Microstructure evolution and mechanical strength evaluation in Ag/Sn/Cu TLP bonding interconnection during aging test
Introduction
The third-generation wide-bandgap semiconductors such as SiC and GaN have emerged as potential substitutes for traditional Si semiconductors, due to their excellent properties especially the stable electrical performance at high temperatures [1]. Particularly, SiC chip is able to work stably even at temperature up to 600 °C [2], [3]. That calls for suitable bonding approaches that are able to extract the best performance from SiC semiconductors and withstand high temperature in rigorous environments at the same time.
High-Pb solders are no longer preferred and gradually fading out of the industry since the ban of using Pb in consumer electronic products in 2006 [4]. Au-based solders represented by Au80-Sn20 solder have excellent properties such as good thermal conductivity and possibility for fluxless application [5]. However, the high cost and high melting temperature (280 °C) of these alloys limit their further development [6]. Nano‑silver sintering is a promising high temperature packaging method with advantages in good electrical conductivity and increasable melting point, but the porosity and Ag migration in bondlines will induce reliability issues during service [7], [8], [9], [10], [11].
One of the most promising methods, transient liquid phase (TLP) bonding has been developed for decades to address the problems when operating at high temperatures [12], [13]. TLP bonding requires completely consumption of the low-melting-point metal and formation of high-melting-point intermetallics. So far, many systems have been proposed such as Ag-Sn [14], [15], Cu-Sn [16], Au-In [17], Cu-In [18], Ni-Sn [19] and Au-Sn [20]. Among them, the most commonly used are Ag-Sn and Cu-Sn systems. Sn is the primary element in solders, Cu is the most common metallization on substrates, and Ag is usually used as the outermost deposition layer on a chip. In a TLP bonding process, after the sandwiched Sn has been completely consumed, the interconnection will be composed of Ag-Sn IMCs and Cu-Sn IMCs. What's more, in some cases, Ag is also used as a surface finish on Cu metallization layer [21]. After this thin layer of Ag is consumed, the remnant Sn will further react with Cu and thus forming an interface between Ag-Sn IMCs and Cu-Sn IMCs as well. It may arouse serious concern that whether this phase constitution will be vulnerable during long-term operation in high temperature due to the heterogeneity of different IMCs.
Only few studies have been reported on the reliability evaluation in Ag/Sn/Cu TLP bonding interconnections. Lin et al. [22] investigated the Sn/Ag/Cu interfacial reactions and found out that the Ag3Sn layer could retard the Sn/Cu interdiffusion and reduced the growth rate of the Cu–Sn IMCs, and the applied strain would enhance the growth rate of Cu6Sn5 phase, inducing a thicker Cu6Sn5 layer than the ordinary one. However, no information regarding the mechanical reliability and the microstructure evolution was included in this study when subjected to long-term service in high temperature conditions. In fact, the reliability of Ag/Sn/Cu interconnections is of great importance for the operation of power devices, though our attention has always be drown to accelerating reactions in a TLP bonding process. Shao et al. [23] investigated interfacial reaction and mechanical characterizations for Cu/Sn/Ag system TLP bonding at temperatures ranging from 260 to 340 °C and found out that during shear test, cracks initiated in the pores and mainly propagated along the Cu6Sn5/Ag3Sn interface.
In this study, microstructure evolution and phase transformation during aging test were studied in detail to further evaluate the reliability during long-term service in high temperature.
Section snippets
Experimental
Cu plates with a dimension of 10 mm × 10 mm × 3 mm and Ag plates with a dimension of 7 mm × 7 mm × 1 mm were ground and ultrasonically cleaned in acetone to remove oxidation and oil contamination. SAC0307 solder paste was stencil printed between Cu and Ag boards. The sample, as schematically illustrated in Fig. 1 (a), was placed on a heating plate and reflowed in air atmosphere at the temperature of 250 °C.
In order to maintain a suitable thickness of Sn layer, a pressure of 5 MPa was applied to squeeze out the
Microstructure evolution and diffusion behaviors during aging
The cross-sectional SEM images of the as-reflowed Ag/Sn/Cu TLP bonding interconnects were shown in Fig. 2. The IMCs layer was mainly composed of Ag3Sn and Cu6Sn5. Fig.2 (a) shows an earlier state in Ag/Sn/Cu reaction when Sn islands still exist. During this process, the Ag3Sn and Cu6Sn5 scallops impinged and merged with each other and thereafter produce a boundary without defects between Ag-Sn IMCs and Cu-Sn IMCs after Sn islands were consumed, as shown in Fig. 2(b). Fig. 3 shows the
Conclusion
- 1.
The microstructures of the as-reflowed and aged Ag/Sn/Cu TLP bonding interconnects were investigated. No defects such as voids were observed at the interface between Ag-Sn IMCs and Cu-Sn IMCs after reflow and aging test.
- 2.
The phase composition at the interface of as-reflowed samples was Ag/Ag3Sn/Cu6Sn5/Cu and transformed to Ag/(Ag)/ζ-Ag/Cu3Sn/Cu after aging test. Different characteristics of diffusion between Cu and Ag atoms were found in Cu/Ag/Sn TLP sample.
- 3.
The average shear strength of
Acknowledgement
This work is financially supported by the National Natural Science Foundation of China (no. 51375116) and the Science and Technology Project of Shenzhen (no. JCYJ20160318095308401).
Conflicts of interest
None.
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