The reliability study of selected Sn–Zn based lead-free solders on Au/Ni–P/Cu substrate
Introduction
The importance of soldering in electronic packaging is increasing due to the rapid growth of the electronics industry in recent years. Traditionally, binary Sn–Pb solders are extensively used in the electronic packaging [1]. With the advent of area array packaging concepts (flip chip and ball grid arrays), usage of solders in die attach is increasing sharply [1]. In a ball grid array (BGA) microelectronics component, the solder balls are attached to the Si, and the entire assembly is processed through a reflow oven. The solder ball melts and forms a joint between the solder ball pad of the substrate and the ball itself [1]. The performance of solder alloys has become one of the crucial roles in die attach application. Well-developed information of Sn–Pb solders including mechanical properties, physical properties, microstructural evolution at the solder interface, corrosion resistance and process variables is available in the literature [2], [3], [4], [5], [6]. However, legislation restricts and/or decreases the use of Pb due to environmental and toxicological concerns. The importance of lead-free solders is increasing nowadays, and some good reviewed articles are also available in the field of lead-free solders [1], [7].
The eutectic Sn–9Zn alloy appears to be an attractive one with a relatively low melting temperature of 198.5 °C [8]. However, the wetting angle on copper substrate for Sn–Zn solders is high when the flux is used for Sn–Pb solders [9], [10]. The use of a stronger flux in Sn–Zn alloy soldering may become a potential problem in the application of these alloys due to the removal of flux after soldering. Additionally, the usage of the flux should be decreased and/or prohibited in many advanced soldering processes. For example, the use of flux in both flip chip and BGA packaging should be minimized due to the difficulty of removal the flux after soldering [1].
In addition to flux soldering of electronic components to copper substrates, there is an alternative way with no or less flux usage in soldering [11], [12]. The pure copper substrate is first electroless plated with a Ni–P layer, and subsequently coated with a thin layer of Au. With the aid of a thin Au protective layer on the electroless Ni–P plated copper substrate, most solder alloys can be successfully soldered in air with no or mild flux. Additionally, the surface Au layer can protect the substrate from corrosion and oxidation, and the Ni layer provides an effective diffusion barrier to inhibit the detrimental growth of Cu–Sn intermetallics [1], [7], [11], [12], [13]. This Au/Ni metallization has become increasing common in microelectronic packaging [1], [7]. Many publications on Sn–Zn–In and Sn–Zn–Ag solders are available, but very few works are focused on Sn–Zn based solders on Au/Ni–P metallized copper substrate without flux usage [1], [7], [9], [10], [13]. Therefore, both the wetting behavior and interfacial reaction kinetics of Sn–Zn based solders on such Au/Ni–P metallized copper substrate without flux usage need further study in order to apply these solders in electronic packaging.
The melting behavior of solder alloys plays a crucial role in electronic packaging [1]. The melting point of the most popular Pb–Sn eutectic solder is 183 °C, and most soldering processes are performed at temperatures between 200 and 220 °C [14]. Therefore, it is preferred that liquidus of new developed lead-free solders is close to 180 °C. Since both Ag and In are important melting point depressants (MPDs) in Sn–Zn based solders, this research concentrates on Sn–Zn based alloys with various amounts of Ag and In additions. The purpose of this study is focused on melting behavior of Sn–Zn based solders, wetting characteristics, coefficients of thermal expansion (CTE), microstructure and long-term reliability of the selected Sn–Zn based solder on Au/Ni–P metallized copper substrate in order to evaluate application of the Sn–Zn based solders in die attachment.
Section snippets
Experimental procedures
High purity oxygen-free copper, 0.5 mm in thickness, was first electroless plated with a Ni–P layer with the thickness of 5.4 μm, and followed by a thin Au plating of less than 0.1 μm thickness. Samples of 10 g master alloys were prepared from high purity element pellets (>99.9 wt.%) by vacuum arc remelting with the operation voltage of 60 V and 70–80 A. In order to measure the melting range of various solders, thermal analyses of solders were performed with a SETARAM Labsys® DSC (differential
Thermal analysis of the Sn–Zn based solders
Table 1 summarizes the DSC analysis of some Sn–Zn based solders upon heating cycles. It is reported that some degree of undercooling was observed in cooling cycle, and the degree of superheating upon heating cycle can be decreased by using a slower heating rate in the experiment [15]. Therefore, the heating cycle with the heating rate of 1 °C/min was chosen in the study of melting behavior. As discussed by Yoon et. al., there are three measured temperatures, Ts, To and TL, in their study [16].
