings that small additions of Fe may increase the elastic compliance and plastic energy dissipation ability of the solder. This specific improvement is believed to be a significant factor in better drop impact performance of the solder alloy [6,7]. Electronic products and assemblies are deployed for a variety of applications and subject to diverse extreme environment conditions. Thus far, there are still many discoveries to be made concerning the corrosion resistant behavior of low silver content lead-free alloys. Generally in most applications, the solder alloy is directly exposed to air moisture, industrial contaminant and oceanic environments (sodium chloride ions). It was also testified that during packaging, solders were exposed directly to air moisture, underfill materials and other industrial pollutants which are all corrosion agents. In order to demonstrate high reliability, solder materials must be studied for its resistance and behavior towards such corrosion agents [8,9]. Various mechanical properties will significantly be affected after prolonged exposure to this corrosive environment. Corrosivity is dependent on the presence of chloride ions which cause pit nucleation, the presence of humidity which cause galvanic corrosion and the drying phase which trap chloride ions beneath the corrosion by-products. Sodium chloride (NaCl) is identical to the properties of seawater and is commonly used to
2045
N.I.M. Nordin et al. / Microelectronics Reliability 54 (2014) 2044–2047
simulate its corrosive effects in electronics devices. Corrosion rate is proportional to the metal etched per year. The unit for the measurement is millimeter per year (mmy) or inch per year (iny). One of the methods in measuring the corrosion rate of a metal is to expose the sample to the test medium and measure the loss of weight of the material as a function of time. There are many types of corrosion processes in materials. Uniform corrosion, pitting, galvanic and intergranular corrosion are all likely to happen to binary, ternary, quaternary and other compositions of solder alloys. In the case of galvanic mechanism, this is a possible occurrence because each metal has its own electrical potential, as alloys are composed of a few metals. When the material is electrically connected and placed in an electrolyte, the metal with the higher negative potential becomes the anode and the more positive becomes the cathode. The metal that acts as the anode will corrode quickly. The flow of electric current continues until the potentials are equal between both electrodes [10,11]. A previous study has demonstrated that lead-free solders are more susceptible to corrosion than leaded solders [9]. This is despite the main element in the solder alloys, tin (Sn), is known for the passivity film that forms on its surface, and the added elements Ag and Cu are acknowledged for their stable structures [9,12,13]. These elements react with each other and form intermetallics, of Ag3Sn and Cu6Sn5. These compounds are well distributed in the matrix and also at the interface of the solder/pad joint. Generally IMCs are chemically stable, and presumably insoluble in etchants, making them stable against corrosion. However, these IMC precipitates react with the base material Sn to form galvanic mechanism. It was reported that dissolution of Sn from solder matrix was actually accelerated by the presence of Ag. The formation of Ag3Sn compound the reaction as its electrode possesses potential close to Ag’s potential value [9,14,15]. Shifting to leadfree solders are likely to create solder joint reliability issues, as the greater susceptibility of lead-free solders toward corrosion is attributed to galvanic corrosion induced by the presence of Ag (see Table 1). The use of the solder alloy is essential for the interconnection and packaging in the electronics products assembly. Extensive researches were and are still being actively conducted globally to find formulations that will work optimally in all conditions. In this work, the corrosion performance of SAC105 with minor addition of Fe was investigated. The corrosion behavior of the materials exposed in 5 wt% NaCl solution was examined. Previously, Shnawah et al. have demonstrated the improvement in the mechanical properties of these formulations compared to SAC105 and SAC305 formulation, where the addition of Fe to SAC105 was found to enhance the elastic modulus of this bulk solder alloy.
2. Experimental procedure
(a)
(c)
(b)
(d)
Element
CK OK Sn L Cu K Ag L Cl K
0hour (atm%)
96hour (atm%)
31.79 14.59 39.52 14.1
25.72 35.43 34.7 3.48 0.67
-
Fig. 1. Solder ball surface and magnified view for SAC105 (a) and (b) 0 h and (c) and (d) 96 h.
(a)
(c)
(b)
(d)
Element
Solder ball specimens of Sn–1Ag–0.5Cu (SAC105) and Sn–1Ag– 0.5Cu–0.5Fe (SAC105–0.5Fe) were used in this study. The alloys were prepared and fabricated into spheres with an average diameter of 1 mm using a laboratory-based ball fabricating apparatus. After cleaning and screening, the solder spheres were ready for the ball attachment on pads to form solder joints. The solder balls were board-assembled on 1.5 mm thick, high glass transition temTable 1 Electrode potential of elements [10].
CK OK Sn L Cu K Ag L Cl K
EDX 0hour (atm%)
96hour (atm%)
46.41 14.59 33.28 5.72 -
27.35 32.87 26.6 9.35 3.83
Fig. 2. Solder ball surface and magnified view for SAC105–0.5Fe (a) and (b) 0 h and (c) and (d) 96 h.
