Effects of minor Fe, Co, and Ni additions on the reaction between SnAgCu solder and Cu
Introduction
In the past few years, the SnAgCu family of solders has obtained a wide acceptance as a replacement for the PbSn eutectic solder in electronic applications. At present, relatively fewer activities are undertaken by researchers worldwide to develop another solder family to replace the PbSn eutectic solder. Instead, the current main research thrust in lead-free solder alloy development is on enhancing or fine-tuning the various properties of SnAgCu through adding minor alloying elements. For example, Ni was evaluated for its potential as a minor alloying element to Sn-based lead free solders [1], [2], [3], [4], [5], [6], [7], [8], and so were Ge [2], [5], Fe [9], Co [6], [8], [9], Zn [5], [10], [11], Bi [12], Mn [5], Ti [5], Si [5], and Cr [5]. These previous studies point out minor alloy additions can strengthen the solder performance in many different respects. One of the more noteworthy alloying elements is Ni. It was shown Ni addition to Sn3.5Ag (3.5 wt.% Ag, balance Sn) in amounts as minute as 0.1 wt.% could substantially hinder the Cu3Sn growth during soldering as well as during the following solid-state aging [1], [7]. The Cu3Sn growth had been linked to the formation of micro voids, which in turn increased the potential for brittle interfacial fracture [13]. Recently, it was shown that drop test performance increased for solders joints with just a small amount of Ni addition (<1 wt.%) [14].
The objective of this study is to examine whether Fe and Co additions have a similar effect as Ni does. Emphasis is placed on a systematic comparison study on the effect of Fe, Co, and Ni additions. Further, the level of alloy addition is reduced to as low as 0.03 wt.% so the minimum effective addition of each element can be assessed. Solders with simultaneous Fe and Ni addition as well as simultaneous Co and Ni addition are also prepared to examine whether there is any interaction between the alloying elements.
Section snippets
Experimental
Solder balls with six different compositions, Sn2.5Ag0.8Cu, Sn2.5Ag0.8Cu0.03Fe, Sn2.5Ag0.8Cu0.03Co, Sn2.5Ag0.8Cu0.03Ni, Sn2.5Ag0.8Cu0.03Fe0.03Ni, and Sn2.5Ag0.8Cu0.03Co0.03Ni (all compositions in weight percentage), were prepared from 99.999% purity elements. The solder ball compositions were verified using ICP-AES (inductively coupled plasma atomic emission spectroscopy), and it was estimated the reported compositions had a maximum 0.005 wt.% uncertainty. For a given solder ball composition,
Reaction during reflow
Fig. 1 shows the backscattered electron micrographs for the samples with different alloy additions that were reflowed for one time. The Cu6Sn5 phase is clearly visible in all cases. At this stage, the Cu6Sn5 phase was the only phase noted at the interface, and Cu3Sn was not observed. In the following, Sn2.5Ag0.8Cu will be denoted as SAC, and Sn2.5Ag0.8Cu0.03Fe will be denoted as SAC0.03Fe, etc. Comparing Fig. 1(a) to Fig. 1(b–f), one notes adding Fe, Co, or Ni to SAC made the amount of Cu6Sn5
Reaction during reflow
This study showed doping the SAC with a small amount of Fe, Co, or Ni would not change the type of the reaction product with the Cu substrates. The reaction product was always Cu6Sn5 during a typical reflow. The Cu3Sn phase only formed when the reflow time became very long [1] or during the solid-state aging. This differed from the reaction between Ni and the Cu-doped Sn-based solders, which was very sensitive to the Cu concentration [19], [20], [21], [22]. Under this situation [19], [20], [21]
Summary
- (1)
In the study of multiple reflow using the Sn2.5Ag0.8Cu, Sn2.5Ag0.8Cu0.03Fe, Sn2.5Ag0.8Cu0.03Co, Sn2.5Ag0.8Cu0.03Ni, Sn2.5Ag0.8Cu0.03Fe0.03Ni, and Sn2.5Ag0.8Cu0.03Co0.03Ni solder over Cu substrate, Cu6Sn5 was the only reaction product for all the different solders used.
- (2)
Reflows using the solder without doping produced a thin, dense layer of Cu6Sn5. Adding Fe, Co, or Ni transformed the microstructure into a much thicker Cu6Sn5 with many small trapped solder regions between the grains.
- (3)
The amount of
Acknowledgments
This work was supported by the National Science Council of R.O.C. through grant NSC-95-2221-E-002-443-MY3. The EPMA analysis performed by Ms. S. Y. Tsai of National Tsing Hua University is also acknowledged.
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2021, Materials Today CommunicationsCitation Excerpt :In contrast, although there is a little difference in thickness, the interfacial IMC layers in the multi-alloyed solder/Cu joints are thicker but show a loose porous structure (see Fig. 8(b)-(e)), and the Ni and Sb elements can be detected in the IMC layers, because Ni can replace some Cu atoms in the Cu6Sn5 and Sb can dissolve into the interfacial IMCs [3,10,19]. Formation of a porous interfacial IMC structure should be related with the addition of Ni, and similar phenomenon has been reported at the interfaces between Cu and Ni-containing solders by early literatures [20,21]. The rough IMC/solder interface will increase the mechanical bonding between the solder and the IMC layers, which will further affect the fracture behavior of the solder joint.