Conclusions
- 1.
Based on the current experimental results, an appropriate flux is necessary for using most Sn–Zn based solders in microelectronic packaging.
- 2.
The solidus and liquidus temperatures of Sn–(6.8–9.8)Zn–(3.1–4.1)Ag solders are between 193.1 and 216 °C, which are higher than that of the Pb–Sn eutectic solder. With minor addition of Ag into Sn–Zn based solders, there is little change in their melting range. On the contrary, the melting point of Sn–Zn based alloys can be effectively decreased by In
Acknowledgements
The authors gratefully acknowledge the financial support of this study by the National Science Council (NSC), Republic of China under NSC grant 90-2216-E- 259-003.
References (19)
- et al.
Lead-free solders in microelectronics
Mater. Sci. Eng. R
(2000) - et al.
Solder joint fatigue models: review and applicability to chip scale packages
Microelectron. Reliab.
(2000) - et al.
Characterization of solder interfaces using laser flash metrology
Microelectron. Reliab.
(2002) - et al.
Stacked solder bumping technology for improved solder joint reliability
Microelectron. Reliab.
(2001) - et al.
Development of fluxes for lead-free solders containing zinc
Scripta Mater.
(1999) - et al.
Thermodynamics-aided alloy design and evaluation of Pb-free solders, Sn–Bi–In–Zn system
Acta Mater.
(1997) - et al.
Effect of solder creep on the reliability of large area die attachment
Microelectron. Reliab.
(2001) - et al.
Interface diffusion in eutectic Pb–Sn solder
Acta Mater.
(1999) - et al.
Thermal fatugue behaviour of J-lead solder joints
Microelectron. Reliab.
(1997)
Cited by (43)
Experimental determination and high-throughput calculation of the interdiffusion coefficient matrix and atomic mobility in Ag-rich fcc Ag–Sn–Zn alloys
2023, Calphad: Computer Coupling of Phase Diagrams and ThermochemistryA critical review on performance, microstructure and corrosion resistance of Pb-free solders
2019, Measurement: Journal of the International Measurement ConfederationCitation Excerpt :But Zn made solder is subjected to corrosion and oxidation [64,41–44] and therefore, Sn-Zn cannot be best alternative in replacing Pb-Sn solder alloy. Till today, many researches have been conducted to find the best alternative to Sn-Pb [71–89]. Among others, Sn-Ag-Cu seems to be the most promising one as it has low cost and low melting point.
Experimental investigation and thermodynamic calculation of the Co–Sn–Zn ternary system
2018, Journal of Alloys and CompoundsCitation Excerpt :However, in view of its adverse impact on both the health and environment, the use of Pb has been restricted by law [3], and therefore, the development of alternative lead–free solder alloys is indispensable. In this regard, Sn–Zn solders have been acknowledged to be potential candidates as substitutes for Pb–based solders [4–6]. Eutectic Sn–Zn alloys have a low melting temperature of 198 °C (similar to that of the Sn–Pb solder (183 °C)), show better mechanical strength and are cheaper.
Alloying influences on low melt temperature SnZn and SnBi solder alloys for electronic interconnections
2016, Journal of Alloys and CompoundsFormation and growth of intermetallic phases at the interface in the Cu/ Sn-Zn-Ag-Cu /Cu joints
2015, Journal of Alloys and CompoundsCitation Excerpt :For the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joint, no Cu3Sn phase is observed. The formation of two individual layers in the soldered interface (Fig. 2) could be interpreted as follows: since the Gibbs free energy (ΔG) for Cu5Zn8 (−31.44 kJ/mol) [8,24] is much lower than that (−27.94 kJ/mol) of the AgZn3[8] at 200 °C, the Cu5Zn8 IMPs layer, which is in a metastable state at high temperature [25,26], forms first during soldering [18]. Also, the diffusivity of Sn in Cu–Sn alloys is given by kSn = 1.90 × 10−10 cm2/s at 300 °C, and that of Zn in Cu–Zn alloys is kZn = 2.70 × 10−10 cm2/s at 300 °C [27].
Solid-State Interfacial Reaction between Eutectic Au-Ge Solder and Cu/Ni(P)/Au Metalized Ceramic Substrate and Its Suppression
2015, Journal of Materials Science and Technology