Metal
Electrode potential, E0 (V)
Fe Sn Cu
0.44 0.1375 +0.34
Anodic
Ag
+0.7996
Cathodic
perature (Tg), FR4 printed circuit boards with organic surface finished (OSP) copper pads. The peak in the temperature profile of soldering was around 260 °C and the reaction time above liquidus
2046
N.I.M. Nordin et al. / Microelectronics Reliability 54 (2014) 2044–2047
Fig. 3. Cross section optical micrograph of (a) SAC105 and (b) SAC105–0.5Fe.
Fig. 4. Cross X-ray diffraction of (a) SAC105–0.5Fe.
was about 40 s. For the pretreatment, the salt spray test was compliant to JESD22-A107B JEDEC standard [16]. The test specimens were placed in an enclosed chamber to provide exposure towards 5 wt% of sodium chloride (NaCl) aqueous solution at 35 °C for 96 h. The salt deposits on the surface of the specimens were removed by a gentle rinse of deionized water and left to dry. The as-soldered specimens were mounted in cold-setting epoxies for 24 h. Samples were cross-sectioned and polished to reveal the solder/pad interface. The interfacial microstructures and compositional information were examined by a Scanning Electron Microscope (SEM) equipped with an Energy-Dispersive X-ray (EDX) system. 3. Result and discussion After 96 h of treatment, the surface of the solder spheres demonstrated a degree of surface roughness. Numerous corrosion pits and penetrated areas were observed (see Figs. 1 and 2). Different incidences and patterns were observed on the surfaces of the base
(SAC105) and element added alloy. SAC105 possessed thicker tinoxide layer compared to the Fe added SAC105. This information can be seen by referring to the EDX analysis results (Figs. 1 and 2). The Cl penetration through the material surface was higher in Fe added SAC105. As the ion concentrations detected were higher than those detected in SAC105. Through visual inspection, it is evident that superficial quality degradation of the element added alloys was more significant than the base alloy. Lead-free solders are known for its susceptibility towards corrosion. This is due to the inherent potential difference between all metals within the alloy compositions. This fact therefore explained the impact of corrosion as more significant on the Fe added SAC105. Numerous small holes due to material spalling were seen on the surface of Fe added alloys (see Fig. 2(c)). These incidents were clearly observed from the top view of the samples. EDX analysis detected Sn, O, C, Ag and Cl within the corrosion spot scanned. The atomic concentrations before and after the test measured by EDX were displayed in Fig. 2. The presence of carbon may be due to the accumulation of contaminants. After exposure in NaCl for 96 h and exposure to air for 24 h, Sn content decreased by 20%, which can be due to the material falling off; Ag content were detected about 9%, indicating that the corrosion attacks tin while Ag remained within the solder bulk. As reported, the Ag3Sn remained in the matrix where it was initially located. This leaves Sn as the only element which is affected and corroded away from the solder bulk region [12]. Thus literally, the earlier-mentioned scenario could be similar to the IMCs formed for the investigated Fe added alloy, where the hard IMC and more noble elements stayed in locations where they were initially formed. This could suggest the degraded surface performance of the element added alloy studied in this paper. Since addition of Fe was proven to modify the IMC formation and the main metal of the base alloy (see Fig. 3). As referred to Table 1, Ag and Cu are inert than Sn and Fe. The similar scenario applied to their IMCs.
Fig. 5. Cross-section from an adjacent SAC105–0.5Fe treated at 96 h, arrows indicate corroded area.
N.I.M. Nordin et al. / Microelectronics Reliability 54 (2014) 2044–2047
The minor addition of Fe, though maintaining the formation of Ag3Sn and Cu6Sn5, will introduce FeSn2 and form larger b-Sn (see Fig. 3(b)). Previously, it was proven that the electrode potential of Ag3Sn was similar to Ag. This produced reaction that accelerates the dissolution of Sn into the corrosive media, from the solder matrix [12]. The dissolution reactions were further enhanced when Fe addition into SAC105 formed large b-Sn. A condition that enlarges the area of reactions between Ag and its IMC with tin. In addition, Cu6Sn5 which were identified to form a fine interbranch spacing were suspected to provoke localized strain at the boundaries with the Sn-rich matrix. This feature could be related to the susceptibility of this alloy to corrosion [14] (see Fig. 2(c) and (d)). Conforming to the results from EDX, XRD detected the phases discussed earlier (Fig. 4). There were many FeSn2 phase detected within the scanned area together with other IMCs common to SAC105 solder alloy. Occurrence and intensity of Sn detected were also indicative that the elements were as mentioned, large in size. It is crucial to study the corrosion impact of solder alloys. Localized corrosion which penetrate the material surface are likely to induce crack propagation. The microstructure of most lead-free solder materials consist of grains boundaries, which are significantly different from leaded solder that is constructed of lamellar microstructure. The initiated crack will potentially propagate throughout the bulk solder or in a more detrimental case, could enter the solder joint area and cause joint reliability issues. The localized corroded region is in close proximity to the joint at the pad (see Fig. 5). In this specific condition, if the material experiences continuous stress such as thermal or vibration excitations, this may lead to joint discontinuity. 4. Conclusions Even though Sn is protected by a stable passivation layer of SnO and exhibits relatively good resistance to corrosion, there is still potential for localized corrosion to occur. A mechanism to prove the corrosion path follows a certain Sn grain orientation and the opportunities to investigate the corrosion within SAC105 with element addition are still widely open. Some important causes to be considered are galvanic coupling and intergranular effects. The structure of SAC solder alloy is Sn with the precipitation and inclusion of Cu and Ag that form or suppress IMCs. Literally, certain IMC could accelerate dissolution of Sn into the corrosive solution due to galvanic coupling or create instability between grains through intergranular state. Pretreatment by 5% NaCl salt spray is an aggressive humidity acceleration test condition. Even though the study had a short test duration, it managed to highlight the implications of the potential risk factors caused by a corrosive atmosphere. Further investigations on the end-use conditions and environment is important for a long-term reliability of interconnect using this materials. Specifically: (1) Superficial quality degradations were seen on element added alloy compared to the base solder alloy.
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(2) Principally, the high humidity condition test the overall solder alloy corrosion performance, as different alloys behave differently due to the reaction between the main metal, its alloying components and also the formed IMCs. (3) SAC105–0.5Fe displayed a rough surface by top view inspection on solder bump. Numerous small areas of material spalling were observed. Fe addition maintains Ag3Sn whilst forming larger b-Sn. This combination of elements encourages material dissolution.
Acknowledgements The authors acknowledge the financial supports provided by UMRG Fund (RP003A-13AET and RP003D-13AET) and High Impact Research (Grant No. UM.C/25/1/HIR/MOHE/ENG/29). Authors would also acknowledge PQRE, MIMOS for providing the testing facility. References [1] Abtew M, Selvaduray G. Lead-free solders in microelectronics. J Mater Sci Eng R 2000;27:95–141. [2] Frear DR, Vianco PT. Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys. J Metall Mater Trans 1994;25(7):1509–23. [3] Glazer J. Microstructure and mechanical properties of Pb-free solder alloys for low-cost electronic assembly: a review. J Electron Mater 1994;23:693–700. [4] Kang SK, Sarkhel AK. Lead (Pb)-free solders for electronic packaging. J Electron Mater 1994;23:701–7. [5] Deng X, Piotrowski G, Williams JJ, Chawla N. Influence of initial morphology and thickness of Cu6Sn5 and Cu3Sn intermetallics on growth and evolution during thermal aging of Sn–Ag solder/Cu joints. J Electron Mater 2003;32(12):1403–13. [6] Shnawah DA, Said SM, Sabri MFM, Badruddin IA, Che FX. Novel Fe-containing Sn–1Ag–0.5Cu lead-free solder alloy with further enhanced elastic compliance and plastic energy dissipation ability for mobile products. J Microelectron Reliab 2012;52:2701–8. [7] Zhang L, He CW, Guo YH, Han JG, Zhang YW, Wang XY. Development of SnAgbased lead free solders in electronics packaging. J Microelectron Reliab 2012;52:559–78. [8] Dezhi L, Paul PC, Changqing L. Corrosion characterization of tin–lead and lead free solders in 3.5% NaCl solution. J Corros Sci 2008;50:995–1004. [9] Liu B, Lee TK, Liu KC. Impact of 5% NaCl salt spray pretreatment on the longterm reliability of wafer-level packages with Sn–Pb and Sn–Ag–Cu solder interconnects. J Electron Mater 2011;40:2111–8. [10] CRC handbook. Chemistry and physics; 1990. [11] Harsyanyi G. Irregular effect of chloride impurities on migration failure reliability: contradictions or understandable? J Microelectron Reliab 1999;39:1407–11. [12] Fubin S, Ricky L. Corrosion of SnAg–Cu lead-free solders and the corresponding effects on board level solder joint reliability. In: Proceeding of the 56th electronic components and technology conference; 2006. p. 891–8. [13] Wu BY, Chan YC, Alam MO. Electrochemical corrosion study of Pb-free solders. J Mater Res 2006:21–62. [14] Osorio WR, Peixoto LC, Garcia LR, Garcia A, Spinelli JE. The effects of microstructure and Ag3Sn and Cu6Sn5 intermetallics on the electrochemical behavior of Sn–Ag and Sn–Cu solder alloys. Int. J Electrochem Sci 2012:6436–52. [15] Hayes SM, Chawla N, Frear DR. Interfacial fracture toughness of Pb-free solders. J Microelectron Reliab 2009;49:269–87. [16] JESD22-A107B JEDEC. Salt atmosphere; 2004.